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Thin Solid Films, 102 (1983) 1-46 ELECTRONICS AND OPTICS

Review Paper

TRANSPARENT CONDUCTORS--A STATUS REVIEW

K. L. CHOPRA, S. MAJOR AND D. K. PANDYA Physics Department, Indian Institute of Technology, New Delhi 110016 (India)

(Received November 30, 1982; accepted December 7, 1982)

CONTENTS

1. Introduction 2. Deposition techniques

2.1. Evaporation 2.1.1. Post-oxidation of metal films 2.1.2. Reactive evaporation 2.1.3. Activated reactive evaporation 2.1.4. Direct evaporation

2.2. Sputtering 2.2.1. Reactive sputtering of metallic targets 2.2.2. Sputtering from oxide targets 2.2.3. Ion beam sputtering

2.3. Reactive ion plating 2.4. Chemical vapour deposition 2.5. Spray pyrolysis 2.6. Dip technique 2.7. Chemical solution growth 2.8. Other techniques

3. Structural, electrical and optical properties 3.1. Tin oxide

3.1.1. Undoped tin oxide films 3.1.2. Antimony-doped films 3.1.3. Fluorine-doped films 3.1.4. Other dopants

3.2. Indium oxide 3.2.1. Undoped indium oxide films 3.2.2. Tin-doped films 3.2.3. Other dopants

3.3. Zinc oxide 3.3.1. Undoped zinc oxide films 3.3.2. Indium- and aluminium-doped films

3.4. Cadmium stannate 3.5. Other transparent conducting oxides

0040-6090/83/0000-13000/$03.00 © Elsevier Sequoia/Printed in The Netherlands

2 K.L. CHOPRA, S. MAJOR, D. K. PANDYA

4. Salient features of electro-optical properties 5. Conduction mechanisms

5.1. Carrier concentration 5.2. Mobility

6. Optical effects 7. Applications of transparent conductors

7.1. Solar cells 7.1.1. Semiconductor/insulator/semiconductor solar cells 7.1.2. Transparent electrodes for heterojunction solar cells 7.1.3. Antireflection coatings for solar cells

7.2. Heat mirror 7.3. Reflector-absorber tandem for photothermal conversion 7.4. Opto-electronic devices 7.5. Thin film resistors 7.6. Gas sensors 7.7. Wear resistance applications 7.8. Other applications

8. Concluding remarks Acknowledgments References

Non-stoichiometric and doped films of oxides of tin, indium, cadmium, zinc and their various alloys, deposited by numerous techniques, exhibit high transmittance in the visible spectral region, high reflectance in the IR region and nearly metallic conductivity. The electrical as well as the optical properties of these unusual materials can be tailored by controlling the deposition parameters. These transparent conductors have found major applications in a vast number of active and passive electronic and opto-electronic devices ranging from aircraft window heaters to charge-coupled imaging devices. In this status review we present a comprehensive and up-to-date description of the deposition techniques, electro-optical properties, solid state physics ofthe electron transport and optical effects and some applications of these transparent conductors.

1. INTRODUCTION

The simultaneous occurrence of high optical transparency (more than 80%) in the visible region and high electrical conductivity (about 103 D- 1 cm- 1 or more) is not possible in an intrinsic stoichiometric material. Partial transparency and fairly good conductivity may be obtained in thin films of a variety of metals. The only way to obtain good transparent conductors is to create electron degeneracy in a wide band gap (greater than 3 eV) oxide by controllably introducing non-stoichiometry and/or appropriate dopants. These conditions are very conveniently obtained in oxides of cadmium, tin, indium, zinc and their alloys in thin film form, prepared by a number of deposition techniques. Ever since the first report of transparent

TRANSPARENT CONDUCTORS 3

conducting CdO films prepared by the thermal oxidation of sputtered cadmium by Badeker x in 1907, the technological interest in transparent conductors has grown tremendously. Numerous techniques for depositing several very useful transparent conducting oxides, notably antimony-doped tin oxide (ATO) or fluorine-doped tin oxide (FTO) and tin-doped indium oxide (ITO) have been developed, some at a large-scale production level. A host of electronic, opto-electronic and mechanical applications based on transparent conductors has emerged. These thin film devices include the following: resistors; transparent heating elements for aircraft and automobile windows; antistatic coatings for instrument windows; heat-reflecting mirrors for glass windows and incandescent bulbs; antireflection coatings; selective absorber components in solar heat collectors; gas sensors; electrodes for liquid crystal, electrochromic and ferroelectric photoconductor storage and display devices; CdS/Cu2S and amorphous hydrogenated silicon (a-Si:H) solar cells; semiconductor/insulator/semiconductor (SIS) heterojunctions; protective and wear-resistant coatings for glass containers.

With increasing sophistication of the active and passive devices based on transparent conductors, the need for improved electrical and optical properties and their understanding has been recognized for some time. In the last several years, the deposition techniques have undergone many changes, mainly as a result of the work of international research groups led by Bube, Bunshah, Manifacier and Chopra. In addition, an understanding of the solid state physics of these materials has begun to emerge. As a result, it is now possible to tailor make various transparent conductors with a range of properties. A maximum solar transmittance of about 85%-95% with a minimum resistivity as low as about 7 x 10- s t) cm is achievable, and still further improvements in these parameters are expected to take transparent conductors into the semimetal regime.

The early work until 1955 in this field has been reviewed by Holland 2. Reviews by Vossen 3 and Haacke 4 cover work till the mid-1970s. A more recent but very brief review has been given by Manifacier s. Major developments in this field as regards the deposition techniques, properties of the films so obtained and their understanding have been covered in scattered literature. This review--essentially a status report--aims at providing a comprehensive and up-to-date picture of this very challenging field of transparent oxide conductors.

2. DEPOSITION TECHNIQUES

A variety of thin film deposition techniques have been employed to deposit transparent conducting oxides, such as tin oxide (TO) doped with antimony (ATO) or fluorine (FTO), indium oxide (IO) doped with tin (ITO), zinc oxide (ZO) doped with indium (IZO) and cadmium stannate (CTO). Since the electrical and optical transport in these films depend strongly on their microstructure and stoichiometry and the nature of the impurities present, each deposition technique with its associated parameters yields films of different properties.

Depending on the deposition technique, the substrate can have a significant influence on the properties of the films. Glass substrates are most commonly used. The low temperature processes such as magnetron sputtering, ion beam sputtering and reactive ion plating allow the deposition of the oxide films onto polymer


substrates such as plastic, Mylar and poly(ethylene-terephthalate). The thermal expansion coefficient of the substrate affects 6' 7 the electrical properties of these films, especially in quenching processes such as spraying and dipping, in which intrinsic stresses are induced within the film. The thermal expansion coefficient has also been shown to affect the preferred orientation of crystallites in ZnO films a.

In processes which involve high deposition temperatures, the effect of the diffusion of alkali ions from the glass substrate to the film becomes extremely important 9-~2. The cations act as p-type doping agents and compensate the native donors. The problem is severe in the case of soda-lime glass substrates, which have a high sodium content (about 10-15 wt.% Na20).

Recently, the presence of sodium chloride microcrystals on the film surface has been observed ~3 in SnO2 films grown by chemical vapour deposition (CVD) on soda-lime-silica glass. To overcome the problem due to alkali ions a special cleaning procedure for the glass substrates must be adopted, which involves leaching with nitric acid to dissolve the alkali on the surface, or an inert interfacial layer of another oxide must be deposited.

In Sections 2.1-2.8 the commonly used techniques are discussed with the emphasis on the role of various deposition parameters.

2.1. Evaporation Vacuum and also reactive evaporation using metallic or oxide sources have

been employed to deposit various transparent conducting films. The important control parameters are the evaporation rate, substrate temperature, source-to- substrate distance and oxygen partial pressure.

2.1.1. Post-oxidation of metai films Thin films of TO 1~16, IO 17 and ZO 18 have been prepared by the oxidation of

the respective metal films. The conductivity and the transparency are controlled primarily by the oxidation temperature, which is typically in the range 350-500 °C. The resistivity plotted as a function of thickness shows a minimum (typically at about 500A for TO films is) and increases thereafter, possibly as a result of the incomplete oxidation Of thicker films.

2.1.2. Reactive evaporation Reactive evaporation of TO19, 102o. 21 and ITO 22 films has been achieved by

the vacuum evaporation of the corresponding metal or alloy in an oxygen atmosphere onto substrates heated to about 400°C at rates of about 100-300 A min-1. An increase in substrate temperature results in a decrease in carrier concentration 21 while the mobility 21 increases, possibly owing to better crystal- linity2X. 22. The resistivity of these films decreases sharply with increasing oxygen partial pressure and begins to increase slowly beyond Po2 ~ 10-4 Torr 22

2.1.3. Activated reactive evaporation In the activated reactive evaporation (ARE) method, the reaction between the

evaporated species and the gas is activated by establishing a thermionically assisted plasma in the reaction zone. A dense plasma is generated in the reaction zone by employing a thoriated tungsten emitter and a low voltage anode assembly. The technique has been used to deposit excellent quality transparent conductors of ITO 22-24, ATO 25 and ZO 26. Typical conditions for the growth of ITO 23 films by ARE are as follows: source, an alloy of composition In-15wt.%Sn; reactive gas


composition, Ar-15~oO2; pressure during deposition, about 10-4 Tort; deposition rate, about 500 ~ min- 1 ; substrate temperature, about 370 °C.

2.1.4. Direct evaporation Thin films of IV 2~-31, ITO 29'31-35 and ZO 36 have been deposited by

thermal2a. 29.31.32 or electron beam 3°' 34-36 evaporation of oxide sources. Owing to its high melting point TO has been deposited by electron beam 12' 37.38 and flash 39 evaporation methods. When pure or mixed oxides are evaporated, they reduce and form opaque films of lower-oxides. Transparent conducting oxides are hence obtained either by the post-oxidation of lower oxide films 29'3°'32'37 or by the introduction of oxygen 2s' 31,33-35 during evaporation.

The kinetics of the oxidation of suboxides and also the dependences of the resistivity and the transmission on the substrate and oxidation temperatures have been studied for ITO films 29' 32. The opaqueness of the as-deposited films has been ascribed to the presence of free indium in an In20 3 matrix, which results from the reduction of In20 species that are present in the vapour phase. The post-oxidation of the free indium is via a diffusion-controlled process and the final electrical and optical properties of the films are determined by the relative rates of oxidation and diffusion of the indium.

In the case of oxidation during deposition, the substrate temperature and the oxygen partial pressure are the main control parameters for the process; they have been studied by Mizuhashi 31 who obtained good quality ITO films at an oxygen partial pressure of about 10 -4 Torr and a substrate temperature of about 400 °C with a rate of deposition of about 50 ,~ min- 1.

In contrast, Hamberg et al. as have shown for electron-beam-deposited ITO films that the resistivity and visible absorption decrease sharply with increasing substrate temperature, resulting in the deposition of highly transparent and conducting films at temperatures as low as 150-200 °C. The rate of deposition in this technique is typically in the range 50-150 ~ min- 1.

From the above it can be concluded that, to obtain highly transparent and conducting films by evaporation, higher partial pressures of oxygen (sufficient to oxidize almost all the indium and/or suboxide species) in combination with correspondingly adequate substrate temperatures and fast evaporation rates (to suppress the dissociation of In203) are necessary.

2.2. Sputtering Sputtering is one of the most extensively used techniques for the deposition of

transparent conducting oxide films. Both reactive and non-reactive forms of d.c. and r.f. sputtering and, recently, magnetron and ion beam sputtering have been used.

2.2.1. Reactive sputtering of metallic targets Thin films ofCdO 4°~s, TO 49-5't, ATO 54-5~, 10 40. 52, 58-62, ITO4S. s4, 57.63-71

and CTO 72-74 have been reactively sputtered. Ar-O2, N2-O 2 or 02 are generally used as the sputtering gas, although in some c a s e s H 2 has been added to reduce the target and/or film 75. Typical conditions for the d.c. reactive sputtering of a TO film are as follows: sputtering pressure, 10- 2 Tort; reactive gas mixture, Ar: O2 (1:1); d.c. voltage, about 1-3 kV; discharge current, about 10 mA for a tin cathode 5 cm in diameter. The reactive sputtering rate is typically in the range 100-300 ~ rain-1 when an Ar-O 2 mixture is used and is about 50 ~ rain- 1 for pure 02.

6 K. L. C H O P R A , S. MAJOR, D. K. P A N D Y A

As-sputtered films are usually 5°' 52.55 (although not always 49' 53) amorphous, especially if deposited on unheated substrates. Elevated substrate temperatures or a post-deposition heat treatment results in polycrystalline films. These films usually exhibit strong preferred orientation effects, which depend primarily on the sputtering rate and sputtering pressure 49' 5~

The dynamics of the reactive sputtering process has been studied in detail and the conditions which determine the appearance of various phases (tin, SnO and SnO2) in the film, and hence control the stoichiometry, have been identified 49' 54

In order to obtain the optimum transparent conducting properties, the reactively sputtered oxide films normally require a post-deposition heat treatment in either oxidizing 4°' 48.50.25.64 or reducing 58' To ambients, depending on the initial sputtering conditions.

2.2.2. Sputtering from oxide targets Sputtering from oxide targets to form transparent conducting oxide films

renders a far better control over the stoichiometry, obviating in most cases the post- deposition heat treatment step. Highly transparent and conducting films of TO76-8o, ATOSl-85, iO86, ITOS7 lo4, ZOLO5 lO9 and CTO 11° 11s have been

deposited by this technique. Typical conditions for the growth of ITO films 92 are as follows: target composition, In203-15mol.~oSnO2; argon pressure, 10- 2 Torr; 02 pressure, (3-4) x 10- 5 Tort ; argon flow rate, 20 cm 3 min - 1 at standard temperature and pressure; r.f. power, 550 W; substrate temperature, 450 °C. The deposition rate is generally in the range 100-200 A, min-1, although higher rates are obtained by employing a magnetron sputtering arrangement 1 oo- lo 5.

Hot-pressed, pure or mixed oxide targets are generally used. The target's porosity determines its susceptibility to contamination or reduction. Hence porous targets usually need an extensive presputtering or sputtering in an oxidizing ambient 9°. Significant improvements in crystallinity 1°2, mobility 92'1°z, visible transmittance92, :o: and IR reflectance ~°~ have been reported due to the addition of oxygen in the sputtering mixture over a very narrow range of pressure (typically, an 0 2 pressure 0f(3-4) x 10- 5 Torr and an argon pressure of 10- 2 Torr92).

Films deposited onto unheated substrates tend to be amorphous ~°2' 11o. 1~ and increasing the substrate temperature improves the crystallinity and grain size and decreases the density of structural defects, leading to higher mobilities s6' 91. It should be noted that the optimum values of the substrate temperature obtained by various workers vary over a wide range from 150 to 500 °C.

The application of a negative bias to the substrate can increase the purity of the film by resputtering poorly bonded surface atoms, resulting in improved electrical and optical characteristics 86' lo7.

2.2.3. Ion beam sputtering Ion beam sputtering from oxide targets has been successfully employed to

deposit films of TO 5°, ITO 119"12° and MoO 3 121. Unlike r.f. sputtering, this technique involves minimal intrinsic heating and electron bombardment and hence constitutes a low temperature deposition method which can be useful for several applications of transparent conductors. Fan ~ 19 has studied the effect of the oxygen partial pressure on the electrical and optical characteristics of ITO films deposited onto Mylar substrates at low temperatures (about 80°C). Under optimum conditions of deposition from a target of composition In203-12mol.~oSnO2, films of


resistivity p ~ 5.5 x 10 -4 ~ cm, visible transmittance T ~ 80% and IR reflectivity of about 909/0 have been obtained.

2.3. Reactive ion plating Reactive ion plating is a promising alternative to the high temperature

deposition processes and has been employed to deposit films of IO 122-125 , ITO 12.--128, fluorine-doped IO 129'13° and CTO 126 at temperatures as low as room temperature 12°-12s. The. technique is essentially a reactive evaporation 122.123,12 s. t 27.129 or magnetron sputtering 123-126 ofmetals or alloys, aided by a low power r.f. discharge (about 100 W) directed towards the substrate. The resulting bombardment of energetic ions or atoms provides energy to the surface of the substrate and of the growing film, obviating the need for substrate heating. The presence of ionized reactive species also facilitates their reaction with the evaporated or sputtered atoms. The influence of various control parameters such as negative substrate bias, residual gas composition and deposition rate on the properties of oxide films has been studied in detail123.126.127. The resistivity exhibits a minimum with increasing substrate bias. Typical deposition rates for evaporation and magnetron sputtering are 1000 /~min -1125'127 and 100-500 /~min -1126 respectively.

2.4. Chemical vapour deposition Chemical vapour deposition (CVD) consists of a surface reaction, on a solid

surface, involving one or more gaseous reacting species. Metallic oxides are usually deposited by the vaporization of a suitable metal-bearing compound (which is volatile, thermally stable at a temperature sufficiently high to produce an adequate vapour pressure and thermally unstable at higher temperatures of deposition-- criteria generally fulfilled by organometallic compounds) and its in situ oxidation with 02, H20 or H202 131.02, N2 or argon are generally used as carrier gases.

Thin films of TO 131-149, ATO 9, IO t 5°-152, ITO 1 s3-15s and ZO s' 1Sl. 156 have been deposited by CVD. Organometallic compounds suitable for the CVD of in20 3 150-153 are uncommon and expensive. However, SnO 2 films have been prepared by the pyrolysis of tetramethyltin {32' 141, dimethyltin dichloride 13s and dibutyltin diacetate 9, 137 and by the pyrolysis t 32,139, 14.2,145, 1,t.6 and hydrolysist31-133.136.14o of both stannous and stannic chlorides. ZnO films have been deposited by the oxidation of zinc propionate TM or diethylzinc s' 156. Some heterogeneous reactions leading to the formation of SnO 2 are

SnCl4(g) + O2(g) "~' SnO2(s ) + 2C12(g)

(CH3)4Sn(g) -I- 802(g ) --~ SnO2(s ) + 6 H 2 0 ( g ) + 4CO2(g )

SnCl4(g ) + 2H20(g ) --, SnO2(s ) -t- 4HCI(g)

The mechanism of film formation via the above reactions has been thoroughly examined by Ghoshtagore 1a2. In all cases, the reaction of SnCI4 or (CH3)4Sn has been found to take place at the surface with an adsorbed oxygen atom or water molecule (Rideal-Eley mechanism). Some uncertainties still remain over the hydrolysis reaction 131-133. In pyrolysis, homogeneous volume phase reactions affect the deposition rate significantly at higher temperatures, whereas no such


evidence was observed for the hydrolysis reaction 132. The homogeneous volume phase reaction is highly undesirable for all CVD processes and may result in a haze 13a on the film surface. Tabata et al. 138 have used a reflecting high speed gas stream CVD to remove this haziness.

The main control parameters are the gas flow, the gas composition, the substrate temperature and the geometry of the deposition apparatus. The deposition rate depends mainly on the gas flow rate and substrate temperature. Typical rates are about 300-1000 /~min -1, although much higher rates have also been reported i 36,138. As expected, the film conductivity decreases 1 a 1,141 with increasing 02 or H20 concentration. The substrate temperature is generally in the range 350-400 °C. Higher temperatures favour better crystallinity131.139 and larger grain size 131'139'14°, resulting in higher mobilities 131. The geometry of the deposition system is of the utmost importance, especially for large-scale applications. Recent developments in this direction are significant 135,136. Kalbskopf 136 has described a system with a deposition rate of about 1 tams -1 producing TO films with 19 ~ 3 x 10 -4 ~ cm and T ~ 80~o.

In general the quality of transparent conductors prepared by CVD has been slightly inferior to that of sputtered or reactively sputtered conductors. Better results are expected with the plasma-assisted CVD process. Ghandi et al.l ~ 7 have reported highly transparent films of SnO2 deposited at room temperature and based on the reaction of r.f.-plasma-activated oxygen with tetramethyltin.

2.5. Spray pyrolysis Spray pyrolysis 158'159 involves spraying of a solution, usually aqueous,

containing soluble salts of the constituent atoms of the desired compounds onto heated substrates. The hydrolysis reaction mentioned earlier is normally involved. Whether or not the process can be classified as CVD depends on whether the liquid droplets vaporize before reaching the substrate or react on it after splashing 16°. Several workers have used preheating (temperature, about 200-500 °C) of the sprayed droplets to ensure vaporization of the reactants before they undergo a heterogeneous reaction at the substrate 39" 161,16i. The technique is very simple and is adaptable for mass production of large-area coatings for industrial applications. Various geometries of the spray set-ups are employed 15s' 159,163,164, including an inverted arrangement 163,164 in which larger droplets and gas phase precipitates are discouraged from reaching the substrate, resulting in films of better quality.

Transparent conducting films of T O 39' 161-163,165-169, ATO 11,169-178 and FTO 6,1 o. 164,179-184 have been deposited by spraying an alcoholic solution usually of SnCI4 and sometimes of SnCI 2 172, SnBr4 16a or tetrabutyltin 168. Antimony doping is achieved by adding SbC13 to the spray solution. Fluorine doping is achieved by adding NH4F and sometimes trifluoroacetic acid 1° or HF 164. Similarly, InCla is used as a source of indium in I O '185"186 o r I T O 10'172'179'184'186-189 films. Tin doping is achieved by adding SnCI4. Figure 1 shows the variation in the dopant concentration in the solution with that in the film for three cases. In SnO2: Sb films on borosilicate glass, the antimony concentration in the film is about 5~o-50~o less than that in the solutionla2. In the In2Oa:Sn films on silicon the tin concentration in the film is identical with that in the solution 1°. However, in In20 3 :Sn films on Pyrex, the dopant concentration in the film is higher than that in the solution 1°. No


"~i t ! 0.10 o I ' I ~ '

° o o05 o.,o . 0 2 0

00PANT CONCENTRATION IN FILM (at.rotio) Fig. 1. Dopant concentration in the spray solution vs. dopant concentration in the film: O, antimony-to- tin ratio in SnO2: Sb films on borosilicate glass; O, tin-to-indium ratio in In20 a: Sn film on silicon; x, tin- to-indium ratio in In203 :Sn film on Pyrex. (After refs. 10 and 182.)

satisfactory explanation for this variation with the nature of the substrates has been found. In SnO2:F films the fluorine concentration in the film is generally much lower than that in the solution s' to, 181 (typically 0.8 at.% corresponding to 65 at.% in solutiont81). This has been attributed to the escape of thermal decomposition products of NH4F into the atmosphere. Films of ZnO 190-192, ZnO:in(iZO)193 and ZnO:AI 194 have been deposited using solutions of ZnCI2 or Zn(CH3COO)2. Films deposited from acetate solution exhibit better optical quality. Aluminium or indium doping in these films is achieved by adding the corresponding chloride. CTO195.196 and zinc stannate 197 films have been prepared by spraying appropriate combinations of SnCI4-CdCI2 and SnC14-Zn(CHaCOO)2 respectively.

In all these cases, H20 is the most common convenient oxidizing agent, but it is usually helpful to dilute the solution by adding ethanol or, in some cases, methano118o, propano1169.181 or butyl acetate lss'lsT. These solvents have high volatilities and low surface tensions, and they yield comparable results. The addition of pyrogallol promotes reduction 172. The addition of a small quantity of HCI lO,161.162,176.195 OF CHaCOOH 193 improves the film quality 193'195. N2, O2, argon or air are generally used as carrier gases and yield comparable results.

Important control parameters are the nature and temperature of the substrate, the solution composition, the gas and solution flow rates, the deposition time and the nozzle-to-substrate distance. Viguie and Spitz 16° have investigated the role of the substrate temperature in determining the exact mechanism of film formation, which in turn affects the film microstructure. Films grown at low temperatures (about 300°C) are amorphous 1°'16z. Higher temperatures yield polycrystalline filmslO. 194. When the substrate temperature is increased, the grain size increases, but the preferred orientation effects 162' 164 are found to decrease. The variation in the non-stoichiometry of films with the substrate temperature passes through a maximum and correspondingly a minimum is observed in the resistivity for pure SnO 2 films 39. As a typical case, the variations in carrier concentration N, mobility p and resistivity p with substrate temperature for sprayed FTO films are shown in Fig. 2. The increase in resistivity at high temperatures is attributed to substrate contamination by alkali ions and/or an improvement in stoichiometry. Above a

10 K. L. CHOPRA, S. MAJOR, D. K. PANDYA

certain temperature of deposition, the optical transmission remains weakly affected, although higher temperatures favour slightly better transmission is°. The optimum substrate temperature is about 450-550 °C.

8

z 3,00

// \ J o/// \\\

~0C 5 0

SUBSTRATE TEMPERATURE (% I

2,.

J 4 4 J

Fig. 2. Variations in the resistivity (/\), carrier concentration (IS]) and mobility ((3) with substrate temperature for sprayed FTO films (fluorine concentration in the solution, 65 at.%). (After ref. 181.)

The sheet resistance initially decreases with increasing carrier g a s 39 o r liquid 180 flow rate but saturates later. Typically, a gas flow rate of about 5-101 min- ~ at a pressure of about 0.5 kgf cm- 2 with a solution flow rate of about 5-20 cm 3 min- 1 are commonly used. The film growth rate depends on all these parameters and is typically in the range 1000-2000/~ min- ~ ; however, rates as high as 5000 ,~ min - are achievable. The nozzle-to-substrate distance also affects the film quality.

The size of the droplets and their distribution affect the uniformity of the surface and the transmission of the films la4. The typical droplet size is between 1 ~tm and a few microns. To achieve better uniformity in droplet size, Viverito et al. 19s have used an improved pyrolytic system in which the solution is dispersed to form a fog and, with the aid of suitable obstructions, larger droplets are returned to be recycled. Recently, Blandenet et al. ~84 have used an ultrasonic atomizing system to produce a far narrower distribution in droplet size compared with that in pneumatic atomization, leading to better surface uniformity and better optical transmission in In20 3:Sn and SnO2:F films. In addition, an airless spray technique ~99 has been used to deposit TO films of high transparency and conductivity (Rsh ~ 10 O/I-q; T ~ 85%).

2.6. Dip technique

The dip technique z°° zoz consists essentially of inserting the substrate into a solution containing hydrolysable metal compounds and pulling it out at a constant speed into an atmosphere containing water vapour. In this atmosphere, hydrolysis and condensation processes take place. Finally, the films are hardened by a high temperature cycle to form transparent metal oxides. Any hydrolysable compound including halides or nitrates but preferably metallo-organics are suitable for this


process. A typical reaction in the case of metal alkoxides is

AI(OR)3 + 3H20 , AI(OH)3 + 3ROHT

2AI(OH)3 hea t A I 2 0 3 + 3H20 T

The important control parameters are the viscosity of the solution, the pulling speed and the firing temperature. The rate of heating also needs to be controlled to avoid cracking of the films.

The technique has been used commercially to deposit large-area coatings (about 10-12 m 2) of various metal oxides including SnO 2 and In20 3 for a multitude of applications 2°2. It has also been used to deposit mixed oxides such as SiO2_TiO 2 2ol whose refractive index could be varied over the range from 1.45 (SiO2) to 2.20 (TiO2) depending on the composition. It should be possible to deposit ternary transparent conducting oxides such as CTO and zinc stannate by this technique.

2.7. Chemical solution growth The chemical solution growth technique, well known for chalcogenide films,

has been extended to the deposition of oxide films (Mn203, F e 2 0 3 , ZnO and S n O 2 )

in our laboratory. Metal ions in aqueous solution, under certain conditions, form solid phases of hydroxides M(OH) n or hydrous oxides ( M 2 O n ' x H 2 0 ) which on heating yield the corresponding metal oxides. The major parameters which control the deposition process are the composition of the deposition bath, its pH and its temperature.

Transparent conducting films of TO 2°3 have been deposited that have a resistivity of about 102f~ c m and an average visible transmittance of about 70%. Indium doping in these films is achieved by the diffusion of the metal, resulting in a decrease in the resistivity to about 10-2 f~ cm along with a visible transmittance of about 80%. These films have a carrier concentration of about l02° cm-3 and a mobility of about 3-4 c m 2 V - 1 s - 1.

2.8. Other techniques Several other techniques used to deposit transparent conducting oxide films

include the following. (1) Reactive triode sputtering of In-(10%-20%)Sn alloy targets in an Ar-O2

mixture has resulted in ITO films that have a resistivity of about (6-7) x 10-4 fl cm with an average visible transmittance of about 80% 204.

(2) TO and ATO films have been deposited by the glow discharge decomposition of SnCI2 2os, Sn(CH3)4 206 or SnCI4 206 in oxygen. Best results were obtained for SnO2 films doped with 0.06 at.% Sb and 0.33 at.% C1, which had a resistivity of about 1.77 x 10 -2 f lcm and an average transmittance of about 90% 2o6.

(3) Anodization for TO s° films and screen printing for IO 207, 208, iTO2OS and C d O 2°9 films have also been examined.

3. STRUCTURAL, ELECTRICAL AND OPTICAL PROPERTIES

The structural, electrical and optical properties of transparent conductors

12 K . L . CHOPRA, S. MAJOR, D. K. PANDYA

exhibit a marked scatter. This is not surprising, considering the diverse nature of the techniques employed and the strong influence of the deposition conditions on the properties of these films. Two parameters which are practical measures of the conductivity and transparency are the sheet resistance Rsh and the average visible transmittance T. R,h is related to the resistivity by

p ( t ) 1 R , h - -

t - - ~r(t)t

It should be noted that the dependence of tr on the film thickness t is implicit, through stoichiometric and microstructural changes with changes in film thickness.

The average transmittance of an unsupported film at any wavelength is given by

T~ = (1 - R ; . ) 2 exp( - ct~t)

where R~ is the reflection coefficient and ct~ is the absorption coefficient which in the visible region is primarily due to free carriers.

The phenomena of conductivity and transparency are quite strongly interrelated, leading to a certain trade-off between the two. The exact nature of this interdependence is dictated by the influence of various deposition parameters on the electro-optical properties of these films. However, for films deposited under optimum conditions both Rsh and T depend significantly on the film thickness. Figure 3 shows this dependence for FTO and ITO films.

I r

60

_c o 60 0"- ~ o.... "~" " ~

uw) ~ " c) ~ . .

u'l t~

I I I 0 0.5 1 15

T H I C K N E S S ( N m )

Fig. 3. Variat ions in the sheet resistance ( F T O (©) and I T O (O) films. (After ref. 180.)

100

8o L~ ,u Z

60 ~- ~3 2"

z.C

) and average visible t ransmit tance (- - -) with thickness for

In the following, a brief description of the properties of prominent oxide films will be presented. The well-established electrical and optical parameters of these films are listed in Table I. Figure 4 presents graphically the transparent conducting properties of some of these oxides in terms of their sheet resistance and average visible transmittance.

TA

BL

E 1

EL

ECTR

ICA

L A

ND

O

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14 K . L . C H O P R A , S . M A J O R , D . K . P A N D Y A

1001 ' I ' I 1 I

| , , o

( y) (Spat)

/ \ t t / r ,,0 /(e-Beam) 901-1T_0_.(Im-T ~ .i- 0 '~ IZ0 ZO(ARE)

; I"~' ~ 7 ; ,0 4ssp~,)t • u / .~sp~y) ! I •ZTo Z i_ oATO • W, ~ ( Ion P~t9. )

• (E V(:Np.) ( EV{XP.]I4iH, ~ , ATO CTO /

tj') ( ~ t t . ) • ~ , (Spray} • Z 80 ITO Q: ( Ion ~ a m ) • m--

Cd Sn 03 (Sprcty)

70 J . I J I 10 20

1 , 1 , 1 , 1 1 1 " ] . 1 . 1 , 1 , 1 , 1 " 1 '

TO (CVD)

I n203 :F • (Ion Pltg.} AT0

(SPut t )

ZO ( MQg. %putt )

,•t•O (Spra~)

CTO (Spray) /

SnO 2 • In (Sol Gr.)

i I J 1 I i , I L 1 ~ 1 , I . l . t , l , l , l , l , I 4o 60 ao lo0 14o

SHEET RESISTANCE ( o h m l o )

I~K)

Fig. 4. Values of R,h and T typically obtained for some of the useful transparent conducting oxides prepared by various techniques. The usual abbreviations for the different techniques have been used. The bars show the fluctuations in comparable data reported by various workers.

3.1. Tin oxide TO (SnO2) is the first transparent conductor to have received significant

commercialization. A vast body of literature, including a comprehensive review 21°, exists on the preparation 147" 211, defect structure 21 z. 213 and electrical and optical properties213-216 of single-crystal TO.

3.1.1. Undoped tin oxide films Under opt imum conditions of deposition undoped TO films are generally

polycrystalline (grain size, about 200-300 A 162.164,169) and retain the tetragonal rutile structure va' 162,169 of bulk SnO 2. The existence of a preferred orientation of the crystallites and its dependence on substrate temperature 139' 162 and on oxygen partial pressure 49 (in the case of sputtering) is well established, although reports on the exact nature of the orientation lack consistency. As already mentioned, the presence of the SnO phase 49' s 3 and the stoichiometry53 in reactively sputtered films depend on several deposition parameters. For films prepared by C V D or by spraying, the reports on the presence of the SnO phase are quite conflictinglO. 14o. 16z, 194

The n-type conductivity of undoped SnO 2 is primarily due to its non- stoichiometry, but in'films deposited from chlorides the incorporation of chlorine ions into the lattice 14°'14a also contributes to the conductivity. SnO2 films are degenerate semiconductors, typically with a carrier concentration N ~ 1019-1020 cm -3, a mobility /~ ~ 5-30 c m 2 V -1 s -1 and a resistivity p ~ 10-3 -10 -2 f~cm (corresponding to a sheet resistance of about 100-500 f]/F]). Shanthi et al. 217 have reported that annealing of SnO2 films at 400 °C in oxidizing or reducing ambients causes a large change in p while N remains weakly affected. Figure 5 shows the typical behaviour of p on annealing in vacuum and air. It is concluded that the chemisorption or desorption of oxygen, primarily from the grain boundaries, controls this behaviour and accounts for the lack of thermal stability of the electrical parameters.

T R A N S P A R E N T C O N D U C T O R S 15

E L-

t--

t~ o:

' I I ' A 3.4 E , ( .__

10 ~ • ~ 3.c

~ 'L6

8 ~ ~ 22

4 ~

I 4'

0 | i I , I ~ I , I 1 0 0 2 0 0 3 0 0 400

TEMPERATURE (%)

Fig. 5. The variat ion in resistivity with temperature for spray-deposited undoped SnO2 films on anneal ing in vacuum (e ) and air (©) . The inset shows the variat ion in resistivity with temperature for S n O 2:Sb films on anneal ing in air. (After ref. 217.)

Pure SnO 2 films exhibit a direct optical band gap of 3.87-4.3 eV as reported by various workers 19.144,169,17 s. The visible and near-IR transmittance is high (about 80%), with a plasma edge reported at about 3.2 Inn 194. The effective mass m* of conduction electrons is in the range (0.1-0.2)m= 163,169. The refractive index is fairly constant in the visible region and ranges typically between 1.8 and 2.0 39.166,167,169

The hardness of TO films on Moh's scale is 7-8 (as for topaz) 18a. The films are extremely stable under typical environmental conditions and extremely resistant to chemical etching. In fact, the films are etched only by nascent hydrogen produced by reaction of zinc powder with HCI. Their adhesion to glass or porcelain is very high (about 200 kgf cm-2)166; hence the film is possibly attached to the substrate by chemical bonds.

3.1.2. Antimony-doped filrns Doping of SnO2 with antimony results in a larger grain size (about 600 A) with

no change in the lattice parameters 169. Clearly, the dopant is incorporated substitutionally. Doping also affects the preferred orientation of crystallites 170.171. The doping of SnO2 with antimony gives rise to a donor level at 35 meV z 14. Figure 6 shows the variations in p and T with antimony concentration for sprayed SnO2:Sb films. The resistivity decreases initially with increasing dopant concentration but starts increasing at still higher concentrations. This is attributed to impurity scattering 21s, precipitation of Sb205 17s or an increased disorderS3,169-171. The optimum antimony concentration ranges between 0.4 and 3m01.% 169-171.17s,176. However, for reactively co-sputtered SnO2:Sb films Lehmann and Widmer 57 have obtained an optimum value of 7 mol.% Sb in SnO2.


Typically, ATO films have N > 102°cm -3, /~,,~ 15-30 c m 2 V - l s -1 and p ~ 10-3 fl cm (Rsh ~ 20-100 ~/[-]) with T ,,~ 80%-90%. Randhawa et al. 25 have reported the best results for these films (p ~ 5 x l0 -4 D cm; T ~, 95%) prepared by ARE. Shanthi et al.217 have observed an annealing behaviour similar to that of pure S n O 2 films, except that the changes in electrical parameters take place over a much smaller scale (see Fig. 5, inset), showing that these films have improved thermal stability.

Antimony doping in S n O 2 films results in a large shift in the fundamental absorption edge 169,17s, 1 s2, which is attributed to the Moss-Burstein effect 2~9 (Fig. 7). The increase in carrier concentration due to doping also shifts the plasma edge to shorter wavelengths, typically from 3.2 ~tm for pure SnO 2 to 1.3 ~m for SnO2 films doped with 3 at.% Sb 169. The effective mass of conduction electrons also increases from 0.1me for pure SnO2 to 0.29me for films with 3 at.% Sb (N ~ 5 x 102o cm-3). Such a large change in m* indicates non-parabolicity of the conduction band. _

Typical spectral dependences of n and k for TO and ATO films are shown in Fig. 8. A decrease in n takes place on doping 169. At high doping levels T decreases 9, 2~. 182 (see Fig. 6). The IR reflectance is about 75%-85% (around 5 lam), which is not very high, possibly as a result of smaller mobilities. The normal thermal emittance e± (at 100 °C) of ATO films is about 0.17 183

3.1.3. Fluorine-doped f i lms Fluorine-doped SnO2 films are polycrystalline (grain size, about 400/~ s. 181)

and retain the rutile structure with no change in lattice parameter 181. These films exhibit strong orientation effects, with (200) planes parallel to the substrate. Figure 9

24

20

o v

~8 bo

4

F-CONCENTRATION IN SOLUTION ( o r . * / * )

20 40 60 80 I I I 1

\

' \ X

x \

I I 1 t I 2 4 6 8 10

S b - C O N C E N T R A T I O N IN SOLUIION ( a l . % )

80

kU

20

5.0

~ 4 5

c9

~ 4 o

0 3 5 00 10

J i k l i i ~ J

0 5

N 213 ( 101/cnT 2 )

Fig. 6. Variations in resistivity ( ) and visible transmittance ( - - - ) with dopant concentration for sprayed SnO2: Sb (O) and SnO 2 :F (O) films. (After refs. 169 and 181.) Fig. 7. Variation in the optical band gap with carrier concentration, showing the Moss-Burstein shift in sputtered In203:Sn (O) and sprayed SnO2:Sb (Q) films. (After refs. 93 and 182.)


~o

I i I - -

\

20 1 / o - - - o - - - - -o- . . . . -o 20 (a) 0

"/ EJI~ ~

/ z (b)

o ° x (200) I 0 ~---~ cz: (310~(211)

y, (c) o

0.2 0 G 1 0 ~ c 1B ~O0 80 60 40 20

PHOTON ENERGY ( ¢ V ) ~ 2 0 (DEGREES)

Fig. 8. Spectral dependence of the refractive index (- - -) and extinction coefficient ( ) for sprayed TO (O), 3at.%Sb-doped TO( x ) and 65at.%F-doped TO (@) films. The dopant concentration refers to that in the solution. (After refs. 169 and 181.)

Fig. 9. X-ray spectra of(a)-(o) SnO2:F films about 0.5 pxa thick sprayed at substrate temperatures of(a) 350 °C, (b) 400 °C and (c) 500 °C and (d) for SnO2:F film 1.5 pm thick deposited at 500 °C. (After ref. 5.)

shows the X-ray diffractogram for SnO2: F films sprayed at various temperatures ~. It is clear from Fig. 9(d) that for thick samples (about 1.5 pro) the preferred orientation of crystallites is significantly diminished.

Figure 6 includes the dependence of p and T on fluorine concentration in sprayed SnO2:F films' s,. These films generally have higher mobilities (about 25- 50 cm 2 V- 1 s- ') than TO or ATO films. Manifacier et al. '79 have reported a slight decrease in p when SnO2 is doped with fluorine. N is typically about 5 x 102°-1021 cm -3 with p ~ 4.6 × 10 -4 ~crn and T ~ 80%-90%. Whereas no change in # has been observed .79 on heat treating these films at about 400°C in argon or air ambients, a large decrease in # has been reported 6 as a result of heat treatment in vacuum at 400 °C. In general, annealing in air produced only marginal changes in N,

and p. Highly transparent and conducting films of FTO have been prepared by spray

pyrolysis. Figure 10 shows the spectral dependence of R and T for a sprayed FTO film with a sheet resistance of 8 [~/[:] * 8,. The visible and near-IR reflectance is about 10%-15%. The absorption in the visible is small (about 2%-3%). Figure 8 includes the spectral dependence of n and k for FTO films, showing a behaviour similar to that of ATO films. The plasma edge shifts with increasing N (Fig. 11) and is typically positioned at about 1.5-2.0 pmt s 1., s3. The effective mass of conduction electrons is around (0.3-0.33)mc is,. These films exhibit high IR reflectances (about 80%-90% between 5 and 10 pro) 'sL , e3 and a low ex ~ 0.13 *83 (at 100 °C).

18 K . L . CHOPRA, S. MAJOR, D. K. P A N D Y A

loG!

80

z 6C

< 4 0

i-.

20

I I I r ! I , |

I I t , ' I I i l

0.3 1.0

I ' | . . . . I I '

/ - " \

i

'\ I t i#/1 ' [

/ / /1, S "~, //

7 60 u

Z

~.)

20

Fig. 10. Typical spectral dependence of the reflectance R and transmittance T for FTO ( (--.--) and IZO (- - -) films on glass substrates.

I , , , , l I , , 2.0 5.0 10.0 2 0 5 0

WAVELENGTH (lUre)

), ITO

~C

'~ 60

20

r . . . . i , . . . .

v / ....

,:,1 / ,'

i

' ~ "1 /It #/#l#

. . . . . .

0.5 I 2 ~ 10 ° ~vl~ tENOI~4 barn)

I00

80

60 F

20

Fig. 11. Transmit tance T and reflectance R spectra of F T O films showing the shift in p lasma reflection edge to shorter wavelengths with increasing carrier concentration: - - - , N = 2 .5× 102o cm -3, 2p=2.4pm; . . . . ,N=3.7×lOi°cm-a, 2p=l.9pm; ,N=4.6×102°cm-3,2p=l .6pm; "'', N = 4.9 x 1020 cm -3, 2p = 1.5 Inn. (After ref. 181.)

3.1.4. Other dopants Numerous other dopants have been tried in SnO2 films. These include

phosphorus~41,170, 220, arsenic22 l, indium s ~, 170, ~ 71,174, thallium 170, tellurium222, tungsten222, chlorine 16s, bromine 16 a and iodine 16 s. These dopants do not affect the conduct ivi ty significantly, which is not entirely unexpected. However, the addit ion of phosphorus and arsenic to SnO2 films deposited by C V D has shown encouraging results220, 221 and deserves further investigations.


3.2. Indium oxide Unlike for TO, only limited information is available on the fundamental

material parameters of IO (In203) single crystals 62, 223-22 s. However, commercially it has been a more widely used transparent conductor, owing to its slightly superior electrical and optical properties.

3.2.1. Undoped indium oxide films IO films are generally polycrystalline with a cubic bixbyte structure 23' 58, 86 in

the bulk material and with little ohange in the lattice parameter (a = 10.118 A). Muller 5s has reported a typical grain size of about 100 A for reactively sputtered films. A (100) or (111) preferred orientation has generally been reported in these films which have been obtained by various techniques 24' 60, a6.

The superiority of IO over TO films as transparent conductors is largely due to the higher mobility in IO. IO films prepared by various techniques have mobilities in the range 10-75 cm 2 V- 1 s- 1, with N ~ 10t9-102° cm- 3, leading to p E 10- 3 fl cm. Better results are obtained after a reducing heat treatment which improves the conductivity SS, 179. A subsequent oxidizing heat treatment has been found to result in a decrease in conductivity 5 s, 179.

Of special note is a recent report by Pan and Ma 28, who have deposited pure In203 films by evaporating a mixture of In203-10wt.%In in an oxygen ambient at 340 °C. The as-deposited films had/~ ~ 74 cm 2 V-1 s-1, p ~ 2 × 10 -4 f~ cm and T ,~ 90%. These are the best values obtained for any undoped transparent conductor and rival those for the ITO system very closely.

IO films exhibit a direct optical band gap which lies between 3.55 and 3.75 eV as reported by several workersSS' 61. ~2.18 s and which has been shown to increase with increasing carrier concentration 61. The optical transmission in the visible and near- IR regions is about 75%-90% 21,28,86. The refractive index in the visible region ranges between 1.9 and 2.08 58's6. Muller s9 has reported an effective mass m* = 0.3me for the conduction electrons.

3.2.2. Tin-doped films ITO films retain the bulk structure of In203 but exhibit a slight increase in the

lattice constant (10.118 A < a < 10.31 A) which has been found to depend on the deposition parameters sT' s7, 92, l s9. The grain size ranges typically between 400 and 600 ~, 69. These films also exhibit a strong (100) or (111 ) orientation depending on the deposition conditions 24'57'87'9°'91' 100. However, the electrical and optical properties appear to be independent of the orientation effects 91,92.

The resistivity of these films decreases initially with increasing tin concentration but starts increasing at still higher concentrations, thus showing a behaviour similar to that of ATO films. These films have N ~ 1021 cm- 3 and/z lies in the range 15-40 cm 2 V -1 s -1 leading to p in the range from 7 × 10 -5 to 5 × 10 -4 fl cm. The effect ofheat treatment in various ambients on the electrical and optical properties of sputtered ITO films has been investigated by several workers 69' sT, 94.9s. Manifacier et al. 179 have observed a small but significant change in R,h for sprayed ITO films after annealing in argon or air at 400 °C, which also corroborates the contribution of oxygen vacancies 96 to the conductivity of even highly doped ITO films.

The direct optical band gap of ITO films is generally greater than 3.75 eV and exhibits an N 2/3 dependence (see Fig. 7) owing to the Moss-Burstein shift32.92.93.179. The optical transmission in the visible and near-IR regions is high

20 K.L . CHOPRA, S. MAJOR, D. K. PANDYA

(see Fig. 10), reflection (average visible reflectance, about 10%) being the primary source of photon loss. The absorption in the visible region is typically about 2~ 34, 35. The refractive index in the visible, as obtained by various workers, exhibits a marked scatter between 1.8 and 2.1 66,70,91,93,180. n also depends on the deposition parameters, especially the substrate temperature153.18o. Because of their high N, these films exhibit a plasma edge around 1.2-1.8 ~tm and a sharply rising IR reflectance (Fig. 10), mainly due to the higher mobility. The IR reflectance is high (about 90~ at about 5 ~tm) and correspondingly the thermal emittance is low, typically about 0.1 (at 80 °C). The conduction band effective mass also depends on N 93 and is in the range (0.28-0.43)m e 59,93,187. As is clear from Fig. 4, ITO films with Rsh ~ 2-5 O/I-] and T ,~ 90% have been obtained by various techniques.

ITO films generally etch more easily than TO films. Some of the etchants suggested are sulphuric, chromic and phosphoric acids and mixtures of hydro- chloric and acetic acids 9°. 0.1 M oxalic acid 9° and HCI 67 have also been used. However, the chemical stability of ITO is quite adequate for many sophisticated applications.

3.2.3. Other dopants Other dopants which have been studied in In20 3 films include titanium 186,

antimony s7'186, lead 87 and fluorine 129'13°. Films of In2Oa:F (about 2.3 at.%) prepared by reactive ion plating by evaporating indium metal in an O2-CF4 discharge have yielded Rsh ~ 40 O/[~ with T ~ 80~o.

3.3. Zinc oxide ZO (ZnO) is a technologically important material. The electrical and optical

properties of pure and also doped bulk ZnO have been studied 226 extensively and reviewed 227. Thin films of ZnO have been prepared by various techniques. ZnO has one of the largest electromechanical coupling coefficients of all non-ferroelectric materials, resulting in its extensive use as transducers in surface acoustic wave devices and microwave delay lines. The bulk of the early work, especially on sputtered ZnO films, has been connected with these devices, with an emphasis on surface morphology and preferred orientation effects, and the electrical and optical properties have been neglected.

3.3.1. Undoped zinc oxide films ZO films retain the bulk wurtzite structure and are composed of columnar

crystallites 191 with grain sizes156,193.194 in the range 50-300 ~. Sputtered ZO films have a strong c axis orientation, perpendicular or parallel to the substrate depending mainly on the substrate material, while the degree of preferred orientation depends on the deposition parameters 1°8'1°9. Films prepared by CVD 8 and spray pyrolysis 191.193 also exhibit a preferred c axis orientation.

Undoped ZO films deposited by Webb et al.l o5 using r.f. magnetron sputtering in an Ar-H 2 mixture at T~ ~ 75 °C had a typical as-deposited resistivity of about 2 x 10- 3 O cm (N ~ 1020 c m - 3;/~ ~ 8 c m 2 V - 1 S- 1) and T ~ 90%. Significantly higher mobilities have been obtained for ZO films deposited by ARE 26, leading to p ~ 6 x 10-4-10-3 f~ cm and T ~ 90%. CVD a' 156 and spray pyrolysis 191,194 have also been used to deposit transparent conducting ZO films. The sprayed films, after annealing at about 400 °C in an H2 ambient 191 or vacuum 194, have p ,~ 10- 3-10- 2 O cm (N ~ 1018-1019 cm- 3; p ~ 20 cm 2 V- 1 s- 1) and T ~ 80~-90~o.


Roth et al. 22s have recently observed an interesting phenomenon of band gap narrowing in ZO films prepared by various techniques. All films were n type with N between 5 x 1017 and 2 x 1020 cm- 3. The narrowing appears at about 2 x 1019 cm- a and has been attributed to the onset of a semiconductor-metal transition resulting from the merging of the donor and conduction bands.

Oxygen chemisorption effects at the surface and the grain boundaries of polycrystaUine ZO films affect the electrical parameters significantly 156. Investiga- tions to throw light on these effects are needed.

3.3.2. Indium- and aluminium-doped films ZO films doped with trivalent cations such as indium 19a and aluminium 194

have been spray pyrolysed in our laboratory. Doping has been achieved by adding AICI 3 or InCl a to the zinc acetate spray solution. The as-deposited films are polycrystaUine (grain size, about 100 A) and retain the wurtzite structure. Whereas undopcd ZnO films exhibit a strong preferred orientation with the c axis perpendicular to the substrate, no such orientation effects are observed for indium- doped films. High conductivity and transparency have been achieved in indium- doped films without any post-deposition heat treatment. As-deposited IZO films (thickness, 0.6-0.8 ~tm) with N ~ 5 x 1020 cm-3,/ t ~ 15 cm 2 V- 1 s- 1, p ~ 8 x 10-4 fl cm (R~h ~ 10-15 D/[I) and T ~ 80% (see Fig. 10) have been obtained.

3.4. Cadmium stannate Cd2SnO4 and CdSnOa are two known phases of CTO. Both these compounds

possess an orthorhombic crystal structure and are wide band gap n-type semiconductors. Although the fundamental material parameters are not well known, sufficient information is available for its use as a transparent conductor.

CTO (Cd2SnO4) films prepared by r.f. sputtering of a powdered Cd2SnO 4 target 1 lo-11 s are amorphous 115 if deposited onto unheated substrates. Poly- crystalline films are obtained at elevated temperatures 112. The temperature needed for crystallization depends on the substrate surface 112. The crystal structure of sputtered CTO films prepared from Cd2SnO4 powder or Cd-Sn alloy (reactive sputtering72-74) targets is cubic spinel112 rather than orthorhombic. The sputtered films generally contain CdO and CdSnOa phases, depending strongly on the sputtering conditions. Heat treatment of sputtered CTO films in an Ar-CdS atmosphere at 600-700 °C results in a decrease in CdO content in the film, along with an increase in the lattice parameter (from 9.167 to 9.189 ~)112. A possible explanation is that cadmium dissociates from the CdO and diffuses into the Cd2SnO4 lattice to form interstitial donors; this is corroborated by an accompany- ing increase in carrier concentration 1 lo. In sprayed films x95, the CdSnO a phase is formed at a substrate temperature T~ ,~ 700-750°C whereas at T~ >i 800°C the Cd2SnO4 phase is deposited. The quality and reproducibility of the films is strongly dependent on the cadmium-to-tin ratio in the spray solution.

CTO films prepared by r.f. sputtering of an oxide target in pure 02, when heat treated in an Ar-CdS ambient, show an increase in N from 1019 to 1021cm-a 11o,112. /~ is typically in the range 30-50 c m 2 V - l s -1 leading to p ,~ 1.5 x 10 -4 f~ cm along with T ~ 85% 1 lo. Miyata and coworkers have reported reactive sputtering of Cd-Sn alloy 72-74 or Cd2SnO 4 114 targets in an Ar-O 2 mixture, resulting in films with p ~ 6 x 10-4-10- a D cm and T ~ 85% which do not


require any post-deposition heat treatment. Spray-deposited Cd2SnO4 and CdSnOa films have yielded sheet resistances of about 100f~/V1 and 10t2/vq respectively, with a good visible transmittance (about 80~) ~95.

An increasing conductivity in CTO films, whether it is caused by adjusting the sputtering conditions or by a heat treatment, invariably results in a large shift in the fundamental absorption edge 73' 11 o. 115 (typically from 2.1 to 2.9 eV), improving the visible transmittance significantly. This large shift is attributed to the Moss- Burstein effect and appears most likely to be composed of contributions from Cd2SnO4 and CdO present in the films 112. Nozik 11~ has reported an abnormally low effective mass of 0.04me (N = 4x 1018 cm -3) which increases to 0.11me for N ~ 1 0 2 ° c m -3. The refractive index in the 0.5-1txm range is approxi- mately 2 11 O, 115

These films exhibit a plasma edge at about 1.3-1.5 ~tm, a high IR reflectance (about 90~ at 5 lam) and a correspondingly low emissivity ~11 of about 0.1-0.12, which look quite attractive for heat mirror applications. The chemical l~s and thermal 112 stabilities of CTO films are very good, but not much is known about their mechanical properties.

3.5. Other transparent conducting oxides There are several other binary and ternary metallic oxides which have the

potential to become viable transparent conductors. One such system, Bi20 3 3, has been used to provide nucleation-modifying layers for thin gold and silver films, resulting in their improved conductivity and transmittance. However, the transparent conducting properties of Bi20 3 have not been explored. MoO 3 films 121 have been prepared by ion beam sputtering with p ~ 100 f~ cm and T ~ 70~. Other materials which deserve attention are binary oxides of titanium, tungsten, rhodium and rhenium. Among the interesting ternary oxides are NaxWO 3 229 and zinc stannate 197. Films of zinc stannate have been deposited by spray pyrolysis in our laboratory. These films are transparent but have a high resistivity of about 10 f~ cm. However, we feel that a judicious choice of the deposition conditions should yield good transparent conductors.

4. SALIENT FEATURES OF ELECTRO-OPTICAL PROPERTIES

So far we have given the data on electro-optical properties of various transparent conductors. Now we present the principal common features of these properties which throw light on the electron and optical transport processes in these films.

(A) The electrical conductivity, optical transparency in the visible region and IR reflectivity are interrelated (see Figs. 3 and 21) and are markedly dependent on the deposition conditions, in particular on the substrate temperature and oxidation processes that occur during or after deposition.

(B) Both the electron transport properties 6,sT'97,9a,l°3 and the optical constants are thickness dependent in ultrathin films. This variation may arise due to (a) the presence of oxygen-rich surface layers sT' 9s, (b) increased mobility resulting from increased grain size 6'1°3 and (c) change in the preferred orientation of crystallites with thickness and, consequently, variation in the refractive index in the


case of highly anisotropic materials (e.g. SnO2). The available and reliable information in this area is rather meagre. At thicknesses of about 1000 A or more (depending on the deposition conditions) the resistivity reaches a steady value. However, the optical transmittance continues to decrease slightly (see Fig. 3) partly because of the finite, though small, absorption coefficient and partly because of scattering from surface roughnesst so for thicker films.

(C) All conducting oxides are n type with a high carrier concentration of about 101 s_1021 cm- 3 and a low mobility of about 1-50 cm 2 V- 1 s- 1. In the cases where it is known, the effective mass is small (about (0.1-0.3)m, for pure and doped SnO2 films and about (0.28-0.43)m~ for pure and doped In20 3 films). The marked dependence of the effective mass on the carrier concentration 93' 169 (and hence the Fermi level) suggests non-parabolic bands.

For large N (> 102° cm-3), both N and p change little with temperature, suggesting a degenerate semiconductor behaviour. The temperature dependence of # increases with decreasing N. Figure 12 shows a typical dependence of N and # on the reciprocal temperature lIT for pure TO and ATO films. At relatively low values of N (~< 1019 cm-3) the mobility is thermally activated and follows ss' 152,169 the Petritz 23o type of relation

r-,/2 { . = }

where OB is the activation energy and corresponds to the grain boundary potential in cases where the mobility is determined by the grain boundary scattering model.

(D) The efficiency of doping depends on both the dopant and the matrix oxide. Figure 13 shows the theoretically expected (on the assumption that every dopant

d'

'E v

,g

oooo-o.o-o-o.-o~>

1 /.. j t o/~

O a t %

i

is ;E12 0

z 12 ~>

~E l e

o ~

er4 S w

e r

tO oI °I*

I , , -

L____

i J i i i J I ~ I 2 ~. 6 B i0 t2 0 - - 5 15

t03/T (K';~ DOPANT CONCENTRATION (at,°lo)

20

Fig. 12. Temperature dependence of the carrier concentration (D) and mobility (O) of sprayed TO and ATO films. The dopant concentrations indicated refer to those in the solution. (After ref. 169.)

Fig. 13. Theoretically expecte0 (T) and experimentally observed(E)valuesofthecarrierconcentration vs. t h e dopant concentration (in the film) for sprayed In203 :Sn ( ) and SnO2 :Sb (- - -) films. (After refs. 179 and 194.)


atom contributes one free electron) and the experimentally observed values of the carrier concentration N plotted against the dopant concentration (in the film) for In2Oa:Sn and SnO2:Sb films 179' 194. At low dopant concentrations, the observed carrier concentration in In203:Sn films is higher than that expected from the dopant contribution alone. This implies that the stoichiometric disorder also contributes to the carrier concentration in doped In20 3 films. In SnO2 :Sb films, N is always less than the expected value, implying that the dopant dominates the production of carriers. However, the exact role of the "inactive" dopants is not known and needs further investigation.

(E) The electron transport parameters of non-stoichiometric oxide films are affected markedly by different dopants. Shanthi and coworkers 181' 182 have carried out detailed investigations of dopant effects in sprayed TO films. Figure 14 shows the variations in N and p with antimony concentration for TO films doped with antimony and with a 65 at.% fixed fluorine content plus antimony. Similarly, Fig. 15 shows the dependences of N and p on fluorine concentration for TO films doped with fluorine and with a 1.4 at.% fixed antimony content plus fluorine. Although both antimony and fluorine provide shallow donor states, there is a significant difference in mobility variation with dopant concentration in ATO and antimony- plus-fluorine-doped TO films, emphasizing that the roles of antimony and fluorine in governing the electrical and optical properties of TO films are not similar.

In the case of SnOz:Sb films (see Fig. 14), N increases initially with antimony concentration up to about 6 at.% and then starts decreasing. Similar variation has been observed for In203 :Sn 179,186.187 films. This has been explained in terms of an increased disorder at high dopant concentrations 169-171,179. According to K6stlin et al. ~87, a tin ion which is surrounded by In20 3 can act as a donor, but if one of the nearest-neighbour sites around this primary tin is also filled with a second tin ion

I l l

I o 3O

v

S ~

2 ( \ ~ / \ ' - \ "

i On I ~ J ~7-------9 0

0 2 z 5 ~ 1O

Sb CONCENTRATO~I (at:. '% )

Fig. 14. Variations in the carrier concentration (

'E

5

3 G

- - _ _ _ 2 _ _ ~>

• l , I , , i , I J I J L , ' 8 20 40 60 80 ;00

F CONCENTRATION lot %)

) and mobility (- - -) with antimony concentration (in the solution) for sprayed TO films doped with antimony (O) and with 65 at.% fixed fluorine content plus antimony (Q). (After ref. 182.)

Fig. 15. Variations in the carrier concentration ( ) and mobility (- - -) with fluorine concentration (in the solution) for sprayed TO films doped with fluorine (O) and with 1.4 at.% fixed antimony content plus fluorine (0). (After ref. 182.)


then their donor action is compensated. However, in FTO films (Fig. 15) no such decrease in N is observed for high fluorine concentrations. This could result from the low concentration of fluorine ions incorporated in the film (about 0.8 at.To) even when the fluorine concentration in the film is as high as 65-80 at.%. It is also clear from Figs. 14 and 15 that the behaviour of N for antimony-plus-fluorine-doped films as a function of antimony or fluorine concentration is consistent with its behaviour for ATO or FTO films respectively.

The effect of antimony doping in both ATO and fluorine-plus-antimony-doped TO films (see Fig. 14) is very different (especially at high doping levels) from that of fluorine doping. In this case, at high doping levels (more than 6 at.9/o) of antimony, falls sharply, possibly owing to the formation of clusters by segregated atoms. Similar observations have been made for phosphorus-doped TO films 22° and doped IO filmst s6 However, the sharp decrease in # at higher phosphorus concentrations has been attributed to a loss of crystallinity. In In203, although the mobility is dependent on the dopant used (e.g. tin, titanium, antimony), in all cases it shows similar behaviour, i.e. an initial increase to a maximum followed by a sharp fall at higher dopant concentrations.

Figure 16 shows the dependences of N and # on indium concentration for IZO films prepared by spray pyrolysis 193. N increases with dopant concentration till it saturates to a value of about 3 x 102o cm -3 at about 2 at.To In. However, # decreases initially with increasing indium concentration and then, after passing through a minimum, increases sharply to saturate to around 12 cm 2 V- 1 s - 1.

(F) The fundamental absorption edge of the pure oxide films is similar to that of the corresponding bulk material. The optical gap increases on doping, which is consistent with the Moss-Burstein shift. The optical transitions are direct in most cases. In the visible region, the transparency is limited by the surface reflectance

'E v

1~0'

, , , ,~ T i ~ 0

* ) 10 0 -

o t LO

, I , I ,~ I 0.1 1 2 v

DOPANT CONCENTRATION (at.%)

Fig. 16. Variations in the carrier concentration (C)) and mobility (e) with indium concentration (in the solution) for sprayed IZO films. (After ref. 193.)

26 K . L . C H O P R A , S. MAJOR, D. K. P A N D Y A

(about 10%-15%) for the film-air interface and by a weak absorption (~t ~ 1 0 2 - 1 0 3

cm - 1). The scattering contribution is negligible (about 19/o-2%) for good films. The near-IR region shows some absorption due to the free electrons. A plasma reflection edge is observed in the IR which is followed by a high reflectance region. The position of the edge can be shifted by doping (see Fig. 11). The steepness of the rising reflectance curve depends on the electron transport parameters (N, # and m*).

5. C O N D U C T I O N MECHANISMS

Stoichiometric transparent bulk oxides are known to be wide band gap insulators. Energy band investigations for SnO 2 231,232 and In20 3 233 indicate a multiplicity of conduction and valence bands which are in general non-parabolic and allow both direct and indirect optical transitions.

5.1. Carrier concentration The high conductivity of pure oxide films is due to the high carrier (electron)

concentration, since the mobility in these films is considerably lower than that in the corresponding bulk material. The high electron concentration is attributed to (a) deviation from stoichiometry (or defect structure) and (b) a doping effect leading to controlled valence semiconductors 226,234, 2 3 5 . The stoichiometric deviation may be due to anion vacancies or excess interstitial cations. The exact nature of the defect structure in most cases is still uncertain 53,174.21 o, 2 ~ 2, 2 ~ 3, 226, 234- 236. In the case of TO TM, by combining x moles of SnO with 1 - x moles of SnO 2 the non- stoichiometric oxide Sn # +~ _xSn 2 + x 0 2 - 2 - x is formed, which contains x moles of oxygen vacancies with x moles of Sn 2 + free to donate 2x moles of electrons for conduction. Alternatively, if interstitial tin atoms are to be held responsible for the enhanced conductivity the non-stoichiometric oxide may be represented as S n 4 + l - x S n 2 + x O 2 - 2 - 2x'

The replacement of a higher valency cation by an impurity in the oxide films increases the n-type conductivity, while that of a lower valency cation produces a hole which acts as a trap in the n-type semiconductor and decreases the conductivity. Similar effects occur if the anion sites are doped with higher or lower valency impurities. This doping effect is due to a controlled valence mechanism 234, 235. The ATO234 or FTO ~ 79 can be represented in the ionic notation as Sn4+~_xSb~+xO2-2e ~ and Sn4+O2-2_xF-~e x respectively. However, in tin- doped In20 3 both substitutional tin and oxygen vacancies contribute to the high conductivity 96"x79, and the material can be represented as In2_xSnxO3_2: , 96. It should be noted that the effective doping effects are obtained when the ionic radius of the dopant is the same as or smaller than that of the host ion and, further, the dopant oxide does not form compounds or solid solutions with the host oxide.

5.2. Mobility The observed low mobility and its dependences on N and the temperature can

be explained in terms of scattering mechanisms such as ionized impurity or grain boundary scattering. The mobility data for sputtered In20 3 films has been interpreted by Muller 58 in terms of scattering by ionized imperfections. Similarly, Noguchi and Sakata 2 ~ have shown that/~ and N data for evaporated InzO 3 films are


in accord with the relation of Johnson and Lark-Horovitz 237 for ionized impurity scattering, although their experimentally observed values of# are much smaller than those expected theoretically. Similarly, thermoelectric power measurements on sprayed SnO2 films have led Imai 238 to conclude that ionized impurity scattering is the dominant scattering mechanism. However, the conclusion is questionable as the experimental values of the thermoelectric power measured at room temperature do not agree well with the computed values. Moreover, in the case of sprayed TO films, Shanthi et al. 169 have calculated the contributions due to ionized impurity scattering and optical lattice scattering using formulae 239 for degenerate semiconductors and found that the contribution of these two scattering mechanisms is very small. The mobility due to ionized impurity scattering has also been calculated using the Brooks-Herring formula 24°. The computed value of /~i ('~ 150 cm 2 V- 1 s- t) is much higher than the measured values. Hence the grain boundary scattering mechanism is considered to be dominant.

The initial increase in # with antimony or fluorine concentration is attributed to a decrease in the grain boundary potential due to an increase in the grain size. However, in both FTO and antimony-plus-fluorine-doped TO films (see Fig. 15) the increase in # with fluorine concentration and its subsequent saturation (in the case of FTO films) to values which are considerably higher than the maximum value of # obtained in ATO films can also be partly attributed to the modulation in the grain boundary potential ~bB due to the incorporation of fluorine atoms into the grain boundaries. The filling of oxygen vacancies by fluorine atoms is also possible so that the scattering is considerably reduced 6. Another possible explanation has also been given for the occurrence of larger # values for fluorine doping than for antimony doping. In an ionic lattice such as SnO2, the electron wave is concentrated at the positively charged tin atoms. The replacement of a tin atom by antimony will have a larger effect in disturbing the free motion of the electrons 6.

A simple grain boundary scattering process should not be expected in micropolycrystalline materials with high impurity and interface trap densities. This problem has been reviewed by Orton and Powell T M and Chopra and Dutta 242 for the case of polycrystalline semiconductors. These reviewers observe that a grain boundary trapping model is more appropriate to understand the mobility variation in, for example, ZnO films. Following this model, the conduction band profile can be represented by one of three types shown in Fig. 17. If No is the doping level (so that the Debye length is LD(= (ekT/Noe2)I/2)), N is the carrier concentration, N t is the trap density, nt is the density of charge trapped in the surface states, I is the grain size and Et is the trap energy, the expected behaviour in the three cases (Fig. 17) is

(a) N N°N¢ exp /~ = #o t

Nt

(b) N = (2n) 1/2 NoNe exp N t ' - - No k T

# = #o exp ( )-k~--

(c) N = N O # = #o exp(--,-~-'~ \ K 1 /


........... tEr

F

10; i F ~ i [

= . ,

ee~e~8 : 0.0 ~.5 eV

:3- I0 ~

E

0 ° " ~ ]

(a) (b) (c) lOOO / T {"K-') Fig. 17. Conduction band profile through grains of an n-type film for three cases: (a) n t ~. Nol, L D > 1/2; (b) n t ~ Nol, L D < 1/2; (c) n t < Nol. L D ( = (ekT/Ne2) 1/2) is the Debye length of the carriers. (After ref. 241.) Fig. 18. Temperature dependence of the mobility in sprayed IZO films with various carrier concentrations:curve a, 5.5 x 101 s cm-3; curve b, 6.9 × 1019 cm- 3; curve c, 1.9 x 1020 cm-3. (After ref. 193.)

The case (c) of a fiat band inside the grains explains the observed thermally activated mobility behaviour as shown in Fig. 18. According to model (b), the number of trapped charges at the grain boundary increases with increasing N in the grains, resulting in a linear increase in the barrier height to reach a maximum when all the traps are filled (/~ reaches a minimum). At higher doping levels, N increases further and the regime of case (c) takes over whereby thegra in boundary potential 4, B decreases as a result of the decrease in the depletion layer width. This behaviour is observed in indium-doped ZnO films as shown in Fig. 19. The grain boundary potential has been determined from the l I T 1/2 versus 1 / T plots of Fig. 18. In samples containing higher carrier concentrations # becomes almost temperature independent, showing that q~a is negligible. It should be noted that the observations on ZnO films are very similar to those made by Seto 243 for polycrystalline silicon. The minimum in/~ in the present case occurs at relatively high N ( ~ 6 x 1019 cm - 3 ) . This appears to be due to a high trap density (about 9 x 101 a cm - 2) resulting from a very small grain size (about 50-100~) in these films. The weaker temperature dependence of the mobility that has been shown to occur at low temperatures (see Fig. 18) has also been observed by Roth and Williams 156 in ZnO films. This has been attributed to the mobility's being dominated by tunnelling at low temperatures.

It is clear from the preceding discussion that an acceptable quantitative explanation for the low mobility in oxide films has not yet emerged. The temperature dependence of N and # is obviously determined by the physical, metallurgical and electronic structure of the grain boundary and by the grain size. This point is very dramatically illustrated by the data for ZnO films and indicates further the need for a simultaneous study of structural and electrical properties.


4.00

I o | 3.00

r . i

~2.00 m o x

1.00

7 / I

1 I I01g 1019 1020

CARRIER C O N C E N T R A T I O N ( c ~ 3)

).08

).06

0.01,

0.02 ~( n,, o

-00

Fig. 19. Variations in the mobility (0) and grain boundary potential (&) with carrier concentration for sprayed ZnO :In films: @, ~ , data for annealed samples. (After ref. 193.)

6. OPTICAL EFFECTS

The high optical transparency of these oxides in the visible and near-IR regions of the solar spectrum is a direct consequence of their being wide band gap semiconductors (Eg >~ 3.0 eV). Their fundamental absorption edge generally lies in the UV and shifts to shorter wavelengths with increasing carrier concentration. This shift is due to the filling of the states near the bottom of the conduction band and is well known as the Moss-Burstein shift 219. Assuming parabolic bands, this shift AE can be described for a free-electron Fermi gas as I s7

= h2 (3N~ 2/3 AE 8m*vck n J (1)

where m*vo is the conduction band effective mass, which also includes the contribution from the valence band curvature. This behaviour has been observed for ATO and ITO films (see Fig. 7).

Figure 20 shows typical transmittance, reflectance and absorptance spectra for an ITO film. The visible and near-IR regions are the transparent region for these films. The transmission in this region is limited by several factors: (1) reflection losses which include both specular and scattered (diffuse) components (this backward scattering (about 1%-2%) is primarily due to surface roughness and increases with increasing thickness lS°); (2) absorption (about 1%-2%) s'35 in the film, which is primarily due to free carriers; (3) variations in transmittance that, depending on the thickness of the film, may occur due to interference phenomena with the result that an average visible transmission must be described. The transmitted part of the incident energy also has specular and scattered (diffuse) components. This forward scattering (about 5%) is due to inhomogeneities in the film in the form of unreacted


; 2‘ 4 a- d +.-

30-

WAVELENGTH ()~m)

Fig. 20. Typical reflectance. R and transmittance T spectra (---, specular; -, total) and the absorptance A curve (‘. .) for a sprayed IT0 film.

or partly reacted chemical species generated during the complex pyrolytic process, trapped gases, segregated impurity atoms or other oxide phases. _

In the near-IR region, the free-carrier absorption becomes important. The optical phenomena in this region can be satisfactorily explained on the basis of classical Drude theory 244 For free electrons in metals the complex dielectric . constant is given by

E* = (n-iQ2 = E, - Ne2

~~tn*(u~ - iw/( 5)) (2)

where E, is the high frequency dielectric constant, m* is the electron effective mass and (5) is the averaged relaxation time. n and k are the optical constants which determine the reflectance and absorptance spectra of the film.

Applying this result to transparent conductors with high carrier concentrations (about lOi’-10” cmm3) in the short wavelength approximation (i.e. m(t) D 1) which holds good for SnO, and In,O, we can write the expression for the real part of E* as

Ne2 E, = & -__I 03 .50m*co2 (3)

In addition, the photon loss due to free-carrier absorption is given by

A h2e3Nt

(4)


Under the condition er = 0, eqn. (3) yields the plasma wavelength _

. f + om'V ]2 = ) (5)

The absorption peak in Fig. 20 is due to the plasma resonance. For wavelengths larger than 2p, ~r is negative; hence the plasma becomes reflective. This is the onset of the highly reflecting IR region of these films. The observed shift in the plasma resonance frequency with N for FTO films (see Fig. 11) is in agreement with eqn. (5). Obviously, the magnitude of 2p (i.e. of N) decides the range of near-IR transparency for these films.

In the IR region it can be shown for these films that the IR reflectance is given by 2+s

4eoC 1 R = 1- (6)

e Ntl~

Comparison of the loss equations (4) and (6) reveals that both the losses cannot be minimized simultaneously. While IR loss decreases with increasing Nt, the visible loss increases. Combining eqns. (4) and (6), we obtain

( 2e ~21 (7) A(1--R) = \~cm*#/ n

which is independent of Nt. A large value of m*# thus implies a large IR reflectance and also a sharper transition from low to high reflection. This is an important conclusion for the heat mirror applications of these films.

It should be noted that high IR reflectivity implies low thermal emissivity as a consequence of Kirchhoff's law. We should therefore expect a relationship between emissivity and IR reflectivity and hence sheet resistivity R,h through the dependence on N and g. For example, the data for ITO films (Fig. 21) show this interdependence which follows the relation 2+5

e = l -- (1 + 2eoCR+h)- 2 (8)

1.0

u elJ • • I

• • • • _..

0.8

0.2 .,,"

0 J i ~ I I 10 10 2 10 3 10 4 10 s

SHEET RESISTANCE (ohm/o)

Fig. 21. Variation in the thermal emittanc¢ at 80 °C with sheet resistance of a doped IO coating on a glass substrate: , best-fit curve according to the equation e = 1 - (1 + 0.0053R,h)- 2. (After ref. 245.)


7. APPLICATIONS OF TRANSPARENT CONDUCTORS

A multitude of applications exist in which transparent conductors are involved in active or passive roles. Devices of the former type require a detailed knowledge of the electrical and optical parameters and impose stringent requirements on them. However, in the latter type of application, generally as transparent electrodes, a knowledge of the sheet resistance and the average visible transparency suffices. Figure 22 illustrates the inherent trade-off between these two parameters for a few typical cases. However, a properly chosen combination of R,h and T, which takes into account this inherent trade-off, can be quite useful for quick comparisons. This has led to different definitions of the figure of merit for transparent conductors.

I [ I I I

60

-- 50

~ 20

o 65 70 75 80 85 90

TRAN.9.'IITTA NCE (%)

Fig. 22. Typical variation in the sheet resistance and average visible transmittance for spray-pyrolysed ITO (e), FTO (O) and IZO (A) films of various thicknesses (0.1-0.5 I~m) prepared under optimized conditions.

Haacke 246 has defined the figure of merit as

TlO ~br c = - - = at exp( -- 10ctt)

Rsh

For a given material, q~TC is a function of thickness alone. A value of 10 for the index of T has been chosen to set the value of the visible transmittance T at 90% for that thickness which yields a maximum value of ~bTC. This is a more practical figure of merit than that of Fraser and Cook 9° (FTc ----- T/Rsh), which weighs too much in favour of the sheet resistance. For a constant sheet resistance (R,h = 1~at), ~bTC varies as exp{--(~t/a) x constant} which implies that a large value of the fundamental quantity tr/~t will result in large ~TC"

For free-carrier absorption it can be shown that tr/ct = (~c3n/22e)la2m .2, and since for semiconductors #-,~ (m*) -x we can conclude that a good transparent conductor must possess a high mobility and a low effective mass 4.

A thickness-independent figure of merit has also been defined as 247

ct F = - R s h l n T = -

o


This definition is useful for making a comparison between two transparent conductors with the same thickness.

In Sections 7.1-7.8 we describe some important applications of transparent conductors.

7.1. Solar cells 7.1.1. Semiconductor/insulator/semiconductor solar cells Heterojunction solar cells consisting of a wide band gap semiconductor

(usually an oxide semiconductor) mated to a much narrower band gap (active) semiconductor have gained considerable prominence during the past few years. The performance of these devices is strongly controlled by the presence of a thin interfacial insulator layer. Heterojunctions with this interfacial layer constitute SIS solar cells (Fig. 23(a)). SIS junctions possess many features which make them an attractive alternative to the conventional silicon homojunction diode for photovoltaic applications t 94.

,AL ° , , , ° - - - i m W [] m m LAYE~O0~O.¢) ~".". ; : : '~: ' ' ; : ' : ":~',~(:;'~'':"' '~'~'::~:~::','I CONOUC'rOR O000-)O00d')

• ~:;,:.. :., .,: ;:..'=..'.. • . . . . . .~..~', , , : .., ..... :. :. "; BASE SEMICONDUCTOR I: ." ..,. ::.,. '., .,,'.:: t ,. ~l... :.. ,:...... ,~ :'I I~."."~";~ ;:'; ':':.::", ..'.:':' -"'."'": ".'!'.;::}~'~I • ~ . ~ : . - ~ " . . : ' . ' . : " , ) " ~" " . : : " ' . , ' t ' - "-~'~' L:..~ . . . . . . . . . . . . . . . . :.~.,,, ,.,,...--....?-~

(a) l l l . . H , nnlH,mm.m,n|nnn|mnnmw.m .) METAL ELEC1ROOE

Cu2S (O.Spm) ]

Cu FOIL ---~"

(b)

V / / / / / / / / / / / / / / / / / A - - GLASS

h:- '4 . . . . . . . . . . . .

k ~ \ \ \ \ \ % ~ \ x x xxx'xxx~x'xxx'~ S n O x : F ( 0 .6 ~ m )

V / / / / / / / / / / / / / / / / A G. s

(p) o-SiC; H (OOtJ~m)

Sn 0 2 ~ a-Si:H(O.Spm) GLASS ~ = -$i)H (~OSpm)

(c)

Fig. 23. Schematic cross sections of(a) an SIS solar cell, (b) a CdS/Cu2S heterojunction solar cell and (c) an a-SiC: H/a-Si: H solar cell.

The choice of the top oxide layer depends on its work function and the nature and electron affinity of the base semiconductor. The theory of SIS junctions has been discussed in detail and the conditions for optimum performance have been outlined 24s-25°. Both single-crystal and polycrystalline silicon have been used to form junctions with transparent conducting oxides, yielding high efficiency cells. The performance of these devices has been reviewed recently 251.

Both ITO and TO films have yielded comparable results, the air mass (AM) 1 efficiencies of cells fabricated on single-crystal silicon being in the range


10~-15~ 251. Ion-beam-sputtered ITO/SiO2/p-Si solar cells have yielded high efficiencies r/~ 13~-16~o 120.252. Sprayed TO/SiO2/n-Si solar cells have also yielded ~/~ 14~o z 53. The SIS solar cells based on polycrystalline silicon have yielded ~/~ 10~o-12~ 251. An ITO/a-Si solar cell (where a-Si denotes amorphous silicon) has also been fabricated (r/~ 1.2~) 254. Other systems which have yielded respec- table efficiencies (about 8~o-9~) are electron-beam-deposited ITO/p-CulnSe2 255, sprayed ZnO/p-CdTe 256 and r.f.-sputtered ITO/p-CdTe 256. Ion-beam-deposited ITO/p-InP 257 has yielded active area efficiencies of about 14.4~. Other promising systems are ZnO/SiOx/p-Si 26 and MoO3/p-Si 121

7.1.2. Transparent electrodes for heterojunction solar cells Thin films of ITO and FTO have been extensively used to provide a

transparent conducting base for back-wall heterojunction solar cells, yielding efficiencies of about 6 ~ - 8 ~ 179,258. A schematic diagram of the structure of a sprayed CdS/Cu2S solar cell deposited onto FTO is shown in Fig. 23(b).

Both TO and ITO conducting films are used as transparent electrodes in the p-i-n type (where i denotes an insulator) as well as the heterojunction type of a-Si- based solar cell 259-261. Figure 23(c) shows schematically the structure of an a-SiC: H/a-Si: H (where a-SiC: H denotes amorphous hydrogenated SiC) heterojunction solar cell yielding an AM 1 efficiency of about 7.5~ 260. Efficiencies as high as about 9.1~ and about 10~o have recently been announced by Energy Conversion Devices Inc. and RCA Laboratories respectively for p-i-n structures with p layers consisting of a graded composition of the type a-Si:C:N:H.

711.3. Antireflection coatings for solar cells Both ITO and TO have refractive indices in the range 1.8-2.0, which makes

them useful as antireflection coatings on silicon solar cells 24% 262. Chambouleyron and Saucedo 262 have spray deposited TO film onto n ÷/p silicon solar cells to obtain antireflection effects at the silicon-air interface. They have also shown that the presence of a TO layer reduces the surface recombination velocity, leading to a large increment in the short-circuit current of the cells.

7.2. Heat mirror The spectral selectivity of transparent conducting oxides makes them extremely

useful as heat mirrors 263. Heat mirror coatings are of considerable interest in controlling the temperature of closed spaces with windows and of devices (such as solar cells exposed to solar radiation). Moreover, these coatings could be used to keep the heat confined within a closed space with windows in a cold environment, or in a solar collector 245' 264.

It is clear that the selectivity requirements for different applications will be different. Whereas the conservation of space heating would require the window material to reflect back the thermal IR from within, the conservation of space refrigeration would require the window materials to reflect the near-IR range (above 0.8 l~m) of incident solar radiation. The latter requirements are also applicable to solar cell devices. The presently available transparent conducting oxide films with plasma edges at about 1.5 lam are well suited for keeping heat radiation confined in a building but offer few advantages for keeping out the IR radiation of the sun. For the latter application it is necessary to develop materials with a plasma edge at about 0.7 ~tm.


Heat mirror coatings are also useful in making a considerable saving in the energy required to heat the filament of incandescent lamps and reducing the unwanted heat radiation in the area illuminated by the lamp. In a conventional lamp, the tungsten filament is at 2650 K and emits about 90% heat and 10% visible radiation. Normally both heat and light pass through the glass envelope. The application of a heat mirror coating on the glass envelope of the lamp will result in the reflection of the thermal part back to heat the filament and only the visible part will be transmitted (Fig. 24, inset). Thus the power consumed to heat the filament is reduced considerably and a cool visible illumination is received. To have an effective heat mirror coating, the plasma edge should be at 0.7 pm. Frank et al. z4s have studied the usefulness of In203:Sn films for such an application. Figure 25 shows the

I00 , , ,

VlS ( 1~ O l " 5 i O l ) ~

-~ 80 >w I-- ~ $iOl, 0.I tim

l~03.1i.m GLASS

I / I I 7%0 60 .0 80 90 100

2650K 1 - T~F F (%)

Fig. 24. Dependence of the effective visible transmittance and 1 - - TEFF ~6s° g (non-transmitted radiation of the filament at 2650 K) on the thickness of the InlO3 coating; the carrier concentration in the In203 is 15 x 10 l° cm- 3 and the thickness of the additional SiO2 film is 0.1 pm (After ref. 245.)

I00 I00

i : • I " , , 4- I i\ / ".,

tt!i DTA / . 7 ~Q ' I i so i U i I~.AJICK~ EMISSION U z I ~ .; i V," I I seecti luM (zl iso' lo z

..,,o - Piil i :t. V 't, ~u . e . • ". ~, !: ,t ' \ I \ "..

20 ~ ~i ~ ~ I \ ' l0

" ~ I i ' " ' " " " ....L 0 0 ( 2 03 0.#. 0.5 0.7 1.0 2 3 4 s ? I0

WAVE LENGTH ( j im)

Fig. 25. Spectral variations in the transmittance and reflectance of two heat mirror coatings for incandescent bulbs: , In203 coating with a protective 0.I pm layer of SiOi; - - . --, TiO2/Ag/TiOz coating. Also shown are the Planck emission spectrum for a 2650 K body (filament) and the visual sensitivity spectrum (V). The shaded area shows the overlap between these two spectra which is a measure of the useful radiation for illumination emitted by the lamp filament. (After ref. 245.)


spectral transmittance and reflectance profiles of such a coating with N ~ 1.5 x 1021

cm- a (2p ~ 1.2 lam) and an antireflection ovcrlayer of SiO 2 0.1 pm thick. Also shown in the figure are the 2650 K Planck spectrum and the visual sensitivity curve. Figure 24 shows a plot of the calculated effective visible transmittance TEFF ws (obtained by calculating the weighting functions for equal log 2 intervals from Fig. 25) against the effective non-transmitted filament radiation I - TEFF2650 K (which is a measure of the IR reflectance (rejection)) as a function of the In203:Sn layer thickness. Since a gain in one parameter causes a loss in the other, an optimum thickness must be used.

Because of the overlap of the filament's Planck spectrum and visual sensitivity curve and the non-availability of ITO layers with 2p < 1 pm, it becomes difficult to separate the visible and IR radiations. This problem does not arise when ITO layers are used in a sodium lamp 265, because the sodium emission is well separated from the 540 K heat radiation spectra 245. A better coating for an incandescent lamp is a TiO2/Ag/TiO2 multilayer stack 266. The transmittance and reflectance profiles for this stack are also shown in Fig. 25. It is clear that with the help of this coating the IR radiation can be completely eliminated. The position of the higher transmission edge and reflection step can be varied by adjusting the thickness of the three layers 266. The use of this multilayer has been shown to result in a saving of 50~ in electricity for the same light output.

A new type of transparent heat mirror has been reported by Fan et al. 229 which consists of a transparent conducting microgrid formed by etching an ITO film. For square openings of about 2.5 pm separated by lines about 0.6 ~tm wide the visible transmittance increased from 80~o to 90~. Although 65~o of the area of the film was etched, the IR reflectance only decreased from 90~o to 83~.

7.3. Reflector-absorber tandem for photothermal conversion Metal-semiconductor tandems are used as selective absorbers for photo-

thermal conversion, the metal being responsible for the low thermal emittance of the system because of its high IR reflectivity. However, the system suffers from reflection loss at the semiconductor-air interface and has a somewhat higher emittance than that of the pure metal surface. In Fig. 21 we saw that transparent conductors also possess low emittances. As a further result of their visible-near-IR transparency they can be used as the top layer in the tandem, resulting in low IR emittance and obviating the need for a metallic substrate. In addition, these layers will also act as antireflecting coatings for the semiconductor, thus suppressing the reflection loss. Therefore, a coating of a transparent conductor on any semiconductor absorber surface will yield a selective absorber tandem. Photothermal conversion systems based on highly conducting oxide films have been fabricated, which include

I A| REJkR Y2 03 INSULATING ~ ELJECTROOIE (0.2~m) LAYERS ( 0 . 4 N m ) ~ " ~ " - " w ~ ~ ~ \ ~ ~ . ~ . ~ . ~ \ I

I::'.:~"."."~:.::;" : " " t'."'-'-':~":" "t"::,'":..':L

ITO TRANSPARENT - - - ~ ~ . . w - . . _ _

EI.EC TRO DE (0.2ji~rn) .~ .~ Ik

LLIMINISCENT LAYE]I~ (O.GHm)

G L A S S

Fig. 26. Schematic cross section of-an a.c. electroluminescent structure based on ZnS: Mn film and using ITO as the front electrode. (After ref. 950


ITO/Si 267 (~t =0.85; e = 0.07), (SnO2:F)/black enamelled steel ls3 (~t =0.92; 81oo = 0.15), Cd2SnOa/Si 111 (ct > 0.9; e ,~ 0.1) and (SnO2:F)/black molyb- denum/(SnO2: F)/quartz s (~t = 0.92; e5oo = 0.17-0.20).

Another application of these materials in photothermal conversion is as antireflection coatings. We have used SnO2 :F and ZnO:In films in our laboratory 26s to serve as antireflection coatings on PbS films in the following configurations: (SnO2: F)/PbS/stainless steel (ct = 0.93; e 1 oo = 0.15) and (ZnO:In)/PbS/stainless steel (~t = 0.92; eloo = 0.16). In these configurations, the emittance of the system is decided by the combined reflectance behaviour of the oxide layer and the metal substrate.

7.4. Opto-electronic devices Thin films of TO and ITO have found wide-ranging applications in numerous

opto-electronic devices. They have been extensively used as transparent electrodes in various display devices, whi.'ch include electrochromic displays 269, light-emitting diodeslSl, lss and liquid crystal displays 27°'271. For small alphanumeric displays, etching of a transparent conductor by photolithographic techniques is required. Therefore ITO is preferred in these applications since it can be etched more easily than TO. One of the many important applications where these films are used as front electrodes is in electroluminescent devices. Figure 26 shows a schematic cross section of an a.c. electroluminescent structure based on ZnS:Mn 95.

These films have also been used in high sensitivity photodetectors 272, vidicons 2 ~ 3 and ferroelectric photoconductor image storage and display devices 274. Visible imaging devices have been fabricated using ITO transparent conducting gates. A typical device consists of ITO gates on an oxide or an oxide--nitride insulator on n-Si 67. The high transmittance and low refractive index of the transparent layer results in a high quantum efficiency of about 60%-80% in the 0.4-1.0 ~tm wavelength range. These values are similar to those for photodiodes at larger wavelengths but are significantly higher in the blue and green regions. However, there is a considerable improvement over the behaviour of conventional polycrystalline silicon gate devices.

Transparent conducting oxide films have also been used as transparent gates on charge injection devices (CIDs) 2~s and charge-coupled devices (CCDs) 2~6. Large (78 K cells) self-scanned high density CID imager arrays have been fabricated using chemically vapour-deposited ATO films 275. ATO was preferred over ITO because of its higher temperature stability and slightly better optical transmission at shorter wavelengths. In the case of imaging in CCD area arrays, the device requires a conducting electrode gate over the active area.' The use of ITO has facilitated the fabrication of front-side-illuminated CCD area arrays 276, shown schematically in Fig. 27.

It has recently been shown that ITO films are quite resistant to laser damage and offer significant potential for use in Pockels cells, which are used as electro-optic shutters in high energy solid state laser systems 99.

7.5. Thin film resistors The basic considerations for the choice of a resistor material are a low

temperature coefficient of resistivity (TCR) and a fairly high degree of stability. Pure


PHOS t DOPED 8000 a SILO% I i

UNOOPEO 5 ILOX

CHANNEL

SILICON N-YWE IO-2OJ.L. CM 100

Fig. 27. Cross section of the imaging section of a time delay and inte~ation CCD using TO gate technology. (After ref. 276.)

TO films of sheet resistance about 1000 Q/Cl have been found to be stable in an oxidizing ambient up to about 500 “C, In addition, antimony-Dodd films exhibit a better stability than that of undoped films 277 It has been shown that the reversible . change in the resistivity of boron- and antimony-doped SnOz films depends on the dopant concentration and the film thickness 278. Depending on these parameters, the TCR can be controlled to vary between 0 and 500 ppm “C- ’ at room temperature. As a consequence of these attractive properties, highly stable TO resistors277’27Q are being manufactured commercially.

1.6. Gas sensors The increasing interest in areas such as pollution control, detection of

hazardous gases and monitoring of combustion processes has lent prominence to gas-sensing devices. Solid state gas sensors based on metallic oxide thin films have become attractive owing to their simplicity and portability. Thin films of SnO, 280-283 and ZnO 284 have shown promising results in the detection of gases such as CO, C02, H,, H2S, alcohols and hydrocarbons. The use of SnO, films as smoke sensors has also been reported recently285~286.

The device consists typically of a metal oxide film on an insulating substrate with two metal electrodes. These films exhibit a change in electrical conductivity when a change in the ambient occurs, and this property forms the basis of gas detection. Figure 28 shows typical H, and H,S response curves of a commercial SnO, sensor (Taguchi sensor from the Figaro Engineering Co., Japan) operating in air at 500 K 281. Zeolite molecular sieves (3 A pore size) were used to discriminate between the effects of H2 and H2S. Similar filter discrimination techniques can be used for other gases.

The change in conductivity can be either due to direct transfer of electrons from


the adsorbed gas to the oxide semiconductor T M or due to a reaction of the adsorbed gas with previously chemisorbed surface oxygen 2sl'2s3. The sensitivity and the response time of these devices have been found to depend strongly on the film deposition conditions2S2.

,o

z 1 0 b'3

10 2

I I

a

I ; I lO lOO lOOO

GAS CONCENTRATION (ppm]

Fig. 28. H2 (curve b) and H2S (curves a and c) responses of a commercial SnO2 sensor. Curves a and b were obtained with zeolite molecular sieves present; curve c was obtained with no zeolite. (After ref. 281.)

7.7. Wear resistance applications In order to reduce damaging of glass containers during manufacture or filling, it

is desirable to apply a low friction wear-resistant coating to the glass surface. Various coating materials such as silicones, various waxes and glycols are known for this purpose. However, the wear resistance of these lubricating films is nearly always increased if an oxide film (known as the hot end coating) is present between the glass surface and the lubricating layer. Chemically vapour-deposited TO films 2sT'2ss have been successfully employed for such applications.

7.8. Other applications Transparent conductors have been extensively used as transparent heating

elements on the glass windscreens of automobiles, aeroplanes and ships to combat the formation of ice and mist +°' 2s9. These films are also used to remove electro- static charges that accumulate on the glass windows of instruments +°' t66. Other specific applications include touch-sensitive switches and microwave shielding. Interesting cold-cathode properties of SnO2 thin films have also been observed 29°. TO films have also been used in the fabrication of thin film field effect transistors29~, 292. The polarized memory effect observed in the Se-SnO2 system 293 could also lead to interesting applications.

8. CONCLUDING REMARKS

(1) Although the interrelated electrical and optical properties of transparent conductors can be tailored to suit the whole range of applications, a more significant challenge is to obtain metallic conductivity without sacrificing the optical transparency. This means that we must have a much better understanding of the scattering


processes, the role of dopants, the trap states at the grain boundaries and the microstructure. It is through this information that we should be able to improve the carrier mobility by a factor of 2 or more, so that the electrical resistivity can be brought down to a level of about 25 fi cm. This is not an impossible task today in view of the micro- and nano-analytical facilities available to help in materials synthesis.

(2) Transparent conductors will continue to play an increasingly important role in electronic and opto-electronic devices. Further efforts are required to introduce such dopants and impurities which will make the transparent conductors “active” to electric fields and illumination. This is a totally unexplored area.

(3) With an appropriately tailored plasma reflection edge and IR reflectivity, the optical selectivity of transparent conductors can play a very important role in saving and/or conserving energy in the heating and cooling of buildings and in a significant improvement of the light emitting efficiency of incandescent tungsten bulbs. It may very well be necessary to develop new improved materials and appropriate technologies for this purpose.

ACKNOWLEDGMENTS

We are grateful to Jagriti Singh, S. Kumar, Bharat Bhushan and D. Chowdhry for their assistance in the preparation of the manuscript.

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