ORIGINAL PAPER

Transparent conducting zinc oxide as anti-reflection coating depositedby radio frequency magnetron sputtering

R Das1* and S Ray2

1School of Applied Science, Haldia Institute of Technology, Haldia 721657, WB, India

2Energy Research Unit, Indian Association for the Cultivation of Science, Kolkata 700 032, WB, India

Received: 06 July 2010 / Accepted: 09 February 2011 / Published online: 1 March 2012

Abstract: Highly transparent (visible transmission above 90%) and conducting ZnO:Al thin films with strong anti-

reflection property have been prepared by reactive radio frequency (rf) magnetron sputtering under Ar ? H2 ambient,

substrate temperature at 300�C. H2 ratio in the Ar and H2 gas fed from 0 to 40%. The electrical resistivity and sheet

resistance of ZnO:Al film are 2.8 9 10-4ohm-cm and less than 10X/h respectively. The % reflectance (R) is significantly

small (2% [ R [ 0.5%) in the wavelength range 1,000–1,500 nm and the refractive index (n) of the same ZnO:Al film is

1.49 at k = 1,100 nm. All ZnO:Al films have \002[ orientation with good surface topography.

Keywords: Magnetron sputtering; Transparent conducting ZnO:Al thin film; Broadband near infrared anti-reflection

coating; Hydrogen ambient; Telecommunication application

PACS No.: 77.55hf; 78.20.Ci; 78.40.-q

1. Introduction

Reflective and antireflective (AR) optical coatings have

been developed for a variety of applications, e.g., for optical

and electro-optical systems in telecommunications, medi-

cine, military products and consumer products. Now a days

physical vapor deposition (PVD) processes play dominant

role in the fabrication of high quality optical interference

coatings. E-beam evaporation is the preferred deposition

method for large-scale applications. The biggest challenges

are: (i) Identification of a material with suitable refractive

index. (ii) Selection of a stable and volatile precursor spe-

cies allowing the deposition of high quality films with low

absorption, low stress, low surface roughness, good stability

at high humidity, and (iii) High precision thickness control.

AR coatings have been widely used in glass lenses, eye-

glasses, lasers, mirrors, solar cells, IR diodes, multipurpose

broad and narrow band-pass filters, architectural and auto-

motive glass and displays such as cathode ray tubes, as well

as plasma, liquid crystal and flat panel displays [1–5]. Most

common material for a single layer coating is MgF2, which

has relatively low refractive index of 1.38 at visible wave-

length compared to glass. Several workers reported about

the double layer and multilayer coatings such as MgF2–

ZnS, MgF2–TiO2, and SiO2–SiN. Chopra et al. [6] exam-

ined the deposition of ZnSMgF2-SiO, ZnS-Na2AlF, and

GeZnS composite thin films and their multilayers. Belkind

et al. [7] used two tilted planar magnetrons for co-sputtering

some oxides (A12O3–SiO2, Al2O3–TiO2, ZrO2/TiO2) and

nitride TiN–ZrN. Sankur and Gunning [8, 9] used e-beam

deposition to produce mixed TiO2–SiO2, and discussed

changes in the microstructure of the deposited films. Cevro

[10] discussed about Ta2O5–SiO2 composite AR-coating in

near infra-red region. Shivalingappa et al. [11] described

e-beam technique for deposition of CeO2–SiO2 thin films.

Several researchers have worked on transparent con-

ducting ZnO films and reported the material properties for

different applications such as in photo-electronic devices

[12], displays thin film solar cells and recently in the fab-

rication of light emitting diodes [13]. Kluth et al. [14] and

Jin et al. [15] have developed ZnO:Al films suitable for

energy efficient windows of microcrystalline silicon solar

cell and as well as for the application as back reflector for

light trapping in solar cells. No scientific information

regarding anti-reflection properties of ZnO film has been

reported till date.

� 2012 IACS

*Corresponding author, E-mail: rajesh_eru2003@yahoo.com

Indian J Phys (January 2012) 86(1):23–29

DOI 10.1007/s12648-012-0010-9

In this paper, we report the development of ZnO:Al

films on glass substrate prepared by reactive sputtering

using H2 as reactant gas and its Anti-reflection property in

the specific wavelength region. To prepare aluminium

doped zinc oxide films, rf-Magnetron Sputtering technique

is preferred rather than other chemical vapor deposition

(CVD) methods due to its uniform ion damage free films

over a large area with precise thickness control and high

rate of deposition [16–20]. Detail studies have been done

on the influence of hydrogen ambient on ZnO:Al films

properties. In this paper, attempt has been made to correlate

optical properties of ZnO:Al films with its electrical,

microstructural and surface properties.

2. Experimental details

Aluminium-doped zinc oxide films were deposited on

Corning 7059 glass using an rf-planar magnetron sputtering

system (Nordiko, UK) by both non-reactive and reactive

sputtering under different conditions. A sintered ceramic

disc of ZnO:Al2O3 (2 wt%) was used as the target. During

non-reactive sputtering inert gas like Ar was used in the

deposition chamber. In case of reactive sputtering hydro-

gen gas was incorporated with Ar gas in different propor-

tion as reactant. During each deposition the chamber

pressure was kept constant with the help of a throttle valve

and the gas flows were maintained by mass flow control-

lers. Substrates of Corning 7059 glass were placed parallel

to the target surface at a distance of 7 cm. The rf-power is

applied between two capacitively coupled electrodes.

Before each deposition, the base pressure inside the

deposition unit was brought down to 4 9 10-4 Pa by

water-cooled oil diffusion pump through a liquid nitrogen

trap. ZnO films were deposited at rf-power = 100 Watt,

chamber pressure = 0.4 Pa, and substrate temperature at

300�C.

The film thickness was measured using a surface profi-

lometer. The electrical resistivity of the films was mea-

sured at room temperature by the Van der Pauw method.

The carrier concentration and carrier mobility of the film

were estimated by Hall Effect. A double-beam UV–VIS-

NIR spectrophotometer combined with a photo multiplier

tube (PMT) and a Peltier-cooled PbS IR detector (ISR-

3100 Integrating Sphere with 60 mm diameter was

attached) was used to measure the optical transmittance

and diffuse reflectance spectra of the transparent conduct-

ing oxide (TCO) thin films in the wavelength range 220–

1,800 nm in 1 nm interval with the light beam of 8� angle

of incidence. Refractive indices of the films were measured

by Ellipsometry (J.J.Woollam, USA). The data acquisition

(w and D) versus wavelength in the range 250–1,100 nm

was done for three incident angles 56�, 59� and 62� for

each point. The orientations of the crystallographic planes

as well as the microstructural analysis were investigated by

X-ray diffraction (XRD) (Philips) using monochromatic

Cu–Ka radiation (35 K/20 mA) with wavelength 1.54 A.

The detail study of surface topography was performed by

atomic force microscopy (AFM) measurement using a

NanoScope III instrument with an etched silicon cantilever

having a tip radius of 10 nm and 35� apex angle. Data were

taken in air ambient in tapping mode. Total scans were

extended over areas of 2 9 2 lm. The elemental analysis

was done by energy dispersive spectroscopy (EDS) with

20 kV operating voltage attached with scanning electron

microscope (SEM) (JEOL JSM 6700F).

3. Results and discussions

3.1. Electrical properties

The dependence of electrical resistivities of ZnO:Al films

with rf-power and chamber pressure prepared under dif-

ferent gas ambients have been studied in detail and dis-

cussed elsewhere [21]. The lowest resistivity of ZnO:Al

films prepared at Ar atmosphere was 6.7 9 10-4 ohm-cm

under the optimum deposition condition (rf-power = 100

Watt, chamber pressure = 0.4 Pa, Ar-flow = 50 sccm and

substrate temperature at 300�C). ZnO films were also

prepared under Ar ? H2 plasma with same deposition

conditions using different hydrogen dilution ratio (CH).

The lowest resistivity (q) and sheet resistance (Rsh) of ZnO

film prepared under Ar ? H2 ambients were 2.8 9 10-4

ohm-cm and 6.8X/h respectively. It is interesting to note

that initially with increase of hydrogen dilution in the

deposition chamber, the resistivity of ZnO films decreases

and then increases with further increase of CH (CH = [H2/

(Ar ? H2)] 9 100%). Here the film thicknesses of ZnO

films are lying between 800 nm to 900 nm.

The carrier concentration of different ZnO films as

estimated from Hall effect measurements, varies from

4.8 9 1018 to 2.3 9 1021 cm-3 as the films were prepared

under Ar and Ar ? H2 ambients deposited under optimum

conditions. The Hall mobility of the films also decreases

from 23.9 to 8.5 cm2/V.s. The values of electrical param-

eters are shown in Table 1 and discussed below. The

decrease of the resistivity of ZnO:Al film when prepared

with CH B 10% may be consistent with hydrogen remov-

ing oxygen and thereby increasing the zinc–oxygen ratio in

the deposited films. These results in the formation of either

oxygen vacancies or interstitial zinc giving rise to donor

levels [22]. Oxygen vacancies are deep donors in ZnO and

should therefore not contribute significantly to the carrier

concentration. An alternative intrinsic donor is interstitial

zinc, which is a shallow donor but has higher formation

24 R. Das, S. Ray

energy than oxygen vacancies [23], which may increase the

carrier concentration. Therefore, the increase of carrier

concentration into the ZnO film may be attributed due to

incorporation of hydrogen, which is shallow donor level

and used for high n-type doping of ZnO [24]. Hydrogen

behaviors on ZnO as a donor have been reported by several

experimentalists [25–27]. Webb et al. [28] achieved higher

conductivity in sputtered zinc oxide films by introducing

hydrogen into the argon sputter gas. They observed that the

resistivity initially decreases with increasing added

hydrogen up to some optimum condition and then starts

increasing in higher hydrogen partial pressure. Here, it has

been observed from the experimental results that the ZnO

film deposited with higher CH [ 10% values, becomes

more resistive and growth rate becomes slower due to

simultaneous plasma etching during deposition. The

increase in resistivity beyond CH = 10% is due to decrease

in carrier concentration. Hydrogen may act as an ampho-

teric impurity in high hydrogen partial pressure incorpo-

rating as H- (an acceptor) in ZnO:Al film [24]. As a result,

ZnO:Al films deposited at higher CH values may show

increased compensation from an increase in the density of

acceptor levels resulting from changes in the growth

characteristics [29] and hence increases the resistivity.

3.2. Surface properties and elemental analysis

Figure 1a–f presents AFM images showing the 2d- and 3d

topography of ZnO films deposited under Ar and Ar ? H2

ambients.. Clearly the Ar ? H2 deposited ZnO film is

much rougher than the Ar deposited ZnO film.

Quantitative measurements of the root mean square

roughness values have been listed in Table 1. The mini-

mum and maximum values of surface roughness are 2.30

and 9.96 nm, respectively, for Ar and Ar ? H2 deposited

ZnO films. Significant variations in surface feature of dif-

ferent ZnO films are evident from the micrograph. With

incorporation of H2 in Ar as sputtering gas surface

roughness of ZnO films gradually increases to 9.96 nm but

with gradual increase of hydrogen dilution in deposition

chamber, the film growth rate decreases.

Elemental analysis of ZnO films deposited under Ar and

Ar ? H2 ambients, studied by EDS attached with SEM, are

shown in Table 2. The data acquisition has been done for 7

different spots/sample for 2.5 min of each spots. The

results for different spots are almost equivalent for a par-

ticular sample. There is a large increase of the Zn–O ratio

with increasing H2 content.

3.3. Optical properties

Optical transmittance and reflectance spectra of ZnO:Al

films deposited under Ar, and Ar ? H2 ambients are dis-

played in Figs. 2 and 3a, b, respectively. The average

transmittance of all ZnO:Al films in the visible range is

found to be above 90% shown in Fig. 2. Curve-1 shows the

optical transmittance of ZnO film deposited under Ar

ambient. Curve-2 shows the transmittance curve of the

ZnO film deposited with CH = 5% and here the transmis-

sion peak maxima has shifted towards shorter wavelength

side. For the ZnO film deposited with CH = 10% values

the whole spectrum (Curve-3) is shifted towards shorter

wavelength side. All the ZnO films provides an excellent

UV shielding as the absorption edge in the lower wave-

length side is at k = 300 nm. Important thing is that the

band gap absorption edge has been shifted towards lower

wavelength side for ZnO:Al films deposited under

Ar ? H2 ambient. In the longer wavelength side the

transmission of ZnO:Al films are different for different

films deposited under different conditions. In the case of

ZnO:Al film deposited with CH = 10%, the percentage

transmission is negligible at 1,400 nm compared to 60%

transmission at the same wavelength in case of Ar depos-

ited ZnO film. It is evident that initially the transmission in

IR-region decreases with the increase of CH value, but with

further increase of CH values the % transmission increases

in IR-range. At very low hydrogen dilution i.e. at

CH B 5%, carrier generation increases in the ZnO film but

not too high. On the other hand, it has been observed that

Table 1 The electrical, optical parameter and surface roughness of ZnO:Al films prepared under Ar and Ar ? H2 with different dilution ratio at

300�C

Sputtering

gas ambient

Film resistivity

Ohm-cm

Hall mobility

(l) (cm2/V.s)

Carrier concentration

ne (cm3)

Refractive index at Surface roughness

(nm) drms (nm)500 nm 1,100 nm

Ar 6.7 9 10-4 23.9 3.9 9 1020 1.46 1.71 2.28

Ar ? H2 with different hydrogen dilution (CH = [H2/(Ar ? H2)] 9 100%)

CH = 5 5.5 9 10-4 13.4 6.3 9 1020 1.45 1.52 6.75

CH = 10 2.8 9 10-4 9.7 2.3 9 1021 1.39 1.49 8.26

CH = 20 8.3 9 10-3 9.3 5.3 9 1019 1.48 1.47 9.24

CH = 40 6.7 9 10-2 8.5 4.8 9 1018 1.49 1.46 9.96

Transparent conducting zinc oxide 25

with excessive increase of hydrogen concentration (CH) in

the gas mixture IR-transmission of these films increases

again. In this case, for the film deposited at CH C 40%

visible transmission slightly decreases and the films

become slightly blackish. From elemental analysis of ZnO

films deposited under Ar and Ar ? H2 ambients (depicted

Fig. 1 AFM images (2-d and 3-d topography) of ZnO films deposited under a, b Ar only and Ar ? H2 ambient with hydrogen ratio of c, d 10%

and e, f 20%

26 R. Das, S. Ray

in Table 2) performed by EDS study it is evident that there

is a large increase of the Zn–O ratio with increasing H2

content (Table 2). It may suggest the presence of metallic

zinc precipitations in the films. This would be consistent

with the observation that the films become ‘‘slightly

blackish’’ for higher H2 content. Here the film thicknesses

are lying between 800 and 900 nm. This is consistent for

the similar types of optical transmission spectra.

Figure 3a, b show the diffuse reflectance spectra of

uncoated Corning-7059 glass and ZnO:Al films. Important

feature has been observed from the reflectance spectra of

transparent-conducting ZnO films shown in Fig. 3a. Curve-

1 in Fig. 3a shows the reflectance spectrum of corning

glass in the wavelength range 850–1,800 nm. Generally

typical uncoated glass reflects nearly 4% in the entire solar

spectrum under normal incidence condition. But, here the

measured optical reflectance of Corning-7059 glass is

average 7–9% over the entire solar spectrum for the light

beam with 8� angle of incidence condition. But in the case

of ZnO film deposited under Ar ? H2 ambient with

CH = 10% values, the %reflectance is very small (lying

below 2%) in the above mentioned wavelength range

shown in the Fig. 3a indicated by curve-2. Especially, the

average reflectance is lying within 0.5–2% in the wave-

length region from 1,230 to 1,470 nm. The reflectance in

ZnO film deposited with CH = 10% is also lower value in

visible region compared to uncoated glass. In Fig. 3b the

curve-1 and curve-2 show the reflectance spectra of ZnO

film deposited under Ar and Ar ? H2 ambient with

CH = 5% respectively and significant reduction of reflec-

tance in the above mentioned wavelength region in near

infrared (NIR) region is observed. In both cases, the

reflectance minima have been shifted. ZnO:Al films

deposited under higher hydrogen dilution (i.e., CH = 20%

or 40%) have no significant reduction of reflectance has

been observed. In the case of ZnO film deposited with

CH = 10%, it is evident that both transmission (T) and

reflection (R) decreases in the near IR-region and this

Table 2 Summery of EDS result of as-deposited ZnO:Al films under

Ar and Ar ? H2 ambient

Element Composition (wt%) of ZnO:Al films

Ar Ar ? H2 (CH = 10%) Ar ? H2 (CH = 20%)

O K 47.87 44.18 40.08

Al K 2.36 2.46 2.39

Zn K 49.77 53.37 57.53

Zn–O ratio 1.04 1.21 1.44

200 400 600 800 1000 1200 1400 1600 1800 2000

0

20

40

60

80

100

3

1

5

4

3

2

1

% T

ran

smit

tan

ce

Wavelength (nm)

Fig. 2 Transmission spectra of ZnO films deposited under (1) Ar

ambient and Ar ? H2 ambient with (2) CH = 5%, (3) CH = 10%, (4)

CH = 20% and (5) CH = 40%

800 1000 1200 1400 1600 18000

2

4

6

8

(b)

(a)

10

12

14

2

1-Uncoated Glass

% R

efle

ctan

ce

Wavelength (nm)

800 1000 1200 1400 1600 1800

Wavelength (nm)

0

2

4

6

8

10

12

14

2

1

% R

efle

ctan

ce

Fig. 3 a Reflectance spectra of (1) Corning glass and (2) ZnO:Al film

deposited under Ar ? H2 ambient with CH = 10%; b Reflectance

spectra of ZnO:Al film deposited under (1) Ar ambient and (2)

Ar ? H2 ambient with CH = 5%

Transparent conducting zinc oxide 27

behavior can be explained in terms of the classical Drude

theory [30] since doped ZnO is degenerate semiconductor.

In the near IR-region, the decrease in both R and T are due

to free carrier absorption. The high transmission in the

visible range is understood from the fact that ZnO is direct

band gap semiconductor with high band gap.

The refractive index (n) of the ZnO film deposited under

Ar ? H2 ambient decreases with increase of wavelength

and its value is 1.49 at 1,100 nm whereas it is still higher

value (n *1.71 at 1,100 nm) for Ar deposited ZnO film

listed in Table 1. Generally the refractive index of ZnO

film lies in between 1.95 and 2.05. But, the slight variation

of refractive index of ZnO films are due to change in

materials properties as well as compositional change in

ZnO films deposited with different CH values. The decrease

of refractive index (n) of ZnO with increasing carrier

concentration is well known effect and has already been

shown by a number of authors [31].

3.4. Structural study

The X-ray diffraction patterns of ZnO:Al films prepared in

Ar and Ar ? H2 plasma are shown in Fig. 4 with the

preferential growth along \002[. Others have reported

similar predominant orientation in the\002[direction and

formation of hexagonal structure. This indicates hexagonal

wurtzite structure with the c-axis perpendicular to the

substrate surface. The crystallite size of ZnO film can be

obtained by using the Scherrer’s equation d = 0.89k/

bCosh, where k is the wavelength of X-ray, b is the full

width half maxima (FWHM) and h is the angular peak

position measured in degree. The crystallite size of ZnO

films prepared under Ar and Ar ? H2 gas ambient is 198 A

(for curve 1) 170 A (for curve 3) and 150 A (for curve 3),

respectively, as estimated from \002[ peak. Here it is

evident that the crystallite size is found to increase as

FWHM decreases with the change of gas ambients. The

crystallite size of ZnO:Al films becomes smaller while

deposited by Ar ? H2 gas ambients and further reduction

was also observed due to further increase of CH values. In

Ar ? H2 atmosphere a significant amount of energetic

hydrogen species are present during sputtering. These

reactive hydrogen species could remove weekly bound

oxygen or interstitial metal atoms in the depositing film,

even react with the growing clusters containing interme-

diates such as ZnxOy, adsorbed O, and reduced Zinc atoms

which results the stoichiometric variation in the film. The

weekly-absorbed oxygen and possible reduced interstitial

metal atoms in ZnO may be removed or re-sputtered by

reactive hydrogen species. This effect of reactive hydrogen

species reduces the crystallite size and hydrogen leads to

more non-stoichiometric film.

4. Conclusions

Transparent conducting ZnO:Al thin films with high anti-

reflection property have been developed by reactive RF-

magnetron sputtering using H2 as reactant gas with appro-

priate proportion with Ar. The different material properties

correlated with antireflection properties of ZnO films have

been found out. The transmittance of ZnO film depends on

the film resistivity, optical band gap and refractive index of

the material. But the reflectance in the visible to near infrared

region strongly depends on the surface roughness and

matching of film thickness with refractive index. The

refractive index of the anti reflecting ZnO:Al film is 1.49 at

1,000 nm and the film thickness is 420 nm. This ZnO film

deposited with CH = 10% with high anti reflection property

has higher surface roughness value. On the other hand,

ZnO:Al film deposited under Ar ambient has higher refrac-

tive index and more flat surface. All the ZnO:Al films are

\002[oriented. This antireflection property can be used in

thin film solar cell to minimize the reflection loss of incident

light in the front surface of the electrode of thin film solar

cell. The high anti-reflection property of ZnO:Al film in the

wavelength range 1,000–1,500 nm can be used for operating

at 980 and 1,480 nm wavelengths of pump laser modules

used in optical fiber telecommunication systems.

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20 40 60 80 100

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3

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<004>

<004>

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<002>

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