2008 International Students and Young Scientists Workshop "Photonics and Microsystems"

Investigation of electrical and optical properties ofTi02:Pd, Ti02:(Eu,Pd) and Ti02:(Tb,Pd) thin films

Karolina Sieradzka, Jaroslaw Domaradzki, Danuta Kaczmarek, and Bartosz MichalecFaculty ofMicrosystem Electronics and Photonics, Wroclaw University ofTechnology

Janiszewskiego 11/17,50-372 Wroclaw, Polan{iEmail address:karolina.sieradzka@pwr.wroc.pl

II. EXPERIMENTAL PART

Fig. 1. The structures for electrical characterization which were carriedout in the following configuration: a) planar, at the thermoelectrical and1-V measurements, b) vertical, at the I-V measurements.

transparent oxide p-n heterojunction based on TOS thin filmswith two, different types of electrical conduction. At present,one exist a wide possibility of potential optoelectronicapplication such heterojunction (UV diodes [5], transparenttransistors [6], solar cells [7] etc.).

Substrate:Si

IDS thin films

IDS thin films

Substrate:Si02, Coming 7059)

b)

a)

Investigated samples were fabricated by magnetronsputtering. The Ti02 matrix was stechometric andnanocrystalline [8]. The dopants in prepared Ti02:(Eu, Pd) andTi02:(Tb, Pd) ternary thin films were very carrefully selectedto be in similar amount with binary Ti02: Pd thin films. Hence,the thin films consist of the following compositions: Ti02: Pd(6 at. % and 23 at. % of Pd), Ti02:(Eu=0.9 at. %, Pd=5.8 at. %)and Ti02:(Tb=0.6 at. %, Pd=9 at. %). Thin films weredeposited on glass and silicon substrates. The amount ofdopants was estimated with energy disperse spectrometer.

To determine the level of transparency and the cutoffwavelength of the thin films, transmission measurements in thespectral range from 300 nm to 1000 nm were performed.

The d.c. electrical characterization of TOS thin films wasrealized by using two, different configuration: planar (Fig. 1 a)and vertical (Fig. 1b). Moreover, to estimate the electricalparameters TOS thin films metal electrodes were deposited onthe their surface (in the case of vertical measurements theelectrodes were deposited also at the backside of siliconsubstrate).

I. INTRODUCTION

It was known on the basis of available literature thattitanium dioxide is one of the materials which are especiallyused for fabrication of thin films for different electronicsapplications. It is because posseses many favourable properties,as: excellent optical transparency, high refractive index andthermal stability. For example, these properties make it usablefor smart windows, liquid crystal display devices, [1] etc. Ti02

has a wide gap (above 3 eV [2]) what indicates their dielectricnature in the room temperature. In a result of this, d.c.resistivity of undoped Ti02 is about 108 Qcm [3]. Additionalhigh electrical permittivity make that such thin films can beapplied for production of capacitors [4].

While, to get a thin films based on Ti02, which will betransparent for the visible light and additionallysemiconductive in a room temperature, it is necessary infabrication of the TOS materials to modify the Ti02 propertiesthrough incorporation of particular dopants in the form of (forexample) Eu, Tb, Pd elements. Except using of a single TOSthin films in the coatings application, one able to do also the

Abstract - Integration of electronics and photonics causesgreater requirement for development of electrical andoptical activated thin films for a new field of applicationknown as transparent electronics. One of the key materialswhich is necessary for production of functional elements intransparent electronics devices are transparent oxidessemiconductors (TOSs). The TOSs can be applied in manypractical applications, for example as electrodes in liquidcrystal displays or as a coatings in smart windows, opticaldevices, solar cells etc.In this work subject of research are the thin filmsfabricated based on Ti02 as a matrix with particulardopants (Pd, Eu, Tb). The thin films were prepared bymodified high-energy magnetron sputtering method anddeposited on various substrates. The electrical and opticalproperties of the samples were characterized by means ofoptical transmission spectroscopy, thermoelectrical andcurrent to voltage (I-V) measurements. It has been shownthat incorporation of selected dopants into Ti02 matrixallows us to obtain the thin films with resistivity of about 60Ocm in the room temperature, good transparency, equal to50 % and desired type of electrical conduction p Ti02:(Tb,Pd) or n Ti02:(Eu, Pd) in the room temperature.

1-4244-2554-9/08/$20.00 ©2008 IEEE

74 2008 International Students and Young Scientists Workshop "Photonics and Microsystems"

The thickness of prepared thin films was 540 nm forTi02:Pd (for various amount of Pd), 396 nm forTi02:(Eu, Pd) and 420 nm for Ti02:(Tb, Pd) thin films. 500

1(1<)440 380 320

dielectric

Fig. 3. The results of electrical resistivity measured by four-point probe methodfor Ti02doped with Pd; Tb, Pd and Eu, Pd

2,0 2,2 2,4 2,6 2,8 3,0 3,2 3,4

100011 (1{1)

TABLE 1THE RESULTS OF SEEBECK COEFFICIENT GETTING FROM THERMOELECTRIC

MEASUREMENTS FOR TI02: Po, TI02:(Eu, PD) AND TI02:(TB, Po) THIN FILMS[10]

TOSTi02:(Tb,Pd) ••••••••

••••• ••••• Ti02:Pd (6 at. 0/0)

-------------------Ti02:(Eu,P~). • • • •

•••••••••••_. _. _. !~~:~~ ~~~ ~..~.-

........ 104

E(.) 103

a"--'"

0. 102

101

10° L..-I~----L---&.........L........a..........I...-...I....-..a....-a..........&----'-----I...---I....""'"

Comparison of Seebeck characteristics of prepared thinfilms are presented in the table 1. As one can see, Tb dopantresults in conversion of the type of electrical conduction fromelectron to hole conduction and Seebeck coefficient is about120 ,.,.,V/K in the room temperature. Eu dopant also causesincreasing of Seebeck coefficient value comparatively to ownPd and it is about -80 ,.,.,V/K in the room temperature. This thinfilm has electron type of conduction.

Seebeck coefficient Type ofSample

[p.tV/K]electricalconduction

Ti02: Pd (Pd=6 at. %) -11 n

Ti02: Pd (Pd=8.4 at. %) -8 n

Ti02:(Eu=0.9 at. %, Pd=5.8 at. %) -80 n

Ti02:(Tb=0.6 at. %, Pd=9 at. %) 120 P

6050 I------II----~~------,..~-~-____t

40

III. RESULTS AND DISCUSSION

Fig. 2. Transmission characteristics of Ti02 thin films doped with Pd; Tb, Pdand Eu, Pd.

A, (nm)

100-----------------,

20

01-l*~~~~~~----,.~~.....:.,...___,_---r"_4

300 400 500 600 700 800 900 1000

80

In Fig. 2 the transmission spectra of Ti02:Pd thin filmswith different amount of Pd dopant have been presented.The region above solid horizontal line on the plot markedat 50 % of T'A is the acceptable region of good transparencyfor TOS materials. As one can see from Fig. 2, doping withPd of the Ti02 matrix caused clearly decreasing of thetransmission coefficient (T'A) in comparision with undopedTi02• It is caused mainly by presence of Pd dopant,because previous investigations shown that doping withlanthanide didn't get worse of the transmission coefficient[9]. The thin film doped with 23 at. % Pd didn't performrequirement of the good transparency and was eliminatedfrom further analysis. Besides, fundamental absorptionedge of TOS thin films was shifted from about 330 nm forundoped Ti02 in longer wavelength range for doped thinfilms.

From electrical point of view, Pd dopant makes possibleconsiderable decreasing of electrical resistivity until to valueabout 60 Qcm in the room temperature (Fig. 3). Therefore, Pddopant makes possible to get a thin films of TOS type in theroom temperature. While, lanthanide dopants in the form of Euand Tb elements causes different behaviours. The additionaldoping with Eu results in a bit better electrical conduction,when the Tb dopant gave increasing of resistivity until aboutfour order, but the thin film still belongs to TOS.

To confirm that obtained thin films of TOS type can formheterojunction with silicon substrate, they were examined bycurrent to voltage measurements in vertical configuration. InFig. 4 the I-V characteristics for selected thin film ofTi02:(Eu,Pd) have been presented. The shape of recordedcharacteristics is strongly non-linear what indicates theformation of heterojunction at the interface of TOS and siliconsubstrate. Moreover, in a semilogarithmic plot is visible thetemperature influence on I-V curves and when the temperaturewas increased the I-V characteristics were shifted.

2008 International Students and Young Scientists Workshop "Photonics and Microsystems" 75

-8

-;-p

-4 oUM

<]

4

T=304 KT=308 KT=320 KT=330 KT=339 KT=350 K

8

[9]

[10]

of Ti02 thin films obtained by high-energy reactive magnetronsputtering, Applied Surface Science, vol. 254, pp. 4396-4400, 2008D. Kaczmarek, J. Domaradzki, A. Borkowska, A. Podhorodecki, J.Misiewicz and K. Sieradzka, Optical emission from Eu, Tb, Ndluminescence centers in Ti02 prepared by magnetron sputtering,Optica Applicata, vol. 37, no. 4, pp. 433-438, 2007K. Sieradzka, J. Domaradzki, A. Borkowska, D. Wojcieszak, D.Kaczmarek, XIV Krajowa Konferencja KOWBAN'2007,Modelowanie mechanizm6w przewodnictwa w strukturach zcienkimi warstwami tlenk6w p6lprzewodnikowych, SzklarskaPor~ba,pp.221-226,2007

Fig. 4. I-V characteristics of Ti02:(Eu,Pd) thin films on Si-p at differentambient temperature

IV. CONCLUSIONS

In this work it has been shown that incorporation of dopantsinto Ti02 matrix gives TOS thin films with resistivity p - 60Qcm in the room temperature and also with good transparencyabove 50 %. The fabricated thin films displayed desired type ofelectrical conduction p for Ti02:(Tb, Pd) or n for Ti02:Pd andTi02:(Eu, Pd) in the room temperature.

ACKNOWLEDGEMENT

This work was financed from the statute sources (No. 343554) given by Polish Ministry of Science and Education.

Authors would like to thank Eugeniusz L. Prociow fromour Faculty for thin films performance.

REFERENCES

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[2] V.Diebold, The surface science of titanium dioxide, Surface ScienceRepor~,voI.48,pp.53-229,2003

[3] Zakrzewska K., Rozprawy - Monografie, Wydawnictwa AGH,Krak6w, 2003

[4] M. D. Stamate, On the non-linear I-V characteristics of dcmagnetron sputtered Ti02 thin films, Appl. Surf. Sci., vol. 205, pp.353-357,2003

[5] H. Kawazoe, M. Yasukawa, H. Hyodo, M. Kurita, H. Yanagi, andH. Hosono, P-type electrical conduction in transparent thin films ofCuAl02, Nature, vol. 389, pp. 939-942,1997

[6] T. Kamiya, H. Hiramatsu, K. Nomura, and H. Hosono, Deviceapplications of transparent oxide semiconductors: Excitonic blueLED and transparent flexible TFT, J. Electroceram., vol. 17, pp.267-275,2006

[7] A. Kudo, H. Yanagi, K. Veda, H. Hosono, H. Kawazoe, and Y.Yano, Fabrication of transparent p-n heterojunction thin film diodesbased entirely on oxide semiconductors, Applied Physics Letters,vol. 75, or 18, pp. 2851-2853, 1999

[8] R. Wasielewski, J. Domaradzki, D. Wojcieszak, D. Kaczmarek, A.Borkowska, E. L. Prociow, A. Ciszewski, Surface characterization