the Netherlands Key words: Sputtering Deposition; ZnO; Resistivity; Transmittance

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1 Advanced Materials Research Online: ISS: 1-95, Vols , pp 7-75 doi:.0/ 011 Trans Tech Publications, Switzerland Electrical and Optical Properties of Indium and Aluminum Doped Zinc Oxide Films Prepared by RF Magnetron Sputtering Luting Yan 1, a, J.K. Rath, R.E.I. Schropp 1 School of science, Beijing Jiaotong University, Beijing 00, P.R. hina anophotonics, Debye Institute for anomaterials Science, Utrecht University, 50 TA Utrecht, the etherlands a ltyan@bjtu.edu.cn Key words: Sputtering Deposition; ZnO; Resistivity; Abstract. ZnO: In (IZO, wt % In O ) and ZnO: Al (AZO, 1wt % Al O ) films were deposited on orning glass substrates by RF magnetron sputtering. The samples were either prepared on unheated substrates and post annealed in at different temperatures, or prepared at elevated temperatures. Electrical, optical and structural properties were investigated as a function of deposition temperature and annealing temperature. Increasing the substrate heater temperature would lead to a decline in the electrical conductivity of IZO films, while AZO films showed unchanged performance in the substrate heater temperature range of Post annealing appears to be an effective way to improve the electrical properties of both IZO and AZO films without sacrificing transparency. In this work, AZO films have higher conductivity and light transmission than IZO films. Introduction Transparent conductive oxides (TO) are essential in various types of thin film solar cells, dye-sensitized solar cells and polymer solar cells. The quality of TO has an important influence on obtaining satisfactory solar cell performance. High-quality TO has a high transmittance and conductivity, and has to be very stable during preparation and operation of the solar cells. Tin doped indium oxide (ITO) is one of the widely used TO. ompared with ITO, doped zinc oxide thin films show many advantages such as easy fabrication, high stability in hydrogen plasma, and nontoxic nature. Moreover, the raw materials are relatively cheap and abundant [1]. In this work, IZO and AZO films were deposited by RF magnetron sputtering at different substrate heater temperature. The samples prepared without heating were post annealed in a oven for about one and a half hours at different temperatures. Electrical and optical properties were investigated as a function of substrate heater temperature and annealing temperature. Experimental Preparation IZO and AZO films. IZO and AZO thin films were prepared on orning glass substrates by RF magnetron sputtering in the deposition system Salsa at different substrate heater temperature (in short, ), U(indicating the unheated condition and about 90 because of the effect of the sputter plasma ), 150,00,50, 00 and 50. Two ceramic targets, ZnO (In O, wt %) and ZnO (Al O, 1 wt %) were used. Oxygen (0.1%) was used in the gas flow together with argon. The working pressure during the deposition of the films was maintained at 0. Pa (IZO) and 0. Pa (AZO) separately. Power was controlled at 00 W. The deposition time of IZO and AZO films were 000 s and 100 s, respectively, to keep the film thickness at around 00nm, although some fluctuation in deposition rate occurs as a function of substrate heater temperature. IZO and AZO thin films prepared at the unheated condition were annealed in a oven for about one and a half hours at different annealing temperature ( ). Methods for haracterization. The carrier concentration, Hall mobility and resistivity were obtained from Hall effect measurements (PhysTech GmbH RH00). The reflectance and transmittance of the layers were measured by Eta-Optik mini R-T spectrometer. All rights reserved. o part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-/05/1,:5:)

2 Advanced Materials Research Vols Results and Discussion The effects of on the electrical properties of IZO and AZO. Fig.1 shows the variation of resistivity, carrier concentration and Hall mobility of the IZO films prepared at different. The resistivity is a combination of mobility and carrier concentration. With the increase of, the value of carrier concentration decreases except at the point of 00. The mobility is very small and fluctuates with the change of. The highest value of mobility appears at 150.The resistivity grows with increasing. / is an important parameter to characterize TO film [], but it is believed in this work, / is more suitable, which not only reflects a relative value of /, also embodies an absolute value of. If / is high, it indicates that the film has a high mobility and a low carrier concentration that fulfills the demand for a low resistivity and high IR transmission. The highest / of.1-0 cm 7 /V /s at 150 (see Table1) indicates that the best IZO film could be prepared at a relatively low substrate temperature. Resistivity - (Ω cm) U Set substrate heater temperature 1 0 arrier concentration 0 (cm - ) 7 5 Mobility(cm V -1 s -1 ) Resistivity - (Ω cm) U Set substrate heater temperature arrier concentration 0 (cm - ) Mobility(cm V -1 s) Fig.1.Electrical resistivity, carrier concentration and mobility of Fig..Electrical resistivity, carrier concentration and mobility of the IZO films prepared at different. the AZO films prepared at different. ompared with IZO films, AZO films have higher mobility and lower resistivity (see fig. ). With the increase of, the values of carrier concentration showed periodic fluctuations and have the highest value of.09 0 cm - at 00. The mobility first increases then slightly decreases to a platform in the temperature range of 00-00, and finally decreases quickly when further increasing. The resistivity showed a stable plateau in the temperature range of is a turning point where carrier concentration and mobility drop dramatically, leading to a maximum resistivity of.17 - Ω cm. Table1 shows / ratio of AZO films has a maximum of cm 7 /V /s at 150, but changes little in the temperature range of , which showed a wide suitable temperature range to prepare AZO films. Table1. / ( -0 cm 7 /V /s ) ratio of IZO and AZO films prepared at different ( ) U / (IZO) / (AZO) The effect of on the electrical properties of IZO and AZO. When the annealing temperature is higher than 00, carrier concentration and electron mobility of the IZO films are simultaneously increased (fig.). Table show the highest / ratio of 9-0 cm 7 /V /s for IZO films appeared at 00. Through post-annealing, grains may grow and grain boundaries become smaller, which can explain the increased conductivity. For the AZO films (fig.), the trend in carrier concentration and Hall mobility as a function of annealing temperature are similar with IZO films. With increasing annealing temperature, the conductivity of AZO films improves, but the amount of the increase is not as large as for IZO film and the change of / ratio is not obvious (see table ). The average value of

3 7 Advanced Engineering Materials the / ratio for AZO films is much higher than for IZO films, which shows that the AZO films have better conductivity and transparency than the IZO films. Resistivity - (Ω cm) U As-deposited Annealing temperature ( ) 1 1 arrier concentration 0 (cm - ) 1 1 Mobility(cm V -1 s -1 ) Resistivity - (Ω cm) U As-deposited Annealing Temperature arrier concentration 0 (cm - ) Mobility(cm V -1 s -1 ) Fig.. Electrical resistivity, carrier concentration and mobility of the IZO films at different Fig.. Electrical resistivity, carrier concentration and mobility of the AZO films at different Table. / ( -0 cm 7 /V /s ) ratio of IZO and AZO films prepared at different ( ) U / (IZO) / (AZO) U = 150 = 00 = 50 = U =0 =00 = wavelength (nm) wavelength (nm) Fig. 5. spectra of IZO film at different Fig.. of IZO film at different The effect of and on the optical properties of IZO and AZO films. From transmittance spectra of IZO films prepared at different and annealed at different (fig.5 and fig.), it can be seen that transmittance increased in the range of shorter wavelengths (lower than 700 nm) and decreased at longer wavelength range ( nm) with increasing. With increasing, the transmittance in the longer wavelength range (00-00 nm) is increased. To AZO films (fig. 7 and fig. ), transmittance can be increased by increasing, but it changed little with increasing. ompared to IZO films, AZO films have higher transmittance, especially at shorter wavelength. s of AZO films exceed at 00 nm, but for IZO films, it is only close to U =00 =50 = U =0 =00 = Wavelength (nm) Wavelength(nm) Fig.7. spectra of AZO film at different Fig.. of AZO film at different

4 Advanced Materials Research Vols Summary. IZO and AZO films were deposited on orning glass substrates by RF magnetron sputtering. To IZO films, increasing will lead to a decline in the performance of conductivity. The resistivity of AZO films showed a stable platform in the temperature range of There were a sharp decline in the performance of conductivity, when further increase to 50. of AZO films can be partially improved through increasing. Post annealing is an effective way to improve the electrical properties of IZO and AZO films without sacrificing transparency. ompared with IZO (In O, wt %) films, AZO (Al O, 1wt %) films have higher conductivity and higher light transmission, and are more suitable for solar cells. Acknowledgments The work was supported by Huygens hina scholarship, ational atural Science Foundation of hina (090701) and School Foundation of Beijing Jiaotong University(009JBZ019-).The authors are grateful to Yanchao Liu, J.W.A.Schüttauf and T.M.M.Bakker for their helpful discussion. References [1] Minami T. Mater. Res. Soc. Bull. 5(), (000) p. [] Yanchao Liu. 0 Dissertation (ISB ) p7.

5 Advanced Engineering Materials.0/ Electrical and Optical Properties of Indium and Aluminium Doped Zinc Oxide Films Prepared by RF Magnetron Sputtering.0/