A Study of High Quality Al-doped ZnO Thin Films Grown at Low Temperature by Pulsed Laser Deposition
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A Study of High Quality Al-doped ZnO Thin Films Grown at Low Temperature by Pulsed Laser Deposition R. K. Gupta1, K. Ghosh1, S. R. Mishra2, and P. K. Kahol1 1 Physics, Astronomy and Materials Science, Missouri State University, Springfield, MO, 65897 2 Department of Physics, The University of Memphis, Memphis, TN, 38152 ABSTRACT Highly conducting and transparent Al-doped ZnO (AZO) thin films, which are oriented along c-axis and have wurtzite structure, were grown on quartz substrate at low temperature by pulsed laser deposition. The techniques of x-ray diffraction (XRD), Raman spectroscopy, atomic force microscopy (AFM), optical transmission spectroscopy (OTS), electrical resistivity, and Hall Effect were used to study the effect of growth temperature and oxygen pressure on the structural, electrical transport, and optical properties of these films. The optical transparency in all the films is high and does not change much with oxygen pressure and growth temperature. However, electrical parameters such as resistivity, carrier concentration, and mobility strongly depend on both oxygen pressure and growth temperature. The temperature dependence resistivity measurement indicates semiconducting behavior of all the films. A detailed study indicates that the films which are highly conducting and transparent correspond to an optimum temperature of 200 ûC and an oxygen pressure of 5 × 10-7 bar. Higher transmittance of the AZO films compared with pure ZnO and ITO and comparable mobility make us to suggest that Al-doped ZnO is an excellent material for optoelectronic applications. INTRODUCTION Organic light emitting diodes (OLEDs) are potential candidates for use in the next generation of display technology [1]. The OLEDs use a transparent conductive oxide (TCO) layer as anode. Although indium tin oxide (ITO) is most widely used as a TCO, indium is relatively scarce in the earth’s crust and thus production cost of ITO is relatively high [2]. Zinc oxide (ZnO) is a non-toxic and inexpensive material compare to ITO [2]. It is a II-VI n-type semiconductor with a band gap of approximately 3.3 eV at room temperature and its electrical conductivity is mainly due to excess zinc at interstitial sites [3]. Its electrical properties can be modified by doping with cationic or anionic substitution [4]. For example, cationic doping of ZnO films with trivalent atoms such as Al, In, and Ga leads to increased electrical conductivity. Aluminum doped zinc oxide (AZO) films show good optical transmittance in the visible and near-infrared region. Because of such unique properties, AZO films can be used as a transparent conducting electrode in devices [5]. A number of fabrication techniques such as sputtering [6], chemical vapor deposition [5], spray pyrolysis [7], and pulsed laser deposition [8] have been used to grow ZnO films. The pulsed laser deposition (PLD) technique has many advantages such as: (a) It has the ability to maintain target composition in the deposited thin films; (b) High quality films can be deposited at room temp
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