Surface Characterization of Ga-doped ZnO layers
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Surface Characterization of Ga-doped ZnO layers J.D. McNamara,1 J.D. Ferguson,1 M. Foussekis,1 I. Ruchala,1 M.A. Reshchikov,1 A.A. Baski,1 H. Liu,2 V. Avrutin,2 and H. Morkoç2 1 Department of Physics, Virginia Commonwealth University, Richmond, VA 23284 2 Department of Electrical and Computer Engineering, VCU, Richmond, VA 23284 ABSTRACT Epitaxial ZnO layers heavily doped with Ga (GZO) were grown at 400 °C under metaland oxygen-rich conditions in terms of metal-to-reactive oxygen ratio by plasma-assisted molecular beam epitaxy (MBE). Several atomic force microscopy (AFM) techniques were used to characterize the surface morphology and electrical properties of these GZO films in ambient conditions. Local I-V spectra indicate that layers grown under both O-rich and metal-rich conditions are highly resistive until a relatively high voltage sweep (±12 V) is used. After removal of an insulating surface layer, conduction is possible at lower voltages, but eventually the film resistivity increases and it again becomes insulating. In addition to local I-V spectra, local charge injection and subsequent surface potential measurements were used to probe surface charging characteristics. For charge injection experiments, a reverse-bias voltage is applied to the sample while scanning in contact mode with a metallized tip. The resultant change in surface potential due to trapped charge is subsequently observed using scanning Kelvin probe microscopy (SKPM). The layers deposited in a metal-rich environment demonstrate the expected behavior, but the O-rich layers show anomalous negative and positive charging. Finally, surface photovoltage (SPV) measurements using above-bandgap UV illumination were performed. The GZO layers produce SPV values of 0.4 to 0.5 eV, where the films deposited in an O-rich environment have slightly higher SPV values and faster restoration. INTRODUCTION ZnO heavily doped with Ga (GZO) has attracted a great deal of attention as a promising transparent conducting oxide (TCO) for electrode applications in solar cells and light-emitting diodes (LEDs) [1]. Desirable TCOs should show high transparency and conductivity, along with a high mobility of charge carriers [2]. However, surface electrical effects introduced during growth can limit the efficiency of TCOs [3]. To better understand the role of oxygen supply on GZO surface behavior during MBE, several GZO films were grown under varying oxygen pressures to investigate their surface conductivity and electrical behavior. Previous studies in our group have shown that films grown under metal-rich conditions have superior structural, electrical and optical characteristics compared to films deposited under O-rich conditions [4,5]. Here, we investigate surface-related behaviors such as local I-V, charge injection, and surface photovoltage using atomic force microscopy (AFM) techniques. EXPERIMENTAL GZO layers were prepared by radio frequency (RF) plasma-assisted MBE on a-plane sapphire. Initially, a thin ZnO buffer layer (~10 nm) was grown at 300 °C to provide a smooth
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