Effect of Growth Conditions on Electronic and Structural Properties of GZO Films Grown by Plasma-enhanced Molecular Beam
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Effect of Growth Conditions on Electronic and Structural Properties of GZO Films Grown by Plasma-enhanced Molecular Beam Epitaxy on p-GaN(0001)/Sapphire Templates H.Y. Liu1, V. Avrutin1∗, N. Izyumskaya1, M.A. Reshchikov2, S. Wolgast3, C. Kurdak3, A.B. Yankovich4, A. Kvit4, P. Voyles4, Ü. Özgür1, and H. Morkoç1,2 1
Department of Electrical and Computer Engineering, Virginia Commonwealth University, Richmond, VA 23284; 2Physics Department, Virginia Commonwealth University, Richmond, VA 23284; 3Physics Department, University of Michigan, Ann Arbor, MI 48109; 4 Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, WI 53706
Abstract: We report on a strong effect of p-GaN surface morphology on the growth mode and surface roughness of ZnO:Ga films grown by plasma-assisted molecular-beam epitaxy on p-GaN/c-sapphire templates. A range of ZnO:Ga surface morphologies varying from rough surfaces with well defined three-dimensional islands, capable to enhance light extraction in light-emitting diodes, to rather smooth surfaces with a surface roughness of ~ 2 nm suitable for vertical-cavity lasers can be achieved by controlling the surface morphologies of p-GaN. Optical transmittance measurements revealed high transparency exceeding 90% in the visible spectral range for ZnO:Ga with both types of surface morphology. INTRODUCTION ZnO doped with Ga and Al (dubbed in the literature as GZO and AZO) has recently attracted considerable attention due to the possibility to replace indium tin oxide (ITO) for electrode applications in optoelectronic and photovoltaic devices where both high conductivity and high transparency are required to improve conversion efficiencies [1]. GZO/AZO has an advantage over ITO in GaN-based devices since ZnO and GaN share the same wurtzite crystal structure with a small lattice mismatch of ~1.8% that enables epitaxial growth of ZnO electrodes with improved carrier mobility and optical transparency. In addition, GZO/AZO exhibits several other advantages over ITO, such as low cost, abundance of the raw materials, superior thermal conductivity, transparency, and thermal stability [2]. Comparative analysis of properties of various transparent conducting oxides, including GZO/AZO, ITO, and fluorine-doped tin oxide, can be found in elsewhere [3]. In addition to the high conductivity and high transparency required, the surface morphology of GZO/AZO is also an important factor to be considered, which could have considerable effect on device performance. For example, rough surface is desired to improve the photon extraction efficiency and consequently the external quantum efficiency in GaN-based light emitting diodes (LEDs). High efficiency of epitaxial GZO with a rough surface morphology as transparent contact to the top p-type GaN layer of InGaN LEDs has been proved by comparing its performance with the LEDs ∗
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having the same structure but utilizing Ni(5-nm)/Au(5-nm) as p-contact [4]. The GZO-LEDs showed 50% higher external quantum efficiencies at the s
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