Properties of Epitaxial ZnO Thin Films for GaN and Related Applications
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H. Shen*, M. Wraback*, J. Pamulapati*, S. Liang**, C. Gorla**, and Y. Lu** *U.S. Army Research Laboratory, Sensors and Electron Devices Directorate, AMSRL-SE-EM, 2800 Powder Mill Rd., Adelphi, MD 20783-1197 [email protected] "**Dept. of Electrical and Computer Engineering, Rutgers University, Piscataway, NJ 08855-0909 Cite this article as: MRS Internet Nitride Semicond. Res. 4SI, G3.60 (1999) ABSTRACT In this paper, we present a detailed study of high quality (110) ZnO films, epitaxially grown on R-plane sapphire substrates by metal-organic chemical vapor deposition (MOCVD). The epitaxial relationships are (112 0) ZnO//( 0112) A12 0 3 and [000 11 ZnO//[ 0 111] A120 3 as confirmed by X-ray diffraction (0-20, and 4)-scan) and high-resolution cross-sectional transmission electron microscopy (HR-TEM). Low temperature photoluminescence (PL) indicates the ZnO thin films are almost strain free. Optical absorption and reflection measurements with linearly polarized light indicate a strong optical anisotropy. The polarization rotation towards the C-axis associated with the optical anisotropy is utilized to demonstrate an optically addressed ultra-fast, ultraviolet light modulator. INTRODUCTION High quality zinc oxide (ZnO) films are useful for many applications. One of the more recent applications is the use of ZnO as a substrate for growth of GaN based materials[ 1-3] since the lattice mismatch between GaN and ZnO is relatively small. Due to a lack of low cost ZnO substrates, high-quality ZnO buffer layers on other substrates are of particular interest. In addition, it is also possible to develop UV lasers and modulators from these ZnO films.[4,51 The most common technologies used for depositing ZnO films are laser ablation and sputtering.[61 Improvements have been made through triode sputtering and other new deposition techniques. Despite these advancements, there are still some problems associated with sputtering. Metal-organic chemical vapor deposition (MOCVD)[7,8] is an alternative technique for growth that has advantages such as chemical and thermodynamic dependent growth, control at the atomic level, large area deposition, and the possibility of different in situ doping processes. While most research has focused on ZnO films grown on (0001) oriented sapphire substrates (i.e. C-plane A120 3), less research has been done on ZnO grown on other planes. In this paper we report a detailed study of the properties of ZnO grown on (0112) oriented sapphire (R-plane). G 3.60 Mat. Res. Soc. Symp. Proc. Vol. 537 ©1999 Materials Research Society
SAMPLE GROWTH The samples used in this study were grown by MOCVD. R-plane sapphire was used as the substrate. The precursors used were DiEthyl Zinc (DEZn - (C2H5 )2Zn) and 02. Typical growth conditions are as follows: chamber pressure of 20 to 50 torr; a growth temperature of 250 to 600 'C; and a total carrier gas flow from the top of 5000 to 15000 sccm. The thickness of the sample is 1 jtm[9]. STRUCTURAL PROPERTIES Figure 1 shows the x-ray 0-20 scans from the sample. From the scan we found
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