Effect of Remote Hydrogen Plasma Treatment on ZnO Single Crystal Surfaces
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M3.9.1
Effect of Remote Hydrogen Plasma Treatment on ZnO Single Crystal Surfaces
Yuri M. Strzhemechny Center for Materials Research, The Ohio State University, Columbus, OH 43210, U.S.A. John Nemergut Department of Electrical Engineering, The Ohio State University, Columbus, OH 43210, U.S.A. Junjik Bae1 Department of Physics, The Ohio State University, Columbus, OH 43210, U.S.A. David C. Look Semiconductor Research Center, Wright State University, Dayton, OH 45435, U.S.A. Leonard J. Brillson Department of Electrical Engineering, Department of Physics, and Center for Materials Research, The Ohio State University, Columbus, OH 43210, U.S.A. 1
Currently at Center for Quantum Devices, ECE Department, Northwestern University, Evanston, IL 60208, U.S.A. ABSTRACT We have studied the effects of hydrogen plasma treatment on the defect characteristics in single crystal ZnO grown at Eagle-Picher by chemical vapor transport. Depth-dependent cathodoluminescence (CL) spectra, temperature-dependent (9-300 K) and excitation intensity-dependent photoluminescence (PL) spectra reveal significant changes resulting from unannealed exposure of n-type ZnO to a remote hydrogen plasma. Low temperature PL spectra show that this hydrogen exposure effectively suppresses the free-exciton transition and redistributes intensities in the bound-exciton line set and twoelectron satellites with their phonon replicas. The resultant spectra after hydrogenation exhibit a new peak feature at 3.366 eV possibly related to a neutral donor bound exciton. A simple thermal analysis of the activation energy for the 3.366 eV line yields 5-10 meV. Hydrogenation also produces a violet 100 meV-wide peak centered at 3.16 eV. Remote plasma hydrogenation produces similar changes in room-temperature CL spectra: nearband edge emission intensity increases with hydrogenation. Furthermore, this new emission increases with proximity to the free ZnO surfaces, i.e., with decreasing the energy of the incident electron beam from 3.0 down to 0.5 keV. Subsequent annealing at 450 ºC completely restores both the PL and CL spectra in the sub-band gap range. The appearance of a new bound-exciton feature at 3.366 eV with H plasma exposure, the near-surface nature of the spectral changes, and the reversibility of spectral features with annealing indicate a direct link between H indiffusion and appearance of a shallow donor.
M3.9.2
INTRODUCTION ZnO has emerged as a promising new semiconductor for optoelectronic and hightemperature, high power microelectronics, yet its electrically active defect properties are relatively unexplored. The role of hydrogen-related defects has attracted significant attention recently because of their potential of meeting practical challenges in tailoring ZnO properties. Today there is a substantial experimental evidence of the various ways hydrogen affects key features of ZnO: enhancement of charge carrier parameters and optoelectronic properties, as well as control of deep levels. For example, it was shown that hydrogen improves conductivity [1], [2]
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