Characterization of Non-Polar Surfaces in HVPE Grown Gallium Nitride
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0955-I09-05
Characterization of Non-Polar Surfaces in HVPE Grown Gallium Nitride Kun-Yu (Alvin) Lai1, Judith A. Grenko2, V. D. Wheeler2, Mark Johnson2, E. A. Preble3, N. Mark Williams3, and A. D. Hanser3 1 Electrical and Computer Engineering, North Carolina State University, Raleigh, NC, 27695 2 Materials Science and Engineering, North Carolina State University, Raleigh, NC, 27695 3 Kyma Technologies, Inc., Raleigh, NC, 27617
ABSTRACT Non-polar surfaces of HVPE grown GaN were characterized by cathodoluminescence (CL), scanning electron microscopy (SEM), and secondary ion mass spectrometry (SIMS). Both of a- and m-plane GaN were prepared by growing thick GaN along the c-axis, and cutting in transverse orientations. The exposed non-polar surfaces were prepared by mechanical polishing (MP) and chemically mechanical polishing (CMP). Non-uniform luminescent characteristics on a- and m-plane GaN were observed in CL images, indicating a higher concentration of impurities in the area of more luminescence. CL spectra from the bulk samples revealed two peaks: 364 nm and 510 nm, related to band edge and impurity defects respectively. The detection by SIMS confirmed that oxygen was inhomogeneously incorporated during the growth of thick GaN layers. Surface qualities of a- and m-plane GaN were also investigated. The lower optical intensities from a-plane GaN at low acceleration voltages indicated more surface damages were introduced during polish. The optical intensity difference from the two samples was reduced at higher acceleration voltages. Similar CL intensities at low acceleration voltages from a- and mplane GaN substrates prepared by CMP indicated improved surface qualities. INTRODUCTION Non-polar GaN substrates have recently drawn much research interest due to its potential to improve the light efficiencies and high-frequency responses in optical and electronic devices [1, 2, 3]. Epilayers for commercially available III-nitride devices are grown along the polar [0001] direction and preserve wurtzite structures. It has been found that electric fields on the order of MV/cm are generated along the [0001] direction in wurtzite III-nitrides by piezoelectric (PZ) and spontaneous (SP) polarizations [4, 5]. PZ polarization is induced by the strain from the lattice mismatch between substrates and epilayers, while SP polarization comes from the lack of inversion of symmetry in wurtzite structures. These substantial electric fields pull the electrons and holes towards opposite sides in quantum wells, resulting in the reduced overlap between wave functions. The reduced overlap adversely affects GaN-based optical devices in 2 ways: i) decrease of the luminous efficiency [6] ii) blue-shift of the transition energy with increased bias [7, 8]. For electronic devices, polarity effects not only delay the frequency responses of the carriers in the channels of GaN-based heterostructure field effect transistors HFETs [3],the strong fields also induce surface and interface traps, which are responsible for the current collapse and persistent
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