Defect reduction in (11-20) a-plane GaN by two-step epitaxial lateral overgrowth
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Defect reduction in (11-20) a-plane GaN by two-step epitaxial lateral overgrowth X. Ni1, Ü. Özgür1, Y. Fu1, N. Biyikli1, H. Morkoç1, and Z. Liliental-Weber2 1 Dept. of Electrical Engineering, Virginia Commonwealth University, 601 W Main St., Richmond, VA, 23284 2 Lawrence Berkeley National laboratory, Berkeley, CA, 94720
ABSTRACT We report a two-step growth method to obtain uniformly coalesced epitaxial lateral overgrown a-plane GaN by metalorganic chemical vapor deposition (MOCVD). By obtaining a large wing height to width aspect ratio in the first step followed by enhanced lateral growth in the second step via controlling the growth temperature, we reduced the tilt angle between the advancing Ga-polar and N-polar wings for improved properties. Transmission electron microscopy (TEM) showed that the threading dislocation density in the wing area was 1.0×108cm-2, more than two orders of magnitude lower than that in the window area (4.2×1010cm-2). However, a high density of basal stacking faults, 1.2×104 cm-1, was still observed in the wing area. Near field scanning optical microscopy (NSOM) at room temperature revealed that the luminescence was mainly from the wing regions with very little contribution from the windows and meeting fronts. These observations suggest that due to significant reduction of threading dislocations radiative recombination is enhanced in the wings. INTRODUCTION In c-axis-oriented hexagonal GaN system, the spontaneous and strain induced piezoelectric polarizations produce strong electric fields, which cause spatial separation of electrons and holes in quantum wells that are used for active regions in optical devices. Such a separation increases the radiative lifetime1 at the expense of the quantum efficiency,2 and also results in a red shift of the emission. One approach to overcome this problem is to grow m-plane or a-plane hexagonal GaN, which are nonpolar. Recent studies on a-plane AlGaN/GaN quantum wells 3 , 4 and related LEDs 5 have already demonstrated the absence of polarization-induced electric field. In order to realize high-performance nitride devices, epitaxial lateral overgrowth (ELO) method has been used to reduce the density of threading dislocations in a-plane GaN using MOCVD. 6 , 7 However, one should consider the wing tilt, which was shown to be an important matter for c-plane GaN ELO. 8 In this paper, we investigate the effects of growth parameters on overgrown a-plane GaN, and report on structural and optical characterization for the optimized GaN ELO samples grown by MOCVD. EXPERIMENT The (1120) a-plane GaN films were grown on (1 102) r-plane sapphire substrates.9 A 1.5 µm-thick a-plane GaN film with a low temperature GaN nucleation layer was used as the ELO template. An approximately 100 nm-thick SiO2 layer was grown on the a-plane GaN template by remote plasma enhanced chemical vapor deposition. Using conventional photolithography and buffer oxide etch (BOE), a striped mask pattern was transferred to SiO2. The pattern consisted of 4 µm-wide open windows and
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