Observation of dislocation etch pits in epitaxial lateral overgrowth GaN by wet chemical etching
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Observation of dislocation etch pits in epitaxial lateral overgrowth GaN by wet chemical etching T. C Wen, S.C. Lee, H. S. Chuang, C. H. Chiou, and W. I Lee Department of Electrophysics, And Microelectronics and Information Systems Research Center, National Chiao Tung University, HsinChu, Taiwan, Republic of China ABSTRACT This work investigates dislocation etch pits in epitaxial lateral overgrowth (ELO) GaN by wet chemical etching. A mixture of H2SO4 and H3PO4 was used as a dislocation etchant, and SEM and AFM were employed to observe the surface topography. For the as-grown sample, SEM images present the flat, smooth surface without any pits or hillocks. After the chemical etching, hexagonal shaped etch pits were observed at the edge of ELO GaN. AFM observation of etched ELO GaN displayed high densities of etch pits clustered in the “window” region and the coalescent line of two growing fronts. In contrast, the overgrowth region was nearly free of etch pits. Moreover, we observed that different sizes of etch pits dominated in “window” region and coalescent region. This implied different types dislocations dominated in these regions. INTRODUCTION Owing to a significant lattice mismatch between GaN and sapphire, GaN films grown on sapphire contain dislocation densities of 108-1010 cm-2 [1]. Dislocations are believed to limit the fabrication of high quality nitride-based devices with a longer lifetime and high performance. Lester et al. indicated that nitride-based light emitting diodes have great tolerance of high dislocation densities [1]. This implies that dislocations in these diodes do not act as efficient nonradiative recombination centers. However, high dislocations densities still influence the device characteristic and performance. Kozodoy et al. found that reverse bias leakage current of p-n junction diodes was three orders of magnitude larger than that in the high dislocation density region [2]. Shiojima et al. revealed that dislocations caused a small area with low Schottky barrier height at the interface [3]. Selective area growth (SAG) and epitaxial lateral overgrowth (ELO) are effective methods for the reduction of dislocation densities in GaN films. Furthermore, this reduction improves the device performance. Nakamura et al. increased the lifetime of their laser diode from 35 to 1150 hours by the ELO method [4]. There are many reports investigating defects in ELO GaN by
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transmission electron microscopy (TEM) [5,6]. However, there have been few reports which investigated dislocations in ELO GaN by wet chemical etching. In this work dislocation etch pits of ELO GaN are observed by wet chemical etching. Wet chemical etching is a conventional method for evaluating dislocations in an III-V compound semiconductor. There have been many observations of dislocation etch pits on GaN surface [7-9]. For example, Shiojima et al. [9] identified that the dislocation etch pit densities observed by atomic force microscope (AFM) were almost the same as the mixed dislocation densities observed by TEM. In this
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