Direct Observations of Atomic Structures of Defects in GaN by High Resolution Z-Contrast Stem
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OF DEFECTS IN GAN
4 3 Y. Xin" 2, S.J. Pennycook2 ,, N.D. Browning', P. D. Nellist , S. Sivananthan', B. Beaumont , JP Fauriel' and P. Gibart4
'Department of Physics, University of Illinois at Chicago, Chicago. IL 60607-7059, USA. 2 Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37381-6031, USA 3 Cavendish Laboratory, Cambridge University,Madingley Road,Cambridge CB3 OHE,UK "4 CRHEA-CNRS, rue Bernard Gregory, 06560 Valbonne, France. ABSTRACT GaN/(0001)Sapphire grown by low pressure MOVPE is studied by high resolution Z-contrast imaging using STEM. First direct observation of the threading dislocation with edge character shows the atomic core structure, which appears to have a similar configuration to the {10-10} surface. The surfaces of the nanopipe walls are on { 10-10} with the terminating layer between the atoms with one bond per pair. In addition, the high resolution Z-contrast image of the prismatic stacking fault confirms the results by conventional HRTEM. INTRODUCTION Since the success in the production of high efficiency light emitting diodes (LED) based on gallium nitride material[1], wide gap nitride semiconductors have attracted much attention recently due to their promising performance in short-wave length LED, blue lasers, and other wide range of electronic-opto applications. Many efforts therefore have been mainly devoted into two parts, one on the improvement of the crystal quality of the nitride layers by different growth methods, the other on the investigation of the less-known electronic and optical properties of these materials by various methods and also by theoretical calculations. The most interesting and intriguing issue concerning GaN lies in the fact that the GaN-based LED is found to be remarkably insensitive to the high density of structural defects such as dislocations in the films, which is most unusual compared with LEDs based on III-V arsenides and phosphides[2]. It is well know that dislocations introduce non-radiative recombination centres in these GaAsP based LED. In this paper, we report the first direct observation of core structures of threading dislocations with edge character in hexagonal GaN, and some other defects, such as prismatic stacking fault, nanopipe.. These observations allow the reasons for the insensitivity to defects to be intimated. EXPERIMENT The GaN films used in this study were grown by metalorganic vapor phase epitaxy (MOVPE), which have been reported in detail in other papers [3]. A brief description of the growth process and general features of the film is presented here. The films were deposited on (0001) sapphire substrate in a low-pressure MOVPE reactor. The substrate was nitridated at high temperature, and the buffer layer grown at 525°C, followed by heat ramping up to 1150'C, and growth proceeded under hydrogen with V/IlI ratio 1400. Conventional TEM images were obtained using Philips CM12 microscope, while the atomic structures of the defects are studied by high resolution Z-contrast imaging [4] on a 300kV scanning transmission el
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