Hollow core dislocations in Mg-doped AlGaN
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Hollow core dislocations in Mg-doped AlGaN D. Cherns, Y.Q. Wang, R. Liu*, F.A Ponce*, H. Amano** and I. Akasaki** H.H. Wills Physics Laboratory, University of Bristol, Bristol BS8 1TL, United Kingdom
*Dept of Physics, Arizona State University, Tempe, AZ 85287-1504, USA **Dept of Mat. Sci and Eng, Meijo University, Nagoya 468-8502, Japan ABSTRACT Transmission electron microscopy has been used to investigate the core structure of threading dislocations in heavily Mg-doped (1020 cm-3) Al0.03Ga0.97N films grown on (0001) sapphire by metal-organic chemical vapour deposition. Evidence is presented that Mg segregates to edge and mixed dislocations, and that these dislocations often have open cores with diameters in the range 1-5nm. The mechanism of hollow core formation and the role of Mg are discussed.
INTRODUCTION Light emitting and electronic devices based on hexagonal (0001)GaN layers often have high densities of threading dislocations, 109 cm-2 or greater, introduced during the early stages of growth. The electronic properties of these dislocations are therefore of great interest. One factor which may affect these properties is whether dislocations have open or closed core structures. This appears to depend on the dislocation type, i.e whether the dislocations are of edge or a-type with Burgers vectors, b = 1/3, mixed or c+a-type with b = 1/3 or screw or c-type with b = . In observations of undoped GaN grown by metalorganic chemical vapour deposition (MOCVD), screw dislocations have been reported as open core with diameters of typically 5-30nm, whereas edge and mixed dislocations, which usually predominate in device structures, have closed core configurations [1]. There is evidence that the core structure is affected by impurities and by n-doping. We have found that screw dislocations in heavily Sidoped material can alternate between open core and closed core configurations [2]. Others have reported that the density of nanopipes in n-GaN depends on the level of background impurities during growth [3]. In this paper we report transmission electron microscope (TEM) studies of dislocations in heavily Mg-doped Al0.03Ga0.97N films grown by MOCVD on (0001) sapphire. It is shown that, in contrast to observations on n-GaN, dislocations of edge and mixed type are found to have open cores with diameters in the range 1-5nm. It is also shown that excess Mg segregates to these dislocations. The possible role of the Mg in generating open core dislocations is discussed. EXPERIMENTAL Studies were carried out on a 5µm thick Al0.03Ga0.97N layer doped with Mg (1020 cm-3). The layer was grown on (0001) sapphire by MOCVD at 1100ºC. The sapphire substrate was patterned with grooves along a direction although the observations
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described here relate to growth on the (0001) seed face. TEM studies were carried out using a 300kV Philips EM430 microscope operating at 250kV on samples prepared in both plan view and cross-sectional orientation. Microanalysis using electron energy loss spectroscopy (EELS) was carried out on a J
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