The Effect of Growth Stoichiometry on the GaN Dislocation Core Structure
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The Effect of Growth Stoichiometry on the GaN Dislocation Core Structure Marcus Q. Baines1, David Cherns1, Julia W. P. Hsu2 and Michael J. Manfra2 H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol, BS8 1TL, UK 2 Bell Laboratories, Lucent Technologies, 700 Mountain Avenue, Murray Hill, NJ 07974
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ABSTRACT Plan-view transmission electron microscopy was used to study the core structures of different dislocations in (0001) GaN layers grown under Ga-rich and Ga-lean conditions by molecular beam epitaxy. In Ga-rich samples at least one third of mixed type dislocations were open-core, and edge dislocations were observed to be closed-core. In contrast, under Ga-lean conditions, all dislocations were observed to be closed-core, and many were associated with pits at the sample surface. High resolution studies of the open core dislocations revealed that many were decorated with a disordered deposit, the origin of which is discussed. INTRODUCTION The growth of GaN devices on substrates such as (0001) sapphire and SiC leads to high densities of threading dislocations, often exceeding 109 cm -2. These dislocations have attracted much attention because of their influence on device properties. An important question which bears on the electronic properties is whether threading dislocations have open or closed core structure. This in part depends on the dislocation type, i.e. whether the dislocations are of a-type (edge dislocations with Burgers vectors b = 1/3< 1120 >), a+c-type (mixed dislocations, b = 1/3< 1123 >) or c-type (screw dislocations, b = ). In undoped material grown by metalorganic chemical vapour deposition (MOCVD), previous studies have shown that screw dislocations can be open-core, with diameters in the range 5-25nm [1]. Open-core screw dislocations have also been observed in n-doped GaN [2]. The work of Liliental-Weber et al indicates that the formation and density of open-core screw dislocations may depend on impurities (e.g. O) and dopants (e.g. Si) [3]. In p-doped GaN, the situation appears to be different. Recently, we reported the first observations of open cores on edge and mixed dislocations in a Mg-doped MOCVD sample. The segregation of Mg to dislocations seemed to be a factor in the creation of open-cores [4]. It has also been shown that core structure is affected by stoichiometry. Hsu et al have compared the structure and electrical properties of dislocations in GaN grown by molecular beam epitaxy (MBE) under Ga-rich and Ga-lean conditions [5]. Transmission electron microscope (TEM) studies, carried out on cross sectional samples, suggested that under Ga-rich conditions, dislocations with a screw component (i.e. b=c or c+a) may have extended cores, with some evidence that the cores were decorated with excess Ga. Under Ga-lean conditions, the dislocations had conventional, closed cores. Using scanning current-voltage microscopy (SIVM) it was found that samples grown under Ga-rich conditions show three orders of magnitude higher reverse bias leakage than those grown und
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