Oxygen Segregation to Nanopipes in Gallium Nitride
- PDF / 6,491,571 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 89 Downloads / 216 Views
0892-FF22-05.1
Oxygen Segregation to Nanopipes in Gallium Nitride
M. Hawkridge and D. Cherns H. H. Wills Physics Laboratory, Tyndall Avenue, Bristol, BS8 1TL, UK. ABSTRACT This paper examines the core structure and composition of threading dislocations in GaN grown by hydride vapour phase epitaxy. Transmission electron microscopy showed that screw dislocations have widely varying core structures from open cores (“nanopipes”) to closed cores, with irregular variations between the two observed along the length of many dislocations. New evidence was found demonstrating that the equilibrium structure of screw dislocations is a closed core configuration. Electron energy loss spectroscopy combined with high resolution imaging showed that 1.7±0.3 monolayers of nitrogen were substituted by oxygen at the surfaces of nanopipes. In contrast, closed core dislocations showed little evidence of oxygen segregation. It is argued that these results support a model where nanopipe formation is controlled by the segregation of oxygen to pits predominately associated with, albeit not exclusively, dislocations. The implication of the results in understanding the electronic behavior of dislocations in GaN is also discussed. INTRODUCTION Threading dislocations in GaN are known to reduce the optoelectronic performance of devices. In order to understand the properties of dislocations in GaN, it is important to understand their core structure and composition. Recent studies have suggested that these are inter-related. Threading dislocations in GaN are of 3 types, with Burgers vectors 1/3 (edge dislocations), 1/3 (mixed dislocations) and (screw dislocations). Our previous work has shown that in nominally undoped GaN grown by metal-organic vapour phase decomposition (MOCVD), screw type dislocations are of open core type, or nanopipes [1]. In MOCVD grown GaN heavily n-doped with Si, the morphology of dislocations are observed to change exhibiting constricted segments (closed cores) as well as more open structures [2]. The total density of nanopipes, including those unassociated with dislocations, have also been observed to increase with increasing concentration of certain dopants that include Si, but also O and Mg [3]. More recent studies have shown evidence of oxygen segregation to nanopipes in MOCVD GaN, although the exact amounts and distribution of the oxygen are unclear [4]. In contrast to screw dislocations, edge and mixed dislocations are usually observed to be closed core type. However, in MOCVD Al0.03Ga0.97N heavily doped with Mg, both edge and mixed dislocations have exhibited open cores on a fine scale [5]. There was strong, albeit indirect evidence, that these open cores were formed by the segregation of Mg by a stress field interaction mechanism, a result supported by recent surface voltage studies [6]. Also, in GaN grown by molecular beam epitaxy (MBE) under Ga-rich conditions we have reported open core dislocations of mixed and screw type [7] that may be decorated by excess Ga. No open core edge type dislocations were observed.
Data Loading...
