Theory of Threading Edge and Screw Dislocations in GaN

Device quality wurtzite-GaN (a) can be grown using metalorganic chemical vapor phase deposition (MOCVD) on (0001) sapphire substrates. The large lattice misfit results in dislocation tangles near the interface but isolated threading dislocations, parallel to c, with densities ,109 cm22 and Burgers vectors c, a, and c 1 a persist beyond ,0.5 mm above the interface [1–3] and thus cross the active region of the devices. An unexpected finding [4,5] is that these dislocations do not lead to a pronounced reduction in the device lifetime of the lightemitting diodes [6] or blue lasers [7]. Recent cathodoluminescence (CL) studies of the yellow luminescence (YL) centered at ,2.2 eV have shown that the YL is spatially nonuniform and can be correlated with extended defects and especially low angle grain boundaries which contain dislocations [8]. On the other hand, atomic force microscopy in combination with CL has led to the conclusion that threading dislocations act as nonradiative recombination centers and degrade the luminescence efficiency in the blue light spectrum of the epilayers [9]. Screw dislocations in a-GaN have elementary Burgers vector 6c and are unusual in often being associated with nanopipes with diameters 50–250 Å [10]. Pure edge dislocations lying on h1010j planes are a dominant species of dislocation in a-GaN grown by MOCVD on (0001) sapphire, occurring at extremely high densities of ,108 1011 cm22. Here we explore the structure and electronic properties of threading screw and edge dislocations in GaN using an ab initio local-density functional (LDF) cluster method, AIMPRO, and a density functional based tight binding method DF-TB which can be used for both clusters