Subhash Mahajan

Gallium nitride epitaxy on (0001) sapphire

Abstract Two-step epitaxy consisting of low-temperature GaN nucleation layers (NLs) and high temperature (HT) GaN overgrowths, deposited on sapphire (0001) by metal-organic chemical vapour deposition, has been examined by transmission electron microscopy and atomic force microscopy. Results indicate that NLs consist of faceted crystalline islands that exhibit a spread in rotation about the (0001) axis. These islands undergo a metamorphosis on annealing and become rounded. The lateral growth during HT deposition of GaN occurs by the attachment of atomic species to steps associated with the rounded islands, resulting in faceted flat-topped islands. Furthermore, growth occurs preferentially in certain regions which evolve into ‘growth patches’. These patches then grow vertically and laterally over the underlying subgrains. The coalescence of these patches produces a continuous GaN layer. The origins of threading dislocations (TDs) in GaN layers have also been investigated. Results show that the majority of TDs do not form during the coalescence of islands as was assumed previously. Instead, two possible sources of TDs have been identified: firstly, growth faults in NLs and, secondly, point defects. Partial dislocations associated with the faults in the NLs constitute the major source of basal plane dislocations, which may develop into TDs by self-glide and climb. It has also been suggested that the mosaic structure observed in fully grown GaN layers is due to elastic interactions between TDs, which glide and climb to form subgrain boundaries.

Influence of AlN nucleation layer growth conditions on quality of GaN layers deposited on (0 0 0 1) sapphire

Abstract The influence of AlN nucleation layer (NL) growth conditions on the quality of GaN layer deposited on (0xa00xa00xa01) sapphire by organometallic chemical vapor phase epitaxy (OMVPE) has been investigated by X-ray diffraction, atomic force microscopy and transmission electron microscopy. Growth pressure, temperature and time were varied in this study. Results indicate that there exists an optimal thickness of the NL is required for optimal growth. Both thin and thick NLs are not conducive to the growth of high-quality GaN layers. Arguments have been developed to rationalize these observations.

Latent Iron in Silicon

It is usually assumed that the iron in iron-contaminated, boron-doped silicon exists as FeB pairs. The iron can be cycled between its interstitial (Fei) and paired (FeB) states with the total density Fei+FeB remaining constant. We have discovered that iron can also exist in other paired states, which we believe to be Fe-vacancy or Fe-implant damage pairs. When these pairs are destroyed and subsequently when FeB pairs form, we have observed an increased density. We refer to the excess iron (ΔFeB) after re-formation as latent iron.

Origins of defect structures in dendritic web silicon

Defects introduced into dendritic web silicon during growth have deleterious effects on electrical properties, and in extreme cases can lead to growth termination. This study used transmission X-ray topography and etch pitting techniques to elucidate the formation of several defect structures in dendritic web silicon. Results indicate that the formation of third dendrites is caused by liquid droplet entrapment within the web. Inverted U-shaped surface features have no correlation with internal dislocation structure, but are probably caused by a decrease in web thickness near the growth initiation. Stringers are formed by the reaction of two dislocations of different Burgers vectors, and the stringer density does not increase with ribbon length. Finally, web curvature is caused by plastic deformation due to a build up of glissile dislocations pinned by surface steps. An understanding of the causes of the formation of these defects can lead to improved processing for more efficient web growth.