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Abstract

In situ spectroscopic ellipsometry study of GaN nucleation layer growth and annealing on sapphire in metalorganic vapor-phase epitaxyWe present the details of GaN nucleation layer grown on ͑0001͒ sapphire substrates below 600°C by metal organic chemical vapor deposition. These films have cubic ͑c-GaN͒ zinc blende structure which starts to transform into a hexagonal ͑h-GaN͒ wurtzite structure upon annealing around 650°C and above. The films deposited above 700°C by pulsed laser deposition directly on sapphire substrate showed the wurtzite structure. Both c-GaN and h-GaN films grow epitaxially on ͑0001͒ sapphire substrates via domain matching epitaxy, where integral multiples of planes match across the film-substrate interface. The c-GaN has the following epitaxial relationship:In terms of planar matching, ͑220͒ planes of c-GaN match with ͑30-30͒ planes of sapphire, and 1 / 3͑422͒ planes of c-GaN match with ͑−2110͒ planes of sapphire in the perpendicular direction. The transformation from c-GaN into h-GaN involves the transformation of ͑220͒ planes of c-GaN into ͑−2110͒ planes of h-GaN and 1 / 3͑422͒ planes of c-GaN into ͑30-30͒ planes of h-GaN, and the epitaxial relationship changes to ͗0001͘ h-GaN ʈ ͗0001͘ sap and ͗−2110͘ h-GaN ʈ ͗10-10͘ sap . In terms of planar matching epitaxy, ͑−2110͒ planes of h-GaN match with ͑30-30͒ planes of sapphire, and, in the perpendicular direction, ͑30-30͒ planes of h-GaN match with ͑−2110͒ planes of sapphire. This epitaxial relationship is known as 30°or 90°rotation. It is interesting to note that relative spacing for c-GaN as well as h-GaN planes remains the same during this transformation because of a͑c-GaN͒ = ͱ 2a͑h-GaN͒ = ͱ 3c͑h-GaN͒ / 2 equivalence between lattice constants of cubic and hexagonal structures. The transformation from cubic to hexagonal structure can occur via insertion or removal of stacking faults in ͕111͖ planes of c-GaN and ͕0001͖ planes of h-GaN. The hexagonal structure is preferred as a template for higher-temperature growth, however, the cubic structure, which is a defective hexagonal with stacking faults in alternate layers, can also provide a template for epitaxy. The role of Shockley partials terminating at the island edges and the dislocations associated with subgrain boundaries in the generation of threading dislocations is discussed.

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We report a pulsed laser annealing method to convert carbon fibers and nanotubes into diamond fibers under ambient conditions.