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 have studied the effect of embedding nanocrystalline Au particles on the electrical and optical characteristics of ZnO films. Au-embedded epitaxial ZnO films were deposited on (0001) sapphire substrates with a pulsed laser deposition technique. The crystalline quality of both the ZnO matrix and Au nanoparticles was investigated by X-ray diffraction and transmission electron microscopy. Composite films were characterized by photoluminescence, optical absorption, and low-temperature electrical resistivity measurements. Photoluminescence spectra of theses films showed a sharp excitonic peak at 3.22 +/- 0.05 eV without any signature of green band emission. Electrical resistivity measurements showed these films to be highly conducting, with a room-temperature resistivity of 3.4 +/- 0.2 m omega-cm.

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We have developed a technique to grow self-aligned epitaxial Cu/MgO films on Si (100) using Pulsed Laser Deposition Method. In this method we deposit a uniform film of Cu/Mg (5-7%) alloy over Si (100) at room temperature using TiN as an intermediate buffer layer. As a result of HRTEM (with spatial resolution of 0.18 nm) and STEM-Z investigations we observed that when this film is annealed at 500°C (in a controlled oxygen environment), in less than 30 minutes time, all the Mg segregates at the top and at the bottom surface of Cu. This is understood to be the consequence of lower surface energy of Mg. At 500°C Mg is quite sensitive to oxygen and thin layer of MgO is immediately formed at the top surface, we also observed a thin layer of MgO at the Cu/TiN interface. Thickness of the upper MgO layer was found to be 15 nm while that of lower layer was 10 nm. MgO underneath layer acts as a diffusion barrier and inhibits the diffusion of Cu in the system. Upper MgO layer acts as a passivating layer and improves the quality of copper against oxidation. Electrical resistivity measurements (in the temperature range 12-300 K) showed MgO/Cu/MgO/TiN/Si (100) sample to be highly conducting. We also observed that the resistivity of the system is insensitive to ambient oxygen environment. Selfaligned MgO (100) layer also provides a means to grow several interesting materials over it. This technique can be used to integrate high temperature superconductors like YBa2Cu3O7 with silicon chip.