Development of group III/nitride core/shell heterostructures by atomic layer deposition on nanorods

A nanostructured LED design has been proposed in which a p-GaN/i-InyGa1- yN/n-GaN double heterostructure is deposited on p-GaN nanorods that are grown on a Si substrate. The design advantages include: 1) a ten-fold increase in active area for photo-generative recombination resulting in an increase in brightness, 2) increase in photon extraction efficiency, and 3) growth on Si wafers that eliminates the need for topside contacts and possible integration with other Si-based technologies. The feasibility of using atomic layer deposition (ALD) was studied both experimentally and theoretically as a means of fabricating the proposed nano-structure LED. The ALD process window for growing thin films of GaN on Si substrates from GaCl3 and NH3 was determined. Optimum ALD growth was obtained with a GaCl3 exposure time of 2-8 sec followed by a 30 sec nitrogen purge, a 10 sec NH3 pulse, and another 30 sec nitrogen purge. One cycle resulted in 2.56 A of growth over the entire ALD process window and ALD films obtained with these conditions were found to be extremely uniform in thickness with many samples having roughness as low as 0.3 to 0.5 nm. These ALD conditions were then applied to growth of GaN on InN nanorods. Randomly-oriented polycrystalline structures were observed for samples grown in the temperature range 565 to 580°C. For growth at a temperature of 595°C the GaN shells were more crystalline but at a cost of decomposition of the underlying InN nanorod. For growth at any temperature in the range 565 to 595°C the InN and GaN domains tended to delaminate and show two separate sets of diffraction spots as shell thickness increased. Analytical models were developed to predict the mechanical stability of core/shell heterostructures fabricated from materials with dissimilar lattice constants. The models predict that there is no thickness of pure InN core and pure GaN shell that yields stable, defect-free shells. However, with alloyed shells, the model predicts a range of thicknesses of cores and shells over which the structures are stable.