Optical Properties of Single-Crystalline ZnO Nanowires on M-Sapphire

ZnO nanowires have been synthesized using a catalyst-assisted heteroepitaxial carbothermal reduction approach on a m-sapphire substrate. Intricate and uniform arrays have been obtained with each nanowire forming an angle ∼30° with the substrate normal. Photoluminescence studies at room temperature for wavelengths between 335 and 620 nm reveal a strong single exciton peak at ∼380 nm (3.26 eV) with accompanying deep-level blueshifted emission peaks at ∼486, 490, and 510 nm. UV resonant Raman spectroscopy has been used to characterize the nanowires at room temperature with multiphonon scattering exhibiting phonon quantum confinement.

Hydride vapor phase epitaxy (HVPE) was performed to prepare thick GaN films. It is found that (1) surface treatment of the sapphire substrate by the Ga+HCl gas just before the growth of GaN film reduces the pit density and improves the crystalline quality of the epitaxial GaN film(2) the photoluminescence (PL) spectrum measured at 4.2 K shows the free A-exciton line and a narrow I2 line, indicating that the GaN crystals prepared in this study are of high purity and high crystalline quality, and (3) the magnitude of the strain of the homo-epitaxially grown GaN on a thick GaN buffer layer thus prepared is less than half that of a hetero-epitaxially grown GaN on sapphire. These results show that high quality, thick single crystals of GaN can be prepared homo-epitaxially using HVPE.


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