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We report on temperature-dependent Hall-effect measurements and secondary ion mass spectroscopy on unintentionally doped, n-type conducting GaN epitaxial films. Over a wide range of free carrier concentrations we find a good correlation between the Hall measurements and the atomic oxygen concentration. We observe an increase of the oxygen concentration close to the interface between the film and the sapphire substrate, which is typical for the growth technique used (synthesis from galliumtrichloride and ammonia). It produces a degenerate n-type layer of ≈1.5 μm thickness and results in a temperature-independent mobility and Hall concentration at low temperatures (<50 K). The gradient in free carrier concentration can also be seen in spatially resolved Raman and cathodoluminescence experiments. Based on the temperature dependence of the Hall-effect, Fourier transform infrared absorption experiments, and photoluminescence we come to the conclusion that oxygen produces a shallow donor level with a binding energy comparable to the shallow Si donor.

Vibrational modes are observed at ambient pressure in O-doped GaN at 544 cm Ϫ1 using Raman spectroscopy. Investigation of these modes with applied hydrostatic pressure reveals the existence of three closely spaced modes that shift in relative intensity with increasing pressure. Notably, transitions between the different modes occur at previously observed electronic transitions associated with the DX-like center behavior of substitutional O on the N site. A simple one-dimensional oscillator model is used to extract approximate force constants; these are consistent with the assignment of the 544 cm Ϫ1 mode to O N and with force constants for C and B dopants in GaP. The relative intensity changes observed at 11 and 17 GPa are assigned to changes in the charge state due to the merging of the ϩ/0 ionization level and the Fermi energy and the transition to DX Ϫ that causes a previously observed drop in free electron concentration, respectively.

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GaN films grown on (0001) 6H–SiC and (0001) Al2O3 substrates using low-pressure chemical vapor deposition with GaCl3 and NH3 as precursors are comparatively explored by optical, scanning tunneling, and transmission electron microscopy. Independent of the substrate material used, the surface of the GaN layers is covered by hexagonally shaped islands. For GaN on 6H–SiC, the islands are larger in diameter (≈50 μm) and rather uniformly distributed. An atomically flat interface is observed for GaN on Al2O3 in contrast to GaN grown on 6H–SiC, where the interface is characterized by large steps. For both substrates, faceted holes (named as pinholes) are observed in near-surface regions of the GaN layers occurring with a density of about 7×108 cm−2. No unequivocal correlation between the density of pinholes and the density of threading dislocations (≈1.6×1010 cm−2 for GaN/Al2O3 and ≈4×109 cm−2 for GaN/6H–SiC) can be found. Rather, different types of defects are identified to be correlated with the pinholes, implying a dislocation-independent mechanism for the pinhole formation. Despite the small lattice mismatch between GaN and 6H–SiC, the pronounced original surface roughness of this substrate material is believed to account for both the marked interfacial roughness and the still existing high density of threading dislocations.