Here we present the first direct observation of the atomic structure of threading dislocation cores in hexagonal GaN. Using atomic-resolution Z-contrast imaging, dislocations with edge character are found to exhibit an eight-fold ring core. The central column in the core of a pure edge dislocation has the same configuration as one row of dimers on the {10-10} surface. Following recent theoretical work, it is proposed that edge dislocations do not have deep defect states in the band gap, and do not contribute to cathodoluminescence dislocation contrast. On the other hand, both mixed and pure screw dislocations are found to have a full core, and full screw dislocation cores were calculated to have states in the gap.

GaN/sapphire layers have been grown by Metal Organic Vapour Phase Epitaxy (MOVPE). An amorphous silicon nitride layer is deposited using a SiH4/NH3 mixture prior to the growth of the low temperature GaN buffer layer. Such a process induces a 3D nucleation at the early beginning of the growth, resulting in a kind of maskless ELO process with random opening sizes. This produces a significant decrease of the threading dislocation (TD) density compared to the best GaN/sapphire templates. Ultra Low Dislocation density (ULD) GaN layers were obtained with TD density as low as 7×107cm−2 as measured by atomic force microscopy (AFM), cathodoluminescence and transmission electron microscopy (TEM). Time-resolved photoluminescence experiments show that the lifetime of the A free exciton is principally limited by capture onto residual donors, similar to the situation for nearly dislocation-free homoepitaxial layers.

Photoluminescence (PL) experiments were carried out at 300 and 12 K to investigate the electro-optical properties of Cd1−xZnxTe grown by molecular beam epitaxy on GaAs substrates. The compositional dependence of the band-gap energy was determined. It has a quadratic dependence on x. The near band edge PL spectra at 12 K show free and bound exciton lines for x=0 and 1 and only broadened bound exciton peaks for other compositions. The bound exciton broadenings are quantitatively explained based on the compositional fluctuations of the cations. The PL line shapes give indications of the high quality of the layers.

The electronic structure of pure edge threading dislocations in metalorganic vapor phase epitaxy grown wurtzite GaN thin films has been studied directly by atomic resolution Z-contrast imaging and electron energy loss spectroscopy in a scanning transmission electron microscope. Dislocation cores in n-type samples grown in N-rich conditions show no evidence for the high concentration of Ga vacancies predicted by previous theoretical calculations. Nitrogen K-edge spectra collected from edge dislocation cores show a sudden and significant increase in the intensity of the first fine-structure peak immediately above the edge onset compared to the bulk spectra. The origin of this increase is discussed.

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