GaN epitaxial layers grown uniformly on Si substrates suffer from randomly distributed thermal cracks. The growth on prepatterned Si substrates is demonstrated as an efficient way to control the geometrical distribution of the thermal cracks. In order to study this effect and to find the maximum crack-free lateral dimension of a GaN patterned unit on Si, a simple procedure termed lateral confined epitaxy (LCE) was developed. This procedure confines the growth of GaN to separate mesas of Si, which are defined on the Si substrate prior to the growth. The growth is performed by a single, continuous metalorganic chemical vapor deposition run. LCE enables the variation of mesa lateral size, while keeping the growth rate nearly unchanged. By performing a set of LCE growth runs of ∼0.7 μm GaN, on Si mesas of varying lateral dimensions, we specified the maximum crack-free range of GaN on Si as 14.0±0.3 μm, for that GaN thickness. A reduction of random crack density is observed with decreasing GaN pattern size and is explained in terms of shear-lag stress distribution.

An exponential dependence of the photoconductivity on the surface photovoltage at GaN layers is predicted theoretically and confirmed experimentally. The prediction is based on the assumption that the material is mainly an ordered polycrystal, consisting of columnar grains. Accordingly, transport is expected to be limited by potential barriers at the grain boundaries, arising from the charge trapped at grain-boundary defects. The observed exponential dependence provides evidence that strongly supports the model by establishing a direct link between the bulk conductivity and the surface potential barrier. The same model is shown to successfully explain several other defect-related findings as well.

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Lateral confined epitaxy ͑LCE͒ of GaN on Si substrates results in a reduction of thermal crack density with decreasing the lateral dimensions of the growth pattern. Below a critical size, crack-free GaN on Si is obtained. The intensity of band-to-band photoluminescence ͑PL͒ peak in LCE GaN is strongly enhanced with respect to uniformly grown GaN on Si. The present study rules out the effect of crack density, internal reflections ͑microcavity effects͒, as well as enhanced light extraction efficiency, and excitation or emission through preferred facets ͑shape effects͒ as the main factor in PL enhancement. It is shown that the reduction in threading dislocation density ͑TDD͒ along the edges of the LCE patterns improves the luminescence efficiency. The relative increase in high quality material ͑low TDD͒ with the reduction of LCE unit size is, thus, the main reason for the enhanced PL intensity.

Electrical and optical isolation of unintentionally doped GaN layers due to the damage created by H+ and He+ ions passing through the layer are demonstrated. As a result of the irradiation, the sample resistance increases by 11 orders of magnitude and the band-to-band photoluminescence (PL) emission is totally quenched. Following annealing (1000 °C, 30 s), the conductivity can be nearly completely recovered, whereas only partial recovery of the PL emission is obtained. A model is proposed which invokes the presence of potential barriers for electronic transport across extended defects as the major factor limiting carrier mobility. Radiation defects increase these barriers, thus affecting the sample resistivity. This model fits the experimental results for both H and He induced damage extremely well and thus proves that defects created by nuclear collisions of the ions traversing the layer are responsible for the observed effects.

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