Thick high quality gallium nitride (GaN) layers presenting a dislocation density reduced to 6×10⁶ cm⁻²were grown by hydride vapour phase epitaxy (HVPE). Scanning electron microscopy (SEM) characterizations, X-ray double diffraction (XRD) measurements, photoluminescence and reflectivity experiments, both at 4.5 K have been carried out to reveal the optical and the structural properties of the GaN epilayers. The strain relaxation was studied as a function of increasing the thickness. Special emphasis has been placed to the control of the parasitic nucleation on the reactor wall upstream the substrate, which consistently appears for long-time growth experiments. The amount of the parasitic deposit was quantitatively assessed by theoretical modelling. Attention was paid to accurate characterizations of 450 μm thick GaN layers obtained via thickening through a regrowth step after growth interruption and strain relaxation on cooling the templates.

 

Fig. 1. Top and cross-sectional SEM images of GaN-HVPE layers. (a) Cross-sectional SEM image for sample S4 (124 μm thick, 4 h growth). In Fig. 1a the straight line at the interface between the MOCVD GaN and the sapphire substrate is only an optical artefact. (b) Cross-sectional SEM image for sample S8 (176 μm thick, 8 h growth). (c) Cross-sectional SEM image for sample S11 (454 μm thick, 11 h growth). (d) Top SEM image for sample S11 (454 μm thick, 11 h growth).

Source: Journal of Crystal Growth

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