Dislocation/grain boundary effects on the thermal conductivity of hydride vapor phase epitaxy grown GaN/sapphire (0001)

We measured 2-3 μm spatial/depth resolution thermal conductivity (K) at 300 K of two n-GaN/sapphire (0001) samples, fabricated by hydride vapor phase epitaxy (HVPE), using scanning thermal microscopy (SThM). The thicknesses of the samples were in the 20-25 μm interval with carrier concentrations in the (1.5-4.0)×10¹⁶ cm^-3 range. Plan-view transmission electron microscopy and atomic force microscopy imaging identified the distribution of dislocations/grain boundaries. On both samples the SThM examination over about 20 μm exhibited sections of relatively high and low thermal conductivities. The highest thermal conductivity results were compared to earlier work on HVPE n-GaN samples, with somewhat similar carrier concentrations and thicknesses, but no grain boundaries. The present study shows that the presence of grain boundaries, which act as additional scattering factors, leads to lower κ values when compared to the samples with no grain boundaries. This thermal conductivity behavior attributed to the existence of dislocations/grain boundaries is similar to previous observations on lateral epitaxial overgrowth GaN material where a correlation between high/low K and low/high threading dislocation densities was established