PAMBE growth of GaN nanowires on metallic ZrN buffers -- a critical impact of ZrN layer thickness on the growth temperature
An impact of thin metallic ZrN layers on Si and sapphire wafers on substrate temperature during MBE growth of GaN nanowires is studied. Using nucleation kinetics of GaN as a sensitive probe we show that a thin ZrN layer strongly increases the substrate temperature, which significantly affects the dimensions and density of the nanowires. To quantify the effect we developed a technique of optical pyrometer calibration that allows reliable determination of emissivity, and thus precise measurement of temperature of substrates with unknown optical parameters, such as ZrN buffers of various thicknesses. Our results show that emissivity of ZrN-coated Si and sapphire wafers differs significantly from the bulk ZrN and increases drastically for films thinner than ~100 nm. Simple calculations indicate that ignoring the influence of the thin film may lead to huge errors in temperature readings and consequently to losing the growth control. Then, we show that we can compensate for the impact of ZrN buffer on substrate temperature and grow identical nanowire arrays on Si substrates with and without ZrN layers. Finally, having identical arrays of GaN nanowires we used X-ray diffraction to compare nanowire arrangements on Si and ZrN/Si substrates with a thin SiN nucleation layer.