GaN evaporation in molecular-beam epitaxy environment

GaN(0001) thick layers were grown on c-plane sapphire substrates by molecular-beam epitaxy using ${\mathrm{NH}}_{\mathrm{3}}.$ The evaporation of such GaN layers in vacuum was studied as a function of substrate temperature. In situ laser reflectivity was used to quantitatively measure the decomposition rate of the GaN(0001) plane. It is nearly zero below 750 °C, increases rapidly above 800 °C, and reaches 1 μm/h at 850 °C. An activation energy of 3.6 eV is deduced for the thermal decomposition of GaN in vacuum. The evaporation rate as a function of the incident ${\mathrm{NH}}_{\mathrm{3}}$ flux was also investigated for different substrate temperatures. A kinetic model is applied for the interpretation of the experimental results.