Efficiency of NH3 as nitrogen source for GaN molecular beam epitaxy

We show that optical reflectivity measurements can be used to evaluate the part of a NH3 flux which reacts with a Ga-terminated GaN surface or with a Ga-flux simultaneously impinging on the surface, as in standard molecular beam epitaxy situation. At least for temperatures not exceeding 700 °C, the ratio of the reacted part of the NH3 flux to the incident flux can be assimilated to the NH3 cracking efficiency. Being nearly zero below a threshold temperature of 450 °C, it increases with temperature but remains low (∼4%) explaining why an exceptionally high V/III flux ratio is necessary to grow GaN using NH3.