GaN Growth Investigations - Effect of RHEED on Growth Rate

10.8 GaN Growth Investigations - Effect of RHEED on Growth Rate

It was observed that the RHEED electron beam pushes the balance point up by an estimated 10-20°C. It was further observed that the growth rate decreases as predicted by the model if the region hit by the RHEED beam is above the balance point. On the other hand it was also noticed that the growth rate slightly increased if the sample was grown exactly at the balance point. This observation needs more attention since it could have been just due to the fact that the RHEED beam was hitting a part of the sample that was slightly below the balance point, while the RHEED beam caused that section to be raised to just the balance point giving the best growth rate possible. It is speculated at this point that the RHEED beam might actually increase the incorporation rate of NH_3, which would cause the growth rate to increase, while above the balance point the evaporation rate of Ga is the dominant effect causing the growth rate to decrease.

All observations mentioned above were made on a GaN film grown on a-plane sapphire, which was hotter at the center by about 20°C than the rim which was at the balance point. The qualitative observations mentioned here were made by analyzing the thickness fringes caused by the different growth rates on the sample due to the substrate temperature gradient. The sample also had definite streaks across the entire diameter caused by the RHEED electron beam, since the sample was not rotated during growth.

Further studies of these phenomena are needed, especially since the RHEED electron beam might not just alter the growth rate, but also the crystal quality. It could be possible that the RHEED electron beam drives out Hydrogen from the GaN lattice as reported by Nakamura et. al. (Ref. 6) and discussed in the introduction chapter, which would be a very welcome effect. Experiments are currently prepared involving the rastering of the RHEED beam across the sample during growth to obtain more information about this effect.

Table of Contents