Enhanced GaN Decomposition at MOVPE Pressures
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re 1. (a) Phase contrast and (b) transmission Normarski images of GaN surface heated for 20 minutes in H2 at a pressure of 150 torr at a temperature of 811 °C. The bar on image (a) indicates a length of 20 µm.
close proximity to with the backside of the susceptor. Temperature measurement in the reactor was found to be reproducible to within 5 °C after 8 months of use. For the decomposition study, pieces of the GaN on sapphire were cleaved and weighed to within 0.1 mg using an analytical balance [7]. Repeated weighing of the GaN on sapphire pieces were reproducible to within 0.1 mg. The pieces were reintroduced into the reactor and heated under varying conditions using either 6 SLM flow of H2 or 3 SLM of N2. Each piece was ramped at 25 °C per minute to the annealing temperature, which ranged from 800 to 1130 °C. After annealing for a set time and cooling, each piece was re-weighed in air to determine the mass loss. If Ga droplets were observed, they were removed by etching in dilute HNO3 and rinsing with DI water. (On some samples which were not etched, the Ga droplets were found to be very stable in air even up to several months, suggesting minimal oxidation of the liquid Ga droplets.) Each piece was then weighed again to determine the weight of liquid Ga. Finally, the piece was annealed at 1080 °C until the remaining GaN was decomposed, leaving only the initial bare sapphire surface. The bare sapphire weight was used to calculate the sapphire area in order to convert the measured weights to kinetic rates (atoms/cm2) [7]. RESULTS After annealing GaN in the absence of NH3, the most notable change in the GaN surface morphology is the appearance of Ga droplets as shown in Fig. 1. For Fig. 1, the GaN surface was annealed at a temperature of 811 °C for 20 minutes in H 2 at a pressure of 150 torr. The Ga droplets are observable as the lighter regions in the phase contrast image (Fig. 1(a)) and as the darker regions in the transmission image (Fig 1(b)). Because the GaN decomposition rate is larger than the Ga desorption rate, the liquid Ga droplets accumulate on the surface and coalesce into larger droplets, similar to the liquid droplet growth mechanism developed by Family and Meakin [9]. In flowing H2 Ga droplets were observed for pressures greater than 22 torr [7] for anneals at 992 °C. Compared to the Ga droplets in flowing H2, the Ga droplets in flowing N2 were barely discernable even at the highest magnification of 1000x. The droplet size increased as both the anneal temperature and the pressure were increased in H2 and in N2 for temperature greater than 1000 °C. For temperatures ranging from 800 - 1000 °C, the H2 pressure had a strong influence on both the quantity of Ga and the Ga droplet size. This is shown in Fig. 2, where the kinetic rates for Ga accumulation (i.e. liquid Ga on the surface), GaN decomposition, and Ga desorption are plotted as a function of pressure. Figs. 2(a) through 2(c) show the rates at anneal temperatures of 992 °C, 902 °C, and 811 °C, respectively. It is clear from Fig. 2(a) that the GaN d
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