Sublimation Sandwich Growth of Free Standing GaN Crystals
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Inlet NHS
Sandwichcell
,Substrate holder
S1r
Source
-Heater -70uartz tubes
Outlet
Fig. 1. Scheme of the growth system used for sublimation sandwich growth of GaN crystals. under atmospheric pressure with the required mass-flow rate. Two W-Re thermocouples have been placed inside the graphite at a small distance to the source and the seed. Hence the measured temperatures give somewhat higher values for the source temperature and somewhat lower values for the substrate temperature. Nevertheless, the experimentally measured temperatures are given in the paper. Sapphire or SiC are used as seed. Either Ga with purity of 99.999 or GaN grown ill our cell from Ga and ammonia at temperature 1250-1300'C have been used as source. Vertical arrangement allows continuous supply of the material into the source and it is shown that stable growth process is possible with the duration of more than 2 hours. X-ray rocking curves, photo- and cathodoluminescence data as well as the morphology of the surface, layer transparency and its color have been used to determine quality of the grown GaN layers. RESULTS AND DISCUSSION In order to estimate the ultimate possibilities of the growth process and to select the optimal growth parameters, an understanding of the growth mechanism and development of a mathematical model are necessary. Evaporation of Ga from the graphite container in the sandwich system in vacuum, argon and ammonia atmospheres at the temperature between 1150-2000'C for both thermal and RF heating of the cell has been investigated. The results are shown in Fig. 2. It can be seen that the rate of transport of Ga in ammonia atmosphere is significantly higher compared to that in argon in the whole temperature range of interest an(1 is close to the rate of Ga evaporation into vacuum (the rate of free evaporation into vacuum is approximately 30 times larger than the transport rate in tile sandwich geometry - see upper line). The reasons for this behavior are not clear. One of the possible explanations is as follows. The reaction of Ga with ammonia on the surface of the source produces H2 and N2 that is 1.5 times more in volume than the amount of the decomposed ammonia. At that small droplets of Ga can arise due to local supersaturation in the gas phase and be transported to the substrate. The observed high intensity of the reaction of Ga with ammonia can lead to the formation of the foam-like mass containing microcrystalline GaN. Another possible mechanism might be formation of highly volatile species containing Ga, N and H atoms. Also, simulation of the gas flow inside the growth cell shows formation of recirculation pattern directed upstream from the source to the seed. This might intensify the rate of evaporation of Ga from the source and its transport to the seed. In reality, all these mechanism might work together and provide the observed extremely high rate of Ga 28
transport between the source and the seed in the sublimation sandwich cell. Further efforts are necessary to clarify this behavior. 1026
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