Our sapphire samples are usually prepared by a supersonic cleaning process involving consecutive baths of Acetone, Methanol, Isoprophen Alcohol, and deionized water. Then the samples are mechanically mounted into an e-beam evaporation system and about 2000 Å of Ti is deposited on the fine grind side as a heat diffuser as discussed earlier in the chapter on MBE system modifications. The EPI ready side of the sample is routinely checked with a Dektak Profiler and typical values are surface roughnesses of 100 Å peak to peak over several hundred microns. Etching will be tried in the future to improve the surface.
After e-beam deposition the samples are rinsed in deionized water and blown dry using N_2. Samples prepared this way have been checked using AFM, and it was found that dust particles seem to deposit quickly on the surface ones the substrate is removed from the clean room. Therefore a new procedure is being tried next time, mounting the samples in the clean room and keeping sample and sample holder under deionized water until they are ready to be inserted into the MBE system, where they will be blown dry with N_2 after being placed into the introduction chamber. The introduction chamber will be flooded with N_2 at the same time to prevent dust from entering this chamber.
Cubic substrates tried so far for growing cubic GaN have been MgO and GaAs. MgO was prepared the same way as sapphire, but this procedure presumably damages the surface (water). GaAs is available EPI ready and no special cleaning procedure is required.
Table 8: Properties of prospective nitride substrates (Ref. 1)
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