An Atomic Force Microscopy Study of the Initial Nucleation of GaN on Sapphire
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Mat. Res. Soc. Symp. Proc. Vol. 395 01996 Materials Research Society
determine conditions for MBE growth of GaN that result in buffer layers with increased grain size as well as determining the appropriate parameters to promote two-dimensional growth. EXPERIMENT The GaN layers for this study were grown at West Virginia University by MBE in a system similar to that described elsewhere [10]. Since we are interested in developing lower temperature growth of GaN by MBE, we have only investigated growth temperatures less than 700 0 C. A standard MBE source provided the Ga flux. A cryogenically-cooled rf plasma source (Oxford Applied Research CARS-25) operating at 500W was used to produce the active nitrogen flux. The layers were characterized using x-ray diffraction and atomic force microscopy (AFM) (Digital Instruments Nanoscope II). RESULTS Several studies [3,4,11] have indicated that the transition between buffer layer and "bulk" film structure occurs in the first 0.4 to 0.5 tim of growth. We grew a series of 3000 A layers to determine both growth rates and growth modes in this transition region. We were also interested in the transition point between Ga-rich growth and conditions which produced Ga condensation as evidenced by the presence of Ga droplets. Figure 1 illustrates our growth rate for several conditions. Above 2.5 x10" 7 Torr Ga and 600 0 C, growth is limited by the amount of active nitrogen present as indicated both by the increase in growth rate with temperature at a fixed Ga flux, and by the relatively constant growth rate at a given temperature for increasing Ga flux. 0 The decrease in growth rate between 2.5 and 5.0 x10- 7 Torr Ga at 660 C is apparently related to a change from a three-dimensional to a two-dimensional growth mode, as discussed later.
0.50 Ga Condensation
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Figure 1. Growth rate of GaN for several temperatures and valus of Ga flux.
Ga Condensation
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S0.25
600 °C
Ga Condensation
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630 C 660 0°C
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Ga Flux (10-7 BEP)
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Early in this study we grew layers using nucleation conditions reported by others [2,5,6,7,12]. We found that exposing the sapphire substrate to an active nitrogen flux resulted in a fine-grained (
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