Desorption Mass Spectroscopy (DMS) and Reflection High Energy Electron Diffraction (RHEED) were used to control the growth of GaN by MBE. The conditions under which the growth rate was limited by the available NH_3 flux or by the available Ga flux were identified. DMS was used to monitor the Ga and H_2 desorption rates from the sample surface as a real time measure of GaN growth rate. RHEED was used to determine the fractional Ga coverage during growth. During Ga limited growth all the available Ga incorporates, while under NH_3 limited conditions the growth rate is reduced with increased Ga flux. This effect is attributed to an increasing fractional Ga coverage of the sample surface blocking available GaN formation sites. Rutherford Backscattering Spectroscopy and Scanning Electron Microscopy were used to verify those results. RHEED shows that NH_3 limited samples exhibit a 2D, weak (2x2) diffraction pattern, corresponding to layer-by-layer growth, while Ga limited samples reveal a 3-D transmission pattern. NH_3 limited growth conditions result in insulating films, while Ga limited growth gives conducting samples. Stylus measurements indicate that Ga limited growth produces much lower macroscopic roughness than for NH_3 limited growth. AFM measurements showed faceting on the Ga limited samples with a platelike growth mechanism similar to that observed in whisker growth. NH_3 limited samples exhibit atomic steps with 30 nm terrace lengths as well as 100 nm pinholes.

The content of the talk was based on a detailed poster given later at the PCSI-24:

Ammonia- vs. Ga-limited Growth of GaN(0001)A by Molecular Beam Epitaxy, R. Held, D.E. Crawford, A.M. Johnston, A.M. Dabiran, and P.I. Cohen, University of Minnesota, PCSI-24, Research Triangle Park, NC, January 12-15, 1997