Reactions of trimethylgallium multilayers on silicon (100)

Abstract

Trimethylgallium (TMGa) is an important source gas for metal-organic chemical vapor deposition (MOCVD) and metal-organic molecular beam epitaxy (MOMBE). In a previous paper Lee et al. (Surf. Sci. 216 (1989) 173) found that at coverages below 2 × 10 14 molecules/cm 2, TMGa decomposes via an intramolecular transfer hydrogen process to yield methane, carbon, gallium, and hydrogen. Here we consider the mechanism of TMGa decomposition on top of this first monolayer of TMGa using temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). It is found that at TMGa exposures > 1 × 10 16 molecules/cm 2, TMGa transiently populates a weakly bound state. The TMGa desorbs from this weakly bound state if the sample is held in vacuum for several minutes at 300 K. However, the weakly bound TMGa will react if the sample is flashed immediately after dosing. The weakly bound TMGa decomposes at 400–600 K to yield dimethylgallium (DMGa (abs)). methane, and CH x groups. The DMGa reacts upon further heating to yield monomethylgallium (MMGa (abs)), methane and additional CH x groups. The MMGa decomposes above 700 K, to yield methyl radicals and gallium. Traces of ethylene are also seen. These results verify that carbon deposition is an intrinsic part of TMGa decomposition on Si(100). In contrast, MMGa can decompose cleanly. Hence, MMGa may be a promising intermediate for MOMBE.