Reflectance–difference studies of organometallic chemical vapor deposition growth transients on (001) GaAs

Abstract

Using reflectance–difference spectroscopy to follow surface coverage on a submonolayer scale, alternating trimethylgallium (TMG) and arsine flows to separate their surface-chemical effects and to eliminate gas-phase interactions, and low (330–430 °C) temperatures to eliminate gas-phase pyrolysis, we isolate and identify the different factors that kinetically limit organometallic chemical vapor deposition (OMCVD) growth on (001) GaAs by their time, pressure, and temperature dependences. When an AsH3-stabilized surface is exposed to TMG, the relative coverage by Ga-containing reacted species increases linearly in time to 80% coverage and exponentially thereafter, and exhibits a pressure dependence of classic Langmuir adsorption-isotherm form. We derive a model that quantitatively describes these features in terms of reversible excluded-volume chemisorption and subsequent irreversible decomposition of TMG at (001) surface lattice sites. Analysis of the data yields −26 and 39 kcal/mol for the chemisorption enthalpy and decomposition barrier, respectively. This model describes kinetically limited atmospheric- and near-atmospheric-pressure GaAs OMCVD growth rates not only for alternating-flow OMCVD below 450 °C but, unexpectedly, also for conventional OMCVD to temperatures at least as high as 650 °C. We show that the wide range of OMCVD activation energies reported for (001) GaAs in the literature, 13–32 kcal/mol, does not represent unusually large scatter but is a result of the actual TMG decomposition rate being determined by a combination of fractional-occupancy and decomposition-probability effects. The surprising success of this model raises serious questions about the importance of gas-phase reactions in GaAs growth by OMCVD.