Thermodynamic approach of AlGaN MOVPE growth at atmospheric pressure

Abstract

AlxGa1-xN epilayers were grown on GaN/sapphire substrate by metalorganic vapor-phase epitaxy (MOVPE) at atmospheric pressure. Different trimethylaluminum (TMA) flow rates were used in order to vary the solid aluminum (Al) molar fraction. In situ laser reflectometry shows that the higher the TMA flow, the lower the growth rate. The Al molar fraction was determined by high-resolution X-ray diffraction and energy-dispersive X-ray spectroscopy. In order to explain the evolution of the growth rate and Al fraction, we have developed a complete thermodynamic model in which all possible parasitic reactions between TMA and ammonia (NH3) have been considered. The experimental results are explained by adding a correction factor γ in the definition of the theoretical Al molar fraction. This factor is related to parasitic reactions. Two principal adducts AlCH3.NH and (AlCH3.NH)3 were predicted to form in a temperature range from 300 to 1000 K with a maximum equilibrium partial pressure at 600 K for AlCH3.NH species. These adducts undergo a thermal decomposition and disappear for temperatures above 700 K and 1000 K for (AlCH3.NH)3 and AlCH3.NH, respectively. We conclude that the parasitic reactions would be avoided or minimized if TMA and NH3 are mixed in a reactor region where the temperature is above 1000 K. Thus, the growth rate and Al incorporation would be better controlled in a MOVPE reactor designed such a way that the mixing of TMA and NH3 flows takes place at the latest, in the hottest region.