Thermodynamic study on the origin of oval defects in GaAs grown by molecular-beam epitaxy

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Abstract
The relationship between oval defect density and GaAs growth conditions for molecular-beam epitaxy is investigated. Oval defects are proved to form continuously during growth, with density dependent on Ga cell and substrate temperatures. The correlation between oval defect formation and chemical reactions occurring in the Ga cell and on substrate surfaces, e.g., Ga-Ga2O3, GaAs-Ga2O3, and C-Ga2O3, including the vaporization of Ga and Ga2O3, is investigated thermodynamically. Ga2O formed by the reactions between Ga and Ga2O3 and between C and Ga2O3 in the Ga cell is determined to be related to oval defect formation. Next, substrate surface reactions are studied to investigate the behavior of Ga2O emitted from the Ga cell. The activation energy related to the reaction suggests that only Ga2O adsorption and desorption occur on the substrate. Thus, Ga2O adsorbed on the substrate determines oval defect formation. Prevention of Ga oxidation by chamber baking suggested by the above investigation results in a decrease in oval defect density by a factor of 10. Here, Ga2O is also proved to be the defect origin. It is formed by the reactions between Ga and H2O and between C, Ga, and H2O remaining even in ultrahigh vacuum conditions. These results suggest that both the oxygen and the H2O remaining in the MBE chamber should be reduced to eliminate oval defects.