STUDY OF SELF-LIMITING GROWTH-MECHANISM IN CHLORIDE ALE

The growth process of chloride atomic layer epitaxy (ALE) using GaCl is studied using the temperature programmed desorption method (TPD), the surface photo-absorption (SPA) method and ab initio molecular orbital calculations. From the TPD measurements, the GaCl adsorption energy is calculated to be 32 kcal mol-1 and 38 kcal mol-1 for Ga-terminated and As-terminated surfaces respectively. In the absence of H-2, the Ga-Cl bond appears to be very stable in the adsorption state. Process simulation using a simple cluster model of (AsH2)2-GaCl indicates that adsorption is completed by forming sigma and pi covalent bonds. and that there is hardly any self-dissociation of the Ga-Cl bond. From SPA measurements in the ALF system with ambient H-2, a strong similarity is found between the spectra of the GaCl-supplying surface and the Ga-covered surface. Based on these results, a model of the chemical process in chloride ALE is proposed where adsorbed GaCl molecules can react immediately with hydrogen and Cl is released from the surface as HCl. A self-limiting mechanism results from the very short residence time of GaCl (approximately 10(-3) s at 450-degrees-C) on the Ga-terminated surface.