Carbonyl sulfide (OCS) as a sulfur-containing precursor in MOCVD: a study of mixtures of Me2Cd and OCS in the gas and solid phases and their use in MOCVD

Abstract

Mixtures of dimethylcadmium (Me₂Cd) and carbonyl sulfide (OCS) have been examined in the gas and solid phases over a wide range of temperatures. No interaction is observed between Me₂Cd and OCS in a 1:1 molar ratio at room temperature in the gas phase, nor is any interaction detected in the solid phase at liquid-nitrogen temperature. On heating the 1:1 mixtures of Me₂Cd and OCS to 250 °C in a sealed vessel, gaseous products are formed. These consist of methane, carbon monoxide and ethane in an approximately 12:2:1 molar ratio, although a large excess of unreacted OCS is also present showing that this compound does not react fully with Me₂Cd at 250 °C. In a flow system at 300 °C only methane and carbon monoxide are formed, in the molar ratio 6:1, although the amount of reaction of the OCS is much less (as evidenced by a higher proportion of unreacted OCS). When the flow reaction is repeated at 450 °C more of the OCS reacts and the proportion of carbon monoxide in the gaseous reaction products is much higher. Using a commercial MOCVD apparatus, high-quality layers of cadmium sulfide are obtained from Me₂Cd–OCS mixtures. Temperatures in the range 350–450 °C lead to somewhat slow growth rates which only reach 1 µm h^–1 when a 200-fold molar excess of OCS over Me₂Cd is used. A small amount of prereaction is observed, but only when H₂ is used as the carrier gas. This is attributed to the formation of very small concentrations of H₂S by reaction of OCS with H₂. The resulting epitaxial layers have good thickness and electrical uniformity. These experiments confirm that OCS may be used as a precursor for the growth of thin layers of CdS by MOCVD. However, the large excess of OCS required here suggests that the compound might be more useful for doping than for the growth of pure layers of CdS.