Reaction chemistry of group IV containing copper chalcogenide semiconductors Cu2MX3 (M = Sn, Ge and X = S, Se)

Abstract

Cu2(Sn,Ge)(S,Se)3 alloys are new semiconductor absorber layers of interest in thin film photovoltaics. This paper presents a new synthesis of Cu2GeS3 and Cu2Sn(S,Se)3 layers from a Cu thin film reacting with two gas phase species, MX (SnS(e), GeS) and X (S, Se). Equilibrium reactions of Cu2MX3 ternaries with their gaseous binaries MX are examined for various annealing temperatures and background pressures. Incorporation of Sn and Ge only occurs when Cu2S(e) is formed and there is a sufficient partial pressure of MX. Observation of a lateral composition gradient across the sample for the SnS and SnSe containing absorbers suggest a limiting factor during the annealing. A theoretical model shows that the generation rate of these species is responsible for this gradient. This result highlights the importance of controlling the vapour pressure uniformity of MX during absorber annealing for the synthesis of homogeneous compound semiconductor layers that contain volatile group IV chalcogenide species, such as kesterite.