Molecular beam epitaxy of n-type ZnS: A wide band gap emitter for heterojunction PV devices

Low-temperature epitaxy of zinc-blende ZnS films on GaAs(001) substrates was demonstrated by compound-source molecular beam epitaxy. The epitaxial relationship between the film and substrate was determined by reflection high-energy electron diffraction, high-resolution X-ray diffraction, and selected area electron diffraction measurements to be ZnS(001)∥GaAs(001). Strain-relaxation of the ZnS lattice occurred within the first 300 nm of film growth. The resistivity of the films could be tuned through the incorporation of an Al impurity dopant. The lowest thin-film resistivity achieved was 0.003 Ω-cm, with corresponding electron carrier concentration and mobility of roughly 4.5×10¹⁹ cm^-3 and 46 cm² V^-1 s^-1, respectively. Al, Ag, and In metals were found to make good ohmic contact to heavily doped ZnS films, whereas ITO and AZO transparent conductive oxides did not. Applications to novel PV devices incorporating low electron affinity absorbers are discussed.