Zinc silicon phosphide as a wide band gap semiconductor for integration with silicon

Abstract

There has been a longstanding need for optically-active materials that can be integrated with Si, both for tandem photovoltaics and for other optoelectronic applications. We focus on ZnSiP2, a ternary III-V analog with 0.5% lattice mismatch with Si and a 2.1 eV band gap, nearly ideal for a top cell on a Si-based tandem device. We have shown that ZnSiP2 has many properties suitable for applications to Si-based tandem photovoltaics using bulk single crystals grown in a Zn flux. Here, we report the first photovoltaic measurements of ZnSiP2 using photoeletrochemistry. We show that ZnSiP2 has excellent photoresponse and high open circuit voltage of 1.3 V, as measured in a photoelectrochemical configuration. The high voltage and low band gap-voltage offset are on par with much more mature wide band gap III-V materials. Photoluminescence data combined with theoretical defect calculations illuminate the defect physics underlying this high voltage, showing that the intrinsic defects in ZnSiP2 are shallow and the minority carrier lifetime is 7 ns. The favorable results obtained from characterization of bulk material encourage the development of ZnSiP2 as a photovoltaic absorber. To pursue this development, we have constructed a thin film growth reactor. This reactor employs a combination of chemical vapor deposition, using silane and phosphine as precursor gases, and physical vapor deposition, using an effusion cell to evaporate elemental Zn. We will present the results of ZnSiP2 film growth on (100) Si substrates. The composition, structure, and morphology of these films have been characterized by energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy, X-ray diffraction and transmission electron diffraction, and electron microscopy, respectively. These promising results represent significant advancement towards implementing ZnSiP2 as a top cell material on Si-based tandem photovoltaics.