Three-step growth of highly photoresponsive BaSi2 light absorbing layers with uniform Ba to Si atomic ratios

Abstract

Barium disilicide (BaSi2) shows great promise as a light absorbing material for solar cell applications. Thus, it is important to form high-quality BaSi2 films with a low defect density. We previously found that the photoresponsivity of a-axis-oriented BaSi2 epitaxial films was enhanced drastically when formed under Si-rich conditions by molecular beam epitaxy. However, the degree of a-axis crystal orientation normal to the sample surface was degraded. In this work, we investigated the origin of this degradation by cross-sectional transmission electron microscopy (X-TEM). It was found that excess Si atoms in BaSi2 films diffused out and precipitated around the BaSi2/Si interface, resulting in the formation of Si layers with an interface roughness of 0.2–0.3 μm when grown under Si-rich conditions. Furthermore, extended defects stemming from such rough interface regions were detected by deep level transient spectroscopy, and they acted as hole trap defects. To achieve high photoresponsivity in BaSi2 films under Si-rich conditions while avoiding such Si precipitation, we proposed a three-step growth method for BaSi2 films. X-TEM observations confirmed uniform Ba to Si atomic ratios throughout the entire layer and smooth BaSi2/Si interfaces. In addition, relative to films grown by the conventional two-step method, the a-axis orientation was improved significantly, and the photoresponsivity increased by approximately five times to reach 0.5 A/W at a wavelength of 800 nm under a bias voltage of −0.1 V. This corresponds to an external quantum efficiency above 90%.