We have fabricated approximately 0.5-lm-thick undoped n-BaSi 2 epitaxial films with various average grain areas ranging from 2.6 to 23.3 lm 2 on Si(111) by molecular beam epitaxy, and investigated their minority-carrier lifetime properties by the microwave-detected photoconductivity decay method at room temperature. The measured excess-carrier decay curves were divided into three parts in terms of decay rate. We characterized the BaSi 2 films using the decay time of the second decay mode, s SRH , caused by Shockley-Read-Hall recombination without the carrier trapping effect, as a measure of the minority-carrier properties in the BaSi 2 films. The measured s SRH was grouped into two, independently of the average grain area of BaSi 2. BaSi 2 films with cloudy surfaces or capped intentionally with a 3 nm Ba or Si layer, showed large s SRH (ca. 8 ls), whereas those with mirror surfaces much smaller s SRH (ca. 0.4 ls). X-ray photoelectron spectroscopy measurements were performed to discuss the surface region of the BaSi 2 films.

p-BaSi 2 /n-Si heterojunction solar cells consisting of a 20 nm thick B-doped p-BaSi 2 epitaxial layer (p ¼ 2.2 Â 10 18 cm À3) on n-Si(111) (q ¼ 1-4 X cm) were formed by molecular beam epitaxy. The separation of photogenerated minority carriers is promoted at the heterointerface in this structure. Under AM1.5 illumination, the conversion efficiency g reached 9.0%, which is the highest ever reported for solar cells with semiconducting silicides. An open-circuit voltage of 0.46 V, a short-circuit current density of 31.9 mA/cm 2 , and a fill factor of 0.60 were obtained. These results demonstrate the high potential of BaSi 2 for solar cell applications. V

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B-doped p-BaSi2 layer growth by molecular beam epitaxy and the influence of rapid thermal annealing (RTA) on hole concentrations were presented. The hole concentration was controlled in the range between 1017 and 1020 cm−3 at room temperature by changing the temperature of the B Knudsen cell crucible. The acceptor level of the B atoms was estimated to be approximately 23 meV. High hole concentrations exceeding 1 × 1020 cm−3 were achieved via dopant activation using RTA at 800 °C in Ar. The activation efficiency was increased up to 10%.

The 730 nm-thick undoped BaSi 2 films capped with 5 nm-thick amorphous Si (a-Si) intended for solar cell applications were grown on Si(111) by molecular beam epitaxy. The valence band (VB) offset at the interface between the BaSi 2 and the a-Si was measured by hard x-ray photoelectron spectroscopy to understand the carrier transport properties by the determination of the band offset at this heterointerface. We performed the depth-analysis by varying the takeoff angle of photoelectrons as 15 , 30 , and 90 with respect to the sample surface to obtain the VB spectra of the BaSi 2 and the a-Si separately. It was found that the barrier height of the a-Si for holes in the BaSi 2 is approximately À0.2 eV, whereas the barrier height for electrons is approximately 0.6 eV. This result means that the holes generated in the BaSi 2 layer under solar radiation could be selectively extracted through the a-Si/BaSi 2 interface, promoting the carrier separation in the BaSi 2 layer. We therefore conclude that the a-Si/BaSi 2 interface is beneficial for BaSi 2 solar cells. Published by AIP Publishing.