Growth of BaSi sub(2) continuous films on Ge(111) by molecular beam epitaxy and fabrication of p-BaSi sub(2)/n-Ge heterojunction solar cells

We grew BaSi sub(2) films on Ge(111) substrates by various growth methods based on molecular beam epitaxy (MBE). First, we attempted to form BaSi sub(2) films directly on Ge(111) by MBE without templates. We next formed BaSi sub(2) films using BaGe sub(2) templates as commonly used for MBE growth of BaSi sub(2) on Si substrates. Contrary to our prediction, the lateral growth of BaSi sub(2) was not promoted by these two methods; BaSi sub(2) formed not into a continuous film but into islands. Although streaky patterns of reflection high-energy electron diffraction were observed inside the growth chamber, no X-ray diffraction lines of BaSi sub(2) were observed in samples taken out from the growth chamber. Such BaSi sub(2) islands were easily to get oxidized. We finally attempted to form a continuous BaSi sub(2) template layer on Ge(111) by solid phase epitaxy, that is, the deposition of amorphous Ba-Si layers onto MBE-grown BaSi sub(2) epitaxial islands, followed by post annealing. We achieved the formation of an approximately 5-nm-thick BaSi sub(2) continuous layer by this method. Using this BaSi sub(2) layer as a template, we succeeded in forming a-axis-oriented 520-nm-thick BaSi sub(2) epitaxial films on Ge substrates, although (111)-oriented Si grains were included in the grown layer. We next formed a B-doped p-BaSi sub(2)(20 nm)/n-Ge(111) heterojunction solar cell. A wide-spectrum response from 400 to 2000 nm was achieved. At an external bias voltage of 1 V, the external quantum efficiency reached as high as 60%, demonstrating the great potential of BaSi sub(2)/Ge combination. However, the efficiency of a solar cell under AM1.5 illumination was quite low (0.1%). The origin of such a low efficiency was examined.