Growth of BaSi₂ continuous films on Ge(111) by molecular beam epitaxy and fabrication of p-BaSi₂/n-Ge heterojunction solar cells

Abstract

We grew BaSi2 films on Ge(111) substrates by various growth methods based on molecular beam epitaxy (MBE). First, we attempted to form BaSi2 films directly on Ge(111) by MBE without templates. We next formed BaSi2 films using BaGe2 templates as commonly used for MBE growth of BaSi2 on Si substrates. Contrary to our prediction, the lateral growth of BaSi2 was not promoted by these two methods; BaSi2 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 BaSi2 were observed in samples taken out from the growth chamber. Such BaSi2 islands were easily to get oxidized. We finally attempted to form a continuous BaSi2 template layer on Ge(111) by solid phase epitaxy, that is, the deposition of amorphous Ba–Si layers onto MBE-grown BaSi2 epitaxial islands, followed by post annealing. We achieved the formation of an approximately 5-nm-thick BaSi2 continuous layer by this method. Using this BaSi2 layer as a template, we succeeded in forming a-axis-oriented 520-nm-thick BaSi2 epitaxial films on Ge substrates, although (111)-oriented Si grains were included in the grown layer. We next formed a B-doped p-BaSi2(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 BaSi2/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.

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

“…These results verify the a-axis orientation deterioration for BaSi 2 films. Similar precipitation of excess Si was reported upon the growth of BaSi 2 on Ge 36 and FeSi 2 films on Si by MBE. 37 We next investigated the defect properties around the BaSi 2 /Si interface in sample H by DLTS.…”

Effects of molecular beam epitaxy growth conditions on grain size and lattice strain in a-axis-oriented BaSi2 films

“…27) Our final goal is to achieve operations of BaSi 2 -pn/Ge-pn tandem solar cells. For this purpose we have also studied the epitaxial growth of BaSi 2 films on a Ge substrate, 28) enabling us to realize flexible solar cells by using mono-like Ge films on an insulator. 29) To achieve solar cells with a high-η requires the formation of high-quality BaSi 2 light absorbing layers with a low defect density.…”

Minority carrier lifetime of Ge film epitaxial grown on a large-grain seed layer on glass

“…Multijunction solar cells have updated the highest conversion efficiency of solar cells. Ge has been used in the bottom cell of the multi-junction solar cells because of its desirable characteristics, such as its narrow band gap (0.66 eV), high absorption coefficient (~10 4 cm −1 at 0.8 eV), and good lattice matching to group III-V compound semiconductors (0.1% lattice mismatch with GaAs) [1,2]. However, bulk Ge substrates are expensive, which has limited their application to special uses.…”

“…One promising approach to reducing the fabrication cost is substituting the bulk Ge substrate with a Ge thin film on an inexpensive glass substrate (softening temperature: ~550 °C) [3]. Additionally, (111)-oriented Ge is favorable for forming silicide solar cell materials [4] and nanowire arrays [5], which may improve solar cell performance. Transferring single-crystal Ge layers to inexpensive substrates is a promising method [6][7][8]; however, there remain difficulties in keeping the process costs low and fabricating in large areas.…”