Ge Layer Transfer To Si For Photovoltaic Applications
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Ge Layer Transfer To Si For Photovoltaic Applications James M. Zahler, Chang-Geun Ahn, Shahrooz Zaghi, and Harry A. Atwater Thomas J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, CA 91125, U.S.A. Charles Chu and Peter Iles Tecstar, Inc., City of Industry, CA 91745-1002, U.S.A. ABSTRACT We have successfully used hydrophobic direct wafer bonding along with hydrogen-induced layer splitting of germanium to transfer 700 nm thick, single-crystal germanium (100) films to silicon (100) substrates without using a metallic bonding layer. The metal-free nature of the bond makes the bonded wafers suitable for subsequent epitaxial growth of layered solar cells at high temperatures without concern about metal contamination of the device active region. Contact mode atomic force microscopy images of the transferred germanium surface generated by the formation of micro-bubbles and micro-cracks along the hydrogen-induced layer-splitting interface reveals minimum rms surface roughness of between 10 nm and 23 nm. Electrical measurements indicated ohmic I-V characteristics for germanium layers bonded to silicon substrates with ~400 Ω cm-2 resistance at the interface. Triple-junction solar cell structures grown on these Ge/Si heterostructure templates by metal-organic chemical vapor deposition show comparable photoluminescence intensity and minority carrier lifetime to a control structure grown on bulk Ge. The use of a molecular beam epitaxy Ge buffer layer to smooth the cleaved surface of the Ge heterostructure has been shown to smooth the rms surface roughness from ~11 nm to as low as 1.5 nm with a mesa-like morphology that has a top surface roughness of under 1.0 nm giving a promising surface for improved solar cell growth on solar cell structures. INTRODUCTION Compound III-V semiconductor layered structures grown on bulk germanium substrates have been used to create high efficiency triple-junction solar cells with efficiencies greater than 30% [1, 2]. However, these are prohibitively expensive for all but space applications. The Ge substrate constitutes a large portion of this cost. Ge/Si heterostructures formed by wafer bonding and layer transfer of a thin crystalline Ge layer by hydrogen-induced exfoliation are being considered as a way to reduce the product cost while maintaining solar cell device performance. By transferring thin, single-crystal layers of Ge to a less expensive Si substrate and reclaiming the donor wafer through a polish process, a single 300 µm thick Ge wafer could serve as a source for transfer of in excess of 100 thin Ge layers. This application requires the bond at the interface of the Ge/Si heterostructures to be covalent to ensure good thermal contact, mechanical strength, and to enable the formation of ohmic contact between the Si substrate and Ge layers. To accomplish this hydrophobic wafer bonding will be used, because the H surface-terminating species that facilitate van der Waals bonding have been shown to be evolvable from the surface in Si/Si bonding systems at
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