Ge layer transfer to Si for photovoltaic applications

Abstract

We have successfully used hydrophobic direct-wafer bonding, along with H-induced layer splitting of Ge, to transfer 700-nm-thick, single-crystal Ge (100) films to Si (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 triple-junction GaInP/GaAs/Ge solar cell structures at high temperatures, without concern about metal contamination of the active region of the device. Contact-mode atomic force microscopy images of the transferred Ge surface generated by hydrogen-induced layer-splitting reveals root mean square (rms) surface roughness of between 10 and 23 nm. Electrical measurements indicate ohmic I– V characteristics for as-bonded Ge 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. An epitaxial Ge buffer layer is grown to smooth the cleaved surface of the Ge heterostructure and reduces the rms surface roughness from ∼11 to as low as 1.5 nm, with a mesa-like morphology that has a top surface roughness of under 1.0 nm, providing a promising surface for improved GaAs growth.