James M. Zahler

High efficiency InGaAs solar cells on Si by InP layer transfer

InP/Si substrates were fabricated through wafer bonding and helium-induced exfoliation of InP, and InGaAs solar cells lattice matched to bulk InP were grown on these substrates using metal-organic chemical-vapor deposition. The photovoltaic characteristics of the InGaAs cells fabricated on the wafer-bonded InP/Si substrates were comparable to those synthesized on commercially available epiready InP substrates, thus providing a demonstration of wafer-bonded InP/Si substrates as an alternative to bulk InP substrates for solar cell applications.

InGaAs/InP double heterostructures on InP/Si templates fabricated by wafer bonding and hydrogen-induced exfoliation

Applications of InP-based materials are numerous, and thus integration of InP on Si may enable realization of powerful integrated III‐V-on-Si systems. InP and its lattice matched quaternary counterpart In12xGaxAsyP12y are direct gap semiconductors, which have high carrier mobilities, therefore finding applications in lasers, multijunction solar cells 1 and high-speed devices. Additionally, they cover the low dispersion and minimum loss wavelengths for optical fiber communication at 1.3 and 1.5mm, respectively, making them attractive materials for fabricating semiconductor lasers and detectors for telecommunications applications. However, InP is mechanically fragile, is not available in large substrates, and is expensive. Integrating InP thin films on Si substrates improves its mechanical strength and may also allow InP integration on large substrates by a process of tiling transferred thin films. Most importantly, a viable approach to InP/Si may enable cost-effective integration of infrared optoelectronic devices with well-established silicon electronics.

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.

Wafer bonding and layer transfer processes for 4-junction high efficiency solar cells

A four-junction cell design consisting of InGaAs, InGaAsP, GaAs, and Ga/sub 0.5/In/sub 0.5/P subcells could reach 1/spl times/AM0 efficiencies of 35.4%, but relies on the integration of non-lattice-matched materials. Wafer bonding and layer transfer processes show promise in the fabrication of InP/Si epitaxial templates for growth of the bottom InGaAs and InGaAsP subcells on a Si support substrate. Subsequent wafer bonding and layer transfer of a thin Ge layer onto the lower subcell stack can serve as an epitaxial template for GaAs and Ga/sub 0.5/In/sub 0.5/P subcells. Present results indicate that optically active III/V compound semiconductors can be grown on both Ge/Si and InP/Si heterostructures. Current voltage electrical characterization of the interfaces of these structures indicates that both InP/Si and Ge/Si interfaces have specific resistances lower than 0.1 /spl Omega/ cm/sup 2/ for heavily doped wafer bonded interfaces, enabling back surface power extraction from the finished cell structure.

Design Approaches and Materials Processes for Ultrahigh Efficiency Lattice Mismatched Multi-Junction Solar Cells

In this study, we report synthesis of large area (gt;2cm2 ), crack-free GaAs and GaInP double heterostructures grown in a multi-junction solar cell-like structure by MOCVD. Initial solar cell data are also reported for GaInP top cells. These samples were grown on Ge/Si templates fabricated using wafer bonding and ion implantation induced layer transfer techniques. The double heterostructures exhibit radiative emission with uniform intensity and wavelength in regions not containing interfacial bubble defects. The minority carrier lifetime of ~1ns was estimated from photoluminescence decay measurements in both double heterostructures. We also report on the structural characteristics of heterostructures, determined via atomic force microscopy and transmission electron microscopy, and correlate these characteristics to the spatial variation of the minority carrier lifetime

Ge Layer Transfer To Si For Photovoltaic Applications

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.

Photocurrent Enhancement in In0.53Ga0.47As Solar Cells Grown on InP/SiO2/Si Transferred Epitaxial Templates

InP/Si engineered substrates formed by wafer bonding and layer transfer have the potential to significantly reduce the cost and weight of III-V compound semiconductor solar cells. InP/Si substrates were prepared by He implantation of InP prior to bonding to a thermally oxidized Si substrate and annealing to exfoliate an InP thin film. Following thinning of the transferred InP film to remove surface damage caused by the implantation and exfoliation process, InGaAs solar cells lattice-matched to bulk InP were grown on these substrates using metal-organic chemical vapor deposition. The photovoltaic current-voltage characteristics of the InGaAs cells fabricated on the wafer-bonded InP/Si substrates were comparable to those synthesized on commercially available epi-ready InP substrates, and had a ~20% higher short-circuit current which we attribute to the high reflectivity of the InP/SiO2/Si bonding interface. This work provides an initial demonstration of wafer-bonded InP/Si substrates as an alternative to bulk InP substrates for solar cell applications.