Charles Chu

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.

Characterizing the process of cast molding microfluidic systems

Cast molding is a simple, low cost microfabrication method which offers the potential to fabricate microstructures in a large variety of polymer materials. We discuss some characterizations of a cast molding technique which exploits the use of polydimethylsiloxane as a mold material. The suitability of this process for microfluidic systems with five different polymers was studied by observing the mold sticking properties, pattern transfer resolution, and effects on surface roughness and surface wettability. The process yielded excellent results for all polymers, suggesting the suitability of cast molding as a general purpose microfabrication technique for polymers. Surface wettability was modified for some polymers.

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.

Laminated microfluidic structures using a micromolding technique

Microfluidic networks suitable for biomedical applications have been fabricated in a variety of polymer materials using micromolding and lamination. By molding several layers of 2D microfluidic patterns in a variety of materials, then laminating together, a wide variety of complex 3D systems with micron sized resolution can be constructed. The process of micromolding and lamination, and the integrated fabrication process for biomedical microfluidic devices is presented. Emphasis is on the practical aspects of fabricating fluidic prototypes in the research laboratory.

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.