| Thin film layer transfer provides a means to realize flexible solar cell devices and complex heterogeneous materials/device integration schemes. As an alternative to grind- or etch-back techniques, which result in the complete destruction of the starting substrate to achieve transfer, this study demonstrates an engineered composite substrate that is readily capable of device layer transfer to an alternative substrate while leaving the starting substrates intact for reuse. Furthermore, this composite substrate is not constrained to homoepitaxial deposition, making it applicable to a wide range of materials systems and device applications.;Fabrication of the composite substrate was achieved by incorporating the techniques of anodic etching, wafer bonding, and hydrogen exfoliation. Silicon handle wafers (p+ or p/p+) are subjected to anodic electrochemical etching in 25% HF electrolyte to create single layer (61% porosity) or double layer (40%/61% porosity) structures, which provide the means for mechanical transfer. The mechanical properties of the porous layers were found to decrease with increasing layer porosity. Upon annealing at typical device growth temperatures, the out of plane lattice parameter undergoes a shift from an initial tensile distortion to a compressive strain due to desorption of species from the porous Si lattice while the in-plane lattice parameter remains registered to the substrate. Additionally, the morphology of the porous silicon films evolves by pore sintering. After annealing, fracture occurs through the single porous silicon layer or at the interface between the porous double layers, enabling thin film layer transfer capability.;Indium phosphide wafers, which have been implanted with hydrogen ions, are then wafer bonded to the porous silicon handle wafers via silicon nitride interlayers. After a two-step annealing process, 0.6 mum layers of indium phosphide are transferred to the handle wafers through hydrogen exfoliation. After chemical mechanical polishing, the transferred InP layers have a surface roughness of 0.5 nm and high crystalline quality, with no detrimental impact due to the presence of the porous Si layer/s.;Metal-organic chemical vapor deposition on the composite substrate shows that residual ion implantation defects present in the InP template layer do not extend into growth layers, and the substrate maintains its high crystalline quality and mechanical integrity. Transfer of the epitaxial layers from the porous silicon handle wafer to an alternative substrate was achieved via fracture through the double porous layer interface. |
