Vapor-liquid-solid grown Cu-catalyzed Si microwire arrays have shown great promise in photovoltaic and photoelectrochemical applications. Integration of GaP and other III-V materials on Si wires is a route to increased performance through the enabling of larger open circuit voltages and tandem or multijunction designs. State of the art work in the field, leveraging careful understanding of Si(001) surface preparation and an atomic layer deposition-like nucleation layer growth, has demonstrated the possibility of metallorganic chemical vapor deposition (MOCVD) of GaP on Si(001) substrates with nearly pristine interfaces free of stacking faults, microtwins and anti-phase domains. In this work, we transfer the atomic layer epitaxy (ALE) nucleation layer optimization to the 3-dimensional Si wire surfaces.
Before growth, planar Si(001), Si(011), Si(112) and Si microwire arrays are chemically cleaned using standard techniques to remove organic and metallic surface contaminants. After a high temperature anneal, ALE nucleation is performed 450°C with alternating pulses of triethylgallium (TEGa) and tertiarybutylphosphine (TBP). After heating the sample to 600°C under TBP overpressure, thicker GaP layers are grown with conventional simultaneous supply of both precursors. Undoped 50 nm thick films grown on planar Si are almost perfectly pseudomorphic (3.7% relaxation) as characterized by high resolution x-ray diffraction. Transmission electron microscopy of cross sections reveal that defects are still present in the GaP layers grown on Si microwires, including twins and antiphase domains. Thin, n-type GaP:Si layers have been grown on p-type Si microwires as a demonstration of a single wire heterojunction solar cell. Realistic materials parameters are used in conjunction with a coupled optical and electrical device simulation implemented in the Synopsys Sentaurus TCAD to assist in the design and interpretation of experimental results.