UC Berkeley

The synthesis of III-V nanowires and pillar-structures on silicon is a promising approach for realizing low cost, large scale III-V photovoltaics. However, performances of III-V nanowire solar cells have not yet been as good as their bulk counterparts, as nanostructured light absorbers are fundamentally challenged by enhanced minority carriers surface recombination rates. The resulting nonradiative losses lead to significant reductions in the quantum yield for external spontaneous emission, which, in turn, manifest as penalties in the open-circuit voltage. In this work, calibrated photoluminescence measurements are utilized to construct equivalent voltage/current characteristics relating illumination intensities to Fermi-level splitting ΔF inside InP microillars. Fermi-level splitting is a good indicator of what voltage can be achieved in a given material. Under 1-sun, we show that splitting can be close to ΔF~0.89eV in undoped pillars. Several strategies to increase the external quantum yield are tested. By cleaning pillar surfaces in acidic etchants the splitting can be increased to ΔF~0.96 eV. Pillars with nano-textured surfaces can yield splitting of ΔF~0.92 eV. Finally, by introducing n-dopants, ΔF greater than 1eV can be achieved, which is due to larger a wider bandgap energy in n-doped Wurzite InP, the increaesed brightness of doped materials, as well as the extraordinarily low surface recombination velocity of InP. These results provide further evidence that InP micropillars on silicon could be a promising material for low-cost, large scale solar cells with high efficiency. Furthermore, strategies for mitigating shunt-path formation in pillar devices have been studied; during the growth of pn-junctions care has to be taken that the core-shell growth mode of the micropillars will not result in a pn-junction where both sides of the junction will be in direct contact with the substrate. It was found that regrowth can be used to prevent such shunt paths between the substrate and the shell of the pillars. Under monochromatic illumination equivalent to 1 sun, open-circuit voltages of larger than 0.604 V could be achieved, which is compares well to other state-of the art results for III-V nanowire solar cells. The discrepancy between the achieved open-circuit voltage and the results from the contactless I-V measurements is explained with unoptimized device design. For further improvement it could be necessary to bury the pn-junction inside higher bandgap materials, and to use n-doped InP material as the light absorber.

Akin to Moore's law in the IC industry, it is believed that next-generation optical components will become more and more integrated onto chips, and perhaps even become integrated with silicon CMOS-electronics. The core-shell growth mode allows for the synthesis of single-crystalline III-V pillar-structures on silicon substrates, offering a pathway for monolithic, CMOS-compatible on-chip integration of active optical components. The viability of the pillar approach has been previously underlined by demonstration of lasers, light emitting diodes, and avalanche-photo-detectors. In this work, the modes of nanopillar lasers are studied in detail; it was found that modes with high azimuthal quantum numbers can exhibit quality factors of the order of 102, which is sufficiently high to achieve lasing at room temperature under pulsed optical pumping. Continuous-wave operation of optically pumped lasers could be demonstrated at 4K temperature as well. The radiation pattern of lasing modes are observed in experiments, and in combination with polarization measurements and Finite-Difference Time-Domain (FDTD) simulations all quantum numbers of the modes could be identified. Strategies to enhance the quality factor for modes with low azimuthal quantum numbers are discussed as well.

The atomically sharp tips makes nanoneedles attractive for applications requiring electric field enhancements such as Surface-Enhanced Raman Scattering (SERS). The possibility of using the nanoneedle structure for Superfocusing was studied in simulations. In FDTD simulations it was found that the needles could allow for electric intensity enhancements of the order of 106, showing that nanoneedles could be a promising structure for delivering optical power to nanoscale dimensions.

Due to the Wurtzite crystal structure nanoneedles exhibit selection rules for nonlinear light generation that are different from selection rules of bulk III-V materials, which exhibit the more common Zincblende phase. Polarization measurements were performed to confirm the Wurtzite selection rules in the pillar material. Potential applications for nonlinear light generation using nanoneedles could be in imaging. Another nanophotonic structure attractive for nonlinear optics are high-contrast gratings. In this work, second harmonic generation from AlGaAs near-wavelength high-contrast gratings was studied in polarization-dependent reflection measurements. The grating design was found to be critical in determining the strength and polarization dependence of the second harmonic signal. The second harmonic response was enhanced by more than 3 orders of magnitude compared to flat AlGaAs surfaces without gratings. These enhancements are due to the diffractive nature of the gratings, specifically their ability to change the direction of light waves.