In this talk, I will discuss electrical and optical properties of infrared photodetectors/solar cells based on square millimeter ensembles (> 1 million) of vertically processed InP/InAsP semiconductor nanowires grown on InP substrates.

   To investigate current generation processes in InP p⁺-i-n photodetectors/solar cells, we fabricated a sample series where the p⁺-segment length was varied, as well as optimized solar cells with 9.3 % efficiency with similar design. The electrical data generally display excellent rectifying behavior with small leakage currents. From spectrally resolved photocurrent measurements, we conclude that the photocurrent generation process depends strongly on the p⁺-segment length. Without p⁺-segment, photogenerated carriers funneled from the substrate into the NWs contribute significantly to the photocurrent. Adding a p⁺-segment decouples the substrate and shifts the depletion region, and collection of photogenerated carriers, to the nanowires, in agreement with sophisticated theoretical modeling. In optimized solar cells, clear spectral signatures of interband transitions in the ZB and WZ InP layers of the mixed-phase i-segments are observed. Complementary measurements of the dependence of the photocurrent on angle of incidence and polarization, as well as electroluminescence, support our interpretations.

    I will also discuss large area photoconductors based on n⁺-i- n⁺ InP NWs with embedded InAsP QWs grown in axial geometry on InP substrates. Spectrally resolved photocurrent measurements and photoluminescence measurements reveal clear interband transitions both in the InP segments and InAsP quantum wells. By increasing the number of incorporated quantum wells in the nanowires, the contribution from intersubband transitions in the quantum wells can be enhanced thereby facilitating detectors/focal plane arrays with a broad detection window from near-infrared to long-wavelength infrared regions.

Finally, I will briefly mention efforts targeting large area p⁺-n-n^-  InP/InAsP avalanche nanowire photodiodes with low electric field in the low-bandgap InAsP segment and high electric field in the InP multiplication region. Besides clear interband photocurrent signals from the InP and InAsP regions, a strong bias-dependent blueshifted photocurrent peak appears at forward bias offering an interesting novel tunability of the spectral window.

The presented nanowire-based photodetectors can potentially be grown on silicon substrates due to the small footprint of the nanowires. Successfully developed, novel detector families of low-cost, high performance broadband detectors for optical communication, thermal imaging and solar cell applications can thus be realized.