In this paper several designs to maximize the absorption efficiency of superconducting-nanowire single-photon detectors are investigated. Using a simple optical cavity consisting of a gold mirror and a SiO2 layer, the absorption efficiency can be boosted to over 97%: this result is confirmed experimentally by the realization of an NbTiN-based detector having an overall system detection efficiency of 85% at 1.31 μm. Calculations show that by sandwiching the nanowire between two dielectric Bragg reflectors, unity absorption (>99.9%) could be reached at the peak wavelength for optimized structures. To achieve broadband high efficiency, a different approach is considered: a waveguide-coupled detector. The calculations performed in this work show that, by correctly dimensioning the waveguide and the nanowire, polarization-insensitive detectors absorbing more than 95% of the injected photons over a wavelength range of several hundred nm can be designed. We propose a detector design making use of GaN/AlN waveguides, since these materials allow lattice-matched epitaxial deposition of Nb(Ti)N films and are transparent on a very wide wavelength range.@en

The interface properties of amorphous silicon (a-Si:H) solar cells have a large influence on the solar cell performance, especially that of the p/i interface. For highly efficient a-Si:H solar cells, a good p/i interface region with low defect density is necessary. In this contribution we report the influence of H2-plasma exposure of the intrinsic layer near the p/i interface on the solar-cell performance. During processing of n-i-p a-Si:H solar cells with PECVD, we exposed the i-layer to a H2-plasma before p-type a-SiC:H deposition. With this treatment, the Voc and FF increased significantly, as well as the external quantum efficiency in the wavelength range from 300 nm to 600 nm. The initial efficiency increases from 6.3% (without H2-plasma treatment) to 8.4% (with H2-plasma treatment for 450 s). We think that this performance enhancement can be explained by a reduction of the defect density in the i-layer near the p/i interface following H2-plasma exposure. Additionally, the exposed subsurface layer has a higher mobility band gap, which can act as graded buffer layer to suppress the band offset between the intrinsic a-Si:H layer and ptype a-SiC:H layer. This band gap increase is confirmed by activation energy measurements.@en