A band-structure engineering approach is proposed to use semiconductor alloys such as SixGe1−x−ySny with a direct bandgap slightly above the indirect fundamental bandgap to demonstrate momentum(k)-space carrier separation of photogenerated carriers in photodetectors to reduce dark current. This approach combines the large absorption coefficient of direct-gap semiconductors with the long carrier lifetime of indirect gap semiconductors and reduces tunneling current due to the large effective masses of the indirect valleys. When the difference of the direct and indirect bandgaps (i.e., the energy difference between the direct and indirect valleys) varies from 0.4 to 3kBT, the direct-bandgap absorption covers a wide wavelength range from 2 to 22 μm for those alloys lattice matched to GeSn virtual substrates and to various commercially available substrates. The same approach can be adopted for other material systems.

Rock-salt lead chalcogenides such as PbTe are of much current interest for the physics study of quantum materials as a topological insulator and practical applications for infrared photodetectors. Heterocrystalline (rock-salt on zincblende) and heterovalent PbTe/CdTe/InSb heterostructures are grown on (001) InSb substrates using a single-chamber molecular beam epitaxy system. Elemental Pb and Te sources are used to independently vary the flux conditions at the heterocrystalline interface in nearly lattice-matched PbTe/InSb and PbTe/CdTe heterostructures. A streaky (1 × 1) surface reconstruction is observed during the growth of thicker PbTe layers on both InSb and CdTe, signifying smooth layer-by-layer growth. The thickness required for smooth PbTe growth on nearly lattice-matched zincblende materials can be minimized with the proper choice of growth conditions, particularly at the heterocrystalline interface. Characterization with x-ray diffraction indicates good crystalline quality, and observations by transmission electron microscopy reveal sharp interfaces between the PbTe and CdTe films.

No abstract

Monocrystalline 1.7 eV Mg0.13Cd0.87Te/MgxCd1−xTe (x > 0.13) double heterostructure (DH) solar cells with varying Mg compositions in the barrier layers are grown by molecular beam epitaxy. A Mg0.13Cd0.87Te/Mg0.37Cd0.63Te DH solar cell featuring abrupt interfaces between barriers and absorber and the addition of a SiO2 anti-reflective coating demonstrate open-circuit voltage (VOC), short-circuit current density (JSC), fill factor (FF), and device active-area efficiencies up to 1.129 V, 17.3 mA/cm2, 77.7%, and 15.2%, respectively. The VOC and FF vary oppositely with the MgxCd1−xTe barrier height, indicating an optimal design of the MgCdTe DHs as a trade-off between carrier confinement and carrier transport. Temperature-dependent VOC measurements reveal that the majority of carrier recombination in the devices occurs outside the DHs, in the a-Si:H hole-contact layer, and at the interface between the a-Si:H layer and the MgxCd1−xTe top barrier at room temperature. Simulation results for the device with the highest efficiency show that the p-type a-Si:H layer and the Mg0.37Cd0.63Te top barrier contribute 1.3 and 2.4 mA/cm2 JSC loss, respectively.

No abstract