Band gap engineering of CaxSr1-xF2 and its application as filterless vacuum ultraviolet photodetectors with controllable spectral responses

Abstract We experimentally demonstrate that the band gap of mixed crystals of calcium fluoride and strontium fluoride (CaF2 – SrF2, CaxSr1-xF2) can be engineered by modulating the composition ratio of CaF2 and SrF2. By increasing the CaF2 content, the band gap increases and the absorption edge is blue-shifted. A range of band gap energies can be obtained between 9.73 eV for pure SrF2 and 10.24 eV for pure CaF2. The first order Raman frequency shift increases linearly from 285.2 cm−1 for pure SrF2 to 327.8 cm−1 for pure CaF2. The ability to manipulate the band gap is maintained even at very low temperatures. Vacuum ultraviolet (VUV) photoconductive detectors are fabricated to explore the effect of varying composition ratios on spectral sensitivity. The spectral response of the photodetectors shifts to shorter wavelengths as the band gap increases. This allows the spectral response to be controlled by appropriately choosing the CaF2 – SrF2 ratio. Using CaxSr1-xF2 sensors also eliminates the need for extra filters to cut off unwanted longer wavelengths as the onset of their absorption occur in the VUV region. The controllable spectral response and filterless feature of CaxSr1-xF2 photodetectors provide an advantage over currently available oxide-, nitride-, and diamond-based photodetectors.