Abstract: Vacuum ultraviolet (VUV) light are important for various applications such as surface treatment, optical cleaning of semi-conductor substrates and sterilization. Constant monitoring of high intensity VUV radiation is required to maintain the high standard of manufactured products in many industrial applications. Currently, VUV detectors based on oxide, nitride and diamond have been developed. However, these detectors require filters to block out deep-UV, leading to the reduction of the sensing area. The band gap of the photodetector material is an important consideration when developing the next generation of VUV detectors as the band gap determines the spectral response of the detector. Here, a filterless VUV detector using mixed crystals of calcium fluoride and strontium fluoride (CaF2 – SrF2, CaxSr1-xF2) will be presented [1]. Generally, fluoride compounds have significantly wider band gaps that allow them to be highly transparent in the deep UV region. Unwanted low energy photons are therefore intrinsically blocked out without having to use filters. We experimentally demonstrate that the band gap of these mixed crystals 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 ability to manipulate the band gap is maintained even at very low temperatures. VUV photoconductive detectors were 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 eliminate 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 the filterless feature of CaxSr1-xF2 photodetectors provide an advantage over currently available oxide-, nitride-, and diamond-based ones.

Keywords: photoconductive detector, vacuum ultraviolet, fluoride crystal, band gap control

Reference [1] K. Suzuki, M. Cadatal-Raduban, M. Kase, S. Ono, Opt. Mater, 88, 576-579 (2019)

Biography: Dr. Marilou Cadatal-Raduban is currently a lecturer of Physics at the School of Natural and Computational Sciences, Massey University in Albany, Auckland, New Zealand. Dr. Cadatal-Raduban is originally from the Philippines where she obtained her Bachelor of Science in Applied Physics (2002) and Master of Science in Physics (2004) degrees from the University of the Philippines-Diliman. Dr. Cadatal-Raduban obtained her Doctor of Philosophy degree (2008) from the Graduate University for Advanced Studies in Japan while doing research on ultraviolet and vacuum ultraviolet lasers and laser materials at the Institute for Molecular Science under the Japan Society for the Advancement of Science (JSPS) scholarship. Dr. Raduban then worked as a postdoctoral fellow then a specially appointed researcher at the Institute of Laser Engineering, Osaka University in Japan (2008-2011) before accepting an academic post at Massey University in Auckland, New Zealand (2012-present). Her current research interests include solid-state vacuum ultraviolet (VUV) laser materials and ultraviolet (UV) lasers and amplifiers, spectroscopy of rare-earth-doped crystal and glass scintillators, and development of radiation sensors based on wide band gap crystals and semiconductor thin films.