Antireflective coating for BiPbSrCaCuO sensors

The applications of high T/sub c/ superconductors as transition edge bolometers or room temperature thermopiles appear most promising at the long infrared wavelengths. However, the sensor performance at these wavelengths is impeded by the large reflectance of superconductor films. In this work, effects of antireflective coating on the performance of BiPbSrCaCuO sensors were investigated. Thermal evaporation was used to deposit semiconductor CdS onto sputtered BiPbSrCaCuO films. It was found that, in order to achieve transparent and uniform CdS coating, it was necessary to maintain the substrate temperature at 180/spl deg/C during the evaporation. The superconductivity of BiPbSrCaCuO appeared to be not degraded by the deposition of CdS. After the deposition of CdS with a nearly optimum thickness, the reflectance of BiPbSrCaCuO sensor was reduced from 0.8 to below 0.4 for wavelengths above 10 /spl mu/m. In addition, the temperature coefficient of the film resistance was seen to increase in the transition region. The increases of film absorptance and resistance were consistent with the change of bolometric responsivity of the coated sensor. The use of Ge semiconductor for antireflective coating was also studied. A reflectance of 0.16 was achieved at wavelengths near 10 /spl mu/m on Ge coated sensors. This small reflectance was in good agreement with predictions derived from the computed optical constants of BiPbSrCaCuO films.