Nanocrystallized CdS beneath the surface of a photoconductor for detection of UV light with picowatt sensitivity

In this study, we demonstrated that the improvement of detection capability of cadmium sulfide (CdS) photoconductors in the ultraviolet (UV) regime is much larger than that in the visible regime, suggesting that the deep UV laser-treated CdS devices are very suitable for low-light detection in the UV regime. We determined that a nanocrystallized CdS photoconductor can behave as a picowatt-sensitive detector in the UV regime after ultra-shallow-region crystallization of the CdS film upon a single shot from a KrF laser. Photoluminescence and Raman spectra revealed that laser treatment increased the degree of crystallization of the CdS and led to the effective removal of defects in the region of a few tens nanometers beneath the surface of CdS, confirming the result by the transmission electron microscopy (TEM) images. Optical simulations suggested that UV light was almost completely absorbed in the shallow region beneath the surface of the CdS films, consistent with the observed region that underwent major crystal structure transformation. Accordingly, we noted a dramatic enhancement in responsivity of the CdS devices in the UV regime. Under a low bias voltage (1 mV), the treated CdS device provided a high responsivity of 74.7 A W(-1) and a detectivity of 1.0×10(14) Jones under illumination with a power density of 1.9 nW cm(-2). Even when the power of the UV irradiation on the device was only 3.5 pW, the device exhibited extremely high responsivity (7.3×10(5) A W(-1)) and detectivity (3.5×10(16) Jones) under a bias voltage of 1 V. Therefore, the strategy proposed in this study appears to have great potential for application in the development of CdS photoconductors for picowatt-level detection of UV light with low power consumption.