CdSe/ZnS quantum dot encapsulated MoS2 phototransistor for enhanced radiation hardness.

Notable progress achieved in studying MoS2 based phototransistors reveals the great potential to be applicable in various field of photodetectors, and to further expand it, a durability study of MoS2 phototransistors in harsh environments is highly required. Here, we investigate effects of gamma rays on the characteristics of MoS2 phototransistors and improve its radiation hardness by incorporating CdSe/ZnS quantum dots as an encapsulation layer. A 73.83% decrease in the photoresponsivity was observed after gamma ray irradiation of 400 Gy, and using a CYTOP and CdSe/ZnS quantum dot layer, the photoresponsivity was successfully retained at 75.16% on average after the gamma ray irradiation. Our results indicate that the CdSe/ZnS quantum dots having a high atomic number can be an effective encapsulation method to improve radiation hardness and thus to maintain the performance of the MoS2 phototransistor.

Bias-dependent photoresponsivity of multi-layer MoS2 phototransistors.

We studied the variation of photoresponsivity in multi-layer MoS2 phototransistors as the applied bias changes. The photoresponse gain is attained when the photogenerated holes trapped in the MoS2 attract electrons from the source. Thus, the photoresponsivity can be controlled by the gate or drain bias. When the gate bias is below the threshold voltage, a small amount of electrons are diffused into the channel, due to large barrier between MoS2 and source electrode. In this regime, as the gate or drain bias increases, the barrier between the MoS2 channel and the source becomes lower and the number of electrons injected into the channel exponentially increases, resulting in an exponential increase in photoresponsivity. On the other hand, if the gate bias is above the threshold voltage, the photoresponsivity is affected by the carrier velocity rather than the barrier height because the drain current is limited by the carrier drift velocity. Hence, with an increase in drain bias, the carrier velocity increases linearly and becomes saturated due to carrier velocity saturation, and therefore, the photoresponsivity also increases linearly and becomes saturated.