High photosensitivity and wide operation voltage in two-dimensional CdS nano-crystal layer embedded a-InGaZnO hybrid phototransistors

We suggest a design for the channel stacking sequence in hybrid phototransistors consisting of an oxide layers and two-dimensional (2D) chalcogenide nano-crystals to develop good electrical performance and a good photoresponse. The hybrid channels of the phototransistors were composed of a photo-insensitive oxide [amorphous InGaZnO (a-InGaZnO)] for effective charge transport and a visible photosensitive chalcogenide (CdS) nano-crystal. Interestingly, the electrical performance, photosensitivity, and recovery properties of the hybrid phototransistor depended strongly on the order of the channel stacking sequence. When the CdS nano-crystal layers were located close to the gate interface in the hybrid channel, significant degradation of the field-effect mobility and recovery behavior was observed, which was due to the increase in gate-interface trap density caused by many surfaces of nano-crystals. In contrast to the negative V_th shift for the a-InGaZnO/2D CdS nano-crystal stacking sequence, the phototransistor using an embedded-absorber channel structure (a-InGaZnO/2D CdS nano-crystals/a-InGaZnO) exhibited a considerably higher off-current under green irradiation, which is attributed to simultaneous realization of effective generation of photocarriers from the CdS nano-crystals and charge transport via the high-mobility upper a-InGaZnO layer. In particular, this hybrid phototransistor exhibited a wide operation voltage window as well as high photosensitivity and photoresponsivity, without any degradation of the oxide thin film transistor's performance.