Single carbon nanotube based photodiodes for infrared detection

The photobehavior of carbon nanotubes (CNTs) has attracted great attention because of their unique cylinder structure and outstanding electrical properties. Much experimental progress toward nanotubes based photodetector has been reported. But it is still unclear whether the photoinduced conductivity change is caused by heating effect or quantum effect for the reported results. Moreover, the sensitivity of the CNT based IR detector needs to be further improved for real applications. In this paper, a single carbon nanotube based photodiode for infrared (IR) detection is constructed by assembling a single CNT to form connections with a pair of electrodes. By forming Schottky contact with an electrode, a semiconductive CNT is assembled into a Schottky diode. The photogenerated electron-hole pairs within the Schottky barrier are separated by an external electric field or the built-in field, producing a photocurrent. Since a semiconductive CNT normally forms Schottky contacts with both electrodes, the photocurrents generated by the two reversely connected Schottky diodes will cancel each other. Experimental results show that, at zero bias, the photocurrent varied from positive to negative as the IR spot center was moved from one electrode to another one. This proved that the photocurrent is caused by the photovoltaic effect in stead of the heating effect. To optimize the performance of the detector, a heterogeneous electrodes structure is designed to maximize the difference between the photocurrents of the two Schottky barriers. Different contact metals are selected such as an Ohmic contact is formed at one electrode and a Schottky barrier is formaed at another electrode. Experimental results show that the signal to dark current ratio of a heterogeneous detector is thousand times higher than the ratio of a homogeneous detector.