Light induced tunnel effect in CNT-Si photodiode Abstract: Negative differential resistance (NDR) - the current is a decreasing function of the voltage - has been observed in the current-voltage curves of several types of structures. We measured the tunneling current and NDR value by illuminating the large area heterojunction obtained by growing Multi Wall Carbon Nanotubes on the surface of n-doped Silicon substrate. In the absence of light, the current flow is null until the junction threshold, beyond which the dark current flows at room temperature with a very low intensity of few nA. When illuminated, a current of tens nA is observed at a drain voltage of about 1.5 V. At higher voltage the current intensity decreases according a negative resistance of the order of MW. In this presentation we report details of NDR characteristics for two tunneling photodiodes which differ in the presence of a Schottky junction on the back (Sample B). Schematic side view of the Silicon substrate produced by the FBK related Sample A, left and Sample B right. Dark current of few nA for Sample A and Sample B. Carla Aramo for the PARIDE Collaboration INFN – Sezione di Napoli – Italy Left – FBK substrate front view; right - SEM image of MWCNT samples growth on the implantation area.

Tunnel effect For both photo-devices, when the drain voltage exceeds the threshold the reverse photocurrent begins to grow linearly until reaching a plateau which is maintained stable for a large voltage range. The most surprising effect is the generation of a significant resonant tunnel-like photocurrent below the junction threshold voltage, completely absent in the absence of light and in the substrate without CNTs. Therefore, resonant tunnel-like current is generated only under light radiation. Its value is proportional to the external quantum efficiency of the device. In the Sample A the light induced photocurrent is higher than in the Sample B, as well as the photo-induced tunnel current. A CNT/Si photodiode has been realized showing a Negative Differential Resistivity below the threshold of 2.4 V. The addition of a Schottky junction on the back of the device reduces the photocurrent but does not erase the tunnel-like effect. The area with negative resistance extends from 1.5 to 2.2 V, with a value of -0.015 MW for Sample A (without Schottky) and -0.57 MW for Sample B (with Schottky). These features, currently still under investigation, suggest a potential use of the device for optoelectronics applications.