High Conversion Efficiency Carbon Nanotube Based Barrier-Free Bipolar-Diode Photodetector.

Conversion efficiency (CE) is the most important figure of merit for photodetectors. For carbon nanotubes (CNT) based photodetectors, the CE is mainly determined by excitons dissociation and transport of free carriers toward contacts. While phonon-assisted exciton dissociation mechanism is effective in split-gate CNT p-n diodes, the CE is typically low in these devices, approximately 1-5%. Here, we evaluate the performance of a barrier free bipolar diode (BFBD), which is basically a semiconducting CNT asymmetrically contacted by perfect n-type Ohmic contact (Sc) and p-type Ohmic contact (Pd) at the two end of the diode. We show that the CE in short channel BFBD devices (e.g. 60 nm) is over 60%, and it reduces rapidly with increasing channel length. We find that the electric-field-assisted mechanism dominates the dissociation rate of excitons in BFBD devices at zero-bias, and thus the photocurrent generation process. By performing a time-resolved and spatial-resolved Mont Carlo simulation, we find that there exists an effective electron (hole) rich region near the n-type (p-type) electrode in the asymmetrically contacted BFBD device, where the electric field strength is larger than 17 V/μm and exciton dissociation is extremely fast (<0.1 ps), leading to very high CE in the BFBD devices.