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

Abstract

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 ends 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 Monte 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.

Electrostatics and quantum efficiency simulations of asymmetrically contacted carbon nanotube photodetector

“…31,32 Because the diameter of (17, 0) CNT is close enough to that of (17, 1) CNT, we believe the electrostatic properties simulated above for (17,0) CNT also applies to (17,1) CNT, so that we can compare the simulation results with the experiment results in Ref. 16. Auger recombination is avoided when we consider a 1 mW Gaussian beam with a radius of 20 µm and a wavelength at 1680 nm which locates in the E 11 exciton peak of a (17,1) CNT.…”

“…For photodiodes based on SWCNTs, p-n junction diodes with split-gate geometry [10][11][12][13][14] and asymmetrically contacted barrier-free bipolar diodes (BFBDs) [15][16][17][18][19] are two of the most widely-studied structures. With regard to the p-n junction diodes, several works have revealed the electrostatic properties and quantum efficiency of the devices.…”

“…As for the BFBDs, our recent experiment has exhibited the correlation between quantum efficiency and channel length. 16 Interpretations have been given in a qualitative way and a quantitative way using Monte Carlo simulation in that work. In the qualitative way, a neutral region of 10 nm was supposed in the shortest channel device, but this assumption may not be accurate enough.…”

“…where D is the diffusion constant, n photon (x) is the number of photon that CNT absorbs per unit time per unit length (we use 8.5 × 10 17 cm 2 as absorb cross section 16 ), η exc is the remaining fraction of exciton oscillator strength when the transfer of spectral weight of excitonic peak to the band-to-band continuum is considered, i.e. η exc = 1 I band /(I band +I exc ), 8 and τ is the lifetime of E 11 exciton in the range of 5 to 120 ps.…”

“…Auger recombination is avoided when we consider a 1 mW Gaussian beam with a radius of 20 µm and a wavelength at 1680 nm which locates in the E 11 exciton peak of a (17,1) CNT. 16 In a steady state, the equation becomes…”