Effective Separation of Photogenerated Electron–Hole Pairs by Radial Field Facilitates Ultrahigh Photoresponse in Single Semiconductor Nanowire Photodetectors

Abstract

We report an investigation on the observation of ultrahigh photoresponse (photogain, 𝐺 𝑃𝑐 >10 6 ) in single nanowire photodetectors of diameter < 100 nm. The investigation which is a combination of experimental observations and a theoretical analysis of the ultrahigh optical response of semiconductor nanowires, has been carried out with emphasis on Ge nanowires. Semiconductor nanowire photodetectors show a signature of photogating where 𝐺 𝑃𝑐 rolls-off with increasing illumination intensity. We show that surface band bending due to depleted surface layers in nanowires induces a strong radial field (~ 10 8 V/m at the nanowire surface) which causes physical separation of photogenerated electron-hole pairs. This was established quantitatively through a self-consistent theoretical model based on coupled Schrodinger and Poisson Equations. It shows that carrier separation slows down the surface recombination velocity to a low value (< 1 cm/s) thus reducing the carrier recombination rate and extending the recombination lifetime by few orders of magnitude. An important outcome of the model is the prediction of 𝐺 𝑃𝑐 ~ 10 6 in a single Ge nanowire (with diameter 60 nm), which matches well with our experimental observation. The model also shows an inverse dependence of 𝐺 𝑃𝑐 on the diameter that has been observed experimentally. Though carried out in context of Ge nanowires, the physical model developed has general applicability in other semiconductor nanowires as well.

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“…The higher scriptR and EQE can be attributed to multiple reasons. It has been shown using Poisson-Schrodinger equation that depleted surface states in a semiconductor NW can give rise to a radial field that inhibits carrier recombination. , Both the photodetector parameters, scriptR and EQE, are directly proportional to the photoconductive gain, G , which is determined by the ratio of the lifetime (τ) and transit time (τ t ) of the charge carriers contributing the current in the external circuit . The device’s asymmetric electrode configuration and shorter NW length reduced the transit time of the photogenerated carriers.…”

Silicon Nanowire Arrays with Nitrogen-Doped Graphene Quantum Dots for Photodetectors

“…In order to obtain better performance in various optoelectronic and photodetection applications many researchers have been focused on texturing the surface of silicon with nanostructures (such as nanocones, NWs, etc.). One dimensional (1-D) SiNWs have enhanced light absorption capability and received increasing research interest because of their higher surface-to-volume ratio, strong light trapping effect, fast charge transport, and high charge collection efficiency by shortening the carrier transport path in comparison to bulk Si. , Thus, changes in the length, diameter, and spacing of SiNWs developed after the etching process reveal improvement of material properties, such as light absorption and scattering, enhancement of surface built-in-field, , electron–hole recombination, quantum confinement, and so forth. This leads to the enhanced application in photodetectors, − photovoltaics high sensitivity sensors, , field-effect-transistors, , thermoelectric devices, super capacitors, and so on.…”

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“…Furthermore, there is a lack of understanding for the appropriate charge transport materials, such as electron transport layer (ETL) and hole transport layer (HTL) materials, optimized for CISe QD-PDs. The electron mobility of typical ETLs is generally 3 orders of magnitude higher than that of general HTLs, causing the charge imbalance and recombination of photogenerated electrons and holes, leading to poor device performance . Thus, the development of high-quality HTLs is in need of high-performance CISe QD-PDs.…”

“…The electron mobility of typical ETLs is generally 3 orders of magnitude higher than that of general HTLs, 33 causing the charge imbalance and recombination of photogenerated electrons and holes, leading to poor device performance. 34 Thus, the development of highquality HTLs is in need of high-performance CISe QD-PDs.…”