Cryogenically probing the surface trap states of single nanowires passivated with self-assembled molecular monolayers

Abstract

Optoelectronically probing the trap state density of single nanoscale devices is a powerful in situ nondestructive technique that is of significance for developing high gain photoconductors by surface engineering. However, the previously demonstrated optoelectronic methods are based on the exponential transient photoresponse assumption and only trap states in a very narrow bandgap region can be probed. In this Letter, we demonstrate a cryogenic technique that is capable of measuring the density of surface trap states in the full half bandgap without the exponential transient photoresponse assumption. The technique is applied to an array of silicon nanowire photoconductors that are fabricated on silicon-on-insulator (SOI) wafer by the top-down approach. Diethyl 1-propylphosphonate (DPP) and hexadecane molecular monolayers are self-assembled on silicon nanowire surfaces as the passivation layer in comparison with dry oxide passivation. The surface trap state density of the dry oxide passivated nanowires exponentially increases from the bandgap center, reaching a peak of ∼5 × 10 cm eV at 50 meV below the conduction band. The defect state density is significantly suppressed after DPP and hexadecane molecules are grafted onto the nanowire surfaces via covalent bonds. The experimental observations are consistent with the density functional theory calculations.

Dynamics of Charge Carriers in Silicon Nanowire Photoconductors Revealed by Photo Hall Effect Measurements

“…The accumulation of excess holes and the deficit of excess electrons in the channel are caused by the localization of excess electrons at surface states and within surface depletion region. Indeed, after the nanowire devices are surface passivated by molecular monolayers, the photoresponse is significantly reduced (also see our recent publication), 17 resulting in a much smaller gain or even no gain at all, although the minority carrier lifetime becomes longer (higher excess minority concentration). 18 The reduced photoresponse leads to no detectable photo Hall signals in experiments (data not shown here).…”

Explicit Gain Equations for Single Crystalline Photoconductors

“…Although excess electrons and holes are generated in pairs, excess minority carriers are often trapped by defects or potential wells (depletion regions for instance) in semiconductors. 19,20 The same number of excess majority counterparts is accumulated in the conduction channel, leading to the experimentally observed high photogain. 14 After the above two assumptions were corrected, we previously derived a gain equation in the same way as the classical gain equation was derived.…”

“…However, high gains in photoconductors are often observed in experiments. ,, The question is where these high gains are coming from. − This is because the classical theory incorrectly made a second assumption that the number of excess electrons and holes contributing to photoconductivity are equal. Although excess electrons and holes are generated in pairs, excess minority carriers are often trapped by defects or potential wells (depletion regions for instance) in semiconductors. , The same number of excess majority counterparts is accumulated in the conduction channel, leading to the experimentally observed high photogain …”

Avalanche Effect and High External Quantum Efficiency in MgZnO/Au/ZnO Sandwich Structure Photodetector

“…Figure 5c depicts that when the MgZnO/Au/ZnO SSPD is excited by 275 and 355 nm illumination, electrons and holes are generated in pairs, and excess minority carriers are often trapped by defects or potential wells (focusing on the interface between ZnO and MgZnO) in semiconductors. [32,33] The same number of excess majority counterparts is accumulated in the conduction channel. These are favorable for PC gain and lead to the experimentally observed high EQE.…”