Direct band gap silicon nanowire avalanche transit time thz opto-electronic sensor with strain-engineering

Abstract

The role of process-induced strain-engineering in inducing indirect to direct band gap transition in Si nanowires (Si-NWs) has been investigated. A very interesting observation reveals that both spatial confinement and process-induced strain induce similar kind of quantum confinement resulting in band gap transition (indirect to direct) in Si-NWs. Further, suitable strain-engineering has resulted in near ballistic transport and subsequently very high electron mobility along nanowire axis has been achieved. Thereafter, the impact of such strain-engineering in tuning peak oscillation frequency, RF power density and opto-electronic properties of Si-NW based laterally-doped pulsed mode Avalanche Transit Time (ATT) oscillators are explored for the first time. The tuning factor is the fraction of insertion of the NWs into the underlying sapphire substrate. For this, a planar ATT device with Si-NWs on Sapphire-On-Silicon (SOS) substrate is proposed. Static, non-linear and thermal analysis are carried out under light and dark conditions, using indigenously developed in-house simulation codes through a self-consistent, quantum drift–diffusion model. The obtained analytical results have been verified with experimental data and considerable agreement has been achieved. The study reveals that the use of Si-NWs has significant technological advantages that include carrier mobility-enhancement and complete band gap transition from indirect to direct. Strain-engineered NW ATTs are revealed to be very much conducive for their useful application as high power (84.25 × 109 W/m2) THz optical switches with excellent thermal stability. For the first time the study reports the suitability of direct band gap pure Si-NW ATTs as potential candidates for application in high power THz opto-electronics.