Terahertz emission from ultrafast lateral diffusion currents within semiconductor devices

Abstract

Single cycle THz emission from unbiased semiconductor devices after ultrafast carrier excitation can be attributed to surge currents on the surface of the device. These currents are due to either drift currents where carriers are accelerated by an internal electric field perpendicular to the surface (surface field effect) or diffusion currents where a separation of charge forms due to electrons and holes having different mobilities (photo-Dember effect). This surface emission is difficult to out couple from the semiconductor device as the emission is parallel to the surface of the semiconductor. This difficulty in out coupling led to a decline in interest for these types of emitters in preference to photoconductive emitters which today are the standard type of emitters used in THz time domain spectroscopy. In recent years a new type of surface emitter based on lateral diffusion currents (lateral Dember currents) has been proposed and demonstrated. This work acted as the initial inspiration for the work described within this thesis. The emission was attributed to net diffusion currents that formed from an initially asymmetrical carrier distribution that formed due to partially masking the pump spot with a metal mask.

Simulations of the situation revealed that diffusion alone cannot account for the observed THz emission from these devices. From this I have extended the mechanism taking into account lateral diffusion currents and dipole suppression under a metal mask. Along with theoretical arguments experimental evidence is given that supports this new theory. These devices are further explored experimentally giving insights into the nature of the emission and how it depends on different pump parameters and external electric fields. Based on this new interpretation I present the design, fabrication, and testing of multiplex emitters that are comparable with commercial photoconductive emitters in both power and band-width.

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Identifiers

ORCID for Vasileios Apostolopoulos: orcid.org/0000-0003-3733-2191

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