Heterostructure Tunnel Diodes for Terahertz and mm-Wave applications

Abstract

The primary work presented in this thesis is the development and improvement of zero-bias detector diodes, for mm-Wave and THz applications. These devices were based on Asymmetric Spacer Layer Tunnel (ASPAT) diodes made on the GaAs and InP platforms. Accurate physical models of two state of the art ASPATs were created, which showed close agreement to experimental results obtained from ASPAT devices grown by Molecular Beam Epitaxy and processed using i-line photolithography techniques. These models were then used to develop two new novel device structures, one based on InP and one based on GaAs, in which quantum wells were added to the conventional ASPAT structures. These new QW-ASPAT devices showed curvature coefficients of 32V-1 and 35V-1 for In0.18Ga0.82As/AlAs/GaAs and In0.8Ga0.2As/AlAs/In0.53Ga0.47As devices respectively. This represents an increase by a factor of over 2.5 from the standard ASPAT devices. The addition of quantum wells also reduced the zero-bias capacitance of both QW-ASPAT devices. The cut-off frequencies were estimated for devices with 4×4µm2 mesa areas, with values of 532GHz being achieved for the In0.18Ga0.82As/AlAs/GaAs devices and over 800GHz for the In0.8Ga0.2As/AlAs/ In0.53Ga0.47As device. These are both substantial improvements over the values achieved for the standard ASPAT diodes. The lower cut off frequencies for the In0.18Ga0.82As/AlAs/GaAs devices are largely due to their inherently higher series resistance. A third novel device structure based upon the In0.18Ga0.82As/AlAs/GaAs QW-ASPAT, was also investigated. This structure used AlxGa1-xAs spacers to reduce the AlAs barrier height under forward bias. This new device structure allowed a zero bias curvature coefficient value of 42V-1 to be achieved. This is higher than the inherent Schottky diode limit of 40V-1¬ and represents the highest zero-bias curvature co-efficient achieved on a GaAs platform to date. The introduction of AlxGa1-xAs spacers also reduced the zero-bias capacitance of the devices when compared with the QW-ASPATs. The cut-off frequencies of 4×4µm2 mesa area devices were estimated with the highest value of 804GHz representing a 256% increase when compared with the standard GaAs ASPAT. As such the new device structure shows promise as a zero-bias mm-Wave and THz detector diode, achieving higher curvature coefficients than the Schottky diode as well as increasing the cut-off frequency of the ASPAT significantly. This thesis also investigated the growth of In0.53Ga0.47As/AlAs ASPATs on GaAs substrates through the use of metamorphic buffer layers. This work is intended to take advantage of the lower cost and easier processing available to GaAs substrates compared to InP, whilst maintaining the advantages of the In0.53Ga0.47As system. These metamorphic ASPATs (mASPAT) showed much lower detection characteristics than standard In0.53Ga0.47As/AlAs/InP ASPAT devices with cut off frequencies of 180GHz for devices with 4×4µm2 mesa areas, mainly due to the large dislocation network in the metamorphic structure.

Date of Award	1 Aug 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Mohamed Missous (Supervisor) & Leszek Majewski (Supervisor)