The Fabrication of GaAs Membrane and GaAs/GaAlAs Heterostructure Field Effect Transistor Devices

Lee, Kim Yang (1987) The Fabrication of GaAs Membrane and GaAs/GaAlAs Heterostructure Field Effect Transistor Devices. PhD thesis, University of Glasgow.

Abstract

The principal aim of the work presented in this thesis was to develop the techniques for fabricating FET devices on 50 nm thick active n+-GaAs membranes. The use of these membranes for electron beam lithography offers the advantage of very high resolution patterning due to the lack of backscattering, and the use of high contrast STEM (Scanning Transmission Electron Microscope) microscopy for very high resolution alignment and analysis. The electron beam lithographic techniques for fabricating nanostructures on thin substrates (principally thin carbon and Si3N4 membranes) are well established in this Department through the effort of previous workers. These techniques were successfully combined with very high resolution alignment to pattern interdigital metal gratings. With 8 nm probe size, alignment accuracy of better than 3 nm was demonstrated and gratings with 24 nm centre-to-centre spacing were fabricated on 60nm thick Si3N4 membranes. The membrane processing techniques were transferred to GaAs and MESFETs successfully fabricated. MESFETs were also fabricated with a fine pitch grating in place of the gate. Electrical characteristics of these devices indicated that surface effects dominated. Comparison between membrane MESFETs and conventional substrated MESFETs permitted an analysis of the effects of the substrate on device operation. Electrical measurements on membrane devices suggest carriers are confined to a thin layer. A theoretical investigation performed in collaboration with Dr. John Davies of this Department indicates that carrier energies are quantised. In order to substantiate this, GaAs membranes were characterised optically using transmission spectroscopy and photoconductivity. The use of membranes permitted a signal corresponding to the higher energy band-gap (L-L band) of GaAs to be resolved. Finally, GaAs MESFETs with gate-lengths of between 0.8mum and 0.2mum were fabricated on a MBE grown heterostructure (n+-GaAs/undoped Ga0.3Al0.7As). These devices showed excellent d.c. characteristics with very high transconductance and low output conductance. However, interface effects which degrade pinchoff characteristics were observed.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Electrical engineering, Materials science
• Date of Award: 1987
• Depositing User: Enlighten Team
• Unique ID: glathesis:1987-77537
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 14 Jan 2020 11:53
• Last Modified: 14 Jan 2020 11:53
• URI: https://theses.gla.ac.uk/id/eprint/77537