Study of transparent indium tin oxide for novel optoelectronic devices
Indium Tin Oxide (ITO) films were deposited on a number of semi-conductor materials using
reactive r. f sputtering technique to form both rectifying Schottky and ohmic contacts. These
contacts were applied in the fabrication of a number of novel optoelectronic devices:
Schottky photo-diodes, transparent gate High Electron Mobility Transistors (HEMTs),
heterojunction bipolar transistors (HBTs) being used as heterojunction phototransistors
(HPTs), light emitting diodes (LEDs) and vertical cavity surface emitting lasers (VCSELs).
A number ofthese novel devices were studied in comparatively greater detail; these were the
Schottky diode and the HPT.
Deposition conditions necessary to produce ITO films with high conductivity and optical
transparency over a wide spectral range were studied and optimised. Separate post
deposition techniques were developed to produce near ideal rectifying contacts and ohmic
contacts with low contact resistance respectively. A thin film of indium (In) was also used to
optimise ITO ohmic contacts to n + - GaAs substrates.
Near ideal Schottky diodes were realised on n-GaAs substrates using aluminium (AI) and
gold (Au) metal contacts. A simulation model was then developed and implemented to study
the behaviour of current transport mechanisms over a wide temperature range. Photodiodes
with ITO as the Schottky metal contact were fabricated and a study comprising of both their
electrical and optical behaviour was undertaken.
Relatively large geometry HBTs and HPTs were fabricated using AIGaAs/GaAs, InGaP/GaAs
and InPlInGaAs systems respectively; the latter devices were first reported as a result of this
study. A comparative study between devices fabricated from these systems were then made.
This was followed by an appraisal of the electrical properties of each of their optical
counterparts which had ITO emitter contacts. The specific photo responsivity and the
spectral responses of these HPTs were analysed. In light of HPTs with transparent ITO
emitter ohmic contacts, a brief examination of the merits of vertical versus lateral
illumination was also made in this work.
Finally a spectral response model was developed to understand and help design
optoelectronic detectors comprising of single layer devices (n-GaAs Schottky photo diodes)
or multiple semiconductor materials (HPTs using AIGaAs/GaAs or InPlInGaAs systems) to
help predict responsivities at a given incident wavelength. As well as material properties of
the constituent semiconductors, this model takes into account the specific lateral and vertical
geometrical dimensions of the device.