1. Introduction

1.1 Genesis

With the demonstrated advantages of an optical fiber telecommunication system over a conventional copper-wire based system, the efficient conversion of an electrical signal to an optical signal and vice versa, is now of utmost importance. Similarly, rapid progress in semiconductor materials growth technology and the emergence of novel techniques in device fabrication has led to a continual improvement in the performance of opto-electronic integrated circuits (OEICs). In addition, with similar maturity in new technologies such as monolithic microwave integrated circuits (MMICs), the commercial viability of direct integration between optical and microwave circuits is becoming more and more likely.

One of the vital components of any high frequency optical system is a large bandwidth, low noise photo receiver which may be realised using monolithic integration of photo diodes with high speed transistors to act as preamplifiers. A wide range of materials and devices, including semi-transparent metal/semiconductor Schottky photo diodes and integrated p-i-n diodes with HBTs have been used for this purpose.

The use of a Schottky diode is ideal for high speed applications where it has many advantages. One of these is the inherent absence of any slow component associated with minority carrier effects. Its usage enables the absorption layer thickness to be engineered to obtain the optimum compromise between external quantum efficiency and detector bandwidth. The latter of these arises from a combination of carrier transit time and RC effects. An inherent disadvantage, however, is the high series resistance and low optical coupling efficiencies obtained which are associated with the thin semi-transparent metal layer. A solution to this problem is to use a layer of Indium Tin Oxide (ITO), a practically transparent and highly conductive material, as the Schottky contact to fabricate both high speed and highly efficient photo diodes.

Renewed interest in using photo transistors as detectors has been aroused world wide, particularly with the advent of Heterojunction Bipolar Transistors (HBTs). Studies using such HBTs with opaque emitter contacts show excellent suitability of these devices as photo detectors in terms of optical performance where signal to noise ratios in excess of 30dB have been obtained. The optical gain of any photo transistor depends on the coupling efficiency, the collection efficiency and its internal gain; in the HPT structure, there lies an inherent trade-off between the speed and the collection efficiency. However, by using a transparent emitter contact, the coupling efficiency can be significantly improved thereby raising the overall gain-bandwidth of the device correspondingly.

Similar arguments could be presented in support of using a transparent ITO contact to a number of other devices such as in transparent gate HEMTs, LEDs and VCSELs. Hitherto, ITO had been the subject of extensive study by material scientists and engineers for a wide range of other applications ranging from anti-reflection coatings to transparent contacts in solar cells. Its usage in the fabrication of microelectronic devices for optoelectronic applications is a relatively new field. Hence, this study essentially represents some of the novel work in this area.

1.2 Aims and Objectives

One of the primary objectives of this work was the application of a transparent ITO contact to a number of optoelectronic devices. These required the development of both Schottky and ohmic contacts. In particular, n-GaAs Schottky photo diodes and the heterojunction photo transistors, both as optical detectors, were studied in detail.

With this goal in mind, the ITO film deposition based on a r.f. reactive sputtering technique was first studied and then optimised. Extensive work involving the calibration of the sputtering machine was carried out to understand the particular effects of various deposition conditions on the ITO films. These films were then characterised to study their electrical conductivity and optical transmittance. A number of different post-deposition annealing techniques were also developed for specific application as either Schottky or ohmic contacts. The use of a thin indium metallic layer between the sputtered ITO and the underlying semiconductor was also studied for its influence on sputter damage, ohmic and Schottky contacts. Work was also carried out to ensure that adequate patterning techniques were developed to realise small geometry devices based on ITO contacts. Similarly, contacts to ITO with pad metals was also studied and assessed.

Once it was established that satisfactory ITO films could be produced, the work was extended to the fabrication of the devices. These films were used to realise novel optoelectronic devices which were characterised and compared to their opaque counterparts.

Schottky diodes with near ideal electrical characteristics were realised on n-GaAs substrates using aluminium (Al) 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. Photo diodes with ITO as the Schottky contact were fabricated and a study comprising of both their electrical and optical behaviour was undertaken. This involved further optimisation work to ensure the sputter damage sustained during the ITO deposition could be first minimised and then the remainder removed to an acceptable level without compromising the rectifying behaviour of the junction, reducing the series resistance and dark currents for photo diode operation. All this had to be carried out while retaining the desired high transparency and high conductivity properties of the ITO film itself.

The next stage of the study involved the use of heterojunction transistors as photo detectors. Large geometry HBTs and HPTs were fabricated using AlGaAs/GaAs, InGaP/GaAs and InP/InGaAs systems respectively with both conventional and transparent ITO emitter ohmic contacts. 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.

A spectral response model was also developed to understand and help design optoelectronic detectors comprising of single layer devices (n-GaAs Schottky photo diodes) or multiple semiconductor materials (HPTs using AlGaAs/GaAs or InP/InGaAs 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 geometric dimensions of the device.

In collaboration with other researchers, two by-products of this study were the first VCSEL and TG-HEMT using ITO as the transparent ohmic and Schottky gate contacts respectively. Some preliminary work was also carried out to produce ITO contacts to visible LEDs which had an emission wavelength of approximately 630nm.

1.3 Summary and Layout of Thesis

In this thesis the background theory and the relevant literature survey are presented in the second chapter. This is followed, in chapter three, by a description of all the processing steps involved in the fabrication of the devices, while chapter four consists the details of the measurement and analysis techniques. Chapter five contains the first set of experimental results of this research; these are the findings concerning the ITO films used in the devices. Finally, chapter six constitutes the second and the larger part of results, namely that of the Schottky diodes and their temperature dependent current transport model, the heterojunction photo transistors, and the spectral response model respectively. The conclusion of the work and a list of suggestions for further work are presented in chapters seven and eight.

A list of publications resulting from this work is given in Appendix A. Appendix B contains the processing steps for liftoff and etch lithography steps. Appendix C has the Hewlett Packard HP4145B Semiconductor Parameter Analyser (SPA) settings for the device parameter extractions while some mathematical derivations are given in Appendix D.

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