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Introduction
Semiconductor technology has developed quite considerably in the past couple of decades and has become the foundation of the modern day electronics industry. These developments have led to reduction in size, increased power and greater reliability of semiconductor based devices. Gallium nitride (GaN), in particular, has emerged as a strong contender in the production of optoelectronic devices. This is due to its ability to operate at high temperatures, voltages and frequencies. It can also operate quite well in caustic environments. All these properties bode well for potential use of GaN in military, space, domestic and industrial applications.
Native defects and impurities can affect the electronic, mechanical, and optical properties of semiconductors. They can either enhance or degrade the material depending on the type of device being fabricated. Defects can enhance devices by acting as recombination centers which increase the switching speeds of diodes and for other high frequency devices. They can be detrimental by reducing the mobility of charge carriers and acting as traps, thus reducing carrier lifetimes. Defects can be introduced to the semiconductor either intentionally or unintentionally during growth, annealing, device fabrication, and other processing methods. In order to study defects and understand their origin, it is common to intentionally introduce them by processing methods such as ion implantation and irradiation.
One of the most important electrical characterisation techniques for studying defects in semiconductors is deep level transient spectroscopy (DLTS). Defects can be identified using this technique by a defect signature, which is composed of the activation energy and apparent capture cross section. DLTS is non-destructive, has a high sensitivity to defects, and can easily distinguish between majority and minority carriers. This makes it a suitable candidate for detecting electrically active defects in semiconductors.
In this study, defects in GaN were detected and characterised by DLTS after various processing techniques. These techniques included exposure of GaN to various radiation effects at low, medium and high energy, namely electron beam exposure, ion implantation and ion irradiation respectively. Defects measured in this study were then compared to defects reported by other studies.
In Chapter 2 of this thesis the properties of GaN are discussed. This includes general properties such as the crystal structure, and optical and mechanical properties. Also, the evolution of the energy-band relationship between the contact of a metal and semiconductor is explained. A discussion on ion-solid interactions conclude the topics of this chapter. Chapter 3 explains general defect theory. The theory of the defect characterisation technique, namely DLTS, is also discussed in this chapter.
1 Introduction
2 Theoretical background
2.1. Introduction .
2.2. Crystal structure .
2.4. Growth
2.5. Metal-semiconductor contacts
2.5.1. Schottky barrier formation
2.5.2. Schottky barrier height determination
2.5.3. Depletion width
2.5.4. Image force lowering
2.5.5. Ohmic contacts
2.6. Current transport mechanisms .
2.6.1. Thermionic emission
2.6.2. Tunnelling
2.7. Ion-solid interaction
2.7.1. Introduction
2.7.3. Nuclear stopping .
2.7.4. Electronic stopping.
2.7.5. Ion damage
3 Defect theory and characterization
3.1. Defect theory
3.2. Defect characterisation .
4 Experimental techniques
4.1. Introduction
4.2. Sample preparation
4.3. Metal deposition
4.4. Annealing
4.5. Irradiation and implantation
4.6. Devise Characterisation
5 Results .
6 Conclusions
