Defects and deep levels in semiconductors

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Some concepts in semiconductor physics

The properties of semiconductors are adequately described in a number of textbooks. For this reason only a short description of the aspects of semiconductor physics that are relevant to this study is given.The main aim is to familiarise the reader with the terminology and the notation used in this thesis. For a more complete introduction to the properties of semiconductors, the reader is referred to textbooks e.g. Ridly (1988) , Sze (1981), Smith (1978), and Henisch (1989).

Metal-semiconductor junctions

A number of early researchers have noted that the current flowing through a metal–semiconductor junction depends on the polarity of the applied voltage. This effect was researched further and later used in point contact rectifiers. Currently, metal-semiconductor junctions are important because they are used in devices as well as tools in the analysis of physical parameters of semiconductors. For this reason, metal-semiconductor junctions have been studied extensively. A number of models have been suggested to explain how these junctions operate. In this chapter, the model proposed by Schottky (1942) will be discussed in more detail. This model describes an ideal case, where the metal and the semiconductor are in intimate contact, without the presence of any interfacial layer or interface states. The Bardeen model describes a more general case where the effects of an interfacial layer and interface states are taken into account (Bardeen, 1947 and Rhoderick, 1988).

The Schottky model

When a metal contact is evaporated onto the surface of a semiconductor, a potential barrier is formed at the metal semiconductor interface. Here, only the case for an n-type semiconductor will be considered. The formation of a Schottky barrier on p-type material occurs similarly.

The ideal case

Figure 2.1 graphically illustrates the formation of a Schottky barrier. Part (a) illustrates the metal and the semiconductor in their isolated, electrically neutral states. Here χ s is the electron affinity of the semiconductor. (The electron affinity of a substance is the energy released when an electron is added to the material – i.e. in this case the difference between the vacuum level and the conduction band edge.) φ m and φ s are the work functions of the metal and the semiconductor respectively. (The work function of a material is the energy required to remove an electron from the material to the vacuum level – i.e.the difference between the vacuum level and the Fermi level.) Here we only consider the case where the work function of the metal is greater than that of the semiconductor, which, in practice, is the most important case. This relationship between the two work functions causes the Fermi level of the metal to be lower than that of the semiconductor, and leads to the formation of a contact with rectifying properties.Now, if the metal and the semiconductor were connected by means of a thin wire, electrons would flow from the semiconductor to the metal due to the difference in work function. Because of this flow of electrons, a positive charge builds up on the surface of the semiconductor, while a negative charge builds up on the surface of the metal, causing an electric field in the gap between the metal and the semiconductor. This electric field opposes the flow of electrons. The equilibrium condition is reached when the Fermi levels of the two materials coincide. This implies that the potential difference between the metal and the bulk of the semiconductor is equal to the difference in their Fermi levels.The negative charge that builds up on the surface of the metal is caused by extra electrons that are accommodated within the Thomas-Fermi screening distance of about 0.5 Å, i.e. within the first atomic layer. In the semiconductor, the positive charge is caused by the removal of electrons. However, the only electrons close to the Fermi level that can be removed are those in the conduction band, which are provided by the ionised donor atoms. Thus, the positive charge in the semiconductor is provided solely by the uncompensated donor atoms, left after electrons have flowed from the conduction band.The concentration of these donor atoms is much lower than the concentration of electrons in the metal.This means that electrons are depleted from the conduction band up to an appreciable depth, w. For carrier densities of 1016 cm–3 the thickness of this so-called depletion layer, is generally in the order of a micron. Because the charge in the depletion region is distributed over a finite distance, the potential changes slowly over the depletion region, and causes the bands to bend upwards as shown in Figure 2.1(b).

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1 Introduction
2 Some concepts in semiconductor physics
2.1 Metal-semiconductor junctions 
2.1.1 The Schottky model
2.1.2 The ideal case
2.1.3 Behaviour of the barrier under forward and reverse bias
2.1.4 Calculation of the electric field in a Schottky barrier
2.2 Defects and deep levels in semiconductors
2.2.1 Emission and capture of carriers from defects
2.2.2 Defect occupation as a function of time
2.2.3 Field dependence of the emission rate
3 DLTS: Deep level transient spectroscopy 
3.1 The DLTS technique 
3.2 Analysis of the DLTS transient
3.2.1 Analogue techniques
3.2.2 Digital processing of DLTS transients
3.3 Differential DLTS 
4 Planning of the digital DLTS set-up
4.1 Acquisition of the capacitance signal
4.1.1 Random noise
4.1.2 Response time, sampling rate and resolution
4.1.3 Periodic noise
4.1.4 Stability
4.2 Selection and characterisation of the instrumentation
4.2.1 Digitiser
4.2.2 Capacitance meter
4.2.3 Pulse generator
4.2.4 Rise and fall times
4.2.5 Sample mounting and temperature control
4.3 Other hardware 
4.3.1 Fast pulse interface
4.3.2 Synchronisation
4.4 Software
5 Experimental
5.1 Introduction
5.2 Sample preparation
5.2.1 Gallium Arsenide
5.2.2 Silicon
5.2.3 Gallium Nitride
5.3 IV and CV characterisation
6 Characterisation of the EL2 and E2 defects in n-GaAs 
6.1 Introduction
6.2 The EL2 and E2 defect levels in n-GaAs
6.3 Experimental 
6.4 Results and discussion
6.5 Conclusions
6.6 Publications
7 Field dependence of the thermally activated emission rate
7.1 Introduction
7.2 Publications
8 DLTS observation of the transformation of bistable defects 
8.1 Introduction
8.2 Experimental 
8.3 Publications:
9 Negative-U defects in n-GaN 
9.1 Introduction
9.2 DLTS of a negative-U defect 
9.3 Negative U properties observed in a sputter-deposition induced defect in n-GaN
9.4 Publications
10 Conclusions 
References 

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