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Interest in the behaviour of metal-semiconductor junctions has existed during the whole period of development of semiconductor devices. Although the point contact device could operate satisfactorily at the required higher frequencies, there was a clear need to improve its reliability and there was a substantial increase in research effort to provide this. An important result of this was the development of technology for producing high purity semiconductors, and subsequently controlled doping.
In addition to improving the reliability and operating characteristics of metal-semiconductor rectifiers, this development made possible the eventual production of the transistor, leading to the rapid and enormous growth in the semiconductor industry. During this period of intensive experimental activity progress was also being made in developing a theoretical understanding of rectification.
In the 1960's there was a great revival of research and development work on Schottky barrier diodes. This activity was encouraged to a considerable extent by the widespread availability of systems allowing controlled evaporation of metal films in a high vacuum, producing contacts which were much more stable and reproducible than the earlier point contacts. As a result of these developments, a greater understanding of metal-semiconductor contacts gradually and further areas of application of Schottky barriers emerged.
During the past twenty five years, components based on Schottky barriers have been increasingly used in microelectronics, and research activity has continued with the aim of obtaining a full understanding of the Physics of barrier formation and of the current transport processes across metal-semiconductor interfaces. These activities have been helped by further advances in vacuum preparation techniques and also by the development of variety of surface sensitive spectroscopy technique. Furthermore, the theoretical methods of investigating the electronic properties of solids and the boundaries between them have advanced significantly with computers increasingly being used both for theoretical calculation and the analysis of experimental data.
The rapid development of the semiconductor industry would not have been possible without the successful research efforts directed towards achieving reliability of contacts and reproducibility of device characteristics. Most of this activity has been related to silicon which is by far the most important semiconductor material, but many other materials including III-V compounds and II-VI compounds, are finding increased and widespread applications and each new material introduces problems concerning the development of a suitable contacting technology. Most recently, with the aid the highly controlled Molecular beam epitaxy (MBE) growth technique allowing the range of potential device applications has been significantly broadened. However, all such application require stable electrical contacts to be formed to the semiconductors involved and although much work has been directed towards the achievement of reliable contacting procedures, for a variety of systems, a detailed understanding of the physical and chemical properties of the metal-semiconductor interface is still far from being fully developed.
1.2 Metal-Semiconductor Contacts
Metal-to-semiconductor contacts are of great importance since they are present in every semiconductor device. They can behave either as a Schottky barrier or as an ohmic contact dependent on the characteristics of the interface.
A Schottky barrier is a metal-semiconductor junction which has rectifying characteristics, suitable for use as a diode. The largest differences between a Schottky barrier and a p-n junction are its typically lower junction voltage, and decreased (almost nonexistent) depletion width in the metal.
Not all metal-semiconductor junctions are Schottky barriers, which rectify current. A metal-semiconductor junction that does not rectify current is called an Ohmic contact. In brief, there are two types of metal - semiconductor junction or contact.
Rectifying contact (Schottky barrier)
Non -Rectifying contact (Ohmic contact)
An ohmic contact is a low resistance junction providing conduction in both direction between the metal and the semiconductor. Ideally, the current through the ohmic contact is a linear function of applied voltage, and the applied voltage should be very small. Since such contacts satisfy Ohm's law, they are usually called ohmic contacts.Â Two general types of ohmic contact are possible: The first type is the ideal non-rectifying barrier, and the second is the tunneling barrier.
A contact between a metal and a semiconductor is typically a Schottky barrier contact.Â However, if the semiconductor is very highly doped, the Schottky barrierÂ depletion region becomes very thin. Â At very high doping levels; a thin depletion layer becomes quite transparent for electron tunneling.Â This suggests that a practical way to make a good ohmic contact is to make a very highly doped semiconductor region between the contact metal and the semiconductor.
Rectifying properties depend on the metal's work function, the band gap of the intrinsic semiconductor, and the type and concentration of dopants in the semiconductor. Design of semiconductor devices requires familiarity with the Schottky effect to ensure Schottky barriers are not created accidentally where an ohmic connection is desired.
There are two ways to make a metal-semiconductor contact look ohmic enough to get signals into and out of a semiconductor (or doing the opposite makes a good Schottky contact).
1. Lower the barrier height
The barrier height is a property of the semiconductor materials. Mostly materials having small value of barrier height is used. Annealing can create an alloy between the semiconductor and the metal at the junction, which can also lower the barrier height.
2. Make the barrier very narrow
One very interesting property of very tiny particles like electrons and holes is that they can "tunnel" through barriers that they don't have enough energy to just pass over. The probability of tunneling becomes high for extremely thin barriers (in the tens of nanometers). It can be done by making the barrier very narrow by doping it very heavily (1019 dopant atoms/cm3 or more).
1.3 Metal/CdTe Interfaces
In case of II-VI semiconductors which are the group of particular interest, CdTe is unique amongst the wide-band gap II-VI compounds in being the only member of this group that can be easily made in both p-type and n-type forms. The increasing interest in solar absorption has created a demand for the characterization of absorbing semi conducting film materials in the visible range for their application in photovoltaic devices. The band gap Eg is the most important parameter in semiconductor Physics. Cadmium Chalcogenide materials have band gaps 1.4 â‰¤ Eg â‰¤ 2.4 eV and reasonable overlap with the solar spectrum. CdTe is a promising base material for solar cells owing to its nearly optimum energy band gap and high absorption coefficient.
Recently, there has been a rapid development in the field of II-VI semiconductors for their use in Photovoltaic devices. Cadmium Telluride belonging to the II-VI group is widely used material for CdS/CdTe hetrojunction photovoltaic devices. It is due to the fact that CdTe have intermediate energy band gap, reasonable conversion efficiency, stability and low cost (Nakayama et al., 1994; Gruszecki and Holmstrom, 1993; Shaalan and Muller, 1990).
Metal/CdTe interfaces play an important role in optoelectronic devices such as infra red detectors and sensors in thermal imaging, solar cells etc (Chand and Kumar, 1995; Dharmadasa et al., 1982; Dharmadasa et al., 1998). However all such application require stable electrical contact to be formed to the semiconductor and although much work has been directed towards the achievements of reliable contacting procedure, a detailed understanding of the physical and chemical properties of the metal-semiconductor interface far from being fully developed. Much of the early work using material of doubtful purity and uncontrolled surface contamination yield results, which could not be reliably reproduced.
A clear understanding of the physical principles underlying the properties of these interfaces is therefore essential in order to develop practical devices based on this semiconductor material. In most of the research work and projects, the simple Schottky model for the metal-semiconductor interface is assumed during selection of the appropriate metals. But the research on metal/n-CdTe has indicated that during formation of Schottky barrier a strong pinning behaviour is observed (Ashok and Giewont, 1985). In barrier height engineering of Schottky junctions, the most common method is the change of doping concentration near the metal/semiconductor interface (Shannon, 1974; Kwok et al., 1987; Jia and Qin, 1990; Averin et al., 1993; Horvath, 1988; Kim et al., 1988; Horvath, 1994).
Schottky barriers at the Metal-Semiconductor (M/S) interface are utilized at present in many advanced solid-state devices like ï§-ray detectors, microelectronics and integrated circuits. However all such application require stable electrical contact to be formed to the semiconductor and although much work has been directed towards the achievements of reliable contacting procedure, a detailed understanding of the physical and chemical properties of the metal-semiconductor interface far from being fully developed. Much of the early work using material of doubtful purity and uncontrolled surface contamination yield results, which could not be reliably reproduced.
In the case of n-CdTe, it has long been known that In can generate an ohmic contact while Au leads to a Schottky barrier. However, in neither case is the junction believed to be abrupt or stable, especially when annealing treatments are used. In fact, the barrier height at the Au-CdTe contact depends on the particular atmosphere in which the device is annealed and there is evidence for the formation of an intermediate Au-Te compound (Dharmadasa et al., 1989). For ohmic contact formation, the diffusion of In into the underlying semiconductor is clearly beneficial as this leads to an enhanced n-type doping in the near-surface region of the CdTe sample. This has the effect of reducing the width of any Schottky barrier, which may be present, due to the existence of surface states. It is well known (Rhoderick and Williams, 1988) that, with sufficiently high densities of surface states, the Schottky barrier height becomes independent of the work function of the metal employed to form the contact. However, if excess doping reduces the width of this barrier, then the junction resistance can be significantly reduced, as electron tunneling becomes the dominant transport mechanism. This is believed to be the mechanism for the formation of In-CdTe ohmic contacts but, while reliance on rapid inward diffusion of In is acceptable for contacting bulk samples or thick layers of CdTe, this is clearly inappropriate in the case of thin, multiple layer devices where deep diffusion of In could result in the modification or shunting of underlying layers (Yousaf et al. 2000).
1.4 Motivation for this work
Various studied has have drawn attention to the important role of the surface stoichiometry to determine the barrier height for CdTe Schottky contacts. This has usually arisen in relation to the effects of different chemical etches prior to the contact formation. For example, the use of hydrazine hydrate or KOH is known to produce a Cd-rich surface and this leads to Au-CdTe barrier height in excess of 0.9 eV (Schmitsdorf et al., 1997) similar to the freshly prepared samples described. On the other hand, a bromine/methanol etch produces a Cd- depleted surfaced and the barrier height for a number of metal (including Au) has been found to be in the region of 0.7 eV (Dharmadasa et al., 1987) clearly this is consistent with the result for Au contact on CdTe layer in which an alternative method (annealing) has been used to generate a Cd deficit in the near surface region.
This research work is mainly concerned with rectifying contacts which provide some useful information about the material characteristics of the MBE grown layers as well as identifying some of the factors which influence the contact properties and their stabilities. Of course, it is to be expected that different methods of surface preparation would generate different device characteristics especially in compound semiconductors such as CdTe for which the surface stoichiometry be significantly changed by chemical etching or annealing. The intention is to correlate these effects with changes in the chemistry of the interface and in the light of the results obtained, to determine the optimum interface structures and contacting procedures for achieving both mechanical and electrical stability. In this study, as inter diffusion effect are enhanced by annealing, therefore annealing has been employed to investigate the effect of any inter diffusion process. The study has involved I-V measurement and stability of the contacts has been investigated by observing changes in these characteristics as a function of annealing time and temperature. Also an alternative technique for fabricating low resistance contacts to thin CdTe epilayers, which involves the use of ion-plated Au, has been used. A drastic change in forward and reverse current has been observed for an ion plated Au-CdTe contacts. Doping dependence of barrier height and ideality factor of Au/n-CdTe Schottky barriers has also been studied by current-voltage characteristics.