Behaviour Of Metal Semiconductor Junctions Biology Essay

Interest in the behavior of metal-semiconductor junctions has existed during the whole period of development of semiconducting material devices. Although the point contact device could run satisfactorily at the needed higher frequences, there was a clear demand to better its dependability and there was a significant addition in research attempt to supply this. An of import consequence of this was the development of engineering for bring forthing high pureness semiconducting materials, and later controlled doping.

In add-on to bettering the dependability and runing features of metal-semiconductor rectifiers, this development made possible the eventual production of the transistor, taking to the rapid and tremendous growing in the semiconducting material industry. During this period of intensive experimental activity advancement was besides being made in developing a theoretical apprehension of rectification.

In the 1960 ‘s there was a great resurgence of research and development work on Schottky barrier rectifying tubes. This activity was encouraged to a considerable extent by the widespread handiness of systems leting controlled vaporization of metal movies in a high vacuity, bring forthing contacts which were much more stable and consistent than the earlier point contacts. As a consequence of these developments, a greater apprehension of metal-semiconductor contacts bit by bit and further countries of application of Schottky barriers emerged.

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During the past 20 five old ages, constituents based on Schottky barriers have been progressively used in microelectronics, and research activity has continued with the purpose of obtaining a full apprehension of the Physics of barrier formation and of the current conveyance processes across metal-semiconductor interfaces. These activities have been helped by farther progresss in vacuum readying techniques and besides by the development of assortment of surface sensitive spectrometry technique. Furthermore, the theoretical methods of look intoing the electronic belongingss of solids and the boundaries between them have advanced significantly with computing machines progressively being used both for theoretical computation and the analysis of experimental informations.

The rapid development of the semiconducting material industry would non hold been possible without the successful research attempts directed towards accomplishing dependability of contacts and duplicability of device features. Most of this activity has been related to silicon which is by far the most of import semiconducting material stuff, but many other stuffs including III-V compounds and II-VI compounds, are happening increased and widespread applications and each new stuff introduces jobs refering the development of a suited contacting engineering. Most late, with the assistance the extremely controlled Molecular beam epitaxy ( MBE ) growing technique leting the scope of possible device applications has been significantly broadened. However, all such application require stable electrical contacts to be formed to the semiconducting materials involved and although much work has been directed towards the accomplishment of dependable contacting processs, for a assortment of systems, a elaborate apprehension of the physical and chemical belongingss of the metal-semiconductor interface is still far from being to the full developed.

1.2 Metal-Semiconductor Contacts

Metal-to-semiconductor contacts are of great importance since they are present in every semiconducting material device. They can act either as a Schottky barrier or as an ohmic contact dependant on the features of the interface.

A Schottky barrier is a metal-semiconductor junction which has rectifying features, suited for usage as a rectifying tube. The largest differences between a Schottky barrier and a p-n junction are its typically lower junction electromotive force, and decreased ( about nonexistent ) depletion breadth in the metal.

Not all metal-semiconductor junctions are Schottky barriers, which rectify current. A metal-semiconductor junction that does non rectify current is called an Ohmic contact. In brief, there are two types of metal – semiconducting material junction or contact.

Rectifying contact ( Schottky barrier )

Non -Rectifying contact ( Ohmic contact )

An ohmic contact is a low opposition junction supplying conductivity in both way between the metal and the semiconducting material. Ideally, the current through the ohmic contact is a additive map of applied electromotive force, and the applied electromotive force should be really little. Since such contacts satisfy Ohm ‘s jurisprudence, they are normally called ohmic contacts.A Two general types of ohmic contact are possible: The first type is the ideal non-rectifying barrier, and the 2nd is the burrowing barrier.

A contact between a metal and a semiconducting material is typically a Schottky barrier contact.A However, if the semiconducting material is really extremely doped, the Schottky barrierA depletion part becomes really thin. A At really high doping degrees ; a thin depletion bed becomes rather crystalline for negatron tunneling.A This suggests that a practical manner to do a good ohmic contact is to do a really extremely doped semiconducting material part between the contact metal and the semiconducting material.

Rectifying belongingss depend on the metal ‘s work map, the set spread of the intrinsic semiconducting material, and the type and concentration of dopants in the semiconducting material. Design of semiconducting material devices requires acquaintance with the Schottky consequence to guarantee Schottky barriers are non created by chance where an ohmic connexion is desired.

There are two ways to do a metal-semiconductor contact look ohmic plenty to acquire signals into and out of a semiconducting material ( or making the opposite makes a good Schottky contact ) .

1. Lower the barrier tallness

The barrier tallness is a belongings of the semiconducting material stuffs. Largely stuffs holding little value of barrier tallness is used. Annealing can make an metal between the semiconducting material and the metal at the junction, which can besides take down the barrier tallness.

2. Make the barrier really narrow

One really interesting belongings of really bantam atoms like negatrons and holes is that they can “ burrow ” through barriers that they do n’t hold adequate energy to merely go through over. The chance of burrowing becomes high for highly thin barriers ( in the 10s of nanometres ) . It can be done by doing the barrier really narrow by doping it really to a great extent ( 1019 dopant atoms/cm3 or more ) .

1.3 Metal/CdTe Interfaces

In instance of II-VI semiconducting materials which are the group of peculiar involvement, CdTe is alone amongst the wide-band spread II-VI compounds in being the lone member of this group that can be easy made in both p-type and n-type signifiers. The increasing involvement in solar soaking up has created a demand for the word picture of absorbing semi conducting movie stuffs in the seeable scope for their application in photovoltaic devices. The set spread Eg is the most of import parametric quantity in semiconducting material Physics. Cadmium Chalcogenide stuffs have band spreads 1.4 a‰¤ Eg a‰¤ 2.4 electron volt and sensible convergence with the solar spectrum. CdTe is a promising base stuff for solar cells owing to its about optimal energy set spread and high soaking up coefficient.

Recently, there has been a rapid development in the field of II-VI semiconducting materials for their usage in Photovoltaic devices. Cadmium Telluride belonging to the II-VI group is widely used stuff for CdS/CdTe hetrojunction photovoltaic devices. It is due to the fact that CdTe have intermediate energy set spread, sensible transition efficiency, stableness and low cost ( Nakayama et al. , 1994 ; Gruszecki and Holmstrom, 1993 ; Shaalan and Muller, 1990 ) .

Metal/CdTe interfaces play an of import function in optoelectronic devices such as infra ruddy sensors and detectors in thermic imagination, 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 semiconducting material and although much work has been directed towards the accomplishments of dependable reaching process, a elaborate apprehension of the physical and chemical belongingss of the metal-semiconductor interface far from being to the full developed. Much of the early work utilizing stuff of dubious pureness and uncontrolled surface taint output consequences, which could non be faithfully reproduced.

A clear apprehension of the physical rules underlying the belongingss of these interfaces is hence indispensable in order to develop practical devices based on this semiconducting material stuff. In most of the research work and undertakings, the simple Schottky theoretical account for the metal-semiconductor interface is assumed during choice of the appropriate metals. But the research on metal/n-CdTe has indicated that during formation of Schottky barrier a strong pinning behavior is observed ( Ashok and Giewont, 1985 ) . In barrier tallness technology of Schottky junctions, the most common method is the alteration 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 i?§-ray sensors, microelectronics and incorporate circuits. However all such application require stable electrical contact to be formed to the semiconducting material and although much work has been directed towards the accomplishments of dependable reaching process, a elaborate apprehension of the physical and chemical belongingss of the metal-semiconductor interface far from being to the full developed. Much of the early work utilizing stuff of dubious pureness and uncontrolled surface taint output consequences, which could non be faithfully reproduced.

In the instance of n-CdTe, it has long been known that In can bring forth an ohmic contact while Au leads to a Schottky barrier. However, in neither instance is the junction believed to be disconnected or stable, particularly when tempering interventions are used. In fact, the barrier tallness at the Au-CdTe contact depends on the peculiar ambiance in which the device is annealed and there is grounds for the formation of an intermediate Au-Te compound ( Dharmadasa et al. , 1989 ) . For ohmic contact formation, the diffusion of In into the implicit in semiconducting material is clearly good as this leads to an enhanced n-type doping in the near-surface part of the CdTe sample. This has the consequence of cut downing the breadth of any Schottky barrier, which may be present, due to the being of surface provinces. It is good known ( Rhoderick and Williams, 1988 ) that, with sufficiently high densenesss of surface provinces, the Schottky barrier height becomes independent of the work map of the metal employed to organize the contact. However, if extra doping reduces the breadth of this barrier, so the junction opposition can be significantly reduced, as negatron burrowing becomes the dominant conveyance mechanism. This is believed to be the mechanism for the formation of In-CdTe ohmic contacts but, while trust on rapid inward diffusion of In is acceptable for reaching majority samples or thick beds of CdTe, this is clearly inappropriate in the instance of thin, multiple bed devices where deep diffusion of In could ensue in the alteration or shunting of underlying beds ( Yousaf et al. 2000 ) .

1.4 Motivation for this work

Assorted studied has have drawn attending to the of import function of the surface stoichiometry to find the barrier tallness for CdTe Schottky contacts. This has normally arisen in relation to the effects of different chemical etches prior to the contact formation. For illustration, the usage of hydrazine hydrate or KOH is known to bring forth a Cd-rich surface and this leads to Au-CdTe barrier tallness in surplus of 0.9 electron volt ( Schmitsdorf et al. , 1997 ) similar to the newly prepared samples described. On the other manus, a bromine/methanol etch produces a Cd- depleted surfaced and the barrier tallness for a figure of metal ( including Au ) has been found to be in the part of 0.7 electron volt ( Dharmadasa et al. , 1987 ) clearly this is consistent with the consequence for Au contact on CdTe bed in which an option method ( tempering ) has been used to bring forth a Cadmium shortage in the close surface part.

This research work is chiefly concerned with rectifying contacts which provide some utile information about the stuff features of the MBE adult beds every bit good as placing some of the factors which influence the contact belongingss and their stablenesss. Of class, it is to be expected that different methods of surface readying would bring forth different device features particularly in compound semiconducting materials such as CdTe for which the surface stoichiometry be significantly changed by chemical etching or tempering. The purpose is to correlate these effects with alterations in the chemical science of the interface and in the visible radiation of the consequences obtained, to find the optimal interface constructions and reaching processs for accomplishing both mechanical and electrical stableness. In this survey, as inter diffusion consequence are enhanced by tempering, hence tempering has been employed to look into the consequence of any inter diffusion procedure. The survey has involved I-V measuring and stableness of the contacts has been investigated by detecting alterations in these features as a map of tempering clip and temperature. Besides an alternate technique for manufacturing low opposition contacts to thin CdTe epilayers, which involves the usage of ion-plated Au, has been used. A drastic alteration in forward and contrary current has been observed for an ion plated Au-CdTe contacts. Doping dependance of barrier tallness and ideality factor of Au/n-CdTe Schottky barriers has besides been studied by current-voltage features.

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