As discussed in subdivision 3.3, harmonizing to thermionic emanation equation for Schottky rectifying tube, at frontward biased, the current can be expressed as ( Sze, 1981 ) :
Valid for V & A ; gt ; 3kT/q
where is the applied electromotive force, Q is the electronic charge, karat is the thermic energy and is the impregnation current defined equation 3.22 as:
Where is the diode country, is the Schottky barrier tallness, is the effectual Richardson invariable and was calculated to be 1.19 x105AK-2m-2 by utilizing a value of 0.11 for the effectual negatron mass/free negatron mass ratio for CdTe. The ideality factor ‘n ‘ was defined in equation 3.24 as:
Saturation current introduced to depict the experimental semi log I-V curve informations from the thermionic emanation theory utilizing ideality equation. The ideality factor N of the rectifying tube was calculated from the incline of the additive part of the semi log I-V curve. Using equation 3.23, the nothing biased barrier tallness was determined from the impregnation current that was obtained from the intercept of the excess plotted additive part with current axis at = 0.
The values for both Ns and are listed in Table 4.1 for junctions at assorted times after formation ( while at room temperature ) and in Table 4.2 for a sample which was subjected to a series of tempering interventions in vacuity at 150C & A ; deg ; .
In Figure 4.1 the logarithmic dependance of Current with forward biased electromotive force is seen to widen over more than five order of magnitude leting ‘n ‘ to be easy deduced from the gradient. Any interfacial oxides layer ensuing from exposure of the semiconducting material surface to the ambiance between growing and metallization would hold the consequence of doing ideality factor a electromotive force dependent parametric quantity instead than a changeless ( Rhoderick and Williams, 1988 ) . The one-dimensionality observed in Figure 4.1 clearly shows that any bing interfacial bed must be undistinguished thickness and value for ‘n ‘ which was deduced from Figure 4.1 being close to 1 indicated the cross barrier conveyance procedure in preponderantly via thermionic emanation.
Harmonizing to Pattabi et Al. ( 2007 ) an ideality factor greater than integrity is by and large attributed to the presence of a bias dependent Schottky barrier tallness. Image forces, burrowing, generation-recombination, interface drosss and interfacial oxide bed are possible factors which could take to a higher ideality factor. The ideality factor represents a direct step of interface uniformity.
This tendency in behaviour due to tempering for sample 228F, with an initial rapid autumn in the barrier height being followed by lower alterations and greater stableness is clearly similar to that observed for sample 228A which remained at room temperature for four hebdomads. It was noted above that this behaviour must be due to chemical reaction or diffusion procedures in the part of the M/S interface. This suggests that the procedures which influence the barrier tallness may be due to some out-diffusion from the inside of the semiconducting material to its surface. Clearly they are non dependent on the presence of the gold bed although some interaction between the Au contact and the implicit in semiconducting material is expected to happen ( Dharmadasa et al. , 1989 ; Van Meirhaeghe et al. , 1991 ) .
Although Au is a p-type dopant in CdTe, the informations in table 4.1 and 4.2 indicates that the alterations in interface features are non dependent on the presence of Au during the procedure of tempering. An alternate account is that there is an outward diffusion of Cd ( likewise taking to the coevals of acceptor provinces near-surface part ) .This reading of the consequences is entirely understanding with the decision reached by Dharmadasa et Al. ( 1994 ) on the consequence of chemical etch interventions. Those etchants which were found to go forth the surface rich in Cd tended to bring forth barrier highs greater than 0.9 electron volts while those go forthing the surface deficient in Cd produced barrier highs which were ~ 0.2 electron volt lower, as found in the instance of the annealed samples studied in this undertaking. Therefore, it is clear that interface reaction lead to a significant alteration in the defect construction in the locality of the junction but farther work will be necessary to find the exact construction of the defects provinces which might be responsible for Fermi degree traping before and after the reaction and the associated decrease in barrier tallness.
5.2 Effect of Ion Plating Technique
Table 4.3 shows informations from I-V features as a map of ion etching clip. A drastic alteration in I-V features of ion-plated Au/n-CdTe Schottky barriers samples ( 228C, 228D, 228E ) was observed. A gradual upward displacement in graphical lines of I-V features of these samples was observed with increasing ion etching clip as shown in Figures 4.2, 4.3, and 4.4.
The consequence of different ion etching clip suggests that a significant denseness of defects has been created below the Au contacts as a consequence of ion barrage of the surface during the plating procedure. The presence of defects in the depletion part, moving as recombination centres, leads to an extra forward prejudice current constituent with an ideality factor of about 2 ( Shah et al. , 2003 )
As can be seen from Figure 4.4, there is a additive relationship between the barrier tallness and ideality factor i.e. the barrier height going smaller as the ideality factor additions. Change in ideality factor indicates that current conveyance mechanisms other than thermionic emanation are present. As this value of N is significantly greater than 2 ( Table 4.3 ) , as would be expected for a bearer recombination mechanism, as discussed earlier, it seems likely that bearer tunneling may besides be playing a function ( Popovic, 1978 ) . It is non expected that a simple tunneling procedure would be runing in the instance of samples with doping densenesss of 1015-1017 cm-3.The cyberspace doping denseness in CdTe was excessively low for burrowing ( it require & A ; gt ; 1017cm-3 ) ( Padovani and Stratton, 1966 ) , but it is possible for negatron to burrow via ladder of closely spaced provinces in depletion part to unite with holes i.e. multi measure burrowing ( Ercelebi et al. , 1990 ; Ou, et Al. 1984, Ay and Tolunay, 2007 ) .
These consequences indicate that the possible consequence of plasma-induced surface defects is that they contribute to the conduction of the contact by moving as fast recombination centres ( Ponon, 1985 ) along with multi measure burrowing centres. This suggests that it might be a utile manner of farming low opposition ( ohmic ) junction utilizing a lower work map metal.
5.3 Effect of Different Doping Concentrations
The ideal I-V features of a Schottky rectifying tube exhibits exponential prejudice dependance as described in subdivision 3.3 can be reduced to
For V & A ; gt ; 3kT/q
The magnitude of this impregnation current is governed by the effectual barrier height i.e. the difference between the conductivity set lower limit ( CBM ) at the surface of Au/n-CdTe and the Fermi degree of the metal ( Au ) .
The value of the barrier tallness can be calculated from the measured impregnation current utilizing equation 3.22:
Deviation from this ideal behavior can be seen in Figures 4.6, 4.7, and 4.8.Those divergences are attributed to image force take downing ( IFL ) , recombination phenomena due to the presence of deep traps and the being of high electric field ( Martin, 1981 ) .Table 4.4 shows barrier tallness and ideality factor ‘n ‘ deduced from I-V features on CdTe Schottky diodes for different doping concentration ranges 2.3-1016-1-1018 cm?3.
Figure 4.10 shows a additive relationship between and ‘n ‘ which is really similar to Fig. 4.5 i.e. for ion plated samples. It has been demonstrated theoretically and by experimentation that the additive relationship between and ‘n ‘ can be attributed to the sidelong inhomogeneties of the barrier tallness in Schottky rectifying tubes. Harmonizing to Tung ‘s theoretical account ( Tung, 1992 ) , the Schottky barrier consists of laterally nonuniform spots of different barrier highs. The spots with lower barrier tallness have larger ideality factors and frailty versa. The presence of traps besides modifies the incline of the forward current and at the same clip the value of the ideality factor, which is higher than integrity for both samples ( low and high doped sample ) ( Koutsouras et al. , 2005 ) .
Figure 5.1: A Conventional diagram demoing the decrease of M/S barrier tallness
due to band-gap narrowing.
With increasing dopant concentration, the breadth of the depletion part ” i.e. given by relation given in Eq.3.11:
at a given prejudice decreases taking to higher electric Fieldss at the interface. Low barrier or effectual barrier height instead than observed, for low and to a great extent doped sample ( 549E, 549F ) , which is the ground for the higher swill under contrary prejudice for doped samples. However, the enhanced recombination rate due to the presence of deep trap degrees besides contributes coevals and recombination consequence and can non be excluded.
With heavier doping, increasing figure of new donor-type energy degrees are created underneath the conductivity set border. Under these fortunes, the givers are so near together that the giver degrees are no longer discrete and non-interacting energy degrees. These are instead debauched unifying together to make an dross bond, and doing band-gap narrowing ( BGN ) of the conductivity set. Obviously, the BGN is the highest near M/S interface, and the lowest in the majority. The effectual M/S barrier tallness is therefore reduced, as shown schematically in Figure 5.1 ( Noor Mohammad, 2004 ) .
The crisp tip of the conductivity set border in contact with the metal is peculiarly lowered, and the new barrier tallness becomes, where is the barrier height without BGN, and is the barrier tallness with BGN. However, a much more opposition arises from the CdTe/InSb junction. It has been shown that there is a possible barrier at this interface, associated with a conductivity set discontinuity of ~0.31 eV ( Van Welzenis and Ridley, 1984 ) .
From a elaborate analysis of I-V features for gold-contacted devices with similar dimensions to those in present survey, effectual opposition value of ~100? have been deduced for the CdTe/InSb junction part ( Sands and Scott, 1995 ) . Harmonizing to the thermionic emanation theory, the contact electric resistance at the M/S contact depends merely on the effectual M/S barrier tallness, as given by ( Sze, 1981 )
( 5.1 )
Where S is the contact country ; q, K and T are electronic charge, Boltzman invariable and temperature severally and is the Richardson invariable ( with a value of ~ 1.2 -105 Am-2K-2 for CdTe ) . is the opposition associated with the forepart metal/CdTe junction. Harmonizing to Yousaf et Al. ( 2000 ) , presuming & A ; lt ; 10? so & amp ; lt ; 0.1 ?cm2 and the corresponding upper bound for effectual barrier tallness is 0.38 electron volt which is consistency with surveies of Al contacts on clean vacuity cleaved surfaces of CdTe which yielded barrier highs of ~ 0.1 electron volt ( Patterson et al. , 1981 ) .
About all the old probe emphasized tunneling as the primary mechanism for low contact electric resistance in n-CdTe. The present survey dose non govern out the importance of burrowing in making low contact electric resistance. However, it demonstrates that, depending on how much is lower than, thermionic emanation, instead than burrowing, may so be the primary cause for low contact electric resistance even in the tunnel contacts. If the surface intervention is really good, and the metal parametric quantity ( e.g. , metal thickness, metal deposition temperature, metal work map, metal combination, etc. ) are optimal, so may be significantly lower than. This, together with BGN and IFL can so play a important function for giving thermionic emanation based low contact electric resistance.
The undermentioned decisions can be reached from the surveies on the effects of tempering clip and temperature, ion plating technique and different doping concentration in scope of 2.3-1016-1-1018 cm?3 on I-V features of the Au/n-CdTe Schottky barrier rectifying tubes samples:
Gold contact formed to n-CdTe by vacuity vaporization output Schottky barriers with initial barrier tallness in surplus of 0.88-0.95 electron volt. This reduced to 0.66-0.68 electron volt in a period of clip which is dependent on temperature. This decrease is found to be accompanied by a partial compensation of the sickly givers in the semiconducting material part near to the contact, a procedure which can be attributed to a discriminatory out diffusion of Cadmium from this part to the contact surface.
It has been shown that the usage of simple vapour deposition on Au on n-type CdTe epilayers gave rectifying behavior with barrier tallness 0.90 electron volt. A drastic alteration in barrier tallness was observed by the usage of ion-assisted plasma procedure, an ion etching clip of 15-20 sec to Au contact. This decrease in barrier tallness is attributed to the plasma- induced surface defects that contribute to the high conduction of the contact by moving as recombination centres along with multi measure degree burrowing centres.
The doping dependance of the barrier tallness and the ideality factor was observed by the consequences of I-V features of Au/n-CdTe Schottky barriers for different doping concentration. The ideality factor additions with increasing bearer concentration and as a consequence barrier tallness decreases. This is due to the consequence of the interfacial bed and interface provinces.
From Comparative survey of ion plated and doped samples of Au/n-CdTe Schottky rectifying tube, a additive relationship between the effectual barrier highs and ideality factors was found which shows that barrier tallness lessenings as ideality factor additions. As a consequence conduction additions. From which it can be concluded that:
When n = 1 so all conveyance of negatron is from the top of the barrier and thermionic emanation current mechanism should be dominant.
When 1 & A ; lt ; n & A ; lt ; 2, so burrowing current mechanism is dominant.
When n = 2, so all conveyance is due to coevals and recombination current.
When N & A ; gt ; 4 so there is non simple burrowing but step degree burrowing occurred.
Consequence of doping in Au/n-CdTe Schottky rectifying tube shows that if n-CdTe is to a great extent doped with important conductivity set flexing near M/S interface, burrowing is possible through metal/CdTe contact. The semiconducting material part at the interface therefore becomes really thin ( i.e. BGN ) leting an unhampered flow of negatrons via tunneling.
Consequence of doping on I-V features of Au/n-CdTe shows that barrier breadth ( tungsten ) decreases with the increasing doping denseness in conformity with ( Eq.3.11 ) .
The chief decision to be drawn from the comparative survey of I-V features of Au/n-CdTe Schottky barriers, formed by the ion-plating procedure and doping procedure, leads us to a much reduced contact opposition. This suggests that it might be a utile manner of organizing stable and low opposition ( ohmic ) junction. To organize stable and low opposition ( ohmic ) junctions, a low work map metal ( e.g. , Al etc. ) may be suited for thin movie MBE grown devices.
5.5 Recommendations for Future Work
The consequences obtained in finding the belongingss of Au/n-CdTe Schottky barriers by I-V features lead to several possible hereafter work that may be conducted:
More elaborate survey is necessary to find the precise nature of the new interface of Au/n-CdTe contact and exact construction of defect provinces.
Many devices need low electric resistance contacts without the load of heavy doping. More elaborate work is required on reasonably doped semiconducting materials to accomplish low resistive contact.
More elaborate work is required to find the belongingss of thermionic emanation ( TE ) based low resistive contacts.