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 3.22, 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 V=0.
In Fig. 4.1 the logarithmic dependance of I 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 Fig. 4.1 clearly show that any bing interfacial bed must be undistinguished thickness and value for ‘n ‘ which was deduced from Fig. 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. 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 150C0.
In order to analyze the stableness of Au-CdTe contacts, the electrical features of a figure of samples were investigated as map of clip after fiction. Table 4.1 gives the information for one of these samples which was studied over a period of four hebdomads. Immediately after fiction it can be seen in Table 4.1 that the barrier height measured 0.88ev. After one hebdomad there was important decrease in the barrier height to 0.80ev as determined from I-V measurings and after two hebdomads at that place was a farther decrease in the barrier height to 0.68eV. At this phase in order to look into the stableness of the measuring system, these measurings were repeated on the following twenty-four hours and, as the Table 4.1 shows indistinguishable features were observed. This confirmed the dependability of the measuring. Subsequent measuring after three hebdomads and four hebdomads indicated a much more stable behaviour of the contact with the barrier height being mentioned in the part 0.67 – 0.68eV.As it was expected that these procedures could be speeded up by increasing the temperature, a figure of sample were studied after tempering or different lengths of clip at 150C & A ; deg ; . Typical sets of I-V consequence are presented in Table 4.2. For this sample ( 228F ) the initial barrier tallness was calculated to be 0.95eV although this is non a dependable value in the position of the initial value of the ideality factor being instead high ( at 1.02 ) . How of all time after the sample was annealed at 150C & A ; deg ; for merely ten proceedingss, there was a important betterment in the ideality factor ( to 1.1 ) and a significant decrease in the measured barrier tallness to 0.75eV. It appears from this that the consequence of a brief annealing intervention was similar to go forthing the sample for a hebdomad or two at room temperature. After the sample was annealed for a 2nd clip ( for 15 proceedingss ) there was further but smaller decrease in barrier tallness to 0.68eV and after a 3rd annealing period ( this clip for 20 proceedingss ) there was an even smaller decrease to 0.65eV.
This tendency in behaviour due to tempering, 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.
In order to supply farther information on the nature of the procedures involved, a 2nd Au contact was formed to try 228F after it had been annealed ( with its first contact in topographic point ) for a sum of 45 proceedingss are antecedently described. The features of this 2nd contact are included in Table 4.2. It is clear that the initial barrier height 0.66eV for this new contact is closer to the concluding ( station tempering ) value for the original contact instead so to the much higher initial ( brittle ) value. 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 ) .The consequence of farther tempering for up to 70 proceedingss is recorded in Table 4.2. Merely little alterations in ideality factor and barrier tallness were observed, bespeaking rather stable behaviour for the new junction similar to that of the original junction after tempering.
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 ~ 0.2eV 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
In order to compare the consequence for Au contacts formed by ion-assisted manner with contacts produced by the usual vaporization process, a figure of samples were given two contacts ( one of each type ) . Fig. 4.2 gives the features for the normal Au contact and Fig. 4.3 gives the features for the ion-plated contact with 15 unsweet ion-etching clip. As expected, the I-V features in Fig. 4.2 are with ideality factor ‘n ‘ 1.2 and barrier tallness ( ) 0.90eV. As expected, the features in Fig. 4.2 are really similar to those shown in Fig. 4.1. However, for the ion-plated contact with 15 unsweet ion-etching clip there is a drastic alteration in both ideality factor ‘n ‘ and the barrier tallness ( ) was found to be 2.2 and 0.69 electron volts severally from I-V features shown in Fig. 4.3. This consequence 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 ( Shochley and Read, 1952 ) .
However, the alteration in the behavior for the ion plated contact with 20 unsweet ion-etching clip is even more drastic than observed in Fig. 4.3. There is a greater addition in both frontward and change by reversal bias current with a really low barrier tallness of the order of 0.45 electron volt and N was determined to be 4.1 observed from features shown in Fig. 4.3. Fig. 4.4 shows the battier highs as a map of ideality factors for these ion plated Schottky rectifying tubes. As can be seen from Fig. 4.4, there is a additive relationship between the barrier tallness and ideality factor, with 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, 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 ) . 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 ) and in add-on to burrowing procedure suggest that this might be a utile manner of farming low opposition ( ohmic ) junction utilizing a lower work map metal.
5.3 Effect of Doping
The ideal I-V features of a Schottky rectifying tube exhibits exponential prejudice dependance as in equation 3.21 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 on the exponentially determined I-V features for normal, low and to a great extent doped InSb substrate in Figures 4.6, 4.7 and 4.8 severally where important inclines are observed for the current under contrary prejudice. 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 ) .
The ideality factors ‘n ‘ and effectual barrier tallness were calculated from I-V features utilizing equation 3.23 and 3.24. The term effectual reflects the fact that the barrier tallness deduced from I-V measurings is lower than the value that should be obtained under inactive status i.e. without bearer injection, and includes the consequence of the image force take downing. Fig 4.11 shows a comparative position of I-V features for these three doped samples. After rating of I-V features, the values of the effectual barrier tallness and ideality factors for three wafers are shown in Table 4.4. A graph between barrier highs and ideality factors of three doped Au/n-CdTe Schottky rectifying tube is shown in Fig 4.9. A additive relationship between ideality factor and barrier tallness can be seen in Fig. 4.9 which is comparable to Fig.4.4. 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 ( Koutsouras et al. , 2005 ) . 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 ) .
With increasing dopant concentration, the breadth of the depletion part W i.e. given by relation 3.11 i.e.
at a given prejudice decreases taking to higher electric Fieldss at the interface. Low barrier or effectual barrier height instead than observed for the to a great extent doped sample ( 549E ) substrate. That is the ground for the higher swill under contrary prejudice for doped samples ( 549F, 549F ) . 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 ( BNG ) of the conductivity set. Obviously, the BNG 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 Fig 5.1.
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
Figure 5.1: Conventional diagram demoing the decrease of M/S barrier tallness due
to band-gap narrowing.
barrier tallness without BNG, 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, 1982 )
( 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-2T-2 for CdTe ) . is the opposition associated with the forepart metal/CdTe junction. Assuming RC & A ; lt ; 10? so ?C & A ; lt ; 0.1?cm2 and the corresponding upper bound for effectual barrier tallness is 0.38 electron volt. This is consistency with surveies of Al contacts on cleen vacuity cleaved surfaces of CdTe which yielded barrier highs of ~ 0.1 electron volt ( Patterson et al. , 1986 ) .
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 plated technique and doping in scope of 2.5-1016-1-1019 cm?3 on I-V features of the Au/n-CdTe Schottky rectifying tubes.
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.
Gold contact formed to n-CdTe by vacuity vaporization output Schottky barriers with initial barrier tallness In surplus of 0.88eV. 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 0.9eV. A drastic alteration in barrier tallness was observed by the usage of ion-assisted plasma procedure, an ion etching clip of 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.
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 leting an unhampered flow of negatrons via burrowing. But existent challenge to accomplishing low resistively contact by utilizing reasonably doped semiconducting materials. Many devices do so necessitate low electric resistance contacts without the load of heavy doping ( Noor Mohammad, 2004 )
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 rectifying tube, formed by the ion-plating procedure and doping consequence, leads to a much reduced contact opposition suggest that this might be a utile manner of farming stable and low opposition ( ohmic ) junction utilizing a lower work map metal ( e.g. , Al etc. ) suitable for thin movie MBE grown devices.