Mathai, Achamma John

Modi, B P

Patel, K D

Pathak, V M

Srivastava, R

IACS Institutional Repository

Iftdian J. Phys. 78 (9), 995-997 (2004)%U P  >G* (oleCharge tra n sp o rt through In -p S i (100) Schottky b a rrie rB P M odi', Achamma John Mathai, K D Patel. V M Pathak and R Snvastava^D e p a r tm e n t  o f  P h y s ic s .  S a r d a r  P a te l  U n iv e r s i ty .  V a llab h  V id y a n a g a i-3 « S  120 . ( i u j a r a t .  In d ia  k S h re e  J a y e n d r a p u r i  A r ts  a n d  .S c ien ce  College. H h a iu c h  3<)2 0 0 1 .  In d iaE  m a il  f js n v ts ia v C « 'y a h o t»  c o mR e c e i v e d  J 6  J u n e  2 0 0 3 ,  m  c e p t e d  2 S  h ' e h n m r v  2 0 0 4A b s t r a c t  S c h o t tk y  b a r r ie r  d e v ic e s  o n  p - ty p e  s i l ic o n  w e ie  f a b r ic a te d  u s in g  in d iu m  a s  c o n ta c t in g  m e ta l  l l a r n c i  h e ig h ts  a n d  id e a li ty  l a c io is  o f thep repared  d e v ic e s  w e re  e v a lu a te d  b y  a n a ly z in g  th e  V -I  d a ta  a t u x im  te m p e ra tu re  I 'hc  r e s u lts  h a v e  b e e n  d is c u s s e d  fo r  th e  ty p e  o f  c h a ig e  u a n s p o i i  T hey have a lso  b e e n  c o m p a r e d  w ith  e x p e c te d  v a lu e s  o n  s o m e  b a s ic  m o d e ls  fo r S B H sK e y w o r d s  S c h o t tk y  b a r r i e r ,  s a t u r a t i o n  c u r r e n t ,  b a r r i e r  h e ig h t ,  id e a l i ty  f a c to rP A C S  N o s .  ' 7 3  3 ()+ y  /  7 3  2 0  rFiven after investigations spreading over more than a century, Schottky devices still elude a clear understanding of the mechanisms and their exact contribution in the overall observed barrier heights. It is known that observed barrier heights are a result of band line up and accompanying charge transfers across the interface. As a result, barrier heights should correspond to that predicted by Mott Schottky model [1]. But observed deviations from the above predictions and the scatter in the reported experimental barrier heights have led to the proposition that more than one sing le m echanism  may be making contribution to the experim entally obtained value. Some influencing factors of these mechanisms [2], found after a critical examination and evaluation of the observed data regarding harrier heights of such contacts, are: (a) intrinsic surface states and metal induced gap states (b) defects, impurities and chemical iBismatch at the interface (c) crystallography, epitaxy and f>nentations at the in te rface  etc. P resence o f spatial inhomogeneities at the interface giving rise to lateral distribution in barrier heights have been proposed as a reason for difference between the observed and the expected values [2]. In this note,report our work on characterisation of charge transport, 'Reality factor and barrier height of indium Schottky diode on p- ^ypesilicon(lOO).In this work, silicon crystal of resistivity around one ohm n^\. and acceptor density around 10*  ^cm"*'^  have been used tot orresponding Authorfabricate In-pSi Schottky diodes, 'fhe crystal was first etched in 4:1:3 solution of HNO^, HF and CH^COOI 1,13) to remove silicon dioxide and impurities. Afler thorough wa.shmg in distilled water, It was quickly dried and loaded in the vacuum sy.stem. A thick aluminium coating of 3000 A was made on the rough side of the crystal by evapoiation and it was heated around 500 in the coating unit itself in vacuum for nearly 30 minutes to make ohmic back contact. The polished surface was again etched in the same solution for~30 sec. After thorough washing and quick drying, it was loaded in a vacuum system to evaporate pure indium through a suitable mask. This evaporation was done in a vacuum of around 10“  ^torr to obtain indium deposition of area 0.089 cm*. The crystal with deposited diodes was mounted on a PCB and suitable contacts were made to ctmnect the diode using coiled silver wire and silver paste (Eltecks (Corporation, Bangalore). This was connected to an arrangement for obtaining V-1 characteristics at 305 K, 288K and 273 K using a voltage source and Keithley electrometer. The temperature variation was done using a cryostatc.The V-I characteristics at all the three temperatures are found to exhibit good rectifying nature. A representative V-I plot at 305 K is shown in Figure 1. The V-l characteristics of the Schottky diodes is representative of the charge transport across the interface. In general, the charge transport across Schottky barriers may belong fully or partly to one or more of the thermionic emission (TE), thermionic field emission (TEE) or pure field© 2 0 0 4 1 ACS996 B P Modi, Achamma John Mathai, K D Patel, V M Pathak and R Srivastavaemission |tunneling (FE)] mechanisms [4|. However, under forward bias condition, tunneling in vSehottky diodes made from nondegenerate semiconductors, as in present case, is ruled out. This leaves thermionic and thermionic field emission as the4 OE-05 3 5E-05 3 Ofc-05 2 5E-05 2 06-05S' 1 56-05 ]1 06-055 0E-060 06+00-5 06-06-0 8 -0 6 -0 4 -0 2V (v o lt)0,2 04F i g u r e  1 . I-V  c h a r a c t c n s t i c s  o f In pvSi S c h o t lk y  c o n la c i  a t 3 0 S  K.possible charge transport mechanisms. A distinction between these two mechanisms can be made with the help of the characteristic energy parameter defined as.N^(1)Here, /?(== h/2n) is the reduced Plank constant, aij* is the effective mass of the charge carriers and f  , is the permittivity of the semiconductor. If 1^)0 ~ AT, then thermionic field emission is expected to be the dominant charge transport mechanism and when ^00 «  kT thermionic emission is expected to be the dominant charge transport mechanism at the temperatures of measurement. In present case, with 10*^ ’ cm "\ comes out to be lower by more than an order of magnitude from kT. Thus in the present case, the thermionic emission is expected to be the dominant mechanism of charge transport across the interface. Therefore, the observed V-I characteristic should follow the equation.= / o [ e x p ( '% ) - (2)with, /q = AA * r -Here, is the saturation current, A is the area of diode. A* is the Richardson constant and is the barrier height. However, chemically-etched practical Schottky diodes are generally known to exhibit voltage-dependent barrier heights and in such cases, the I-V characteristic are expected to follow the equation,/  F — i t (3)Here, n is the ideality factor. Using eq. (3), the barrier heights in the prc.seni ca.se have been evaluated from saturaiinii curren ts ob tained  by p lo tting  varia tion  of function ln[//,/[l-exp(-qiV/<:T')]] with forward bias. Such a plot ioi T = 305K is given in I-igure 2. The ideality factor n is evalualw' by using the equation.n = (yT )[dV /d{\n I)] . (4lF i g u r e  2 . L n  ( / / ( I  - cxp(--f/V7A/))] I's V  chHiactcnsfus of In ,iSi S c h o U k y  c o n t a c t  a t  3 0 5  KThe results of above kind of analysis at the three lempeiatuic are given in Table 1. The results given in Table 1 show that ilu* value of ideality factor is around 1.2 to 1.3.T a b i c  1. B a r r ie r  h e ig h t  a n d  id e a l i ty  f a c to r  fo i In p S i( lO O ) S thoU K v tin'ilt a t  v a r io u s  t e m p e r a tu r e s  ( d io d e  a r e a =  O .O SO cm -)T e m p e r a t u r e( K )B a r r i e r  h c i g h l I d e a l i ty  fa c io t  n3 0 5 0  7 5 1 172 8 8 0  7 3 1 2 02 7 3 0  71 1 2 9The higher values of n is related to the interfacial ettects. most likely from the presence of oxide layer where intcriacial states in equilibrium with the semiconductor may be dominant Most probable reason for the presence of oxide layer at the interface may be insufficient etching of the face on which induiiTi was evaporated or growth of oxide at this face in the time elapsmi^  between etching and deposition of indium. It also indicates that the Schottky diodes might have deviated from the thermion^' emission theory and that other current transport processe*' may also be contributing to the total observed charge transpnii [5.61.Charge transport through In-pSi (100) Schottkv barrier 997In yrder to evaluate the contribution of the tunneling transport ,f charges, we have made a quantitative estimate of the relative importance of the thermionic emission current and the tunneling uncnl for the material used in the present study. As discussed £.'()o calculated from eq. (1) comes out to be lower by more ihan an order of magnitude from kT. Therefore, the influence of ,he tunneling current is insignificant in the I-V measurement of thisSchottky diode.The leakage current may in general, have some influence on the 1-V characteristics of Schottky diodes. Butin our ca,se, shunt conductance is estimated to be 10"^  mho and thus, the leakage ^urient should not have significant influence on saturation cui rents and hence on barrier heights.The recombination current in the depletion region, has been K-poried to be an important factor responsible for the deviation ol the 1-V characteristics from the behaviour described by the Ihermionic emission theory |61. The recombination of the electrons and holes normally takes place through localized ; centers with energies which are near the center of the band gap ,md It IS expected in Schottky diodes with high barriers, at low temperature, and at low forward-bias voltage 14.7]. However, in piesent measurements, the temperatures are not low and is jIm) not very high. Therefore, the recombinatioT> ''urrent Lonlnhution also .seems to be insignificant. Hence, in the case ot present study, the thermionic emission seems to be thedominant charge traasport mechanism and the higher value o f« IS probably related to intcrfacial oxide layer.The barrier height value obtained in pre,seni work, is around0.71 to 0 15 eV and it neither agrees with the expected on M-S model { l.(H eV.) nor with Bardeen model (0..^6 eV ) The values expected on MIGS model is 0.6 eV. Moreover, the reported values of barrier height for ln-p.Si Schottky diixle is around 0 15 eV |8 | Our value is near this value. Therefore, it indicates that intcriacial conditions might be making significant contribution to band line up in present Schottky devices. I lowcver, a more clcai view lequires more precise measurements ovci laiger lemperature ranges.References[ 11 S M .S/e /’/ivwo ol Scmiiomiwhii l ) i r iu  \. (?nd alii) (NcwYoik Wiley baslcrn) ( I W)12| J ft Werm-r and tl Kau .Sduiliky Vonkuts on S iln im  (Spiiiigci .Senes m Electronics and I’liotonics) (cds 1 L J Luy ami P Riissicr Vol. .t2 ( llerliii .Springer Verlag) p H‘) (IW4)|.t| S M S/c V IM  'lirlm olot;v (NewYoik Mdii.iw- Mill) (l‘)')K|(4J E M Rhodciick and R H Williams Mclnl Snnn onilui loi V im iiuh  ( Oxford riaicndon Press) (1988)|5| 1) Doiioval, M Barns and M Zdinial Soliil Sioic I'kv tnm  .t4 I'Ri-'i (1991)[(,1 M Willmci P tm  lU-v B42 5249 (1990)| 7 | A Y C Yu and E H .Snow J A jip l /’/ivs .W .RK)8 (1908)[8 ] M lliro.se. N Altaf and T Ari/iiitii Jpn J Appl I 'h n  9(.t) 260 (1970)

Charge Transport Through In-pSi (100) Schottky Barrier

http://arxiv.iacs.res.in:8080/jspui/bitstream/10821/7260/1/Charge%20Transport%20through%20In-pSi%20%28100%29_By%20B%20P%20Modi.pdf

Charge Transport Through In-pSi (100) Schottky Barrier

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