Sahay, P P

Shamsuddin, M

Srivastava, R S

IACS Institutional Repository

NOTlIndian J.Phys. 68A (4 ) .  3 8 7 - 3 9 2 ( 1 9 9 4 )U P  A1 international journalX-ray irradiation effects on Ni/n-Si MIS Schottky barriers z  a qualitative assessmentP  P  S a h a y  M  S h a m s u d d in *  an d  R  S S riv a s la v aDepartment of Physics, Banaras Hindu University. Varanasi-221 005. India ‘Department of Metallurgical Engineering, Institute of Technology, Banaras Hindu University. Varanasi-221 005, IndiaReceived 17 September 1993, accepted 21 March 1994A bstract : Measurements have been made ot the electrical characteristics of Ni/«-Si MIS (meral-thin interfacial layer-semiconductor) Schottky barriers before and after X-ray irradiation. After X-ray imidiation acceptor-like states have been found to generate somewhere in the energy range 0.33 eV above the top of the valence band edge at the Si-thm Si()2 20 A) interface Apossible mechanism for the generation of these interface states has been proposed on the basis of physico-chermcal models.K eyw ords : MIS Schottky bamers. X-ray irradiation, banier height, acceptor states PACS Nos. : 7 3 .3 0 .+y, 73.40-cT h e  s tu d y  o f  r a d ia t io n  e f f e c t s  o n  s e m ic o n d u c to r  d e v ic e s  is  o f  c o n s id e ra b le  im p o r ta n c e  b ecau se  o f  s te a d y  g r o w th  o f  th e ir  u s e  in  a re a s  w h e re  e x p o s u re  o r  p o te n tia l e x p o s u re  to  h ig h  en e rg y  r a d ia t io n s ,  v iz . in  s a te l l i te s  e x p o s e d  to  n a tu ra l sp a c e  r a d ia t io n s ;  s e n so rs  u se d  in nuclear p o w e r  p la n ts ;  d e fe n c e  e q u ip m e n ts  d e s ig n e d  to  su rv iv e  n u c le a r  e x p lo s io n  ere. D u r in g  the las t d e c a d e  ra d ia tio n  d a m a g e  s tu d y  o n  the  e lec trica l c h a rac te r is tic s  o f  m eta l /se m ic o n d u c h ^  S c h o ttk y  b a r r ie r s  h a s  r e c e iv e d  s p e c ia l  a t te n t io n  by  th e  s e m ic o n d u c to r  r e s e a rc h e r s  [ -6 ). C o n s id e ra b le  w o rk  h a s  b e e n  d o n e  c o n c e rn in g  io n  b o m b a rd m e n t d a m a g e  on  l c  e e c  ch a ra c te r is tic s  o f  S c h o ttk y  b a r r ie r s  [7 -1 1 ].E n e r g e t i c  io n  b o m b a r d m e n t  c a u s e s  la t t ic e  d is o r d e r ,  ion e n t r a p m e n t ,  s u r f a c eamorphization a n d  im p u r i ty  d i f fu s io n  in  ad d itio n  to  th e  d e s ire d  e tc  in g  g ro w thenergy (<  5  k e V )  io n  b o m b a rd m e n t c an  a ffe c t th e  in itia l p h ase s  o f  su b seq u en t g ro w thWmaddres : Deponent of Physic,. S»TLongow..Ins.i.u.e of Eng.n«n„gSi Technology,LongowaM48 106, Sangnir, Punjab, India. ©  1994 lA C S68A (4)-n388 P P Sahay, M Shamsuddin and R S Srivastavaprocesses such as oxidation and silicide formation and metal-Si inter-diffusion [8]. It ha^  been universally observed that ion bombardment results in the generation of donor-like defects close to the Si surface. The positive charge residing in these donor states alters the electrical properties of Schoitky barriers by lowering the barrier height on n-Si and increasing it on p ‘ S i . In this paper, the experimental results regarding X-ray irradiation effects on N\/n- Si MIS Schottky barrier characteristics are presented.Ni/n-Si Schottky barriers were fabricated by vacuum vapour deposition of nickel on n/n^ Si(l 11) epitaxial wafer at ~10"^  ton* pressure. The epitaxial layer thickness used was 7 ^m with a resistivity between 0.4 and 0.65 fltm. Details of fabrication technique have been described elsewhere [12,13]. For X-ray irradiation the device was mounted in the direction of X-ray beam emerging from the window of an X-ray tube of Philips PW 1710 diffractometer. The target of X-ray was copper. It was operated at 30 kV and 20 mA. A|collimated beam of about 1 mm diameter was allowed to pass through the device for 15 min.lThc irradiation was done at room temperature (300 K). Immediately after irradiation daix (I-V) and (C-V) measurements at room temperature were carried out using a HP 41403 nicoammeter and a computer controlled HP 4277A LCZ meter, respectively. \Dark forward (I-V) characteristics of the diode before and after X-ray irradiation are shown in Figure I . The zero-bias barrier height 0/,^ , and the diode ideality factor rj have beenFigure 1. Forward (I-V) characteristics of the diode before and after X-ray irradiatiort.x-ray irradiation effects on Ni/n-Si MIS Schottky harriers etc 389calculated from ( l - \ )  characteristics with an effective Richardson constant equal to 110 A cm“^  K-5 [ 1 4 ]. The results are summarized in Table 1.Table 1. Effects of X-ray irradiation on 0  ^^  77 and at room lemperaiurc (30() K).Diode parameters^ ( c V )rjJV i^enT )^Before irradiation0.762.141 12 xl0>^Alter irradiation for 15 minutes0 79 3.34U2 X 10 ’^The large value (2.14) of rj for the diode before irradiation can be undcrstocxl in terms of the involvement of thin interfacial Si0 2  (* 20 A) layer between the metal and semiconductor, and a large number of interfacial defects (which act as recombination centres) at the interface. The formation of thin S 1 O 2  layer is unavoidable during the fabrication of the device by the conventional technique used in the present investigation. After irradiation rj has increased to 3.34, which may be du  ^to the generation of defects at the Si-thin S 1 O 2  interface by X-ray irradiation. Generation of interface states at the Si-Si02 interface by ionizing radiation in M OS capacitors has been explained by a number of workers [15-19],After X-ray irradiation, the zero-bias barrier height 0/,,, has been found to increase. This may be attributed to either the neutralization of donor states or generation of acceptor-like states. But the neutralization of donor states should improve the diode ideality factor which is not the case here. Hence, the generation of acceptor-like states seems to be the possible cause for increasing 0/,^ , These acceptor-like states seem to generate somewhere in the energy range0.33 eV above the top of the valence band edge. However, it may be noted that the generated acceptor-like states were not very effective to change the diode parameters dramatically after X-ray irradiation. This may be because of the fact that the life time of the centres produced arc too short to see their effect after the radiation is stopped and measurements made.A  possible mechanism for the generation of interface states by X-ray irradiation has been proposed on the basis of physico-chemical models as follows :Due to the inadvertent presence of a thin interfacial Si02 layer, some of the Si-atoms at the Si-thin Si02 interface may be bonded with three Si-neighbouring atoms and one with hydrogen bond [20].On exposing X-ray irradiation H P b  centres, symbolized as H-SiaSia. dissociate according to the reaction:P . +  H .H P .390 P P Sahay, M Shamsuddin and R S SrivastavaThe generation of atomic hydrogen has also been observed within thermal oxide under ionizing radiation [21].The resulting atomic hydrogen encounters either the H Pi, centre or the centre as per the reactions [22]:and H  +  H P ,  H P, HP,(2)(3)These reactions are controlled by the local availability of atomic hydrogen. Eqs. (1) and (2) have been considered as reactions responsible for generation of interface states. (C-V) plots of the diode before and after X-ray irradiation are shown in Figure 2. Tljese plots have been found to be frequency dependent which gives a clear indication of the Validity of interface states model of the diode structure. After irradiation the values of capacitance have been foundFigure 2< (C-V) plots of the diode at different frequencies before and after X-ray irradiation.to decrease. (C ^ -V ) characteristics at low frequency (10 KHz) show (Figure 3) a point of inflection in the forward bias region. This suggests that the deep tr^s at low frequency contribute to the junction capacitance as long as the quasi-Fermi level of electrons intersects the trap levels. When the applied forward bias voltage becomes sufficiently large such thatX-ray irradiation ^ e c ts  on Ni/n^Si MIS Schottky barriers etc 391quasi-fcnni level of electrons lies above the trap levels, the traps do not make any contribution to the junction capacitance. The donor concentration N j  (given in Tabic 1) has been estimated from the deep depiction region of (C'^-V) characteristics at 1 MHz frequency.Figure 3. (C"^-V) characicristics of the diode at different frequencies before and after X-ray irradiation.AcknowledgmentsOne of the authors (P P S) gratefully acknowledges the Council of Scientific and Industrial Research, New Delhi (India) for financial support.References[1] S J Fonash, S Ashok and R Singh Appl. Phys. Lett, 39 423 (1981)[21 S Ashok, T P Chew and B J Baliga Appl. Phys. Utt. 42 687 (1983)[3] R Singh. S J Fonash. S Ashok. P J Caplan. J Shappiro. M Haye-Ali and J Ponpon J Vac Sa. Technol A1 334 (1983)[4] G A Sarov Vacuum  34 1027 (1984)15] XC Mu and S3 Fonash IEEE Bectron Device U u. EDL-6 410 (1985)M S J Fonash Solid State Technol. 28(1) 1.50 (1985)(71 SA shokandAM ogn>Caiii|iero/££££fcrtra"0''''"i^ '' ED W  48 (1984)18] S Ashok. H Knntle and H Beneking Appl- PAys. U n  45 431 (1984)19) S Ashok and B J Baliga J. Appl- Phys S5 1237 (1984)110] S J Fonash Sn/uf Stow 7>cAno/. 28(4) 201 (1985)111] S Ashok IEEE Electron Device Lett. EDL-6 462 (1985)(12] P P Sahay and R S Srivastava Cryst. Res Technol 25 1461 (1990)U3] P p Sahay and R S Srivastava Indian J. Pure Appl Phys. 29 357 (1991)[14] S M Sze Physics o f Semiconductor Devices (New Delhi: Wiley Eastern) Ch 5 (1986)[>51 P S Winokur, J M MeOartity and H E Boesch IEEE Trans. Nucl. Sci NS-23 1580 (1 [>6] F B McLean IEEE Trans. NucL Sci. 27 1651 (1980)[>7] R J Singh and R S Siivasiava Radial. £j|f 60 161 (1982)392 P P Sahay, M Shamsuddin andRS Srivastava[18] Y Nishicka, E F Silva da, (Jr) and T P Ma /  Appl. Phys. 64 3317 (1988)[19] Y Wang, TPM a and RCBarkar>lp/i/. Phys. Lett. 54 2339 (1989)[20] A G Revesz Int. Topical Con}, on The Physics o/SiO; ond Interfaces' ed S T Pantelides (New York Pergamon) p 222 (1978)[21] K L Brower, P M Lenahan and P V Dressendorfer Appl. Phys. Lett. 41251 (1982)[22] K I  Brower and S M Myers Appl. Phys. Lett. 57 162 (1990)

X-Ray Irradiation Effects on Ni/n-Si MIS Schottky Bariers-A Qualitative Assessment

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X-Ray Irradiation Effects on Ni/n-Si MIS Schottky Bariers-A Qualitative Assessment

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