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Growth and properties of aluminium antimonide films produced by hot wall epitaxy on single-crystal KCl
ELSEVIER
Thin Solid Films 312 (L998) I1 I - i 15
Growth and properties of aluminium antimonide films produced by hot wall epitaxy on single-crystal KCl Taminder Singh, R.K. Bedi MaWrial S('icm'e Ltlhorolol31. Department t#'Physit's, Gm'~l Nmmk D(,v Univer.~ity, Amril.~ar 143005, Ira/if Received 22 April 1997; accepted 4 August 1997
Abstract
Alurniniunl antirnonide (AISb) films have been grown by hot wall epitaxy on KCi substrates kept at different temperatures in vacuum o f I X l O .s Torr. The experimental conditions are optimised to obtain better crystallinity of the films. The electrical conductivity. Hall mobility arid carrier concentration arc determined. The films appear to be p-type: thus, indicating holes as dominant charge carriers. Scanning electron microscopy shows an increase in grain size with substrate tempenlture. Transnussion electron micrographs indicate a better crystaltinity of AISb filn,s as compared to tht),.,e grown by thermal co-evaporation, c~) 1998 Elsevier Science S.A. Ke:'wr,'d.v: Atumfltiunt autimonide films: Hot wall epitaxy: KCI suhstrate~,
1. Introduction
Aluminium antimonide (AISb) is a Zinc Blende type semiconductor with an indirect optical gap of 1.6 eV at room temperature. AISb seems to be a promising semicondueling material for high-temperature applications especially for transistors and p - n junction diodes [I,2]. The other applications of AISb include high-energy photon detectors [3], solar cells (for which AISb appear~ to be ideally matched), and for the development of Ga I _, AI ,Sb-based optoelectronic devices [4.5]. In spite of the tact that AISb is closely related to the important direct gap materials lnP and GaAs, it has received little :atention so filr.
""
Dasilva ~t al. [6] have studied the oxidatk,a or AISb films grown by molecular beam epitaxy (MBE). The flash evaporation technique has been erriployed by Richards et al. [7,8] to grow AISb films. Frarlcombe et al. [9] have observed a strong photovoltaic response irl vacuum-deposited AISb t'ihns. The etTect of thickness on the electrical properties of AISb films has also been studied [10]. Laroux et al. [I l] have employed metal organic chelnical vapour deposition (MOCVD) to grow AISb films on insulating substrates. The polycrystalline AISb fihns have been obtained by sputtering in high-purity Argon atmosphere [12]. The AISb t'ihns have also been prepared by liquid phase epitaxy (LPE)[13] and taser annealing [14]. The MBE growth conditions of aluminiunl antirnonide films on (){140-6090/98/$19.(X)
GaSb substrate have been reported by several authors [ 15,16], Chiu and Tsang [ 17] and Waterman et al, [ 18] have attempted to study the reflection high-energy electron diffraction (RHEED) patterns of MBE-grown AISb films. The electronic properties of AISb films are strongly intluenced by their structure, which, in turn, depends upon the deposition conditions and the technique employed. hnprovcment in grain size and reduction of structural defects have been obtained for films deposited under conditions near thermodynamical equilibrium. Hot wall epitaxy (HWE) studleu by Lopez Otero [19] pemfits film deposition very near thermodynamic equilibrium. This technique has already been exploited for lI-V! [20,21] and IV-Vl [22,23] compound semiconductor films. In this communication, we report the structure, electrical and opttcal properties of AISb films prepared by HWE on single-crystal ~CI substrates at different Temperatures. .
2. Experimental
,t
_..~__. . . . . . . . . .
Experimental details of the hot wall set up to deposit AISb fihus have been described earlier [24], High-purity (99,999c;~.) elements aluminiuni arid antimony obtained from Nuclear Fuel Complex, Hyderabad {India) were utilised. The entire system was enclosed in the vacuum chamber of a Hind High Vacuum coating Unit I2AdH with a background pressure of I X l0 ~ Tort, Belore carrying out the deposition, the vacuum chamber was
i t2
7: Si,.k,h.
R.K. Bcrli /
771inS,,lid
baked at 470 K to degas the vacuum equipment, During the degassinlg, the substrate tin6 wall heaters were also switched on. On attaining the desired expeu'inlental condilions, tile sotirce heater was switched tm and the stlbstrate holder positioned on the Lop ot' tile tube It) close tile systenll. Two separate windings were used It) heat the evaporation port and the wall of the cohullU:. A radiant heater was used to heat the substrate ill the tunlperaturc range between 825 and 975 K. The tenlperaturc,~ of the different zones were controlled by PID tenlperature ottotrollers using K-type thermocouple obtained fronl Omega Eng. (USA). The fihlls obtained were anne:tied at 500 1" to stabilise the crystallinlc structure and then kept in wtcuunl to prevent direct contact fronl the atnlospherc, an AISb oxidises in air. This problem wits resolved by depositing a very thin layer of lnSb on the AISb films at room temperaItlre.
The c i r c u i t a l resistivity and the Hall coeMcient of AISb filnls were determined in vacuum (I × 10 4 Torr) using Vander Pauw',.: four-probe technique [25] in Ihe temperature range between 173 and 473 K, Silver contacts were used to connect electrical leads to the films. The optical measurements were carried out at room temperalure using a double beam PYE-UNICAM SP3-300 Spectrophotometer. The transmittance was nleasured inl tile photo-energy range 1.0-2.0 eV. A JEOL JSM-6100 (Japan) scar, ling microscope, and a JEOL TEM-1200 (Japan) electrml microscope were used for structural characterization.
l"ilm.', 3 12 ( 19()fl'~ I 11 .- I 15
Table I
F.xpcrimcntal dcpositioil l~:Uamelers for AlSh filmn l~rel~medby hot wall epit~,xy (FIWI[) S:ullple
KHI Ki 12 KH3
Stnhslrale
lcnll'lel'atltre
Cry.,,l:d line nizu'
Aclivatitm ¢aci*gy
( K)
( I.,Lm )
(eV)
825 t;()<) t;75
1,5 3,b 5,2
0,3 (),2,~ ().21
observed at stibstrate teml+,.'ralure of 975 K. The crystallite size is considerably increased in COml~arison to I'ilulls prepared by thermal co-evaporation technique [28] and HWE grt)Wll I'ihns onl glass substrates [29]. Tile Conlpal+atively improved cryst,'dlite size of AISb fihlls growil by HWE may be attributed to the growth of AISb iayen's tinder conditions as near +.IS possible Lo tllernmdylmnlic equilibrium, which results when the incidenl atOMS of nlolectules have sul'ficient mobility to form ,'t well-ordered structure. The electron diffraction patterns taken at different sites were found to be identical, indicating Ill,it AISb films grow epitaxially on KCI single crystal. Inl the present investigalion, [100] face of KCI single crystal has been used for film preparation. As a result, tile AISb l'ilms obtained are lbund to have [I00] face parallel to the substrate. Larotlx el ai. [I I] also deposited AISb films or| [l I I] CaF+ substrates. Tile electron diffraction patterns indicated the g,'owth of
3. R e s u l t s a n d d i s c u s s i o n
The single-crystal KCI [ I00] substrate is chosen because of its lattice eonlpatibility with AISb. It has been found that KCI has only 2.3c£ lattice misma,'eh with AtSb. Royer [26] has suggested the maximum lattice misnlalch condilion of 15~ for the epitaxial growth. Scllulz [27] has investigated the C d T e / G a A s system with a 14~£• ntisnlatch. The growth of InAs on GaP trY)I] with a 119~ mismatch has also been reported, ill the ease of lattice compatibility between the substrate and filrn material, the l'ilm is expected to have the same orientation as the substrate, Therefore, a KCI single crystal can be used as a sul~strate l'or epitaxial growth o1" AISb films. The purpose of a crystalline st, bstrate is to provide a definite system of nucleation centrcs/sites at very initial stage and play no active role on subsequent t'ilm growth [27]. The Cxl;erimental deposition parameters of AISb films prepared b~ HWE onto KCI single crystal substrate kept at different tempenttures are listed in Table I. Fig. I shows tile scanning electron nlicrograph and electron dil'fntction pattern of an AISb film deposited on KCI single crystal kept at 825 K. It is observed that the films are conlpt)sed ot' well-del'iried crystallites. An appreciable increase in tile grain siz~ is observed with increasing sul~strale ternperattlre. Crystatlites as large as 5.2 /_tnl are
Fig, t, SF,M and ED paltm'n ~1'Alsb I'ihn right,sited ml KCI single cry~tal .",L||~,.MI'~.ll¢.
7'. Sio@, R. K, Ih,di / 77tin Solid I:ilms 312 t ! 9q,~,~ 11 I- I 15 ~
-e-
L
825 K
I 13
30 ~
900K
-*--
825 K
-l--
900K
--o-- 975K U
"
-2
-
.
~
b
20
0 .J
-3 1o ~
J 200
.......
I
300 T(K)
I
_
lO
_
600
p = p,,exp( -
t',,/kT)
where I'~, is tile resistivity activation energy. Observations indicate that the resistivity of the HWE grown AISb films is decreased by a factor of 102 c o n l p a r e d with those prepared by a co-evaponttion technique [28]. This may bc ascribed to the comparatively more ordered structure, and llence smaller number of defects in filnls prepared by HWE. It has also been observed that the electrical rcsisuvity o1' films grown on KCI sttbstrates had low v-flues as compared tt) those grown by HWE on glass sub',,trates [29]. Francombe et al. [9] have Ikmnd corrtpal'ativcly high values of room tempenlture resistivity for AISb films deposited on CaF~ substrate. This indicates thai KC! ix a better subs|rate material than glass and CaF-,. At higher substrate temperatures, tile I'ilms show lower values of the electrical resistivity, indicating tile formation of ordered filnls. Fig. 3 shows the inverse absolute temperature dependence of log of conductivity for AISb films deposited at different subs|rate temperatures. The conductivity of these films increases with an increase in temperature, l)avid et al. [30] observed a striking similarity between the conductivitics of bulk material aild to.evaporated AISb i'ilms. The activation energies obtained fronl the slopes of tog (conductivity) vs. inverse temperature idols in the tclnpcrature range 173-473 K are given in Ttthle i. It is seen that the value tff the activation energy for the t'ilms tlepositcd at 825 K is higher than those deposited at 975 K. Thus, the activation etlct'gy decreases witll an increase
.i
2
J
~
I
3
4
5
....
i.
6
( 10001 T )
Fig. 2. Temllcraturc dependence of electrical resistivity of AISb ill,us deposited 011 KCI subslral¢ kept at (i) 825 K, (it) 91~) K. al|d (iii) ¢)75 K.
epitaxial films, but the lattice match was not good for high-crystalline quality films. Also the SEM showed separated islands on the .gurface of the l'ilms due to the large mismatch ( = l l+J,) b+tween the lattice parameter of the CaF,, substrate and AISb l'ilrns. Fig. 2 shows tile te,npet'ature dependence of the electrical resistivity of AISb films deposited on KCI substrates kept at different teruperatures, it is observed that tile electrical resistivity dt,.creases with an increase in tetnperature. The dependence of resistivity uptm temperature C~,tll be represented by the equation:
~ |
Fig. 3. I.og o1' couduclivily VCl"SUSinverse absohlle tCIllpCl';|lUl'~,
in substrate ten~perature supporting the resistivity measurements, hldicatirlg the fonnation of more ordered systems at higher substrate temperatures. The films are found to be p-type as determined fi'om the Hall probe method, indicatirg that holes are the dominant chmge carriers. Figs. 4 and show the variation of Hall mt~hility and carrier conce~ltration of AISb films against reciprocal temperature, respectively. The mobility of the films is higher by a factor ot"2 than those of HWE grown AISb films on glass ,~ubstrate [29]. The mobility and carrier concentration increase with increase m temperature, showing the predominance of the grain boundary potential ban'ier mechanism in these films. The grain boundary barrier mechanism is based on tlae assumption that grain boundaries have an inherent space charge region due to lattice discontinuity. Tile exponential dependence o f tile mobility attd carrier concentration can be represented by tile equation [31.32] #H =/.t,,exp(q~cl,/kT" ) and = p,,e,,p( - g , / k r )
where ¢~, is the grairl botmdary potential barrier ar+d kt,, is the grain bot, ndary limited mobility in the absence of
25 -o-
B25 K
-~.- 900 t~ --e-
.~ ~';~',t.c.z... =
5"> EU
C',4
975R
20
I
1
2
3
i
1
4
....
i
5
. . . . . . . . . . . . . . . . . . . .
6
(IO001T) i:i~, 4, VarkLlhmof |lall nlol~ilit~. ~,~.ilhhl~cp,e al~,,oiuteteml~L'r~tt|ll¢|~q .,\lSb films.
114
7: Sin qh, R.K. fledi / 771in Snlid l"ihm 312 ~/99,~¢1 I I I ~ 115
-o- 825 K --~- 900 K -e- 975 K
,,,--,, ('3
-e- 825t~
Z
_,-
-o- 975K
100
// ! / //;
¢,q .,..,
,j8
.¢g 9O "d
=1o7 -
1
I
2
I
3
4
•
5
( ~0001 T ) Fig. 5. Varia=itm of cm'n'ier ¢oncentralion with irl~,rer.,4e absolute lenlperltture l'~w AISh films.
~
1
1,0
12
,
1,4 1.6 Energy (eV)
I
is
,
I
z.0
Fig. ~. Speclral vari.'tdon of ¢~ for AlSh lilms.
potential harrier. Ep is the carrier activation energy. The values of q ~ , thus obtained are 0.001 I. 0.0017 and 0.0021 eV for AISb filrns deposited at different substrate temperatures. The absorption coefficient cr was determined from the transmission coefficient T. Neglecting the interference effects, T is determined by [33]
T=
(! - R-')(i
+
exp(a~d) - R:exp( - trd)
where d ix the thickness of the film and R is the rellectivity given by
( n - I)" +KR =
4. C o n c l u s i o n s
(n + i): + K:
where n and K are the refractive index an,/ extinction coefficient, respectively. Generally. K 2
R=~,n-1)=/(. +
I)"
The absorption coeMcient wax determined from measured transmittivity of AISb films prepared by HWE technique onto KCl substrates kept a, different temperatures. The spectral variation of ~ at ,oom temperature was determined over the energy range 1.0-2.0 eV. The absorption edge of AlSb film has been examined in terms of indirect transition using the equation of Bardeen et al. [34], stating that the absorption coefficient ~ is given by
~ht'= B(hv- Ev)
al. [35] for AISb lilms. The increase in bandgap energy with the suhstrate temperatu,'e may be explained by the fact that in the case of fihns deposited at low substrate temperatures, grain boundaries are highly disordered, which give a marked contribution to the absorption for films with low grain sizes; whereas the films deposited at higher substrate temperatures favour the growth of larger grain sizes and stronger orientation ot" the cryslallites giving a less contribution to the absorption.
.'t"
where X = 2 for allowed transition and X = 3 for forbidden transition. Fig. 6 shows the spectral variation of ~ described by X = 2 for the AISb films deposited on KCI substrate. Extrapolating the lines to ( a e h v ) = 0 gives the values of optical gap. It is observed thai the optical gap /:'g lies in the range 1.49-1.522 eV lbr the films deposited at different substrate temperatures. These results are in agreement with those obtained by David et al. [30] and Blunt eL
The grain size as large as 5.2 # m is obtained for the AISb l'ihns grown by hot wall epitaxy on KCI single-crystal substrates. The eleclron diffraction pattern of films indicates epitaxial growth, The electrical resistivity of the HWE-grown films decreases by a factor of 10: in comparison to the co-evaporated fihns, The optical bandgap lies in the range 1.49-1.522 eV and the bandgap energy increases with increase in suhstrate tempe,'att, re. Also, KCI single crystal appears to be a better substrate material than glass and CaF,.
Acknowledgements
The authors wish to thank the Council ol" Scientific and Industrial Research (Government of India), New Delhi for providing financial assistance for carrying out this project. We are also thankful to Dr, Krishan Lal, Scientist, National Physical Laboratory (CSIR) for providing KCI single crystals and helpful discussions. The facilities provided by RSIC, Chandigarh are gratefully acknowledged,
References
{Jl A. ][¢rcLu~, R.R. i tah~r,t.,CLtt,A.|~.. Middclttm. J. I~t,.',,.'m~,,:lwm.Sot:. 1(15(lqSX) 533,
7~ Siugh. R.K. lh'di / Thiu Solid Films 312 { I~,1~t~9 I I I-- 115
12] G, Ziela.,,ek. Archh, E. trhcrtrag. 8 (19541 529, [3] K.M. Yu, A,J. Moll, N. ('hart, W. W~dukiewicz, t', I]~da, Appl, Phys. I,elt. gR~ (1995) 241)6. [4] it.D. l,aw, K. Nakano. I,,R, Tomaslt:l, IEEE J. Quantunl Electron QE-15 (1979) 549. [5] M.B. F*anish C;ipLl,~StII, W.A, [~tl.llllel', S. Sumski, in: H.W, Thim (l:,d.), Pmc. hit. Sympo,,,ium on (;aAs and Rchl|cd ('ottlpoultd.~, Vienll~l. tm, t. ~1' Physics, ],ontloll, 11)81, [6j F.W.O, I)asilva, C. Rai.,,in, M. Nonm~ura. I,. I,ass;tbalerc, Thin Solid Film.,, 20t1 (199L) 33. [71 J.I,. Richards, P.B. }iul'l. I".K. Muller. Single Crystal Films, M.It. l:t'~.tltCOltlllc (ELI.). V~ul Nt'Jslr;md-Reinht~ltl. Ncx~ York, Lt,164. [8] J,L. Richartls. I:',1~. H~u't. I..M. Galhmc. J. AppI. Phys, 37 11963} 3418. [91 M.H. Fr;tnconlbe, A.J. Noreika, S.A. Zeitntan. J.l-. Johnson, Thil| Stflid Films 32 (1976) 259. [1(I] S.M. Pa|cl, A,M. iiir'ldar. Intl. J, Pure AppI, Phys. 21 (1983)418. [1 I] M. l,:lrottx, A.T, ['arli. P. Gibarl. C. Veric, 1. Cry,,I. Growth 18 (1980) 367. [121 N.J. She'.'cit. ,I, Tujcda. ('.M, I~cnclfiml, M, ['ard-ua. S~llitl Slate Colulmm. II (1972) 1619. [13] R. I,itmeb~tch, K.W. Ben/. 55 (1981)531. 114] I,. Baul'ay. R. Andrew. A. Pigeolct, I,.D. 1,audc. "Fifth Solid Film.,, 1)1) (11182) 69. [t5] C. Chang, H. Takaoka. I,.1,. Chailg, I,. Esaki, Appl. i~hys, I,ett. 40 11 I01 (1982) 983. [16] W.T. "rsang, N.A. ()lsxoll, Appl. I:'lly~. l,cll. 43 ([t)bl%F ,'4,.
I 15
[17t T.tt. Chiu, W.T.T.,,ang. J, AppI. Phys. 57(t0} (t~},"¢5)4572, [18] J.R. Walel'llllUl, I'~,V. Shanufl~rt~ok, R.J. Wagner, J, Vac. Sci. Tcchnol. B 11) (2) (1992) 895. [19] A, Lopez Olero, Thin Solid F'ihns 4t.i (t978II t. [2Oi J,F,, Geuthe, R.E, Alderich. Thin Solid Fihns 8 (It,~71) 149. [21] I.S. Athwat. R.K, l~edi, J. Appl. I'hys. 64 (1988) 6345, [22] P.R. Vaya, V. Ramachandran, Solid St;.llC Electron, 27 (1984) 553, [23] J.P. Singh, R,K, Bcdi, J, Appl, Phy.~. 68 (191~0) 277~1, 1241 I.S. Alhwal, R.K. Bcdi, Jpn. J, Appl, Phys, 26 (1987) t413, [25] 1,3. Vander Pauw, Philips Res, Rep, 13 (1958) 187, [26] L. Roycr, Compt. Rend. 194 (1932) 1088, [27] L.G. Schulz, Acta Cryxlallogr, 4 (19511 483. [28] T. Singh. S. Kaur, R.K, Bcdi, in: K. l,al (l:d,), Physics of Semicondtlctol-i)cviccs, Naro,~a, Nc~ Delhi, 1993, p. 511. [29] T. Singh. R.K. Bcdi, m: K, l.al (l:.d,), S¢lnicondt|Cl.~r D¢~.ice.s, Naro,,,a, New Delhi. 1995, p. 409, [301 J,P. David, I.. Capclla, I,. I,aude, S, Malliuuzzi, Re,,'. Phys, Appl. I (196~) 172. [31] R,I.. Pelri,~, Phy,,,. Rex'. IIM 11956) |508, [32] R.H. Bub¢, Atmu. Re','. Mater, Sci. 5 11975) 2111. [33] "1".S, Moss, Optical Properties of Semiconductors, Butlerwurth, LUlldon. 1t)59. [34] J. Bardcen, F.J. Blall, LH. Hall, in: R. Breekeuridge. B. Russel, T, Hahn (Eds.L Pholt~'onducti~'ity Conf.. Wiley, Nc~.~. York, 1956. [35] R.F. Bhlnt, H,P. Fredrik~,c, J.It, Beckcr, W.P, ttosler, Plays,,. Rev, I)6 ( 19541 578.
Thin Solid Films 312 (L998) I1 I - i 15
Growth and properties of aluminium antimonide films produced by hot wall epitaxy on single-crystal KCl Taminder Singh, R.K. Bedi MaWrial S('icm'e Ltlhorolol31. Department t#'Physit's, Gm'~l Nmmk D(,v Univer.~ity, Amril.~ar 143005, Ira/if Received 22 April 1997; accepted 4 August 1997
Abstract
Alurniniunl antirnonide (AISb) films have been grown by hot wall epitaxy on KCi substrates kept at different temperatures in vacuum o f I X l O .s Torr. The experimental conditions are optimised to obtain better crystallinity of the films. The electrical conductivity. Hall mobility arid carrier concentration arc determined. The films appear to be p-type: thus, indicating holes as dominant charge carriers. Scanning electron microscopy shows an increase in grain size with substrate tempenlture. Transnussion electron micrographs indicate a better crystaltinity of AISb filn,s as compared to tht),.,e grown by thermal co-evaporation, c~) 1998 Elsevier Science S.A. Ke:'wr,'d.v: Atumfltiunt autimonide films: Hot wall epitaxy: KCI suhstrate~,
1. Introduction
Aluminium antimonide (AISb) is a Zinc Blende type semiconductor with an indirect optical gap of 1.6 eV at room temperature. AISb seems to be a promising semicondueling material for high-temperature applications especially for transistors and p - n junction diodes [I,2]. The other applications of AISb include high-energy photon detectors [3], solar cells (for which AISb appear~ to be ideally matched), and for the development of Ga I _, AI ,Sb-based optoelectronic devices [4.5]. In spite of the tact that AISb is closely related to the important direct gap materials lnP and GaAs, it has received little :atention so filr.
""
Dasilva ~t al. [6] have studied the oxidatk,a or AISb films grown by molecular beam epitaxy (MBE). The flash evaporation technique has been erriployed by Richards et al. [7,8] to grow AISb films. Frarlcombe et al. [9] have observed a strong photovoltaic response irl vacuum-deposited AISb t'ihns. The etTect of thickness on the electrical properties of AISb films has also been studied [10]. Laroux et al. [I l] have employed metal organic chelnical vapour deposition (MOCVD) to grow AISb films on insulating substrates. The polycrystalline AISb fihns have been obtained by sputtering in high-purity Argon atmosphere [12]. The AISb t'ihns have also been prepared by liquid phase epitaxy (LPE)[13] and taser annealing [14]. The MBE growth conditions of aluminiunl antirnonide films on (){140-6090/98/$19.(X)
GaSb substrate have been reported by several authors [ 15,16], Chiu and Tsang [ 17] and Waterman et al, [ 18] have attempted to study the reflection high-energy electron diffraction (RHEED) patterns of MBE-grown AISb films. The electronic properties of AISb films are strongly intluenced by their structure, which, in turn, depends upon the deposition conditions and the technique employed. hnprovcment in grain size and reduction of structural defects have been obtained for films deposited under conditions near thermodynamical equilibrium. Hot wall epitaxy (HWE) studleu by Lopez Otero [19] pemfits film deposition very near thermodynamic equilibrium. This technique has already been exploited for lI-V! [20,21] and IV-Vl [22,23] compound semiconductor films. In this communication, we report the structure, electrical and opttcal properties of AISb films prepared by HWE on single-crystal ~CI substrates at different Temperatures. .
2. Experimental
,t
_..~__. . . . . . . . . .
Experimental details of the hot wall set up to deposit AISb fihus have been described earlier [24], High-purity (99,999c;~.) elements aluminiuni arid antimony obtained from Nuclear Fuel Complex, Hyderabad {India) were utilised. The entire system was enclosed in the vacuum chamber of a Hind High Vacuum coating Unit I2AdH with a background pressure of I X l0 ~ Tort, Belore carrying out the deposition, the vacuum chamber was
i t2
7: Si,.k,h.
R.K. Bcrli /
771inS,,lid
baked at 470 K to degas the vacuum equipment, During the degassinlg, the substrate tin6 wall heaters were also switched on. On attaining the desired expeu'inlental condilions, tile sotirce heater was switched tm and the stlbstrate holder positioned on the Lop ot' tile tube It) close tile systenll. Two separate windings were used It) heat the evaporation port and the wall of the cohullU:. A radiant heater was used to heat the substrate ill the tunlperaturc range between 825 and 975 K. The tenlperaturc,~ of the different zones were controlled by PID tenlperature ottotrollers using K-type thermocouple obtained fronl Omega Eng. (USA). The fihlls obtained were anne:tied at 500 1" to stabilise the crystallinlc structure and then kept in wtcuunl to prevent direct contact fronl the atnlospherc, an AISb oxidises in air. This problem wits resolved by depositing a very thin layer of lnSb on the AISb films at room temperaItlre.
The c i r c u i t a l resistivity and the Hall coeMcient of AISb filnls were determined in vacuum (I × 10 4 Torr) using Vander Pauw',.: four-probe technique [25] in Ihe temperature range between 173 and 473 K, Silver contacts were used to connect electrical leads to the films. The optical measurements were carried out at room temperalure using a double beam PYE-UNICAM SP3-300 Spectrophotometer. The transmittance was nleasured inl tile photo-energy range 1.0-2.0 eV. A JEOL JSM-6100 (Japan) scar, ling microscope, and a JEOL TEM-1200 (Japan) electrml microscope were used for structural characterization.
l"ilm.', 3 12 ( 19()fl'~ I 11 .- I 15
Table I
F.xpcrimcntal dcpositioil l~:Uamelers for AlSh filmn l~rel~medby hot wall epit~,xy (FIWI[) S:ullple
KHI Ki 12 KH3
Stnhslrale
lcnll'lel'atltre
Cry.,,l:d line nizu'
Aclivatitm ¢aci*gy
( K)
( I.,Lm )
(eV)
825 t;()<) t;75
1,5 3,b 5,2
0,3 (),2,~ ().21
observed at stibstrate teml+,.'ralure of 975 K. The crystallite size is considerably increased in COml~arison to I'ilulls prepared by thermal co-evaporation technique [28] and HWE grt)Wll I'ihns onl glass substrates [29]. Tile Conlpal+atively improved cryst,'dlite size of AISb fihlls growil by HWE may be attributed to the growth of AISb iayen's tinder conditions as near +.IS possible Lo tllernmdylmnlic equilibrium, which results when the incidenl atOMS of nlolectules have sul'ficient mobility to form ,'t well-ordered structure. The electron diffraction patterns taken at different sites were found to be identical, indicating Ill,it AISb films grow epitaxially on KCI single crystal. Inl the present investigalion, [100] face of KCI single crystal has been used for film preparation. As a result, tile AISb l'ilms obtained are lbund to have [I00] face parallel to the substrate. Larotlx el ai. [I I] also deposited AISb films or| [l I I] CaF+ substrates. Tile electron diffraction patterns indicated the g,'owth of
3. R e s u l t s a n d d i s c u s s i o n
The single-crystal KCI [ I00] substrate is chosen because of its lattice eonlpatibility with AISb. It has been found that KCI has only 2.3c£ lattice misma,'eh with AtSb. Royer [26] has suggested the maximum lattice misnlalch condilion of 15~ for the epitaxial growth. Scllulz [27] has investigated the C d T e / G a A s system with a 14~£• ntisnlatch. The growth of InAs on GaP trY)I] with a 119~ mismatch has also been reported, ill the ease of lattice compatibility between the substrate and filrn material, the l'ilm is expected to have the same orientation as the substrate, Therefore, a KCI single crystal can be used as a sul~strate l'or epitaxial growth o1" AISb films. The purpose of a crystalline st, bstrate is to provide a definite system of nucleation centrcs/sites at very initial stage and play no active role on subsequent t'ilm growth [27]. The Cxl;erimental deposition parameters of AISb films prepared b~ HWE onto KCI single crystal substrate kept at different tempenttures are listed in Table I. Fig. I shows tile scanning electron nlicrograph and electron dil'fntction pattern of an AISb film deposited on KCI single crystal kept at 825 K. It is observed that the films are conlpt)sed ot' well-del'iried crystallites. An appreciable increase in tile grain siz~ is observed with increasing sul~strale ternperattlre. Crystatlites as large as 5.2 /_tnl are
Fig, t, SF,M and ED paltm'n ~1'Alsb I'ihn right,sited ml KCI single cry~tal .",L||~,.MI'~.ll¢.
7'. Sio@, R. K, Ih,di / 77tin Solid I:ilms 312 t ! 9q,~,~ 11 I- I 15 ~
-e-
L
825 K
I 13
30 ~
900K
-*--
825 K
-l--
900K
--o-- 975K U
"
-2
-
.
~
b
20
0 .J
-3 1o ~
J 200
.......
I
300 T(K)
I
_
lO
_
600
p = p,,exp( -
t',,/kT)
where I'~, is tile resistivity activation energy. Observations indicate that the resistivity of the HWE grown AISb films is decreased by a factor of 102 c o n l p a r e d with those prepared by a co-evaponttion technique [28]. This may bc ascribed to the comparatively more ordered structure, and llence smaller number of defects in filnls prepared by HWE. It has also been observed that the electrical rcsisuvity o1' films grown on KCI sttbstrates had low v-flues as compared tt) those grown by HWE on glass sub',,trates [29]. Francombe et al. [9] have Ikmnd corrtpal'ativcly high values of room tempenlture resistivity for AISb films deposited on CaF~ substrate. This indicates thai KC! ix a better subs|rate material than glass and CaF-,. At higher substrate temperatures, tile I'ilms show lower values of the electrical resistivity, indicating tile formation of ordered filnls. Fig. 3 shows the inverse absolute temperature dependence of log of conductivity for AISb films deposited at different subs|rate temperatures. The conductivity of these films increases with an increase in temperature, l)avid et al. [30] observed a striking similarity between the conductivitics of bulk material aild to.evaporated AISb i'ilms. The activation energies obtained fronl the slopes of tog (conductivity) vs. inverse temperature idols in the tclnpcrature range 173-473 K are given in Ttthle i. It is seen that the value tff the activation energy for the t'ilms tlepositcd at 825 K is higher than those deposited at 975 K. Thus, the activation etlct'gy decreases witll an increase
.i
2
J
~
I
3
4
5
....
i.
6
( 10001 T )
Fig. 2. Temllcraturc dependence of electrical resistivity of AISb ill,us deposited 011 KCI subslral¢ kept at (i) 825 K, (it) 91~) K. al|d (iii) ¢)75 K.
epitaxial films, but the lattice match was not good for high-crystalline quality films. Also the SEM showed separated islands on the .gurface of the l'ilms due to the large mismatch ( = l l+J,) b+tween the lattice parameter of the CaF,, substrate and AISb l'ilrns. Fig. 2 shows tile te,npet'ature dependence of the electrical resistivity of AISb films deposited on KCI substrates kept at different teruperatures, it is observed that tile electrical resistivity dt,.creases with an increase in tetnperature. The dependence of resistivity uptm temperature C~,tll be represented by the equation:
~ |
Fig. 3. I.og o1' couduclivily VCl"SUSinverse absohlle tCIllpCl';|lUl'~,
in substrate ten~perature supporting the resistivity measurements, hldicatirlg the fonnation of more ordered systems at higher substrate temperatures. The films are found to be p-type as determined fi'om the Hall probe method, indicatirg that holes are the dominant chmge carriers. Figs. 4 and show the variation of Hall mt~hility and carrier conce~ltration of AISb films against reciprocal temperature, respectively. The mobility of the films is higher by a factor ot"2 than those of HWE grown AISb films on glass ,~ubstrate [29]. The mobility and carrier concentration increase with increase m temperature, showing the predominance of the grain boundary potential ban'ier mechanism in these films. The grain boundary barrier mechanism is based on tlae assumption that grain boundaries have an inherent space charge region due to lattice discontinuity. Tile exponential dependence o f tile mobility attd carrier concentration can be represented by tile equation [31.32] #H =/.t,,exp(q~cl,/kT" ) and = p,,e,,p( - g , / k r )
where ¢~, is the grairl botmdary potential barrier ar+d kt,, is the grain bot, ndary limited mobility in the absence of
25 -o-
B25 K
-~.- 900 t~ --e-
.~ ~';~',t.c.z... =
5"> EU
C',4
975R
20
I
1
2
3
i
1
4
....
i
5
. . . . . . . . . . . . . . . . . . . .
6
(IO001T) i:i~, 4, VarkLlhmof |lall nlol~ilit~. ~,~.ilhhl~cp,e al~,,oiuteteml~L'r~tt|ll¢|~q .,\lSb films.
114
7: Sin qh, R.K. fledi / 771in Snlid l"ihm 312 ~/99,~¢1 I I I ~ 115
-o- 825 K --~- 900 K -e- 975 K
,,,--,, ('3
-e- 825t~
Z
_,-
-o- 975K
100
// ! / //;
¢,q .,..,
,j8
.¢g 9O "d
=1o7 -
1
I
2
I
3
4
•
5
( ~0001 T ) Fig. 5. Varia=itm of cm'n'ier ¢oncentralion with irl~,rer.,4e absolute lenlperltture l'~w AISh films.
~
1
1,0
12
,
1,4 1.6 Energy (eV)
I
is
,
I
z.0
Fig. ~. Speclral vari.'tdon of ¢~ for AlSh lilms.
potential harrier. Ep is the carrier activation energy. The values of q ~ , thus obtained are 0.001 I. 0.0017 and 0.0021 eV for AISb filrns deposited at different substrate temperatures. The absorption coefficient cr was determined from the transmission coefficient T. Neglecting the interference effects, T is determined by [33]
T=
(! - R-')(i
+
exp(a~d) - R:exp( - trd)
where d ix the thickness of the film and R is the rellectivity given by
( n - I)" +KR =
4. C o n c l u s i o n s
(n + i): + K:
where n and K are the refractive index an,/ extinction coefficient, respectively. Generally. K 2
R=~,n-1)=/(. +
I)"
The absorption coeMcient wax determined from measured transmittivity of AISb films prepared by HWE technique onto KCl substrates kept a, different temperatures. The spectral variation of ~ at ,oom temperature was determined over the energy range 1.0-2.0 eV. The absorption edge of AlSb film has been examined in terms of indirect transition using the equation of Bardeen et al. [34], stating that the absorption coefficient ~ is given by
~ht'= B(hv- Ev)
al. [35] for AISb lilms. The increase in bandgap energy with the suhstrate temperatu,'e may be explained by the fact that in the case of fihns deposited at low substrate temperatures, grain boundaries are highly disordered, which give a marked contribution to the absorption for films with low grain sizes; whereas the films deposited at higher substrate temperatures favour the growth of larger grain sizes and stronger orientation ot" the cryslallites giving a less contribution to the absorption.
.'t"
where X = 2 for allowed transition and X = 3 for forbidden transition. Fig. 6 shows the spectral variation of ~ described by X = 2 for the AISb films deposited on KCI substrate. Extrapolating the lines to ( a e h v ) = 0 gives the values of optical gap. It is observed thai the optical gap /:'g lies in the range 1.49-1.522 eV lbr the films deposited at different substrate temperatures. These results are in agreement with those obtained by David et al. [30] and Blunt eL
The grain size as large as 5.2 # m is obtained for the AISb l'ihns grown by hot wall epitaxy on KCI single-crystal substrates. The eleclron diffraction pattern of films indicates epitaxial growth, The electrical resistivity of the HWE-grown films decreases by a factor of 10: in comparison to the co-evaporated fihns, The optical bandgap lies in the range 1.49-1.522 eV and the bandgap energy increases with increase in suhstrate tempe,'att, re. Also, KCI single crystal appears to be a better substrate material than glass and CaF,.
Acknowledgements
The authors wish to thank the Council ol" Scientific and Industrial Research (Government of India), New Delhi for providing financial assistance for carrying out this project. We are also thankful to Dr, Krishan Lal, Scientist, National Physical Laboratory (CSIR) for providing KCI single crystals and helpful discussions. The facilities provided by RSIC, Chandigarh are gratefully acknowledged,
References
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