- Email: [email protected]
Growth conditions of InxSey films by molecular beam deposition
,IlateriaA A'cietzce and Lngitwering, Bd (1989) 13-17
13
Growth Conditions ofln.Se,, Films by Molecular Beam Deposition* ,I. Y. EMtLRY. l.. B[~,A H I M - O T S M A N E . M. ,I()L'ANNE, C. JULIEN and M. BALKANSKI
Ix,,horalc, ire d¢ I'to.:iqu,:" de.', Solide',', L#lirersil¢; I:'ier,,e ~,t :llarie ('m,'i,z, A.',',,:;,:'i~; au ('.V/?S, 4 ph:cv .lu',",iet~, t'-75252 I'ari.', ('&hv, 05 (t')a.ce)
( Rccci;'cd No\ ember 8, 19SS: in revised form kL'bruary 28, 19N~))
Abstract
7he growth co/Mid(ms o,/ t/fin ,~liras o/" ltl, Se,. ohtailted by molecular beam deposition have bee, .studied. l:he iq/luence of the .fluxes of ej]itsion elemelzls and the substrate temperature dependenc'ex have bee. iHvestNated by the cltalztcterization ()f the t/fin .fihn structure. X-n 0' patwrns attd R a m a . scattering ,wectra show that ln4Se¢, htSe and ht,Se~ cem be ]ormed in a wide range q/" elem e, m/.llttxes. 1. Introduction The use of intercalation cathodes in secondary batteries has been shown to be a viable route to high energy density, rechargeable energy storage /1]. The most common approach is to use a powder capable of intercalation and to prepare a paste or pellet of this powder to fabricate the positive electrode. The possibility of direct preparation of thin film positive electrodes has been explored 12], and lithium microbattery cells with TiS: [3] or InSe [4] as cathode have recently been realized. Among the different methods of preparation, the purpose of using molecular beam deposition (MBD) for making lithium microbatteries is to control the electrolyte-electrode interface, which appears to be the major limitation in such devices. The molecular beam technique allows one to extend the substrate temperature growth domain of binary compounds which contain a volatile element as the chalcogen atom. Amorphous and polycrystalline In,Se,. thin films have been investigated by several authors 15-81, though only refs. 6 and 8 give studies on the thin film composition. And() and Katsui [61 '::Paper prescnled a! Symposium D on Solid Stale Ionics at the E MP,S Fall Meeting. S|laSbotllg, November 8-IlL 19~8.
0921-5107/8,9/$3.50
havc prepared lnSe thin films with excess selenium content. They have detected diflerent phases such as In, Sc and InzSe; in their films, and singlephase films containing only InSe are obtained at a certain critical ratio. Watanabe and Yamamoto ISJ, using Raman measurements, have shown that lnzSe.~ is predominant in ln~Se I , films, and at x = 0.5 the framework in the structure is crystalline InSe with other complex phases. In this paper we study the growth conditions of ln,Se, thin films obtained by MBD. The influence of the fluxes of effusion elements and the dependence of the substrate temperature have been investigated by the characterization of their structure. X-ray patterns and Raman scattering spectra show that different compounds of the In-Se system can be formed in a wide range of elemental fluxes. 2. Experimental The films are grown by the MBD technique using all MBE 2300 Riber chamber in which an ultrahigh vacuum of 5 x lO 11 Torr is obtained after bake-out at ] 20 °C for 48 h. Indium (6NI and selenium (5N)are placed into individually shuttered molecular beam effusion cells. During the growth, cryopanels surrounding the cells and the deposition chamber are cooled at liquid nitrogen temperature. The total pressure in the reaction chamber increases from the initial vacuum to 2 x 10 ~" Torr, mainly because of the scattering of volatile elements as selenium species. Thin films are deposited on wafers of silicon with (100) orientation or o n amorphous substrates maintained at a temperature in the range 200-400 °C. If we assume that the vapour in the effusion cell is near the equilibrium condition and that the aperture of the cell has an area A, then the total expression for the number of molecules per © Elsevier Sequoia/Prinled in The Netherlands
14
second striking the substrate positioned at a distance d is .I = P A N/d~-( 27r M R T ) ' =
where I' is the pressure, N is Avogadro's number, M is the molecular weight, R is the gas constant and T is the temperature of the cell. T h e most widely used method of determining the beam fluxes is that devised by Foxon and Joyce. who interposed a movable ion gauge between each beam and the substrate {9]. In a first approximation, the relative beam equivalent pressure is equivalent to the flux ratio of the species. T h e indium pressure P~,~ measured by a movable ion gauge is kept constant under the conditions at "1i~,=82()°C and P h ~ = 4 . 5 × l ( ) 7 Torr. T h e selenium pressure f's~, is in the range ( 1 - 2 . 9 ) x 1 0 ~' Torr for a cell temperature 170 < 7[s~,< 185 °C. These partial pressure conditions lead to the growth of In,Se, thin films, and m the following we shall investigate the film formation as a function of the partial pressure ratio R - l s ~ , / t ~ , ,. T h e growth rates are about 560 nm h ~ for flux ratios lower than R = 3 , and 850 nm h ~ for higher values of R. All the films have a thickness of between 2 and 4/~m. T h e X-ray diagrams are recorded using a Philips PX 1820 diffractometer equipped with a Cu K a source (2 = 0.15406 nm). Raman spectra are recorded at room temperature in the quasibackscattering geometry. T h e excitation line at 2 = 5 1 4 . 5 nm of an argon ion laser, a U I000 Jobin-Ywm double monochromator, and a cooled S20-PMT coupled to a computerized photoncounting system are used. Electrical resistivity measurements are carried out using the Van der Pauw method, with deposited indium spots as ohmic contacts. --
)
=
~-
Z
f,
b
g3
--
1,0
a
60
80
28 (degree) Fig. 1. X-ray diffraction diagrams of ln,Se, thin films growu at various substrate temperatures: (a) I~:-- 200 °C. l? =2.6; (biT~=3t)0°C.R=2.5:{c',7~=350°C.R=3.5.
)
3. Results and discussion Figure 1 shows the X-ray diffraction patterns of In,Se,. thin films grown under a partial pressure ratio R - P s~// t ' t,~--~ ~ ~ for three different substrate temperatures. T h e influence of the substrate temperature on the crystallinity is clearly observed. Thin films grown at ~1s = 200 °C (Fig. l(a)) present only a broad band situated at an angle of 22 °, which can be attributed to an amorphous phase. Films grown at 7[>_ 300 °C (Figs. l(b) and l(c)) are polycrystalline, and their X-ray spectra show reflection peaks which can be attributed to the typical (00/} reflections of InSe
/10, 11 J- In Fig. l(b). the weak ~{hkl':, reflections observed are assigned to those of In4Se~ [ 12] and polycrystalline indium, whereas general InSe ( h k l ) reflection patterns are also detected. When the diffraction peaks of Fig. l(c) are indexed as a hexagonal lattice, the indices obey a relation -h+k+l=3n, where /1, k and I are the Miller indices of the hexagonal lattice and n is an integer. This shows that InSe has a rhombohedral symmetry and bekmgs to the V-polytype. In contrast, the fl- and e-polytypes also have a hexagonal lattice, but the indices obey the relation h-k=3n and the peaks are always sharp, it is well known that for energetic reasons, both ~,and e-polytypes can be present at the same time in a crystal and should be evidenced by Raman scattering spectroscopy, as considered next. With increasing substrate temperature, we observe three very different Raman spectra. At ;t[ = 200 °C, the MBD film exhibits a spectrum without sharp peaks (Fig. 2(a)). This figure displays a Raman spectrum with broad peaks and shoulders at 35, 72, 118, 168 and 212 cm E.The appearance of these bands in the region of sharp peaks of the crystallized phase is typical of an amorphous phase. T h e same result has been observed in InSe films prepared by flash evaporation [8]. In an amorphous sample, we can consider that the elementary unit cell has infinite dimension and therefore the Brillouin zone is
15 "~
~
T"'-"""-W
T
"•
i 0
100
200
300 4.00 Wavenumber (crn-1)
Fig. 2. Raman spectra in backscatlering geometry of In,So, thin films obtained under the same conditions as in Fig. I.
reduced to zero. Consequently, optical and acoustic phonon branches are completely folded in the Brillouin zone centre, and in a first approximation the Raman spectrum of an amorphous phase reflects the phonon state density of each lnSe optical branch [13j. At higher substrate temperature, because of the formation of polycrystalline or crystalline phases in MBD films, Raman spectra with narrow peaks are recorded. Figure 2(b) confirms the attribution from X-ray patterns and shows a complex Raman spectrum in which we can identify at least two types of In,Se,. composition: lnSe and In4Se ~ are observed simultaneously, in particular, In4Se~ is clearly identified by the Raman peaks at 104, 131 and 150 cm ~, and InSe is normally characterized by its Raman peaks at 42, 115, 178, 199 and 227 cm ~ [14]. With films evaporated at a substrate temperature of 350 °C, the Raman spectrum is simpler (Fig. 2(c)) and only an InSe single phase is identified. In the two last cases (Figs. 2(b) and 2(c)) we also observe a clear Raman peak at 211 cm Exactly at this frequency, in lnSe single crystal and with the same excitation wavelength, we observe only a shoulder in the higher frequency part of the LO (longitudinal optic) mode at 199 cm ~. in order to explain the intensity enhancement of this peak, we make three remarks, as follows.
(1) The LO polar phonon modes of lnSe are only observed in the resonance condition for a laser line excitation which has its energy very close to the second excitonic absorption peak [151. However, even in this fawmrable experimental condition, the LO mode at 211 cm J remains one order of magnitude smaller than the LO mode at 199 cm ~l 16]. (2) if a fi- or e-polytype is assumed, then according to group theoretical analysis the number of Raman active modes differs [17[ because of a larger elementary unit cell ahmg the c axis. In this case, new modes which arc Davydov pairs are weakly shifted (less than 1 cm ~for optical branches) and poorly Raman active I 18]. In addition, only the folded acoustic branches are observed in the phonon spectrum of layered compounds which exhibit an e-polytype. In InSe the rigid layer mode is observed at 17 cm ~. In our experiment, the optical surface quality did not allow this determination. Nevertheless, the appearance of the peak at 211 cm ~ cannot be explained by the effect of polytype formation. (3) The polar phonons of lnSe are split into four branches: two TO (transverse optic) modes and two LO modes. Two of them, FI I (TO) and F~ * (LO), have a wavevector in the layer plane and are only Raman active in a quasi-backscattcring geometry along an axis parallel to the layer plane. In contrast, the other two, F~ ~(TO) and Fj ~ {LO), have a wavevector along the ~ axis and consequently are Raman active in a quasibackscattering geometry along this direction. The mode at 211 cm ~ corresponds to the LO mode with a wavevector in the layer plane [19j, and its presence in the spectrum of an MBD film can indicate some disorder in the microcrystallite orientation. Figures 3 and 4 show respectively the X-ray diffraction diagrams and the Raman spectra of polycrystalline thin films grown at 7~=400 °C under various partial pressure ratios. At a low pressure ratio ( R = 2 . 2 ) the Raman spectrum shows that the thin films have a mixed composition (Fig. 4(a)): we observe the coexistence of lnSe and in4Se ~ phases, the latter being detected by its low frequency modes at 45, 75 and 104 cm i. At moderate values, 2.4_ 5 we observe by both techniques of charac-
16 ---'r
.......... !
C' g
o o //
o
•
,,
/¢"
B
o
_,
/
.__q
,I ca' ,,~ ~_=
m
I
rag-_
0
20
z
~
60 28 (degree)
i
80
g
200
i
300 400 Substrafe femperafureI%)
t-ig. 5. Schematic phase diagram ~>t growth conditions h u ln,Sg, thin films. T h e full and open circles represent the a m o r p h o u s and polycrystalline phases respectivel_,r. T h e dashed line is the b o u n d a r y between InSe ( zx ?and In ~Se ~ c
Fig. 3. X-ray diffraction diagrams of ln,Se, thin films grown at various partial pressure ratios and at a substratc temperature of 400 °C: (a) R = 2.2: (b) R - 2.6: (c) R = 5.7.
lo0
t
200
300 tOO Wavenumber (cm"i)
Fig. 4. Raman spectra in backscattering geometry of ln,Se, thin films obtained under the same conditions as in Fig. 3.
terization the formation of a selenium-rich compound which is identified as lneSe s. Figure 3(c) shows the X-ray diagram of the y-phase of ln2Se~ grown at R = 5.7; this exhibits a strong line
at 2 0 = 28 ° which is attributed to the {0(16} reflection. The Raman spectrum of Fig. 4(c) shows sharp peaks at 22, 26, 54, 85, 151 and 232 cm which are the characteristic modes of 7-1n2Se~ [20]. However, in the intermediate range, 3 . 5 -< R<-5, one can observe the coexistence of lnSe and 7-1n_,Se> and both the X-ray patterns and the Raman scattering measurements exhibit mainly the features of these two compounds. In Fig. 5, the different phases observed in MBD In,Se,, thin films are reported as a functi(m of substrate temperature and partial pressure ratio. We can determine regions for the amorphous and polycrystalline phases depending mainly on the substrate temperature and domains for InSe and 7-1n2Se 3 polycrystalline thin films for 7~ >-300 °C. The dashed line in Fig. 5 represents the limit between indium-rich and seleniumrich growth conditions as defined by ks<,ls~/.l,.- 1, where ,ls~, and J,, are the selenium and indium fluxes respectively and ks~ is the selenium sticking coefficient. For 300-<7s-<400°C the indium sticking coefficient kin is reasonably assumed to be close to 1 because of its lower equilibrium pressure on InSe at this substrate temperature. Because selenium displays a strongly volatile behaviour, the selenium sticking coefficient depends on the presence of indium, which implies that the partial pressure ratio may increase with Ts in order to maintain the condition ks~.Js~,/4., = 1. F~r indium-rich conditions, the polycrystalline thin films are a mixture of lnSc and InaSe 3 phases at low R values (Figs. l(b), 2(b), 3(a) and 4(a)). For selenium-rich conditions,
17
where ksJs~,/Jin> 1, a partial pressure range exists in which lnSe is grown and excess selenium is re-evaporated from the substrate. At 7~ = 350 °C this range is about R = 3-3.5.
4. Conclusions In this paper we have studied thin films grown by molecular beam deposition of the In-Se system. The formation of amorphous and polycrystalline thin films is strongly dependent on the substrate temperature. Using X-ray diffraction and Raman scattering spectroscopy, the characterizatkm of such films has been conducted as a function of the elemental partial pressures during the evaporation. The determination of the composition and structure shows good agreement between the two techniques. H)r 7k<250 °C an amorphous lnSe phase is obtained. As "/k increases from 300 to 400 °C, InSe and 7-1neSe~ polycrystalline thin films are obtained. The observation of the Raman inactive phonon mode at 211 cm ~ in a backscattering geometry along the c axis may be explained by deorientation of the crystallites. Discussion of this last point remains ()pen.
Acknowledgment This work has been partly supported by the Economic European Community under contract ST 2P-0013-1F(CD).
References I M.S. Whittingham, Science, 192 ', 1976) 1126. 2 C. Julien and M. Balkanski, in J. R. Akridgc ;ed.i, Solid State Microbaneries, in NA TO Adv. Study Inst. Ser., Ser. B, Plenum, New York, 1988. 3 K. Kanehori, K. Matsumoto, K. Miyauchi and T. Kudo, Solid State lonics, 9-10 (1983) 1469. 4 M. Balkanski, C. Julien and J. Y. Emery, in R. R. Hearing (ed.), Extended Abstracts, Int. Meeting on Lithium Batteries, Vancomer PB-31, 1988, p. 246. 5 M. Pcrsin, A. Pcrsin and B. Ccslustka, lhin 3olM l-Thn.~, /2 1972; 117, 6 K. Ando and A. Katsui, l/tin Solid I'ihm. 7(~, 19~; 1 14 I. 7 S. Chandhuri, S. K. Biswas and A. Choudhury, Solid Slaw ( o m m u n , , 53 ¢19851273. 8 1. Watanabe and T. Yalnamoto, J/re. J. ,ll)pl. I'hyv. 24 :1985) 1282. 9 C.T. Foxon and B. A. Joycc, SttU: Sci., CN ', 1~,J77) 293. l0 K.C. Nagpal and S. Z. All. ,Iota ()ysta/logr. L 31 I ~)75 ) 567. I 1 S. Popovic, A. Toncjc. B. Grzcta-Plenkovic. B. Ccluslka and P. Trojko, J. Appl. COstallogr., 12 : 1979) 416. 12 J. H. C. [togg, H. H. Sutherland and 1). J. Williams, Acta ('rystallogr. B, 29(1973) 15911. 13 See e . g . M . H . Brodsky. in M. Cardona icd.), Light Scattering in Solid.s, Springer-Verlag, Berlin. 1975, p, 2115. 14 M. Jouanne and C. Julien, Phys. Status SolMi (b), 11989) in the press. 15 T. Ikari. S. Shigetomi and K. Hashimoto, I'hvs. ,~tatu~ Solidi (b), 111 ( 1982 ) 477. 16 M. Jouanne, C. Julien and M. Balkanski, I'hvs. 5tams Solidi (hi. 144 ( 1987; K 147. 7 T..1, Wiefing and J. L, Vcrble. Ph~. Rev. B, 5 ¢1~)721 1473. 8 1.. N. Alicva, G. 1.. Belenkii, 1. I. Rcshina. E. Y. Salacv and V. Y. Shtemshraiber, 5or. I'hvs. 5olid ,~taw, 21 , 1979; 90. 9 N. Kuroda and Y. Nishina, ,golid ,Stll[e (o]llnlllll., 2¢%' !1978)439. 20 K. Kambas. C. Julien. M. Jouanne, A. Likh~.rman and M. Guittard, l'hvv ,Sntms SoIMi (hL 124 i 1984) K 1115.
13
Growth Conditions ofln.Se,, Films by Molecular Beam Deposition* ,I. Y. EMtLRY. l.. B[~,A H I M - O T S M A N E . M. ,I()L'ANNE, C. JULIEN and M. BALKANSKI
Ix,,horalc, ire d¢ I'to.:iqu,:" de.', Solide',', L#lirersil¢; I:'ier,,e ~,t :llarie ('m,'i,z, A.',',,:;,:'i~; au ('.V/?S, 4 ph:cv .lu',",iet~, t'-75252 I'ari.', ('&hv, 05 (t')a.ce)
( Rccci;'cd No\ ember 8, 19SS: in revised form kL'bruary 28, 19N~))
Abstract
7he growth co/Mid(ms o,/ t/fin ,~liras o/" ltl, Se,. ohtailted by molecular beam deposition have bee, .studied. l:he iq/luence of the .fluxes of ej]itsion elemelzls and the substrate temperature dependenc'ex have bee. iHvestNated by the cltalztcterization ()f the t/fin .fihn structure. X-n 0' patwrns attd R a m a . scattering ,wectra show that ln4Se¢, htSe and ht,Se~ cem be ]ormed in a wide range q/" elem e, m/.llttxes. 1. Introduction The use of intercalation cathodes in secondary batteries has been shown to be a viable route to high energy density, rechargeable energy storage /1]. The most common approach is to use a powder capable of intercalation and to prepare a paste or pellet of this powder to fabricate the positive electrode. The possibility of direct preparation of thin film positive electrodes has been explored 12], and lithium microbattery cells with TiS: [3] or InSe [4] as cathode have recently been realized. Among the different methods of preparation, the purpose of using molecular beam deposition (MBD) for making lithium microbatteries is to control the electrolyte-electrode interface, which appears to be the major limitation in such devices. The molecular beam technique allows one to extend the substrate temperature growth domain of binary compounds which contain a volatile element as the chalcogen atom. Amorphous and polycrystalline In,Se,. thin films have been investigated by several authors 15-81, though only refs. 6 and 8 give studies on the thin film composition. And() and Katsui [61 '::Paper prescnled a! Symposium D on Solid Stale Ionics at the E MP,S Fall Meeting. S|laSbotllg, November 8-IlL 19~8.
0921-5107/8,9/$3.50
havc prepared lnSe thin films with excess selenium content. They have detected diflerent phases such as In, Sc and InzSe; in their films, and singlephase films containing only InSe are obtained at a certain critical ratio. Watanabe and Yamamoto ISJ, using Raman measurements, have shown that lnzSe.~ is predominant in ln~Se I , films, and at x = 0.5 the framework in the structure is crystalline InSe with other complex phases. In this paper we study the growth conditions of ln,Se, thin films obtained by MBD. The influence of the fluxes of effusion elements and the dependence of the substrate temperature have been investigated by the characterization of their structure. X-ray patterns and Raman scattering spectra show that different compounds of the In-Se system can be formed in a wide range of elemental fluxes. 2. Experimental The films are grown by the MBD technique using all MBE 2300 Riber chamber in which an ultrahigh vacuum of 5 x lO 11 Torr is obtained after bake-out at ] 20 °C for 48 h. Indium (6NI and selenium (5N)are placed into individually shuttered molecular beam effusion cells. During the growth, cryopanels surrounding the cells and the deposition chamber are cooled at liquid nitrogen temperature. The total pressure in the reaction chamber increases from the initial vacuum to 2 x 10 ~" Torr, mainly because of the scattering of volatile elements as selenium species. Thin films are deposited on wafers of silicon with (100) orientation or o n amorphous substrates maintained at a temperature in the range 200-400 °C. If we assume that the vapour in the effusion cell is near the equilibrium condition and that the aperture of the cell has an area A, then the total expression for the number of molecules per © Elsevier Sequoia/Prinled in The Netherlands
14
second striking the substrate positioned at a distance d is .I = P A N/d~-( 27r M R T ) ' =
where I' is the pressure, N is Avogadro's number, M is the molecular weight, R is the gas constant and T is the temperature of the cell. T h e most widely used method of determining the beam fluxes is that devised by Foxon and Joyce. who interposed a movable ion gauge between each beam and the substrate {9]. In a first approximation, the relative beam equivalent pressure is equivalent to the flux ratio of the species. T h e indium pressure P~,~ measured by a movable ion gauge is kept constant under the conditions at "1i~,=82()°C and P h ~ = 4 . 5 × l ( ) 7 Torr. T h e selenium pressure f's~, is in the range ( 1 - 2 . 9 ) x 1 0 ~' Torr for a cell temperature 170 < 7[s~,< 185 °C. These partial pressure conditions lead to the growth of In,Se, thin films, and m the following we shall investigate the film formation as a function of the partial pressure ratio R - l s ~ , / t ~ , ,. T h e growth rates are about 560 nm h ~ for flux ratios lower than R = 3 , and 850 nm h ~ for higher values of R. All the films have a thickness of between 2 and 4/~m. T h e X-ray diagrams are recorded using a Philips PX 1820 diffractometer equipped with a Cu K a source (2 = 0.15406 nm). Raman spectra are recorded at room temperature in the quasibackscattering geometry. T h e excitation line at 2 = 5 1 4 . 5 nm of an argon ion laser, a U I000 Jobin-Ywm double monochromator, and a cooled S20-PMT coupled to a computerized photoncounting system are used. Electrical resistivity measurements are carried out using the Van der Pauw method, with deposited indium spots as ohmic contacts. --
)
=
~-
Z
f,
b
g3
--
1,0
a
60
80
28 (degree) Fig. 1. X-ray diffraction diagrams of ln,Se, thin films growu at various substrate temperatures: (a) I~:-- 200 °C. l? =2.6; (biT~=3t)0°C.R=2.5:{c',7~=350°C.R=3.5.
)
3. Results and discussion Figure 1 shows the X-ray diffraction patterns of In,Se,. thin films grown under a partial pressure ratio R - P s~// t ' t,~--~ ~ ~ for three different substrate temperatures. T h e influence of the substrate temperature on the crystallinity is clearly observed. Thin films grown at ~1s = 200 °C (Fig. l(a)) present only a broad band situated at an angle of 22 °, which can be attributed to an amorphous phase. Films grown at 7[>_ 300 °C (Figs. l(b) and l(c)) are polycrystalline, and their X-ray spectra show reflection peaks which can be attributed to the typical (00/} reflections of InSe
/10, 11 J- In Fig. l(b). the weak ~{hkl':, reflections observed are assigned to those of In4Se~ [ 12] and polycrystalline indium, whereas general InSe ( h k l ) reflection patterns are also detected. When the diffraction peaks of Fig. l(c) are indexed as a hexagonal lattice, the indices obey a relation -h+k+l=3n, where /1, k and I are the Miller indices of the hexagonal lattice and n is an integer. This shows that InSe has a rhombohedral symmetry and bekmgs to the V-polytype. In contrast, the fl- and e-polytypes also have a hexagonal lattice, but the indices obey the relation h-k=3n and the peaks are always sharp, it is well known that for energetic reasons, both ~,and e-polytypes can be present at the same time in a crystal and should be evidenced by Raman scattering spectroscopy, as considered next. With increasing substrate temperature, we observe three very different Raman spectra. At ;t[ = 200 °C, the MBD film exhibits a spectrum without sharp peaks (Fig. 2(a)). This figure displays a Raman spectrum with broad peaks and shoulders at 35, 72, 118, 168 and 212 cm E.The appearance of these bands in the region of sharp peaks of the crystallized phase is typical of an amorphous phase. T h e same result has been observed in InSe films prepared by flash evaporation [8]. In an amorphous sample, we can consider that the elementary unit cell has infinite dimension and therefore the Brillouin zone is
15 "~
~
T"'-"""-W
T
"•
i 0
100
200
300 4.00 Wavenumber (crn-1)
Fig. 2. Raman spectra in backscatlering geometry of In,So, thin films obtained under the same conditions as in Fig. I.
reduced to zero. Consequently, optical and acoustic phonon branches are completely folded in the Brillouin zone centre, and in a first approximation the Raman spectrum of an amorphous phase reflects the phonon state density of each lnSe optical branch [13j. At higher substrate temperature, because of the formation of polycrystalline or crystalline phases in MBD films, Raman spectra with narrow peaks are recorded. Figure 2(b) confirms the attribution from X-ray patterns and shows a complex Raman spectrum in which we can identify at least two types of In,Se,. composition: lnSe and In4Se ~ are observed simultaneously, in particular, In4Se~ is clearly identified by the Raman peaks at 104, 131 and 150 cm ~, and InSe is normally characterized by its Raman peaks at 42, 115, 178, 199 and 227 cm ~ [14]. With films evaporated at a substrate temperature of 350 °C, the Raman spectrum is simpler (Fig. 2(c)) and only an InSe single phase is identified. In the two last cases (Figs. 2(b) and 2(c)) we also observe a clear Raman peak at 211 cm Exactly at this frequency, in lnSe single crystal and with the same excitation wavelength, we observe only a shoulder in the higher frequency part of the LO (longitudinal optic) mode at 199 cm ~. in order to explain the intensity enhancement of this peak, we make three remarks, as follows.
(1) The LO polar phonon modes of lnSe are only observed in the resonance condition for a laser line excitation which has its energy very close to the second excitonic absorption peak [151. However, even in this fawmrable experimental condition, the LO mode at 211 cm J remains one order of magnitude smaller than the LO mode at 199 cm ~l 16]. (2) if a fi- or e-polytype is assumed, then according to group theoretical analysis the number of Raman active modes differs [17[ because of a larger elementary unit cell ahmg the c axis. In this case, new modes which arc Davydov pairs are weakly shifted (less than 1 cm ~for optical branches) and poorly Raman active I 18]. In addition, only the folded acoustic branches are observed in the phonon spectrum of layered compounds which exhibit an e-polytype. In InSe the rigid layer mode is observed at 17 cm ~. In our experiment, the optical surface quality did not allow this determination. Nevertheless, the appearance of the peak at 211 cm ~ cannot be explained by the effect of polytype formation. (3) The polar phonons of lnSe are split into four branches: two TO (transverse optic) modes and two LO modes. Two of them, FI I (TO) and F~ * (LO), have a wavevector in the layer plane and are only Raman active in a quasi-backscattcring geometry along an axis parallel to the layer plane. In contrast, the other two, F~ ~(TO) and Fj ~ {LO), have a wavevector along the ~ axis and consequently are Raman active in a quasibackscattering geometry along this direction. The mode at 211 cm ~ corresponds to the LO mode with a wavevector in the layer plane [19j, and its presence in the spectrum of an MBD film can indicate some disorder in the microcrystallite orientation. Figures 3 and 4 show respectively the X-ray diffraction diagrams and the Raman spectra of polycrystalline thin films grown at 7~=400 °C under various partial pressure ratios. At a low pressure ratio ( R = 2 . 2 ) the Raman spectrum shows that the thin films have a mixed composition (Fig. 4(a)): we observe the coexistence of lnSe and in4Se ~ phases, the latter being detected by its low frequency modes at 45, 75 and 104 cm i. At moderate values, 2.4_ 5 we observe by both techniques of charac-
16 ---'r
.......... !
C' g
o o //
o
•
,,
/¢"
B
o
_,
/
.__q
,I ca' ,,~ ~_=
m
I
rag-_
0
20
z
~
60 28 (degree)
i
80
g
200
i
300 400 Substrafe femperafureI%)
t-ig. 5. Schematic phase diagram ~>t growth conditions h u ln,Sg, thin films. T h e full and open circles represent the a m o r p h o u s and polycrystalline phases respectivel_,r. T h e dashed line is the b o u n d a r y between InSe ( zx ?and In ~Se ~ c
Fig. 3. X-ray diffraction diagrams of ln,Se, thin films grown at various partial pressure ratios and at a substratc temperature of 400 °C: (a) R = 2.2: (b) R - 2.6: (c) R = 5.7.
lo0
t
200
300 tOO Wavenumber (cm"i)
Fig. 4. Raman spectra in backscattering geometry of ln,Se, thin films obtained under the same conditions as in Fig. 3.
terization the formation of a selenium-rich compound which is identified as lneSe s. Figure 3(c) shows the X-ray diagram of the y-phase of ln2Se~ grown at R = 5.7; this exhibits a strong line
at 2 0 = 28 ° which is attributed to the {0(16} reflection. The Raman spectrum of Fig. 4(c) shows sharp peaks at 22, 26, 54, 85, 151 and 232 cm which are the characteristic modes of 7-1n2Se~ [20]. However, in the intermediate range, 3 . 5 -< R<-5, one can observe the coexistence of lnSe and 7-1n_,Se> and both the X-ray patterns and the Raman scattering measurements exhibit mainly the features of these two compounds. In Fig. 5, the different phases observed in MBD In,Se,, thin films are reported as a functi(m of substrate temperature and partial pressure ratio. We can determine regions for the amorphous and polycrystalline phases depending mainly on the substrate temperature and domains for InSe and 7-1n2Se 3 polycrystalline thin films for 7~ >-300 °C. The dashed line in Fig. 5 represents the limit between indium-rich and seleniumrich growth conditions as defined by ks<,ls~/.l,.- 1, where ,ls~, and J,, are the selenium and indium fluxes respectively and ks~ is the selenium sticking coefficient. For 300-<7s-<400°C the indium sticking coefficient kin is reasonably assumed to be close to 1 because of its lower equilibrium pressure on InSe at this substrate temperature. Because selenium displays a strongly volatile behaviour, the selenium sticking coefficient depends on the presence of indium, which implies that the partial pressure ratio may increase with Ts in order to maintain the condition ks~.Js~,/4., = 1. F~r indium-rich conditions, the polycrystalline thin films are a mixture of lnSc and InaSe 3 phases at low R values (Figs. l(b), 2(b), 3(a) and 4(a)). For selenium-rich conditions,
17
where ksJs~,/Jin> 1, a partial pressure range exists in which lnSe is grown and excess selenium is re-evaporated from the substrate. At 7~ = 350 °C this range is about R = 3-3.5.
4. Conclusions In this paper we have studied thin films grown by molecular beam deposition of the In-Se system. The formation of amorphous and polycrystalline thin films is strongly dependent on the substrate temperature. Using X-ray diffraction and Raman scattering spectroscopy, the characterizatkm of such films has been conducted as a function of the elemental partial pressures during the evaporation. The determination of the composition and structure shows good agreement between the two techniques. H)r 7k<250 °C an amorphous lnSe phase is obtained. As "/k increases from 300 to 400 °C, InSe and 7-1neSe~ polycrystalline thin films are obtained. The observation of the Raman inactive phonon mode at 211 cm ~ in a backscattering geometry along the c axis may be explained by deorientation of the crystallites. Discussion of this last point remains ()pen.
Acknowledgment This work has been partly supported by the Economic European Community under contract ST 2P-0013-1F(CD).
References I M.S. Whittingham, Science, 192 ', 1976) 1126. 2 C. Julien and M. Balkanski, in J. R. Akridgc ;ed.i, Solid State Microbaneries, in NA TO Adv. Study Inst. Ser., Ser. B, Plenum, New York, 1988. 3 K. Kanehori, K. Matsumoto, K. Miyauchi and T. Kudo, Solid State lonics, 9-10 (1983) 1469. 4 M. Balkanski, C. Julien and J. Y. Emery, in R. R. Hearing (ed.), Extended Abstracts, Int. Meeting on Lithium Batteries, Vancomer PB-31, 1988, p. 246. 5 M. Pcrsin, A. Pcrsin and B. Ccslustka, lhin 3olM l-Thn.~, /2 1972; 117, 6 K. Ando and A. Katsui, l/tin Solid I'ihm. 7(~, 19~; 1 14 I. 7 S. Chandhuri, S. K. Biswas and A. Choudhury, Solid Slaw ( o m m u n , , 53 ¢19851273. 8 1. Watanabe and T. Yalnamoto, J/re. J. ,ll)pl. I'hyv. 24 :1985) 1282. 9 C.T. Foxon and B. A. Joycc, SttU: Sci., CN ', 1~,J77) 293. l0 K.C. Nagpal and S. Z. All. ,Iota ()ysta/logr. L 31 I ~)75 ) 567. I 1 S. Popovic, A. Toncjc. B. Grzcta-Plenkovic. B. Ccluslka and P. Trojko, J. Appl. COstallogr., 12 : 1979) 416. 12 J. H. C. [togg, H. H. Sutherland and 1). J. Williams, Acta ('rystallogr. B, 29(1973) 15911. 13 See e . g . M . H . Brodsky. in M. Cardona icd.), Light Scattering in Solid.s, Springer-Verlag, Berlin. 1975, p, 2115. 14 M. Jouanne and C. Julien, Phys. Status SolMi (b), 11989) in the press. 15 T. Ikari. S. Shigetomi and K. Hashimoto, I'hvs. ,~tatu~ Solidi (b), 111 ( 1982 ) 477. 16 M. Jouanne, C. Julien and M. Balkanski, I'hvs. 5tams Solidi (hi. 144 ( 1987; K 147. 7 T..1, Wiefing and J. L, Vcrble. Ph~. Rev. B, 5 ¢1~)721 1473. 8 1.. N. Alicva, G. 1.. Belenkii, 1. I. Rcshina. E. Y. Salacv and V. Y. Shtemshraiber, 5or. I'hvs. 5olid ,~taw, 21 , 1979; 90. 9 N. Kuroda and Y. Nishina, ,golid ,Stll[e (o]llnlllll., 2¢%' !1978)439. 20 K. Kambas. C. Julien. M. Jouanne, A. Likh~.rman and M. Guittard, l'hvv ,Sntms SoIMi (hL 124 i 1984) K 1115.