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Preparation and characterization of In4Se3 films
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ELSEVIER
Materials Science and Engineering B27 (1994) 53-60
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Preparation and characterization of In4Se 3 films C. Julien, A. Khelfa, N. Benramdane*, J.P. G u e s d o n Laboratoire de Physique des Solides, associ~ au CNRS, Universit~ Pierre et Marie Curie, 4place Jussieu, 75252 Paris Cedex 05, France Received 25 April 1994; in revised form 31 May 1994
Abstract Thin films of In4Se3 were obtained by vacuum evaporation of polycrystalline materials onto substrates at different temperatures Ts. Systematic X-ray analysis was carded out and revealed that the flash evaporation technique can produce stoichiometric films for depositing polycrystalline materials of composition In1.11Se0.89. Optical and electrical characterizations are reported. The energy gap of InaSe 3 films is located between 1.41 and 1.48 eV depending on the substrate temperature. A.c. and d.c. conductivities were measured as a function of Ts. Amorphous films exhibit a T -1/4 dependence of the conductivity which fits well with the Mott model with a density of localized states N(EF)---8.5 x 1018 cm -3 eV- 1. The effect of annealing temperature was also investigated. Hall measurements as a function of temperature show that the predominant conduction mechanism is scattering by grain boundaries in polycrystalline In4Se3 films.
Keywords:Lrl4Se3 films; Thin films; Flash evaporation; Amorphous materials
1. Introduction Increasing interest has been shown recently in the growth of polycrystalline thin films of layered semiconducting compounds of the binary In-Se system, e.g. InSe and In2Se3, because of their applicability in semiconductor technology [1, 2], electrochemical and photovoltaic cells [3-7] and switching devices [8]. The main difficulty encountered in the preparation of these compounds in a single evaporation is the coexistence of several kinds of compounds with different stoichiometries [9]. Also, InSe is thermally less stable than In2Se3. Several authors have studied amorphous and/ or polycrystalline InSe films in which the presence of a small amount of In4Sea has been reported [10-12]. This phase frequently coexists with InSe owing to loss of the more volatile selenium during film growth. It has been reported recently that films with adequate stoichiometry can be prepared by flash evaporation of presynthesized compound [13]. The crystallinity and morphology of films have been studied as a function of the composition of starting material from which reproducible polycrystalline InSe films are formed.
Tetraindium triselenide (In4Se3) is an n-type semiconductor which crystallizes in the orthorhombic structure, space group Pnnm, with unit-cell dimensions a = 1 5 . 2 9 7 A , b = 1 2 . 3 0 8 A and c = 4 . 0 8 1 A . The material is composed of endless interlocking chains running parallel to the c-axis, consisting of fivemembered indium-selenium rings, the chains being cross-linked by strongly bound indium-indiumindium units to form a continuous sheet lying perpendicular to the a-axis [14]. A few works have been devoted to the characterization of In4Se 3 crystals [15-17] but as far as we know there is no study concerning the growth of InaSe j thin films in the literature. In this work, we studied In4Se3 films grown USing a flash evaporation technique and the optimum conditions for the preparation of polycrystalline films were determined. Furthermore, the optical and electrical properties of amorphous and polycrystalline In4Se3 films are reported. Effects of the post-deposition heat treatment on the crystallinity were also investigated.
*Present address: Institute of Electronics, Djillali Liabes University, 22000 Sidi-Bel-Abbes, Algeria. 0921-5107/94/$7.00 © 1994 - Elsevier Science S.A. All fights reserved
SSD10921-5107(94)01111-T
2. Experimentaldetails In4Se 3 films were made on Pyrex glass using a flash evaporation technique. The source materials for films were polycrystaUine materials In2-xSex with composi-
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Materials Science and Engineering B27 (1994) 53-60
tion range 0.89 < x<0.94. The presynthesized powder was prepared by direct fusion of elements in atomic proportions in a vacuum-sealed quartz tube heated up to a temperature of 1200 K for 24 h. The ampoule was cooled down slowly to room temperature and the procedure was repeated two times to improve the homogeneity of the material. Films of dimensions 15 x 10mm 2 were evaporated using tantalum masks and a molybdenum boat on glass substrates maintained at different temperatures ranging from 300 to 680 K in a vacuum of below 10 -3 Pa. For the flash evaporation technique the source materials were powdered and evaporated from a home-made system described elsewhere [18], in which the controlled boat temperature was about 1500 K. The evaporation speed was about 25 nm s- 1. The thickness of evaporated films was in the range 0.3-0.9 # m and was controlled by a quartz crystal monitor which was calibrated using a Tensor profilometer. X-ray diffraction patterns were recorded using a Philips PX1820 diffractometer equipped with a Cu K a source ( 2 = 0 . 1 5 4 0 6 nm). Raman spectra were recorded at room temperature in the quasi-backscattering geometry. The excitation line at 2 = 514.5 nm of an argon ion laser, a U1000 Jobin-Yvon double monochromator, and a cooled S20-PMT coupled to a computerized photon-counting system were used. Optical absorption spectra were measured using a grating monochromator with an attached S 1-PMT. The output signal was detected by a lock-in amplifier. Electrical resistivity measurements, Hall measurements and a.c. measurements were carried out using a five-probe technique. Evaporated indium contact films with indium-pasted leads provided ohmic contacts to the films. The thin film electrical resistance was measured as a function of temperature ranging from 77 to 300 K in an SMC-TBT cryostat using a computerized home-made apparatus. The a.c. electrical conductivity was determind by a bridge technique, recording the real part of the equivalent parallel conductance given by an impedance analyzer H P 4 1 9 2 A in the range 5 H z - 13 MHz.
ticular, InnSe 3 is clearly identified when the partial pressure ratio varies from 2 to 2.7. Using various experimental characterizations, i.e. X-ray diffraction, Raman scattering and IR absorption measurements, the composition and structure of the flash-deposited films were determined as follows. Fig. 1. shows the X-ray diffraction patterns of In4Se 3 thin films grown at various substrate temperatures (curves a-d) compared with the spectrum of crystalline InnSe 3 (curve e). The influence of the substrate temperature on crystallinity is clearly observed. Thin films grown at a substrate temperature below 440 K show only a broad band situated at an angle of 22 ° which is due to the amorphous substrate. Films grown at Ts > 440 K (Fig. 1, curves b-d) are polycrystalline, and their X-ray spectra are dominated by peaks located at 29 ° and 43 ° which are indexed (040) and (002) respectively. The diffraction peaks of Fig. 1 (curve d) are indexed using an orthorhombic lattice [14] for which the indices obey the relation h + k + I = 2n, where h, k and l are the Miller indices of the lattice and n is an integer. Analysis of the X-ray diffraction spectrum of polycrystalline In4Se 3 films is reported in Table 1. All the d(hkl) distances of the X-ray diffraction spectrum are in good accordance with those reported for In4Se 3 single crystal by Hoog et al. [14]. One observes an intensity attenuation of some peaks, such as the (002) line for example, which is attributed to the disordered structure of polycrystalline films.
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3. Results and discussion b
3.1. Growth conditions and structural characterization I
The formation of amorphous and polycrystalline indium selenide films growth by flash evaporation is quite similar to that found by molecular beam deposition (MBD). It has been demonstrated that a relationship exists between the substrate temperature, the partial pressure ratio p=P(Se)/P(In) in the MBD chamber, and the composition of the film [9]. In par-
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30 ~0 50 60 Anqte 2Q (degrees)
70
Fig. 1. X-ray diffraction diagrams of flash-evaporated In4Se3 thin films (a-d) compared with the spectrum of crystalline InaSe3 (e). Thin films were obtained at different substrate temperatures: (a) 300, (b) 440, (c) 490 and (d) 520 K.
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Materials Science and Engineering B27 (1994) 53-60
The influence of the stoichiometry of the starting material on the film structure is shown in Fig. 2. The (004) line is stronger for films grown from polycrystalline flash material Inl.lSe0. 9 than for films obtained from evaporation of Inl.06Se0.94. It has been reported [19] that the variation in peak intensity for In4Se a is due to the large density of faults. This behavior has also been observed by Likforman and Etienne [20]. Figs. 3(a) and 3(b) show X-ray Debye-Scherrer (XRDS) patterns of flash-evaporated In4Se3 thin films grown from In1.1~Se0.89 powder on substrates heated to 440 and 520 K respectively. In4Se 3 film grown at Ts = 440 K exhibits broad rings in the XRDS pattern (Fig. 3(a)). This film is rather strongly structured and the morphology is of powder type. The XRDS pattern of a film deposited at Ts = 520 K shows well defined rings (Fig. 3(b)) which indicate that the films are polycrystalline in nature when the substrate temperature is raised up to 500 K and long-range periodic arrangement of atoms is achieved. These results reveal a textured In4Se 3 film in which the crystallites show a
55
preferred orientation of their lattice c-axis perpendicular to the substrate surface. Polycrystalline films result in XRDS patterns with numerous more or less sharp concentric rings satisfying Bragg's equation. In addition well defined individual spots attributed to (011) and (420) reflections are observed in Fig. 3(b). The value of b-axis lattice parameter can be deduced from this diagram with a reasonable degree of accuracy. It is found that b = 12.280 A for In4Se 3 film grown at Ts = 520 K. With regard to crystal texture, these results show that the b-axis is parallel to the substrate plane. Fig. 4 shows the Raman scattering spectrum of In4Se 3 film grown at Ts=440 K (curve b). Raman active-modes of In4Se3 single crystal occur at 41, 74, 103, 150, 164, 173 and 188 cm -1 (curve a). The vibrational spectrum of In4Sea originates from interlayer and deformation vibrations of the In-Se and Se-Se lattice bonds, but also from interlayer modes which appear at lower frequencies [15]. The energy positions of the band attributed to the In-Se bonds are situated around 173 cm -1. The spectrum of the film displays three broad bands located at 75, 105 and 150
Table 1 Crystallographic data of In4Se3 thin films deposited at Ts = 520 K Thin film data, this work
Crystal data [14] d(hkl) (A)
1/Io (%)
Indiceshkl
d(hkl) (A) l/I o (%)
3.87 3.18 3.07 2.79 2.70 2.42 2.04 1.91
39 2 64 26 20 6 100 48
(011) (301) (040) (430) (231) (141) (002) (800)
3.861 3.186 3.068 2.773 2.709 2.423 2.046 1.905
12 11 100 24 6 8 21 4
b =
,
I
I
10
20
I
30 40 50 60 Angle 2e (degree)
70
Fig. 2. X-ray diffraction diagrams of In.Se 3 thin films grown from different In2- xSex flash materials: (a) x = 0.94 and (b) x = 0.90.
Fig. 3. X-ray Debye-Scherrer patterns of flash-evaporated In4Se3 thin films obtained from Inl.11Se0.89powder deposited at (a) Ts=440 K and (b) Ts= 520 K.
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MaterialsScience and Engineering B27 (1994) 53-60 105
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100 150 200 Frequencq (cmq)
.
250
Fig~ 4. Raman scattering spectra of (a) In4Se3 single crystal and Ts= 440 K.
( b ) I n a S e 3 thin film deposited at
cm- 1. The appearance of these bands in the region of sharp peaks of the single crystal identifies clearly In4Se 3 films in the polycrystalline state (p-In4Se 3). In p-In4Se3, because of the disorder induced by dangling bonds, over-coordination, stacking faults, etc., the elementary unit cell has a large dimension and optical phonon branches are folded in the Brillouin zone center. Thus the Raman spectrum of p-In4Se 3 reflects the phonon state density of each optical branch [21]. We note that at an appropriate deposition rate no amorphous selenium is formed. Such formation is revealed in the Raman spectrum by a high peak at 243 cm- 1 which is characteristic of chains in trigonal Se. 3.2 Optical gap studies After investigations of the morphology and composition of thin films it is apparent that in most cases the degree of disorder and defects present in the material is a function of the substrate temperature. The optical absorption coefficient a - - 6 -1 log (Io/I) of asdeposited In4Sea films of thickness 6 is measured over the spectral range 0.8-2.0 eV at room temperature. Fig. 5 shows the variation in absorption coefficient as a function of phonon energy for In4Se 3 films deposited at Ts = 300 K (curve a) and for film heat treated at 520 K for 40 h. The curve for the as-deposited film on the substrate at low temperature indicates tailing of the absorption coefficient towards the lower photon energies and the roughly exponential tail which is a characteristic of amorphous semiconductors [22]. Such a tail occurs when the density of states near the band edge depends exponentially on energy. For films annealed at T,~= 520 K for 40 h, one observes sharpen-
10
12
1.4
16
18
20
Photon energ4 laY)
Fig. 5. Absorption coefficient as a function of incident photon energy of flash evaporated In4Se3 thin films: (a) amorphous film grown at Ts=300 K and (b) polycrystalline In¢Se3 thin film obtained after annealing at Ta= 520 K for 40 h.
ing of the absorption edge and a shift towards lower energies. Variation of the optical absorption near the fundamental absorption edge has allowed us to determine the optical energy gap. The intercepts of (ahv)2 vs. hv curves extrapolated at a = 0 are taken as the value of optical gap Eg°pt. It is found that Eg°Dr= 1.41 eV for as-deposited films. The optical energy gap increases in the heat-treated film. For polycrystalline In4Se 3 films obtained after thermal annealing at 520 K, an energy gap of 1.48 eV is deduced. This value can be compared with 1.64 eV which is the direct gap energy of single-crystal In4Se3 [23]. The presence of small crystallites in the annealed film is also revealed by the large decrease in tailing of the absorption coefficient of the polycrystalline film. These results are consistent with X-ray diffraction and Raman scattering measurements. Fig. 6 shows the dependence on substrate temperature of the optical energy gap of asdeposited In4Se 3 films. It can be observed that the energy gap slowly increases on heating the substrate up to 600 K. This behavior is attributed to the decrease in the number of defects of structural bonding due to the increase in substrate temperature. This effect is known to result in an increase in optical gap. At Ts = 520 K, the film attains the polycrystaUine state. At temperatures higher than 600 K an abrupt decrease in Eg°pt is observed which is attributed to a change in film stoichiometry. 3.3. Electrical properties In this section, we present results of experimental studies concerning the d.c. and a.c. conductivities to obtain additional information on the nature of the
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Materials Science and Engineering B27 (1994) 53-60
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conduction mechanism in InaSe3 films. Electrical measurements were taken on amorphous (a-In4Se3) and polycrystalline films which exhibit n-type conductivity. Electrical conduction can take place by two parallel processes: (i) by band conduction and (ii) by hopping conduction in the localized states. The former tends to occur at high temperatures, where carders excited beyond the mobility edges into non-localized states dominate the transport, while the latter may be due to carriers excited into localized states at band edges [24], that is,
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6 8 10 IO00/T (K -1)
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Fig. 7. Arrhenius plots of the electrical conductivity of flash evaporated In4Se 3 thin films as a function of the substrate temperature. The inset shows the dependence on substrate temperature of the thermal activation energy in the intrinsic regime.
100
(1)
where ai is the intrinsic conductivity and Oh the hopping conductivity. The intrinsic conductivity is of Arrhenius type expressed as
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where y is a temperature coefficient and Ea is the thermal activation energy for conductivity. A plot of in tri vs. 1/T gives the thermal activation energy E a. Fig. 7 shows the Arrhenius plot of conductivity of In4Se3 films as a function of the electrical conduction. The intrinsic conduction gives the thermal activation energy E~ according to Eq. (2); the values as a function of substrate temperature are shown in the inset. In Fig. 7 the conductivity is shown to be on the order of 10- 5_10-1 S cm- 1 in the range of temperature between 300 and 77 K for a film grown at T, = 440 K. The hopping conduction is observed at low temperature in the range below 160 K. Replots of the conductivity curves in Fig. 7 lead to Mott's relation for thermally assisted hopping conduction [24] (7h
----
172
Z -1/2 exp ( - B I T 1/4)
(3)
where (72 and B are constants. This relation holds quite well at low temperatures as shown in Fig. 8. A marked increase in conductivity over two orders of magnitude is observed when the substrate temperature increases from 400 to 520 K. There have been few investigations
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Fig. 8. Dependence of oT 1/2 on T 1/4 for In4Se 3 thin film deposited at differentsubstrate temperatures.
on the low temperature regime of conduction in indium selenide films to date. Wood et al. [25] observed variable-range hopping-type conduction for amorphous InxSel_ x films with x>0.46. Watanabe and Sekiya [26] studied the T-]/4 dependence of conductivity for amorphous In2Se3 films. The temperature dependence of conductivity in amorphous In4Se 3 films grown at T, = 300 K was investigated. Measurements show that the conductivity is on the order of 10-5-10-2 S cm-1 in the temperature range 300-430 K. Experimental plots of l n ( o T ~/2) against T-1/4 give a straight line. The conductivity is lowered by a few orders of magnitude, and the slope is increased (with B = 1.5 x 102 K 4) in comparison with
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MaterialsScience and Engineering B27 (1994) 53-60
data obtained from films grown on heated substrate. These results suggest that the density of localized states is strongly dependent on the substrate temperature in In4Se3 films. The data of the above measurements are listed in Table 2. It can be seen from these observations that in the films formed at high deposition temperatures the carrier mobility increases with increasing temperature. The carriers show appreciable mobility in the ordered films. The Hall mobility reaches a value of 65 cm 2 V- 1 s- 1 in in4Se3 films grown at Ts = 520 K. In polycrystalline semiconductors the transport of carriers is driven by scattering mechanisms at intercrystallite boundaries rather than by intracrystallite characteristics. Moreover, for compound materials the grain boundaries can be regions of non-stoichiometry which influence the transport properties. By combining the grain-boundary trapping model with a thermoionic-emission mechanism through created potential barriers, Seto [27] has developed a comprehensive theory of transport phenomena in polycrystalline materials which explains most of their electrical properties. The electrical transport properties of indium selenlde films are governed by carrier trapping at the grain boundaries [5]. To simplify the model we assume that polycrystalline In4Se 3 films are composed of identical crystallites of grain size L. We also assume that there is only one type of impurity atom present and that the impurity atoms are totally ionized and uniformly distributed with a concentration N o. The traps are assumed to be neutral initially and become charged by trapping a carrier. Using the above assumptions, an abrupt depletion approximation is used to calculate the energy band diagram in the crystallites. In this approximation all the mobile carriers in a region of width W from the grain boundary are trapped by the trappping states, resulting in a depletion zone. A logarithmic plot of/z BT 1/2 against the reciprocal temperature shows that the mobility/z B follows approximately an exponential law represented by #B = eL( 2 z m * kB T )-1/2 exp(CPB/kB T)
(4)
where ¢PB is the potential barrier height at a grain boundary, m* is the effective mass of a carder, e is the electron charge, and k B is the Boltzmann constant. This
law indicates that charge carriers in In4Se 3 films are scattered by a potential barrier associated with the intergrain boundaries as proposed by Petritz [28]. Fig. 9 shows the variations in In (#B T1/2) for films grown at various substrate temperatures. Using the model of potential barriers for film grown at Ts = 440 K, one obtains (I)B=35 meV, N D = I . S X l 0 1 7 cm -3 and L = 23 nm. The grain size of In4Se 3 obtained from the theoretical Petritz mobility is comparable with the depletion zone width which increases with increasing substrate temperature resulting in a decrease in the potential barrier height. Fig. 10 shows the dependence on frequency of the a.c. conductivity of amorphous In4Se 3 thin films deposited at 350 K as a function of temperature in the range 223-300 K. As one can see, the conductivity increases rather linearly with frequency. Amorphous semiconductors and other disordered systems exhibit an a.c. electrical conductivity o(w) whose frequency dependence can be approximated by o~w)=Aw s
(5)
where s ~ 1 in a wide frequency range. Table 3 reports the values of the power factor obtained from the data in Fig. 10. Many authors have studied this phenomenon, adopting the pair approximation introduced for the first time by Pollak and Geballe [29]. The charge is alternatively exchanged between a pair of states, located at a distance R having a very low probability to interact with other states. The above results can be explained assuming that conduction occurs by phonon-assisted hopping between localized states near the Fermi level E F. According to the Austin and Mott theory [30], the conductivity is given by o ( w ) = ( z / 3 ) e2ka TNF2a-Sw[ln ( vp/W )]4
(6)
where N F is the density of states at EF, Vp is a phonon frequency, and a describes the decay of the localized state wave function. According to Eq. (6), from the plot of or(to)[ln(l,'p/ 09]4 vs. frequency we obtained NF=8.5 X l0 is cm -3 eV-l assuming that a = 0.3 nm-1 taking Vp= 173 cm-1 (5 x 1012 Hz) for amorphous In4Se3 films at room tern-
Table 2 Electrical parameters of In4Se3 thin films as a function of substrate temperature Ts (K)
E a (eV)
ao (S cm- 1)
N (cm- 3)
/aB(cm2 V-1 s- 1)
B (K4)
440 490 520
0.225 0.180 0.164
1314 568 18
4.2 × 1016 6.0 × 1016 2.0 × 1017
23 55 65
46.5 38.3 38.0
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Materials Science and Engineering B27 (1994) 53-60
Table 3 Determination of the power factor s for amorphous In4Se3 thin films calculated using Eq. (5)
,
102
lO~
I
I
I
I
t
I
I
I
I
2
4 6 8 10 IO00/T (K-~I Fig. 9. Temperature dependence of ,uBT 1/2 of polycrystalline In4Se3 thin film deposited at different substrate temperatures: (a) 440, (b) 490, and (c) 520 K.
10-4
I
I
i
~._ 10"5
lO"6
... 10-8 103
= = = 104 105 106 Frequencq (Hz)
59
107
Fig. 10. Frequency dependence of the electrical conductivity of amorphous In4Se3 thill film deposited at Ts = 350 K as a function of temperature; (a) 223, (b) 255, (c) 277, and (d) 300 K.
perature. This value is in good agreement with those for other In-Se films reported in the literature [31-33].
4. Conclusion
The formation of amorphous and polycrystalline In4Se 3 films grown by flash evaporation was investi-
gated. Using various experimental characterizations, i.e. X-ray diffraction, Raman scattering, IR absorption and electrical measurements, properties of the flashdeposited films were determined. The influence of the substrate temperature on crystallirtity is clearly observed. Thin films grown at a substrate temperature of below 440 K are amorphous whereas films grown at T~> 440 K have a polycrystallirte structure. Their X-ray spectrum is dominated by peaks located at 29* and 43* which are indexed (040) and
Temperature(K)
s
223 255 277 300
1.18 1.09 0.90 0.84
(002) respectively. The influence of the stoichiometry of the starting material on the films structure showed that the (004) line is stronger for films grown from polycrystalline flash material Inx.11Se0.89, than for films obtained from evaporation of Inl.06Se0.94 owing to the large density of faults. X-ray Debye-Scherrer patterns indicate that films are textured with crystallites showing a preferred orientation of the lattice c-axis perpendicular to the substrate surface. Determination of the b-axis lattice parameter gives b = 12.280/k for InaSea film grown at Ts--- 520 K The Raman scattering spectrum of InaSe 3 film grown at Ts = 440 K shows that this film is in the polycrystalline state (p-In4Se3). The optical energy gap is deduced from absorption coefficient measurements. It is found that Eg°pt= 1.41 eV for as-deposited films. The optical energy gap increases in the heat-treated film. For polycrystalline In, Se 3 films obtained after thermal annealing at 520 K, an energy gap of 1.48 eV is deduced. At temperatures higher than 600 K an abrupt decrease in eg°pt is observed which is attributed to a change in film stoichiometry. Electrical measurements were taken of amorphous (a-In, Se3) and polycrystalline films which exhibit n-type conductivity. Electrical properties were studied in the two regimes of conduction by band conduction and by hopping conduction in the localized states. The intrinsic conductivity is Arrhenius type with a thermal activation energy E a in the range 0.160-0.225 eV which is a function of substrate temperature. The Hall mobility reaches a value of 65 cm 2 V-1 s-1 in in4Se3 films grown at Ts=520 K. The electrical transport properties of In4Se 3 films are governed by carrier trapping at the grain boundaries. Using the model of potential barriers for film grown at Ts = 440 K, one obtains ~ B = 3 5 meV, N D = I . 5 × 1 0 1 7 cm -3 and L -- 23 nm. The hopping conduction is observed at low temperature in the range below 160 K which exhibits a Z -1/4 dependence of the conductivity. Amorphous InaSe 3 films exhibit an a.c. electrical conductivity o(to) for which the frequency dependence can be approximated by o(to)---Ato ' where s--- 1 in a wide frequency range. Assuming that conduction occurs by phononassisted hopping between localized states near the
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Materials Science and Engineering B27 (1994) 53-60
Fermi level Ev, we obtained Nv---8.5 × 1018 cm -3 eW-1 which is in g o o d agreement with values for other I n - S e films reported in the literature.
References [1] M. Balkanski, Appl. Surf. Sci., 33-34 (1988) 1260. [2] C. Jufien, M. Eddrief, K. Kambas and M. Balkanski, Thin Solid Films, 137(1986) 27. [3] T. Matsushita, T. Nang, M. Okuda, A. Suzuki and S. Yokota, Jpn. J. Appl. Phys., 15(1976) 901. [4] T. Nang, T. Matsushita, M. Okuda and A. Suzuki, Jpn. J. Appl. Phys., 16 (1977)253. [5] J. E Guesdon, B. Kobbi, C. Julien and M. Balkanski, Phys. Status Solidi A, 102 ( 1987) 327. [6] I. Samaras, C. Julien and M. Balkanski, in J. R. Akridge and M. Balkanski (eds.), Solid State Microbatteries, Plenum, New York, 1990, p. 293. [7] C. Julien, I. Samaras, M. Tsakiri, E Dzwonkowski and M. Balkanski, Mater. Sci. Eng., B3 (1989) 25. [8] M. A. Kenawy, A. E E1-Shazly, M. A. Afili, H. A. Zayed and H. A. EI-Zahid, Thin Solid Films, 200 (1991 ) 203. [9] J.Y. Emery, L. Brahim-Otsmane, M. Jouanne, C. Julien and M. Balkanski, Mater. Sci. Eng., B3 (1989) 13. [10] M. Yudasaka and K. Nakanishi, Thin Solid Films, 156 (1988) 145. [11] J. P. Guesdon, C. Julien, M. Balkanski and A. Ch~vy, Phys. Status Solidi A, 101 (1987) 495. [12] B. Thomas and T. R. N. Kutty, Phys. Status Solidi A, 191 (1990) 127. [13] C. Jufien, N. Benramdane and J. P. Guesdon, Semicond. Sci. Technol., 5 (1990) 905. [14] J. H. C. Hoog, H. H. Sutherland and D. J. Williams, Acta Crystallogra. B, 29(1973) 1590.
[15] V. P. Zakharov, V. P. Savchin, I. M. Stakhira and G. E Sheremet, Sov. Phys. Solid State, 23(1981) 1101. [16] V.P. Savehin, Sov. Phys. Semicond., 15 (1981 ) 827. [17] C. Julien, M. Eddrief, M. Balkanski and A. Chrvy, Phys. Rev. B, 46(1992) 2435. [18] C. Julien, A. Khelfa, N. Benramdane, J. E Guesdon, E Dzwonkowski, I. Samaras and M. Balkanski, Mater. Sci. Eng., B23(1994) 105. [19] J. H. C. Hoog and W. J. Duffin, Acta Crystallogr. B, 23 (1967) 111. [20] A Likforman and J. Etienne, C. R. Acad. Sci. (Paris) Ser. C, 275 (1972) 1097. [21] M. H. Brodsky, in M. Cardona (ed.), Light Scattering in Solids, Springer, Berlin, 1985, p. 205. [22] H. P. D. Lanyon, Phys. Rev., 130(1963) 134. [23] A. Khelfa, PhD Thesis, Universit6 E et M. Curie, Paris, 1992, unpublished. [24] N. E Mott and E. A. Davis, Electronic Processes in NonCrystalline Materials, Clarendon, Oxford, 1971, Chapter 7. [25] C. Wood, L. R. Gilbert, C.M. Garner and J. C. Shaffer, in J. Stuke and W. Brenig (eds.), Proc. 5th Int. Conf. on Amorphous and Liquid Semiconductors, Taylor and Francis, London, 1974, p. 285. [26] I. Watanabe and T. Sekiya, J. Non-Cryst. Solids, 97-98 (1987)667. [27] J.Y.W. Seto, J. Appl. Phys., 46 (1975) 5247. [28] R.L. Petritz, Phys. Rev., 110(1958) 1254. [29] M. Pollak and T. H. Geballe, Phys. Rev., 122 (1961) 1742. [30] I.G. Austin and N. E Mott, Adv. Phys., 18 (1969) 41. [31] H. DiGiulo, R. Rella and A. Tepore, Phys. Status SolidA, 100(1987) K35. [32] D. V. Krishna-Sastry and E Jayarama-Reddy, Thin Solid Films, 105 (1983) 139. [33] H. Naito, M. Okuda, T. Matsushita and T. Nakau, Jpn. J. Appl. Phys., 19(1980) L513.
i" , I
ELSEVIER
Materials Science and Engineering B27 (1994) 53-60
B
Preparation and characterization of In4Se 3 films C. Julien, A. Khelfa, N. Benramdane*, J.P. G u e s d o n Laboratoire de Physique des Solides, associ~ au CNRS, Universit~ Pierre et Marie Curie, 4place Jussieu, 75252 Paris Cedex 05, France Received 25 April 1994; in revised form 31 May 1994
Abstract Thin films of In4Se3 were obtained by vacuum evaporation of polycrystalline materials onto substrates at different temperatures Ts. Systematic X-ray analysis was carded out and revealed that the flash evaporation technique can produce stoichiometric films for depositing polycrystalline materials of composition In1.11Se0.89. Optical and electrical characterizations are reported. The energy gap of InaSe 3 films is located between 1.41 and 1.48 eV depending on the substrate temperature. A.c. and d.c. conductivities were measured as a function of Ts. Amorphous films exhibit a T -1/4 dependence of the conductivity which fits well with the Mott model with a density of localized states N(EF)---8.5 x 1018 cm -3 eV- 1. The effect of annealing temperature was also investigated. Hall measurements as a function of temperature show that the predominant conduction mechanism is scattering by grain boundaries in polycrystalline In4Se3 films.
Keywords:Lrl4Se3 films; Thin films; Flash evaporation; Amorphous materials
1. Introduction Increasing interest has been shown recently in the growth of polycrystalline thin films of layered semiconducting compounds of the binary In-Se system, e.g. InSe and In2Se3, because of their applicability in semiconductor technology [1, 2], electrochemical and photovoltaic cells [3-7] and switching devices [8]. The main difficulty encountered in the preparation of these compounds in a single evaporation is the coexistence of several kinds of compounds with different stoichiometries [9]. Also, InSe is thermally less stable than In2Se3. Several authors have studied amorphous and/ or polycrystalline InSe films in which the presence of a small amount of In4Sea has been reported [10-12]. This phase frequently coexists with InSe owing to loss of the more volatile selenium during film growth. It has been reported recently that films with adequate stoichiometry can be prepared by flash evaporation of presynthesized compound [13]. The crystallinity and morphology of films have been studied as a function of the composition of starting material from which reproducible polycrystalline InSe films are formed.
Tetraindium triselenide (In4Se3) is an n-type semiconductor which crystallizes in the orthorhombic structure, space group Pnnm, with unit-cell dimensions a = 1 5 . 2 9 7 A , b = 1 2 . 3 0 8 A and c = 4 . 0 8 1 A . The material is composed of endless interlocking chains running parallel to the c-axis, consisting of fivemembered indium-selenium rings, the chains being cross-linked by strongly bound indium-indiumindium units to form a continuous sheet lying perpendicular to the a-axis [14]. A few works have been devoted to the characterization of In4Se 3 crystals [15-17] but as far as we know there is no study concerning the growth of InaSe j thin films in the literature. In this work, we studied In4Se3 films grown USing a flash evaporation technique and the optimum conditions for the preparation of polycrystalline films were determined. Furthermore, the optical and electrical properties of amorphous and polycrystalline In4Se3 films are reported. Effects of the post-deposition heat treatment on the crystallinity were also investigated.
*Present address: Institute of Electronics, Djillali Liabes University, 22000 Sidi-Bel-Abbes, Algeria. 0921-5107/94/$7.00 © 1994 - Elsevier Science S.A. All fights reserved
SSD10921-5107(94)01111-T
2. Experimentaldetails In4Se 3 films were made on Pyrex glass using a flash evaporation technique. The source materials for films were polycrystaUine materials In2-xSex with composi-
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Materials Science and Engineering B27 (1994) 53-60
tion range 0.89 < x<0.94. The presynthesized powder was prepared by direct fusion of elements in atomic proportions in a vacuum-sealed quartz tube heated up to a temperature of 1200 K for 24 h. The ampoule was cooled down slowly to room temperature and the procedure was repeated two times to improve the homogeneity of the material. Films of dimensions 15 x 10mm 2 were evaporated using tantalum masks and a molybdenum boat on glass substrates maintained at different temperatures ranging from 300 to 680 K in a vacuum of below 10 -3 Pa. For the flash evaporation technique the source materials were powdered and evaporated from a home-made system described elsewhere [18], in which the controlled boat temperature was about 1500 K. The evaporation speed was about 25 nm s- 1. The thickness of evaporated films was in the range 0.3-0.9 # m and was controlled by a quartz crystal monitor which was calibrated using a Tensor profilometer. X-ray diffraction patterns were recorded using a Philips PX1820 diffractometer equipped with a Cu K a source ( 2 = 0 . 1 5 4 0 6 nm). Raman spectra were recorded at room temperature in the quasi-backscattering geometry. The excitation line at 2 = 514.5 nm of an argon ion laser, a U1000 Jobin-Yvon double monochromator, and a cooled S20-PMT coupled to a computerized photon-counting system were used. Optical absorption spectra were measured using a grating monochromator with an attached S 1-PMT. The output signal was detected by a lock-in amplifier. Electrical resistivity measurements, Hall measurements and a.c. measurements were carried out using a five-probe technique. Evaporated indium contact films with indium-pasted leads provided ohmic contacts to the films. The thin film electrical resistance was measured as a function of temperature ranging from 77 to 300 K in an SMC-TBT cryostat using a computerized home-made apparatus. The a.c. electrical conductivity was determind by a bridge technique, recording the real part of the equivalent parallel conductance given by an impedance analyzer H P 4 1 9 2 A in the range 5 H z - 13 MHz.
ticular, InnSe 3 is clearly identified when the partial pressure ratio varies from 2 to 2.7. Using various experimental characterizations, i.e. X-ray diffraction, Raman scattering and IR absorption measurements, the composition and structure of the flash-deposited films were determined as follows. Fig. 1. shows the X-ray diffraction patterns of In4Se 3 thin films grown at various substrate temperatures (curves a-d) compared with the spectrum of crystalline InnSe 3 (curve e). The influence of the substrate temperature on crystallinity is clearly observed. Thin films grown at a substrate temperature below 440 K show only a broad band situated at an angle of 22 ° which is due to the amorphous substrate. Films grown at Ts > 440 K (Fig. 1, curves b-d) are polycrystalline, and their X-ray spectra are dominated by peaks located at 29 ° and 43 ° which are indexed (040) and (002) respectively. The diffraction peaks of Fig. 1 (curve d) are indexed using an orthorhombic lattice [14] for which the indices obey the relation h + k + I = 2n, where h, k and l are the Miller indices of the lattice and n is an integer. Analysis of the X-ray diffraction spectrum of polycrystalline In4Se 3 films is reported in Table 1. All the d(hkl) distances of the X-ray diffraction spectrum are in good accordance with those reported for In4Se 3 single crystal by Hoog et al. [14]. One observes an intensity attenuation of some peaks, such as the (002) line for example, which is attributed to the disordered structure of polycrystalline films.
i
i
i
c-ln4Se 3
e '
i
|
i
d
.c:_
3. Results and discussion b
3.1. Growth conditions and structural characterization I
The formation of amorphous and polycrystalline indium selenide films growth by flash evaporation is quite similar to that found by molecular beam deposition (MBD). It has been demonstrated that a relationship exists between the substrate temperature, the partial pressure ratio p=P(Se)/P(In) in the MBD chamber, and the composition of the film [9]. In par-
i
i
i
i
i a
I0
20
30 ~0 50 60 Anqte 2Q (degrees)
70
Fig. 1. X-ray diffraction diagrams of flash-evaporated In4Se3 thin films (a-d) compared with the spectrum of crystalline InaSe3 (e). Thin films were obtained at different substrate temperatures: (a) 300, (b) 440, (c) 490 and (d) 520 K.
C. Julien et al.
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Materials Science and Engineering B27 (1994) 53-60
The influence of the stoichiometry of the starting material on the film structure is shown in Fig. 2. The (004) line is stronger for films grown from polycrystalline flash material Inl.lSe0. 9 than for films obtained from evaporation of Inl.06Se0.94. It has been reported [19] that the variation in peak intensity for In4Se a is due to the large density of faults. This behavior has also been observed by Likforman and Etienne [20]. Figs. 3(a) and 3(b) show X-ray Debye-Scherrer (XRDS) patterns of flash-evaporated In4Se3 thin films grown from In1.1~Se0.89 powder on substrates heated to 440 and 520 K respectively. In4Se 3 film grown at Ts = 440 K exhibits broad rings in the XRDS pattern (Fig. 3(a)). This film is rather strongly structured and the morphology is of powder type. The XRDS pattern of a film deposited at Ts = 520 K shows well defined rings (Fig. 3(b)) which indicate that the films are polycrystalline in nature when the substrate temperature is raised up to 500 K and long-range periodic arrangement of atoms is achieved. These results reveal a textured In4Se 3 film in which the crystallites show a
55
preferred orientation of their lattice c-axis perpendicular to the substrate surface. Polycrystalline films result in XRDS patterns with numerous more or less sharp concentric rings satisfying Bragg's equation. In addition well defined individual spots attributed to (011) and (420) reflections are observed in Fig. 3(b). The value of b-axis lattice parameter can be deduced from this diagram with a reasonable degree of accuracy. It is found that b = 12.280 A for In4Se 3 film grown at Ts = 520 K. With regard to crystal texture, these results show that the b-axis is parallel to the substrate plane. Fig. 4 shows the Raman scattering spectrum of In4Se 3 film grown at Ts=440 K (curve b). Raman active-modes of In4Se3 single crystal occur at 41, 74, 103, 150, 164, 173 and 188 cm -1 (curve a). The vibrational spectrum of In4Sea originates from interlayer and deformation vibrations of the In-Se and Se-Se lattice bonds, but also from interlayer modes which appear at lower frequencies [15]. The energy positions of the band attributed to the In-Se bonds are situated around 173 cm -1. The spectrum of the film displays three broad bands located at 75, 105 and 150
Table 1 Crystallographic data of In4Se3 thin films deposited at Ts = 520 K Thin film data, this work
Crystal data [14] d(hkl) (A)
1/Io (%)
Indiceshkl
d(hkl) (A) l/I o (%)
3.87 3.18 3.07 2.79 2.70 2.42 2.04 1.91
39 2 64 26 20 6 100 48
(011) (301) (040) (430) (231) (141) (002) (800)
3.861 3.186 3.068 2.773 2.709 2.423 2.046 1.905
12 11 100 24 6 8 21 4
b =
,
I
I
10
20
I
30 40 50 60 Angle 2e (degree)
70
Fig. 2. X-ray diffraction diagrams of In.Se 3 thin films grown from different In2- xSex flash materials: (a) x = 0.94 and (b) x = 0.90.
Fig. 3. X-ray Debye-Scherrer patterns of flash-evaporated In4Se3 thin films obtained from Inl.11Se0.89powder deposited at (a) Ts=440 K and (b) Ts= 520 K.
56
C Julien et aL /
MaterialsScience and Engineering B27 (1994) 53-60 105
b
,
. . . . .
,
,
,
,
,
,o " " •
. " ,.....-"
x5 •a ~
"b
°
°
N ~
•
•
~,aq,j e ~ O ~
10 ,
0
50
100 150 200 Frequencq (cmq)
.
250
Fig~ 4. Raman scattering spectra of (a) In4Se3 single crystal and Ts= 440 K.
( b ) I n a S e 3 thin film deposited at
cm- 1. The appearance of these bands in the region of sharp peaks of the single crystal identifies clearly In4Se 3 films in the polycrystalline state (p-In4Se 3). In p-In4Se3, because of the disorder induced by dangling bonds, over-coordination, stacking faults, etc., the elementary unit cell has a large dimension and optical phonon branches are folded in the Brillouin zone center. Thus the Raman spectrum of p-In4Se 3 reflects the phonon state density of each optical branch [21]. We note that at an appropriate deposition rate no amorphous selenium is formed. Such formation is revealed in the Raman spectrum by a high peak at 243 cm- 1 which is characteristic of chains in trigonal Se. 3.2 Optical gap studies After investigations of the morphology and composition of thin films it is apparent that in most cases the degree of disorder and defects present in the material is a function of the substrate temperature. The optical absorption coefficient a - - 6 -1 log (Io/I) of asdeposited In4Sea films of thickness 6 is measured over the spectral range 0.8-2.0 eV at room temperature. Fig. 5 shows the variation in absorption coefficient as a function of phonon energy for In4Se 3 films deposited at Ts = 300 K (curve a) and for film heat treated at 520 K for 40 h. The curve for the as-deposited film on the substrate at low temperature indicates tailing of the absorption coefficient towards the lower photon energies and the roughly exponential tail which is a characteristic of amorphous semiconductors [22]. Such a tail occurs when the density of states near the band edge depends exponentially on energy. For films annealed at T,~= 520 K for 40 h, one observes sharpen-
10
12
1.4
16
18
20
Photon energ4 laY)
Fig. 5. Absorption coefficient as a function of incident photon energy of flash evaporated In4Se3 thin films: (a) amorphous film grown at Ts=300 K and (b) polycrystalline In¢Se3 thin film obtained after annealing at Ta= 520 K for 40 h.
ing of the absorption edge and a shift towards lower energies. Variation of the optical absorption near the fundamental absorption edge has allowed us to determine the optical energy gap. The intercepts of (ahv)2 vs. hv curves extrapolated at a = 0 are taken as the value of optical gap Eg°pt. It is found that Eg°Dr= 1.41 eV for as-deposited films. The optical energy gap increases in the heat-treated film. For polycrystalline In4Se 3 films obtained after thermal annealing at 520 K, an energy gap of 1.48 eV is deduced. This value can be compared with 1.64 eV which is the direct gap energy of single-crystal In4Se3 [23]. The presence of small crystallites in the annealed film is also revealed by the large decrease in tailing of the absorption coefficient of the polycrystalline film. These results are consistent with X-ray diffraction and Raman scattering measurements. Fig. 6 shows the dependence on substrate temperature of the optical energy gap of asdeposited In4Se 3 films. It can be observed that the energy gap slowly increases on heating the substrate up to 600 K. This behavior is attributed to the decrease in the number of defects of structural bonding due to the increase in substrate temperature. This effect is known to result in an increase in optical gap. At Ts = 520 K, the film attains the polycrystaUine state. At temperatures higher than 600 K an abrupt decrease in Eg°pt is observed which is attributed to a change in film stoichiometry. 3.3. Electrical properties In this section, we present results of experimental studies concerning the d.c. and a.c. conductivities to obtain additional information on the nature of the
C. Julien et al. 1.6
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Materials Science and Engineering B27 (1994) 53-60
101 "--qm)'~1~'~ 025
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conduction mechanism in InaSe3 films. Electrical measurements were taken on amorphous (a-In4Se3) and polycrystalline films which exhibit n-type conductivity. Electrical conduction can take place by two parallel processes: (i) by band conduction and (ii) by hopping conduction in the localized states. The former tends to occur at high temperatures, where carders excited beyond the mobility edges into non-localized states dominate the transport, while the latter may be due to carriers excited into localized states at band edges [24], that is,
•m•
10-5 2
(2)
490K
•
440K
6 8 10 IO00/T (K -1)
4
12
14
Fig. 7. Arrhenius plots of the electrical conductivity of flash evaporated In4Se 3 thin films as a function of the substrate temperature. The inset shows the dependence on substrate temperature of the thermal activation energy in the intrinsic regime.
100
(1)
where ai is the intrinsic conductivity and Oh the hopping conductivity. The intrinsic conductivity is of Arrhenius type expressed as
•
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10"4
Ea/k a T )
520K
"0
._>
.+g
0 U
oi = Oo e x p ( 7 / 2 k B ) e x p ( -
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Ts(K}
° o o
•
~ 10-2
Fig.6. Variation in the optical energy gap Eg°pt of In4Se 3 thin films as a function of substrate temperature.
a=ai + oh
450
400
• •
t.t} 1.2
57
,
i
i
,
I
i
,
,
,
10-1 "T
8 10-2 b 10- 3 K
where y is a temperature coefficient and Ea is the thermal activation energy for conductivity. A plot of in tri vs. 1/T gives the thermal activation energy E a. Fig. 7 shows the Arrhenius plot of conductivity of In4Se3 films as a function of the electrical conduction. The intrinsic conduction gives the thermal activation energy E~ according to Eq. (2); the values as a function of substrate temperature are shown in the inset. In Fig. 7 the conductivity is shown to be on the order of 10- 5_10-1 S cm- 1 in the range of temperature between 300 and 77 K for a film grown at T, = 440 K. The hopping conduction is observed at low temperature in the range below 160 K. Replots of the conductivity curves in Fig. 7 lead to Mott's relation for thermally assisted hopping conduction [24] (7h
----
172
Z -1/2 exp ( - B I T 1/4)
(3)
where (72 and B are constants. This relation holds quite well at low temperatures as shown in Fig. 8. A marked increase in conductivity over two orders of magnitude is observed when the substrate temperature increases from 400 to 520 K. There have been few investigations
10-4
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,
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0.30 T-1/4 (K-1/41
0.35
Fig. 8. Dependence of oT 1/2 on T 1/4 for In4Se 3 thin film deposited at differentsubstrate temperatures.
on the low temperature regime of conduction in indium selenide films to date. Wood et al. [25] observed variable-range hopping-type conduction for amorphous InxSel_ x films with x>0.46. Watanabe and Sekiya [26] studied the T-]/4 dependence of conductivity for amorphous In2Se3 films. The temperature dependence of conductivity in amorphous In4Se 3 films grown at T, = 300 K was investigated. Measurements show that the conductivity is on the order of 10-5-10-2 S cm-1 in the temperature range 300-430 K. Experimental plots of l n ( o T ~/2) against T-1/4 give a straight line. The conductivity is lowered by a few orders of magnitude, and the slope is increased (with B = 1.5 x 102 K 4) in comparison with
58
C Julien et aL /
MaterialsScience and Engineering B27 (1994) 53-60
data obtained from films grown on heated substrate. These results suggest that the density of localized states is strongly dependent on the substrate temperature in In4Se3 films. The data of the above measurements are listed in Table 2. It can be seen from these observations that in the films formed at high deposition temperatures the carrier mobility increases with increasing temperature. The carriers show appreciable mobility in the ordered films. The Hall mobility reaches a value of 65 cm 2 V- 1 s- 1 in in4Se3 films grown at Ts = 520 K. In polycrystalline semiconductors the transport of carriers is driven by scattering mechanisms at intercrystallite boundaries rather than by intracrystallite characteristics. Moreover, for compound materials the grain boundaries can be regions of non-stoichiometry which influence the transport properties. By combining the grain-boundary trapping model with a thermoionic-emission mechanism through created potential barriers, Seto [27] has developed a comprehensive theory of transport phenomena in polycrystalline materials which explains most of their electrical properties. The electrical transport properties of indium selenlde films are governed by carrier trapping at the grain boundaries [5]. To simplify the model we assume that polycrystalline In4Se 3 films are composed of identical crystallites of grain size L. We also assume that there is only one type of impurity atom present and that the impurity atoms are totally ionized and uniformly distributed with a concentration N o. The traps are assumed to be neutral initially and become charged by trapping a carrier. Using the above assumptions, an abrupt depletion approximation is used to calculate the energy band diagram in the crystallites. In this approximation all the mobile carriers in a region of width W from the grain boundary are trapped by the trappping states, resulting in a depletion zone. A logarithmic plot of/z BT 1/2 against the reciprocal temperature shows that the mobility/z B follows approximately an exponential law represented by #B = eL( 2 z m * kB T )-1/2 exp(CPB/kB T)
(4)
where ¢PB is the potential barrier height at a grain boundary, m* is the effective mass of a carder, e is the electron charge, and k B is the Boltzmann constant. This
law indicates that charge carriers in In4Se 3 films are scattered by a potential barrier associated with the intergrain boundaries as proposed by Petritz [28]. Fig. 9 shows the variations in In (#B T1/2) for films grown at various substrate temperatures. Using the model of potential barriers for film grown at Ts = 440 K, one obtains (I)B=35 meV, N D = I . S X l 0 1 7 cm -3 and L = 23 nm. The grain size of In4Se 3 obtained from the theoretical Petritz mobility is comparable with the depletion zone width which increases with increasing substrate temperature resulting in a decrease in the potential barrier height. Fig. 10 shows the dependence on frequency of the a.c. conductivity of amorphous In4Se 3 thin films deposited at 350 K as a function of temperature in the range 223-300 K. As one can see, the conductivity increases rather linearly with frequency. Amorphous semiconductors and other disordered systems exhibit an a.c. electrical conductivity o(w) whose frequency dependence can be approximated by o~w)=Aw s
(5)
where s ~ 1 in a wide frequency range. Table 3 reports the values of the power factor obtained from the data in Fig. 10. Many authors have studied this phenomenon, adopting the pair approximation introduced for the first time by Pollak and Geballe [29]. The charge is alternatively exchanged between a pair of states, located at a distance R having a very low probability to interact with other states. The above results can be explained assuming that conduction occurs by phonon-assisted hopping between localized states near the Fermi level E F. According to the Austin and Mott theory [30], the conductivity is given by o ( w ) = ( z / 3 ) e2ka TNF2a-Sw[ln ( vp/W )]4
(6)
where N F is the density of states at EF, Vp is a phonon frequency, and a describes the decay of the localized state wave function. According to Eq. (6), from the plot of or(to)[ln(l,'p/ 09]4 vs. frequency we obtained NF=8.5 X l0 is cm -3 eV-l assuming that a = 0.3 nm-1 taking Vp= 173 cm-1 (5 x 1012 Hz) for amorphous In4Se3 films at room tern-
Table 2 Electrical parameters of In4Se3 thin films as a function of substrate temperature Ts (K)
E a (eV)
ao (S cm- 1)
N (cm- 3)
/aB(cm2 V-1 s- 1)
B (K4)
440 490 520
0.225 0.180 0.164
1314 568 18
4.2 × 1016 6.0 × 1016 2.0 × 1017
23 55 65
46.5 38.3 38.0
C Julien et aL ,
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Materials Science and Engineering B27 (1994) 53-60
Table 3 Determination of the power factor s for amorphous In4Se3 thin films calculated using Eq. (5)
,
102
lO~
I
I
I
I
t
I
I
I
I
2
4 6 8 10 IO00/T (K-~I Fig. 9. Temperature dependence of ,uBT 1/2 of polycrystalline In4Se3 thin film deposited at different substrate temperatures: (a) 440, (b) 490, and (c) 520 K.
10-4
I
I
i
~._ 10"5
lO"6
... 10-8 103
= = = 104 105 106 Frequencq (Hz)
59
107
Fig. 10. Frequency dependence of the electrical conductivity of amorphous In4Se3 thill film deposited at Ts = 350 K as a function of temperature; (a) 223, (b) 255, (c) 277, and (d) 300 K.
perature. This value is in good agreement with those for other In-Se films reported in the literature [31-33].
4. Conclusion
The formation of amorphous and polycrystalline In4Se 3 films grown by flash evaporation was investi-
gated. Using various experimental characterizations, i.e. X-ray diffraction, Raman scattering, IR absorption and electrical measurements, properties of the flashdeposited films were determined. The influence of the substrate temperature on crystallirtity is clearly observed. Thin films grown at a substrate temperature of below 440 K are amorphous whereas films grown at T~> 440 K have a polycrystallirte structure. Their X-ray spectrum is dominated by peaks located at 29* and 43* which are indexed (040) and
Temperature(K)
s
223 255 277 300
1.18 1.09 0.90 0.84
(002) respectively. The influence of the stoichiometry of the starting material on the films structure showed that the (004) line is stronger for films grown from polycrystalline flash material Inx.11Se0.89, than for films obtained from evaporation of Inl.06Se0.94 owing to the large density of faults. X-ray Debye-Scherrer patterns indicate that films are textured with crystallites showing a preferred orientation of the lattice c-axis perpendicular to the substrate surface. Determination of the b-axis lattice parameter gives b = 12.280/k for InaSea film grown at Ts--- 520 K The Raman scattering spectrum of InaSe 3 film grown at Ts = 440 K shows that this film is in the polycrystalline state (p-In4Se3). The optical energy gap is deduced from absorption coefficient measurements. It is found that Eg°pt= 1.41 eV for as-deposited films. The optical energy gap increases in the heat-treated film. For polycrystalline In, Se 3 films obtained after thermal annealing at 520 K, an energy gap of 1.48 eV is deduced. At temperatures higher than 600 K an abrupt decrease in eg°pt is observed which is attributed to a change in film stoichiometry. Electrical measurements were taken of amorphous (a-In, Se3) and polycrystalline films which exhibit n-type conductivity. Electrical properties were studied in the two regimes of conduction by band conduction and by hopping conduction in the localized states. The intrinsic conductivity is Arrhenius type with a thermal activation energy E a in the range 0.160-0.225 eV which is a function of substrate temperature. The Hall mobility reaches a value of 65 cm 2 V-1 s-1 in in4Se3 films grown at Ts=520 K. The electrical transport properties of In4Se 3 films are governed by carrier trapping at the grain boundaries. Using the model of potential barriers for film grown at Ts = 440 K, one obtains ~ B = 3 5 meV, N D = I . 5 × 1 0 1 7 cm -3 and L -- 23 nm. The hopping conduction is observed at low temperature in the range below 160 K which exhibits a Z -1/4 dependence of the conductivity. Amorphous InaSe 3 films exhibit an a.c. electrical conductivity o(to) for which the frequency dependence can be approximated by o(to)---Ato ' where s--- 1 in a wide frequency range. Assuming that conduction occurs by phononassisted hopping between localized states near the
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Materials Science and Engineering B27 (1994) 53-60
Fermi level Ev, we obtained Nv---8.5 × 1018 cm -3 eW-1 which is in g o o d agreement with values for other I n - S e films reported in the literature.
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