Jul 26, 1993 - periodic table. Each selenium or tellurium atom has six valence electrons, two S and four P electrons. Among them two of the P electrons ...
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Solid State Communications, Vol. 89, No. 12, pp. 1013-1016, 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-1098/94 $6.00 + .00
PP e r g a m o n
0038-1098(93)E0050-8 D E P E N D E N C E OF OPTICAL BAND GAP ON THE COMPOSITIONS OF Se(l_x)Tex THIN FILMS H. EI-Zahed, M.A. Khaled, ~ A. EI-Korashy, z S.M. Youssef I and M. El Ocker ~ University College For Art and Education, Physics Department, Ain Shams University, Hellioplis, Cairo, Egypt 1Faculty of Science, AI-Azhar University, Nasr City, Cairo, Egypt 2 Faculty of Science, Physics Department, Assiut University, Assiut, Egypt
(Received 26 July 1993; acceptedfor publication 19 October 1993 by A.A. Maradudin) Thin films of binary system Se( ) _. 1x Tex (x = 0.2, 0.4, 0.5, 0.8) have been prepared by thermal evaporation under vacuum of 10-~Torr. The thickness of the films were about 1000A. The optical gap is determined as a function of composition. It is observed that the width of optical gap lies between 1.8 and 1.06 eV. The optical gap decreases with increasing Te content. The structure of compositions is transformed from a chain-like structure to a trigonal one depending on Te contents. The validity of the Urbach relation has been proved for compositions of 0.2 < x < 0.5. X-ray diffraction patterns show that polycrystalline films start to appear for x = 0.5 and 0.8. The optical constants (n, k) were determined ellipsometrically in the wave length range 4000-5400 A. 1. I N T R O D U C T I O N C H A L C O G E N I D E glasses have received a lot of attention owing to their potential use in various solid state devices. The common feature of these glasses is the presence of localized state in the mobility gap as a result of the absence of long range order as well as various inherent defects. Recently, the investigation of electron transport in disordered systems has gradually been developed and the investigation of gap states is of particular interest because of their effect on the electrical properties of semiconductors [1, 2]. It has been reported that Se-Te [3] alloys have certain advantages over amorphous Se as far as their use in Xerographic photo-receptors is concerned. Both Se and Te belong to the VI b group in the periodic table. Each selenium or tellurium atom has six valence electrons, two S and four P electrons. Among them two of the P electrons contribute to bonding and construct a chain structure (mordenite). The remaining two electrons become lone pairs. Studies on the band gap of the bulk Se-Te system have been done previously [4]. Yamaguchi [5] studied the structural characteristics of both modification trigonal and mordenite Se(l_ x)Tex. The dependence of Eg on the composition
has been predicted theoretically [5], where it was predicted that Eg mainly decreases by increasing Te content. Such behaviour agrees with experimental measurements obtained for the bulk Te-Se system. It was also reported that the dimensionality of a system influences the way in which Eg varies as a function of x [5]. On the other hand, the study of optical absorption and the determination of the optical parameters of thin films of absorbing semiconductors in the visible spectral range was stimulated by the recent interest in solar absorption. However, not much research has yet been conducted, to the best of our knowledge, on the optical properties of Se(t_x)Tex thin films in the visible spectral region. The aim of this communication is to study the dependence of optical gap and optical constants on the compositions of Se(l_x)Tex thin films (x = 0.2, 0.4, 0.5, 0.8). 2. E X P E R I M E N T A L DETAILS A glassy alloy of Se(l_x)Tex were prepared by a quenching technique. Materials (99.999% pure) were weighed according to their atomic percentages and were sealed in a quartz ampoule in a vacuum of about
1013
1014
OPTICAL BAND GAP ON Se(l_x)Tex T H I N FILMS
10-STorr. The sealed ampoule was kept inside a furnace where the temperature was raised to 700°C. The ampoule was frequently rocked for 10h at the maximum temperature to make the melt homogeneous. The quenching was performed in ice water. Thin films of the glassy alloy Se0_x)Tex were prepared by thermal evaporation at 10 -5 Torr using a Leybold coating unit (constructed in Dr Souror's research laboratory) in which the substrates were kept at room temperature (30°C) at a base pressure of about 10 -5 Torr and using a molybdenum boat. Film thicknesses have been determined using multiple beam Fizeau fringes in reflection [6]. The thickness of the films was about 1000A. Investigations of the structure were carried out using an X-ray diffractometer (Philips model PW 1373). The transmittance (T) and reflectance (R) at normal incidence were recorded using a spectrophotometer P M Q I I (Carl Zeiss). The details of the ellipsometric measurements are given in previous paper [7]. The ellipsometer was calibrated by a Si single crystal parallel to the (1 1 1) face. The uncertainty in optical constants does not exceed 0.02. 3. RESULTS A N D DISCUSSION Obtained values of R and T were used to estimate the optical gap. The method suggested by Demichielis et al. [8] has been used to estimate the width of the optical gap. Near the absorption edge the transmittance can be written in a first approximation as T = (1 - R) exp ( - a t ) ,
R)/T]/t = w(g, T)/t, In (1 - R)/T= w(R, T).
a = [ln(1 -
where Figure 1 represents the linear dependence of al/2w(R, T) on the photon energy (E). Extrapolation gives the width of the optical gap Eg. Such plots indicate that the involved absorption mechanism is an indirect optical transition. This is accepted in noncrystalline systems. This is due to the fact that, in
Seo,sTe02 o Soo.6%.~ . s~o Sreo5 SeoI2T%'.8 .
,.z_~ E 1"2 ._oV..
~
oB
~
a.6
I
2
hv Iev)
Fig. 1. The linear dependence of t~l/2w(R, T) on the photon energy (E) of Se(l _x)Tex thin films.
Vol. 89, No. 12
1.2 0-~
J 0.Z
0.2 04 0.6 0.B
Te%
Fig, 2. The dependence of the optical gap on the composition Se(l _x)Tex thin films. such systems, the wave vector is not conserved due to the lack of translation symmetry. The dependence of the optical gap on composition is shown in Fig. 2. Figure 2 reveals that the optical gap is almost independent of composition for rich Se or Te alloys. At composition Te0.sSe0.5 the width of the gap decreases rapidly from 1.16 to 1.06 eV. The obtained data reveals that the optical gap decreases for increasing Te content. In order to understand the observed results one should consider the structural features of the Se-Te system. It has been established that the S e - T e system exhibits two structures. The chain-like behaviour dominates at high Se contents. Such structural modification is characterized by a wide band gap. Moreover the band gap is not sensitive to composition. This can account for the obtained values of 1.3, 1.28eV for the optical gap (Seo.sTeo.2-Seo.6Te0.4). It is worth noting that at composition Se0.sTe0.3 the optical band gap decreases rapidly. This is most likely due to formation of unstable structures at Se0.sTe0.5. To check this assumption, X-ray diffraction patterns were obtained as shown in Fig. 3 for Se0_x)Te x films. The figure shows that a crystalline phases start to be formed at Se0.sTe0.~. Analysis of the observed pattern allow us to conclude that a pseudobinary S e - T e crystalline phase is segregated. The effect of composition on optical and structural features around Se0.sTe0.5 is now under investigation. However, one can assume that the reduction of the optical gap by increasing Te content is most likely due to the increase of trigonal modification. It seems that at higher Te content the ratio between trigonal and chain-like structures become almost constant. The above arguments allow one to conclude that at high Se content the chain-like structure modification is dominant and at low Se content the trigonal structure is dominated. In both cases the optical gap
Vol. 89, No. 12
OPTICAL BAND GAP ON Se(l _x)Tex T H I N FILMS 9
A
(
B 7
¥
=,6 5
1015
/ ,Seo,sTe02
o.6%.L ° S " •
I
I
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48
I
I
0
I
32
I
2.¢
I
i
J_
r
16
i
__
8
Fig. 3. X-ray diffraction pattern for Se 0 _x)Tex thin films. is almost insensitive to composition. On the other hand, for compositions around Seo.sTeo.5 the optical gap is sensitive to composition. The dependence of extinction coefficient (k) and refractive index (n), in 3.2
H o-.~
n k
2.8 2.4 2.C
1.2
Se Te 0.5 0.2
:%
1.6
/ d
08 0.4
\ ~ I [
/o
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2.8-.~ 2.L c
20
~ 8
1.6
°I"o. -o--oz...oJ
"o
O.z, 1'8
I 2
hv{ev)
the visible region as a function of photon energy has been obtained ellipsometrically. Figure 4 shows variation of optical constants (n and k) for Seo.sTeo.2 and Seo.2Te0.s films as a representative example. For non-crystalline systems, which are characterised by an Urbach edge, the width of the band tail can be estimated by the relation a = a0 exp (hw/Ee) , where hw is the photon energy and Ee is the band tail width [9]. In the investigated range (visible region) it is noticed that both n and k exhibit structure, and n > k indicate low absorption. The logarithm of absorption coefficient a = 47rk/A as a function of photon energy for the three amorphous compositions are shown in Fig. 5. All samples except Se0.sTeo.5 and Se0./Te0.s follow the same pattern where the Urbach edge is observed. The former behaviour is characteristic of a noncrystalline system, while the latter case represents a crystalline system, which agrees with the X-ray diffraction. The observed Urbach behaviour has been used to estimate the width of the band tail (Ee) (see Table 1). The table reveals that the band tail width increases for Se, Seo.sTeo.2 and Se0.6Te0.4, for increasing Te content. It is worth noting that the two compositions Seo.sTe0.5 and Se0.zTeo.s shows no Urbach behaviour which confirm the crystallinity of films of these two compositions.
Table 1. Variation of the width of the band tail (Ee) with composition
1.2, 0-8
S,o.:%.s
Fig. 5. The logarithm of the absorption coefficient (In a) vs the photon energy (h#).
Seo.6Teo./,
I
s%.5Te0 , 5
2'2 2'4 2'6 2'.8 J 3:2
hvlev)
Fig. 4. Variation of both the extinction coefficient (dashed line) and the refractive index (solid line) of Seo.sTeo.2 and Seo.2Teo.s thin films.
Te (%)
Ee
Teo.2 Teo.4 Teo.5 Teo.6 Teo.8
0.025 0.053 0.273 0.050 0.30
1016
OPTICAL BAND GAP ON Se(l_x)Tex THIN FILMS REFERENCES
1. 2. 3. 4.
S.K.M. Dehaldhar & S.P. Sengupta, Ind. J. Pure Appl.. Phys. 17, 422 (1979). W. Beyer, H. Mell and Stuke, J. Phys. Status Solidi (b) 45, 153 (1971). L. Cheung, G.M.T. Foley Fournia & B.E. Springett, Photogr. Sci. Eng. (USA) 26, 245 (1982). K. Shimakawa, J. Noncryst. Solids 43, 229 (1981).
5. 6. 7. 8. 9.
Vol. 89, No. 12
T. Yamaguchi & F. Yonezawa, J. Phy. Soc. Jpn 61, 1240 (1992). O. Tolansky, Introduction to Interferometry, p. 157, Longmans Green, London (1955). M.M. E1 Iker, F.A. Sultan, S.A. Yousef and S.A. E1 Shhar, Phys. Status Solidi (a) 83, 263 (1984). F. Demichels, G. Kanidakis, A. Tagliaferro & Tresso, Appl. Optics 26, 1737 (1987). F. Urbach, Phys. Rev. 92, 1324 (1953).
