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1 Physica B 405 (2010) Contents lists available at ScienceDirect Physica B journal homepage: Incorporation of Bi, Cd and Zn on the optical properties of thin films K.A. Aly a,, N. Afify b, A.M. Aboushly a a Physics Department, Faculty of Science, Al-Azhr Univ. (Assiut branch), Assiut, Egypt b Physics Department, Faculty of Science, Assiut Univ., Assiut, Egypt article info Article history: Received 26 September 2009 Received in revised form 26 December 2009 Accepted 6 January 2010 Keywords: Amorphous semiconductors Optical properties Optical constants X-ray diffraction abstract Different compositions of amorphous and Ge 20 Se 75 M 5 (M=Cd, Bi, or Zn) semiconducting films were deposited onto cleaned glass substrates by thermal evaporation method. The interference transmission spectra T(l) at normal incidence for Ge 20 Se 75 M 5 thin films were obtained in the wavelength range nm. The direct analysis proposed by Swanepoel that is based on the use of the extremes of the interference fringes was used in order to derive the film thickness and the two parts of the complex refractive index (real part, n, and imaginary part, k,). The dispersion of n is discussed in terms of the Wemple DiDomenico single-oscillator model. Furthermore, the optical band gap, E g, has been determined from the absorption coefficient values using Tauc s procedure. The obtained results of E g were discussed in terms of the width of localized state E e. & 2010 Elsevier B.V. All rights reserved. 1. Introduction The common feature of chalcogenide 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. Investigation of electron transport in disordered systems has been gradually developed and the investigation of gap states is of particular interest because of their effect on the electrical properties of semiconductors [1]. The effect of an impurity in an amorphous semiconductor may be widely different, depending upon the conduction mechanism and the structure of the material [2]. Recent experiments revealed that, the addition of impurity with a higher atomic number than the elements in the host material will tend to decrease the optical band gap [3,4]. In recent time, the optical properties of thin dielectric films have been the subject of intense study and significant efforts have been made to develop the mathematical formulation describing the transmittance and reflectance of different optical systems [5 7]. Among the existing methods for determining the optical constants, those based exclusively on the optical transmission spectra at normal incidence have been applied to different crystalline and amorphous materials deposited on transparent substrates in the form of thin films [8 12]. In the present work, the effect of incorporation by Cd, Bi, or Zn on the optical properties of thin films was investigated. The straightforward method proposed by Swanepoel [5,9] based on the use of maxima and minima of the interference fringes in Corresponding author. address: kamalaly2001@gmail.com (K.A. Aly). the transmission spectrum was used for the calculation of the refractive index and film thickness in the weakly absorbing and transparent regions of the transmission spectrum. The absorption coefficient and, therefore, the extinction coefficient have been determined from the transmission spectra in the strong absorption region. 2. Experimental details Bulk chalcogenide glassy samples of and Ge 20 Se 75 M 5 (M=Cd, Bi, or Zn) were prepared by the usual melt quench technique. Thin films were deposited by evaporating bulk and Ge 20 Se 75 M 5 glasses onto cleaned glass substrates kept at room temperature and a vacuum of about Torr by using a conventional coating unit (Denton Vacuum DV 502 A). The evaporation rate as well as the film thickness was controlled using a quartz crystal DTM 100 monitor. The mechanical rotation of the substrate holder during the deposition process is about E30 rpm to produce homogeneous film. The amorphous state of the films was checked using X-ray (Philips type 1710 with Cu as a target and Ni as a filter, l= Å) diffractometer. The absence of crystalline peaks confirms the amorphous state of the prepared samples. The elemental compositions of the investigated specimens were checked using the energy-dispersive X-ray (Link Analytical Edx) spectroscopy. The deviations in the elemental compositions of the evaporated films from their initial bulk specimens were found not to exceed 0.50 at%. Without glass substrate in the reference beam, the optical transmission spectra at normal incidence were measured in the /$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi: /j.physb.20160

2 K.A. Aly et al. / Physica B 405 (2010) wavelength range nm by using a double-beam computer-controlled spectrophotometer (Jusco V-630). The obtained transmittance spectra for different compositions of and Ge 20 Se 75 M 5 (M=Cd, Bi, or Zn) thin films were used to apply the envelope method suggested by Manifacier et al. [8] and Swanepoel [5,9], which is based on the extremes of the interference fringes to calculate the optical constants. 3. Results and discussion 3.1. Calculation of the refractive index and film thickness Manifacier et al. [8] suggest an idea for creating two envelopes of the interference maxima and minima of the transmission spectrum (see Fig. 1). The first, approximate value of the refractive index of the film, n 1, in the spectral region of medium and weak absorption can be calculated by using the following expression [4,5,9]: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffi n ¼ N þ N 2 s 2 ð2þ 400 T M, T m T (α) λ (nm) T (λ) T s (λ) Fig. 1. Transmission spectra for and Ge 20 Se 75 M 5 (M=Cd, Bi, or Zn) thin films. The T M, T m and T a curves according to the text, T s is the transmission of the substrate alone. where N ¼ 2s T M T m T M T m þ s2 þ1 2 where T M and T m are the transmission maximum and the corresponding minimum at a certain wavelength l. Alternatively, one of these values is an experimental interference extreme and the other one is derived from the corresponding envelope [4]. On the other hand, the refractive index of the substrate, s, at each wavelength is p derived from T s (l), using the well-known equation, s ¼ð1=T s Þþ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð1=ts 2Þ 1 [13]. The calculated values of the refractive index, n, using Eq. (2) are shown in Table 1. The accuracy of this initial estimation of the refractive index is improved after calculating, d, as will be explained below. Now, it is necessary to take into account the basic equation for the interference fringes 2nd ¼ ml ð4þ where m is the order numbers (m is an integer for maxima and half integer for minima). Moreover, if n c1 and n c2 are the refractive indices at two adjacent maxima (or minima) at l 1 and l 2 respectively, the film thickness, d 1, can be calculated by using the following expression [4,5,9]: l 1 l 2 d 1 ¼ ð5þ 2ðn c2 l 1 n c1 l 2 Þ It should be noted that, owing to the optical absorption, Eqs. (4) and (5) are not valid at the interference maxima and minima, but are valid at the tangent points referred to previously [5]. A set of d 1 is obtained solving Eq. (3) for each pair of consecutive tangent points. The mean d 1 so calculated is used together with the first refractive indices, to determine the order numbers from Eq. (4): m being an integer for an upper tangent point and a half-integer for a lower tangent point. The accuracy of d can now be significantly increased by taking the corresponding exact integer or half integer values of the, m, associated with each extreme as, M, and deriving a new accurate thickness, d 2, by using Eq. (4). The average value of d 2 found in this way has a smaller dispersion (s1 4 s2). It should be emphasized that the accuracy of the final thickness is better than 1% (see Table 1). Again using the values of n 1 with the exact values of the ordering number, M, and the very accurate value of the thickness, d 2, then, Eq. (4) can be solved for n at each l and, thus, the final values of the refractive index, n 2, are obtained in Table 1. Furthermore, a simple complementary graphical method for deriving the values of m and d, based on Eq. (4) was also used. This expression can be rewritten for that purpose as l 2 ¼ 2d n M l 1 ð6þ where l=0, 1, 2, etc., M 1 is the first extreme. Plotting (l/2) against (n/l) yields a straight line with slope 2d and cut off on the vertical axis of M 1. Fig. 2 shows this plot, in which the values obtained for d and M 1 are displayed for each sample. One can note the deduced values of film thicknesses from Fig. 2 as well as the average value of d 2 listed in Table 1 for all thin film samples under study. Fig. 3 illustrates the dependence of n on the wavelength for four different composition thin films. Major effective role for the change in the n values was related to the change in the concentration of added impurity (Cd, Bi and Zn) at the expense of Se content, and the relative error of Dn/n does not exceed the precision of the measurements DT/T (71%). Now, the values of n 2 can be fitted to a reasonable function such as the two-term Cauchy dispersion relationship, n(l)=a+(b/l 2 ), which can be used for extrapolating the whole wavelengths [14] (see Fig. 3). The least squares fit of the two sets of values of n 2 for the different ð3þ

3 1848 K.A. Aly et al. / Physica B 405 (2010) Table 1 Values of l, T s, T M, T m, n 1, d 1, m, M, n 2, d 2 for and Ge 20 Se 75 M 5 (M=Cd, Bi, or Zn) thin films. Sample l T s T M * T m * n 1 d 1 m 0 M d 2 n d1 ¼ 774; d1 ¼ 37 nm ð4:82%þ; d2 ¼ 769; d1 ¼ 6nmð0:76%Þ d 1 ¼ 697; d1 ¼ 19:22 nm ð2:76%þ;d 2 ¼ 688; d1 ¼ 4:24 nm ð0:62%þ d 1 ¼ 710; d1 ¼ 25 nm ð3:5%þ; d 2 ¼ 719; d1 ¼ 3:34 nm ð0:47%þ d1 ¼ 744; d1 ¼ 26 nm ð3:5%þ; d2 ¼ 743; d1 ¼ 4:58 nm ð0:62%þ The underlined values of transmittance are those given in the transmittance spectra of Fig. 2 and the others are calculated by the envelope method. l/ (l/2) = 1546(n/λ) (l/2) = 1376(n/λ) (l/2) = 1436(n/λ) (l/2) = 1486 (n/λ) - 2 Refractive Index (n) n/λ (10 3 nm -1 ) Fig. 2. The plots of l/2 vs. n/l, in order to determine the film thickness and the first-order number m 1 for and Ge 20 Se 75 M 5 (M=Cd, Bi, or Zn) thin films. samples listed in Table 1 yields n=2.5+( /l 2 ), n= ( /l 2 ), n=2.97+( /l 2 ) and n=2.32+( / l 2 ) for and Ge 20 Se 75 M 5 (M=Cd, Bi, or Zn) thin films Wavelength (nm) Fig. 3. Refractive index dispersion spectra for and Ge 20 Se 75 M 5 (M=Cd, Bi, or Zn) thin films. The solid curves were determined according to Cauchy dispersion relationship [16]. The energy dependence of the refractive index for amorphous materials can be fitted to the Wemple and DiDomenico (WDD) dispersion relationship, which is a single-oscillator

4 K.A. Aly et al. / Physica B 405 (2010) model [15] n 2 ðhnþ¼1þ E 0E d E 2 0 ðhnþ2 ð7þ where c is the speed of light and e the electronic charge. The plots of n 2 vs. l 2, are shown in Fig. 5. The obtained values for, e L, and N/m * are listed in Table 2. where E 0 is the single-oscillator energy and E d the dispersion energy. By plotting (n 2 1) 1 vs. (hn) 2 ) and fitting straight lines as shown in Fig. 4, E 0 and E d can be determined from the intercept, E 0 /E d and the slope, (E 0 E d ) 1. E 0 is considered as an average energy gap to a good approximation, it scales with the optical band gap E g, E 0 E2E g [16]. Fig. 4 also shows the values of refractive index extrapolated to hn=0 for four different compositions. The single-oscillator energy E 0 decreases with the addition of Cd or Bi while the addition of Zn leads to the increase of E 0. Both dispersion energy E d and refractive index n(0) have a reverse manner to E 0 and E g. The WDD model is related to the dispersion energy, E d and other physical parameters of material through the following empirical relationship [15], E d ¼ bn c Z a N e ðevþ where, N c is the effective coordination number of the cation nearest neighbors to the anion, Z a the formal chemical valency of the anion, N e the effective number of valence electrons per anion, and b is a two-valued constant with either an ionic or a covalent value (b i =673 ev and b c = ev, respectively). Therefore, in order to account for the compositional trend of E d, it is suggested that, the observed manner of E d is primarily due to the change in the ionicities (homopolar Se Se bonds are introduced together with extra Se atoms). The values of single-oscillator energy, dispersion energy, static refractive index and the excess of Se Se homopolar bonds for the different thin films are listed in Table 2. Furthermore, both the lattice dielectric constant, e L, and the ratio of the carrier concentration to the effective mass, N/m *, can be investigated by using the slope and intercept of the linear relation [17] n 2 ¼ e L ðe 2 =pc 2 ÞðN=m Þl 2 ð8þ 3.2. Determination of the absorption-, extinction-coefficient and optical band gap Since the values of the refractive index are already known using the Cauchy dispersion equation, the absorbance, w a (l), can be calculated from the interference-free transmission curve, T, (see Fig. 1), using the well-known equation, often used in optical studies, proposed by Connell and Lewis [18] X a ¼ P þ½p2 þ2qt a ð1 R 2 R 3 Þ 1 2 Š Q where P ¼ðR 1 1ÞðR 2 1ÞðR 3 1Þ, Q ¼ 2T a ðr 1 R 2 R 1 R 3 2R 1 R 2 R 3 Þ and R 1, R 2, R 3 are the reflectance of the air film, film substrate and substrate air interfaces: R 1 =[(1 n)/(1+n)] 2, R 2 =[(n s)/(n+s)] 2 and R 3 =[(s l)/(s+1)] 2, respectively. For the high absorption region (ar10 5 cm 1 ), the imaginary part of the complex index of refraction is much less than n, so that the previous expressions to calculate the reflectance are valid. In the spectral region with interference fringes, T a is the geometric mean of T M and T m [5,9], i.e. T a =(T M T m ) 1/2 in the region of strong absorption the interference fringes disappear. In other words, for very large, a, the three curves T M, T a and T m converge into a single curve. Moreover, since d is also known, the relation x a =exp( ad) can then be solved and the values of the absorption coefficient, a, can be determined. In order to complete the calculation of the optical constants, the extinction coefficient, k, is estimated using the values of a and l, using the already mentioned formula k=al/4p. Fig. 6 illustrates the dependence of k, on wavelength for different samples of thin films. The optical band gap will be found from the calculated ð9þ (n 2-1) n (hν) 2 (ev) 2 Fig. 4. Plots of refractive index factor (n 2 1) 1 vs. (hn) 2 for and Ge 20 Se 75 M 5 (M=Cd, Bi, or Zn) thin films λ 2 (10 3 nm) 2 Fig. 5. Plots n 2 vs. l 2 for and Ge 20 Se 75 M 5 (M=Cd, Bi, or Zn) thin films. Table 2 The optical band gap, E g, Urbach s energy, E e, Wemplee DiDomenico dispersion parameters, E 0 and E d, E 0 /E g ratio, the values of the refractive index n(0) extrapolated at hn=0, the excess of Se Se homopolar bonds, the lattice dielectric constant, e L and the N/m* ratio for and Ge 20 Se 75 M 5 (M=Cd, Bi, or Zn) thin films. Composition E g (ev) E e (ev) E 0 (ev) E d (ev) E 0 /E g n(0) Excess Se Se n/m*(10 37 /cm 3 ) e L

5 1850 K.A. Aly et al. / Physica B 405 (2010) Extinction coefficient (K) ln (α) (αhν) 1/2 (cm -1 ev) 1/ values of the absorption coefficient a to its end. It should be pointed out that the absorption coefficient of amorphous semiconductors for the higher values of the absorption coefficient (az10 4 cm 1 ) where, the photon energy dependence of the absorption coefficient for the allowed non-direct transitions can be described by ðahnþ 1=2 ¼ B 1=2 ðhn E g Þ λ (nm) Fig. 6. The extinction coefficient k vs. l for and Ge 20 Se 75 M 5 (M=Cd, Bi, or Zn) thin films hν (ev) Fig. 7. The absorption coefficient in the form of (ahn) 1/2 vs. photon energy (hn) for and Ge 20 Se 75 M 5 (M=Cd, Bi, or Zn) thin films, from which the optical band gap (E g ) is estimated (Tauc extrapolation). ð10þ where B is a parameter that depends on the transition probability and E g the optical energy gap [19,20]. Fig. 7 shows the absorption coefficient in the form of ðahnþ 1=2 vs. hn for different compositions of and Ge 20 Se 75 M 5 (M=Cd, Bi, or Zn) thin films. The intercepts of the straight lines with the photon energy axis yield values of the optical band gap, E g. The obtained values of E g are listed in Table 2, and one can note that, both Cd and Bi (atomic no. 48 and 83, respectively) additions at the expense of Se (atomic no. 34) atoms lead to the decrease of E g that is clear in Fig. 1 (shifts of the measured transmittance to long wavelengths i.e. low energies) while the opposite manner takes place if we replace Cd or Bi by Zn atoms (atomic no. 30). 8.0 In the region of low absorption (ao10 4 cm 1 ), the absorption coefficient a shows an exponential dependence on photon energy, hn, and obeys Urbach s relation [21]. lnðaþ¼lnða 0 Þþðhn=E e Þwhere a 0 is a constant and E e Urbach s energy (the width of the band tail of the localized states in the band gap) as shown in Fig. 8. The deduced values of the E e for different compositions are listed in Table 2. According to Nagel et al. [22] as the width of localized state, E e, increases the value of the optical band gap, E g, decreases i.e. both E g and E e interpret each other. In other words both Cd and Bi lead to the increase of E e, (decrease of E g ) due to the formation of Bi Se and Cd Se (E g =1.7 ev [23 25]) at the expense of Se Se (E g =1.95 ev [29]) and Ge Se (E g =1.85 ev [26]) bonds while the addition of Zn leads to formation of Zn Se (E g =2.7 ev [27 29]) that leads to the decrease of, E e (increase of E g ). Also from Table 2 it is evident that, Tauc s model [30] based on the electronic transitions between the localized states in the band edge tails will be valid for such systems. The obtained results for Ge Se Zn thin films can be interpreted in terms of the Preliminary EXAFS data measured by Choi et al. [31] on Ge Se Zn alloys indicating that Zn is four-fold coordinated up to a zinc content of 20%. This result explains why the addition of this metal does not reduce the band gap of the alloys. In the crystalline phase, ZnSe is a wide band gap semiconductor (E g 2.8 ev) with the zinc-blended structure and one can speculate that here there is dealing with a mixed Ge Se/Zn Se semiconductor system. One would expect a reduction in the band gap only if zinc adopted a higher metallic like coordination rather than being tetrahedrally bonded as in the present system. The observed results in the value of the optical energy gap, E g, with the addition of Cd, Bi and Zd content could be explained according to the Mott and Davis model [35], the width of localized states near the mobility edges depends on the degrees of disorder and defects presented in the amorphous structure. In particular, it is known that unsaturated bonds together with some saturated bonds are produced as a result of an insufficient number of atoms deposited in the amorphous films. The unsaturated bonds are responsible for the formation of some defects in the film [32]. Such defects produce localized states in the amorphous solids. 4. Conclusions 1.6 hν (ev) Fig. 8. Plots of ln(a) vs. (hn) for and Ge 20 Se 75 M 5 (M=Cd, Bi, or Zn) thin films. The allowed non-direct electronic transitions are mainly responsible for the photon absorption in the investigated films. Fitting the refractive indices to the single-oscillator model

6 K.A. Aly et al. / Physica B 405 (2010) (Wemple DiDomenico relationship) results in dispersion parameters that are directly related to the structure of the films. It was found that, the addition of Cd or Bi (high atomic number than Se) leads to the decrease of the optical band gap, E g, and the single oscillator energy, E 0, while the dispersion energy, E d, the refractive index, n(0), the N/m * ratio and the lattice dielectric constant, e L, increase. The obtained results for the addition of Zn at the expense of Se atoms lead to a reverse manner to that observed for the addition of Bi or Cd atoms. Acknowledgment The authors would like to acknowledge Ms. Gh. Abbady assistant lecturer in Phys. Dept., Assiut University for her helpful assistance. Prof. W.W. Bishara, Assiut University is acknowledged for language revision of this work. The Deutsche Gessellschaft für Technishche Zusammenarbeit (GTZ) and the Deutsche Akademische Austauschdienst (DAAD), Germany are acknowledged for partly supporting this work by a grant-in-aid (projects no. PN and PN ). References [1] K.A. Aly, J. Non-Cryst. Solids 355 (2009) [2] M. Kastner, Phys. Rev. Lett. 28 (1972) 355. [3] A.A. Othman, K.A. Aly, A.M. Abousehly, Thin Solid Films 515 (7-8) (2007) [4] K.A. Aly, A. Dahshan, A.M. Abousehly, Philos. Mag. 88 (2008) 47. [5] R. Swanepoel, J. Phys. E: Sci. Instrum. 16 (1983) [6] E.R. Shaaban, M. Abdel-Rahman, Y. El Sayed, Thin Solid Films 515 (2007) [7] J.M. Gonzalez-Leal, R. Prieto-Alcon, M. Stuchlik, Opt. Mater. 27 (2004) 147. [8] J.C. Manifacier, J. Gasiot, J.P. Fillard, J. Phys. E: Sci. Instrum. 9 (1976) [9] R. Swanepoel, J. Phys. E: Sci. Instrum. 17 (1984) 896. [10] M. Hamman, M.A. Harith, W.H. Osman, Solid State Commun. 59 (1986) 271. [11] J.A. Kalomiros, J. Spyridelis, Phys. Status Solidi (a) 107 (1988) 633. [12] E. Marquez, J. Ramirez-Malo, P. Villares, J. Phys. D: Appl. Phys. 25 (1992) 535. [13] F.A. Jenkins, H.E. White, Fundamentals of Optics, McGraw-Hill, New York, [14] T.S. Moss, Optical Properties of Semiconductors, Buttenworths, London, [15] S.H. Wemple, M. DiDomenico, Phys. Rev. B 3 (1971) [16] K. Tanaka, Thin Solid Films 66 (1980) 271. [17] G. Kumar, J. Thomas, N. George, B. Kumar, P. Shnan, V. Npoori, C. Vallabhan, N. Unnikrishnan, Phys. Chem. Glasses 41 (2001) 89. [18] G.A.N. Connell, A.J. Lewis, Phys. Status Solidi (b) 60 (1973) 291. [19] E.A. Davis, N.F. Mott, Philos. Mag. 22 (1970) 903. [20] H. Fritzsche, Philos. Mag. B 68 (1993) 561. [21] F. Urbach, Phys. Rev. 92 (1953) [22] P. Nagel, L. Tichy, A. Triska, H. Ticha, J. Non-Cryst. Solids 59/60 (1983) [23] D. Ham, K.K. Mishra, K. Rajeshwar, J. Electrochem. Soc. 138 (1991) 100. [24] Ph. Allongue, R. Tenne, J. Electrochem. Soc. 138 (1991) 261. [25] A.J. Bard, A.B. Bocarsly, F.R.I. Tam, E.G. Waallon, M.S. Wrightor, J. Am. Chem. Soc. 102 (1980) 373. [26] K. Shimakawa, J. Non-Cryst. Solids 43 (1981) 229. [27] J.M. Dona, J. Herrero, J. Electrochem. Soc. 142 (1995) 764. [28] G.H. Schoenmakers, E.P.A.M. Bakkers, J.J. Kelly, J. Electrochem. Soc. 144 (1997) [29] K. Singh, R.K. Pathak, Electrochim. Acta 39 (1994) [30] J. Tauc, in: F. Abeles (Ed.), The Optical Properties of Solids, North Holland, Amsterdam, 1970, p [31] J. Choi, A. Singh, E.A. Davis, S.J. Gurman, J. Non-Cryst. Solids (1996) 680. [32] N.F. Mott, E.A. Davis, Electronic Processes in Non-Cystalline Materials, Clarendon, Oxford, 1971.