Dispersive optical constants of a-Se100−xSbx films

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Optics & Laser Technology 36 (2004) 35 – 38 www.elsevier.com/locate/optlastec

Dispersive optical constants of a-Se100−x Sbx (lms El-Sayed M. Farag∗ Faculty of Engineering, Basic Science of Engineering Department, Minouya University, Shebin El-Koom, Egypt Received 23 December 2002; received in revised form 9 June 2003; accepted 17 June 2003

Abstract Thin (lms of amorphous Se100−x Sbx (x = 5; 10 and 20 at%) system are deposited on a silicon substrate at room temperature (300 K) by thermal evaporation technique. The optical constant such as refractive index (n) has been determined by a method based on the envelope curves of the optical transmission spectrum at normal incidence by a Swanpoel method. The oscillator energy (Eo ), dispersion energy (Ed ) and other parameters have been determined by the Wemple–DiDomenico method. The absorption coe:cient () has been determined from the re;ectivity and transmitivity spectrum in the range 300 –2500 nm. The optical-absorption data indicate that the absorption mechanism is a non-direct transition. We found that the optical band gap, Egopt , decreases from 1:66 ± 0:01 to 1:35 ± 0:01 eV with increase Sb content. ? 2003 Elsevier Ltd. All rights reserved. Keywords: Amorphous; Se100−x Sbx ; Thin (lm; Optical property

1. Introduction Due to their technological importance, the optical properties of chalcogenide (lms have been the subject of numerous studies. The spectral measurements have been carried out from ultraviolet to near infrared wavelengths [1,2]. The optical absorption constant in the neighborhood of the principle interband absorption edge has been determined by transmission measurements. Both bulk and thin (lm measurements have been combined on a single plot, suggesting that bulk and thin (lm materials diCer greatly in average chemical composition [3]. Chemical disorder could occur because during deposition the surface mobility of the atoms is su:ciently high to allow random, as opposed to ordered bonding. Alternatively, the bonding in the (lm may re;ect the molecular structure of the vapor species [4]. The investigation of the shape and position of the absorption edge as a function of temperature, pressure, site disorder, and alloying is of considerable interest for amorphous semiconductors. Experimental and theoretical investigation of the absorption edge of amorphous semiconductors led to the distinction between two kinds of the optical transitions, direct and indirect transition, between the valence and conduction bands. The direct transition is one in which conservation of energy and momentum are met only by ∗

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considering the incident photon energy and the initial and (nal energy levels of the excited electron [5]. In an indirect transition from valence to conduction band, conservation of wave vector can be achieved only if a phonon is absorbed or emitted in the process of photon absorption. Accordingly, the extrema of conduction and valence bands lie at diCerent values of wave vector of the electrons. It is obvious that these indirect transitions can occur only if there is a breakdown in the selection rule that conserves the wave vector of the electrons. Amorphous chacogenide materials exhibit a wide variety of physical and chemical changes when illuminated by band gap light, the changes may sometimes be reversible on annealing at the glass transition temperature [6]. The application of Sb containing glasses is very limited because of their tendency to crystallize. The stoichiometric composition Sb40 Se60 or antimony triselenide Sb2 Se3 easily crystallizes [7]. The phase diagram for Sbx Se100−x shows immiscibility, but the range x ¡ 40 in this system has an in;ection in the curve of liquidus temperature, which indicates a hidden miscibility gap and the possibility to prepare Sb-containing glasses by fast quenching liquid samples or by deposition of vitreous Sb–Se (lms [8]. The optical transmission and re;ection spectra of the (lms were recorded (at normal incidence) with an ultraviolet/visible/near infrared spectrophotometer. It should be noted that the transmission spectra of the doped thin-(lm samples were those corresponding to homogeneous isotropic weakly-absorbing layers, with uniform thickness, it is also important to mention

E.-S.M. Farag / Optics & Laser Technology 36 (2004) 35 – 38 2.5

1/2 -1 1/2

(αhv)

2. Experimental procedure

2.1 1.9 1.7 1.5 1.3 1.1 0.9 0.7 0.5 1.2

1.4

1.6

1.8 hv (eV)

2

2.2

2.4

opt

Fig. 1. Determination of the optical band gap, Eg , in terms of the Tauc law. 1.8

1.7

opt.

(eV)

1.6

1.5

Eg

The glasses of the Se100−x Sbx system, where x = 5, 10 and 20 at% Sb, were prepared from the pure elements. Antimony (Sb) (Alfa-Aesar, Puratronic, 99.999%) and selenium (Se) (Noranda Advanced materials, photoconductor grade 99.999%) were weighed (5 gm) by an automatic electrical sensitive balance of accuracy 0:0001 gm and placed in precleaned silica ampoles. These were evacuated to a pressure of 10−4 Pa for 30 mins and then sealed. The sealed ampoles were inserted in a furnace held at 1173 K for 10 h. Following heating, the ampoles were quickly immersed in ice water quenching down to a temperature of 0◦ C. Thin (lms of the system were deposited by a thermal evaporation technique onto silicon substrates at room temperature (300 K) using a high-vacuum coating unit (Edwards E 306 A). The working vacuum was kept at about ∼ 2 × 10−4 Pa during deposition. The rate of the (lm deposition was approximately 6 nm=sec. The (lm thickness (t = 1:16 m) was determined using quartz crystal thickness monitor (FTMS) and also interferometrically. The thin (lms structure was amorphous as checked by X-ray diCraction patterns. The transmittance (T ) and re;ectance (R) of a-Se100−x Sbx (lms at normal incidence of light in wavelength range 300 –2500 nm were measured using a double beam spectrophotometer (Cary 2390, Varian) with a re;ection stage [10].

x=5 x =10 x = 20

2.3

3

that in the transparent region, the sum of transmission and re;ection spectra was 100% within experimental error, for all photodoped samples [9]. In the present study we have analyzed the eCect of Sb content on the optical properties of a-Se100−x Sbx binary chalcogenide (lms.

x 10 (m eV)

36

1.4

1.3

1.2

0

2

4

6

8

10

12

14

16

18

20

Sb% Fig. 2. Tauc gap versus Sb content in the a-Se100−x Sbx (lms.

using the following formula [12]: T = (1 − R)2 e−t ;

3. Results and discussion The eCects of the formation of excitation states are most indirectly observed in the optical properties of the amorphous semiconductor in the vicinity of its various absorption edges. These transitions do not conserve the wave vector of the electrons. For amorphous semiconductors the optical absorption obeys the relationship [11] (h ) = B(h − Egopt )2 ;

(1)

where B is a constant and Egopt can be used to de(ne an optical energy gap, although it may represent an extrapolated rather than a real zero in the density of states. The optical transmission and re;ection spectra of a-Se100−x Sbx (lms were recorded at room temperature in the spectral range 300 –2500 nm. The absorption coe:cient was calculated

(2)

where  is the absorption coe:cient and t is the thickness of the (lm. The optical energy gap was obtained from the plot of (h )1=2 versus h as shown in Fig. 1. According to Eq. (1), the intersection of the extrapolated linear portions of the curves shown in Fig. 1 on the h axis give the values of optical energy gap Egopt . The value of optical energy gap of pure Se (i.e. x = 0) is 1:77 eV [11]. The values of the optical energy gap were plotted in Fig. 2 as a function of the Sb content (x). It is shown that the optical energy gap decreases linearly with increasing Sb content, due to an increase in Sb–Se bonds with increasing Sb content, leading to the narrowing of the optical energy gap. Antimony-based chalcogenide glasses are less stable because of the in;uence of the larger atomic radius. The incorporation of Sb into Se chains for concentrations more than 1 at% Sb is a process which partially destroys long chains. There are two diCerent

E.-S.M. Farag / Optics & Laser Technology 36 (2004) 35 – 38 1.2

4.5

Ts 1

x= 5 x = 10 x =20

4.4 x=5

0.8

T%

37

Tmax

4.3 0.6 0.4

4.2

T//in

n

0.2 0 450

650

850

1050

1250

(a)

1450

λ (nm)

1650

1850

2050

2250

4.1 4

2450

3.9 1.2

Ts

3.8

1 x = 10

Tmax

3.7 1000

0.6 0.4

1400

0 450

1600

1800

2000

2200

2400

λ (nm) Fig. 4. Dispersion curves of the mean refractive index, n, for a-Se100−x Sbx (lms.

0.2

650

850

1050

1250

(b)

1450

λ (nm)

1.2

1650

1850

2050

2250

2450

8

Ts -2

(n -1) x 10

x = 20

x=5 x = 10 x = 20

7.5 Tmax

0.8 0.6

T//in

0.4

7

-1

1

T%

1200

T//in

6.5

2

T%

0.8

6

0.2

5.5 0 450

650

850

1050

1250

1450

1650

1850

2050

2250

2450

λ (nm)

(c)

5 0.2

0.4

0.6

0.8

1 2

Fig. 3. Optical transmission spectrum, T (), for a-Se100−x Sbx (lms. The top, Tmax , and bottom, Tmin , envelope curves. Ts is the transmission spectrum of the silicon substrate.

possibilities, double bonding and change of threefold coordination number of Sb to (ve or some local rearrangement of Sb–Se glass chains with increasing Sb content [7]. To calculate the refractive index, n, we used a method suggested by Swanepoel [13]. The optical parameters are obtained by using only the transmission spectrum. According to Swanepoel’s method, which is based on the approach of Manifacier et al. [14] of creating the upper and lower envelopes of the transmission spectrum, the refractive index in the region where the absorption coe:cient, , is ∼ zero, was calculated by the expression [13]
 n = N + N 2 − S 2; (3) where N = 2S



Tmax − Tmin Tmax × Tmin

 +

S2 + 1 ; 2

where S is the refractive index of the silicon substrate (S = 3:4) [15]. Tmax and Tmin are, respectively, the envelope values at the wavelengths in which the upper and lower envelops and the experimental transmission spectrum are tangent, as shown in Fig. 3. The calculated values of the

1.2

1.4

2

(hυ) (eV)

Fig. 5. Wemple–DiDomenico (ts the optical-dispersion data for a-Se100−x Sbx (lms.

refractive index from transmission spectral region 1000 –2500 nm are shown in Fig. 4. It should be noted that the refractive index decreases with increasing Sb content at any given wavelength, due to the eCect of Sb in Se in promoting crystallization, because Sb forms three strong bonds and must be eCective in cross-linking chains [11]. The data on the dispersion of the refractive index, n(), were evaluated according to the single-eCective-oscillator model proposed by Wemple and DiDomenico [16,17]. They found that all the data could be described, to an excellent approximation, by the following formula n2 (h ) = 1 +

E d Eo ; Eo2 − (h )2

(4)

where h is the photon energy, Eo is the oscillator energy and Ed is the oscillator strength or dispersion energy. Plotting (n2 −1)−1 against (h )2 allows us to determine the oscillator parameters, by (tting a linear function to the smaller energy data. All these plots are shown in Fig. 5. The dependence of single-eCective-oscillator parameters on Sb content is shown in Table 1. The Sb content dependence of the static refractive

38

E.-S.M. Farag / Optics & Laser Technology 36 (2004) 35 – 38

Table 1 Optical constants as a function of Sb content of a-Se100−x Sbx (lms

Composition Sb at%

Egopt (eV)

Ee (eV)

Eo (eV)

Ed (eV)

n(o)

Nc

Ne

5 10 20

1.66 1.54 1.35

0.25 0.34 0.34

3.1 3.0 2.85

22.5 22.4 22.38

1.070 1.065 1.062

2.05 2.10 2.20

16.15 15.70 14.97

index n(o) is evaluated from Eq. (4), i.e., n2 (o) = 1 + EEdo as shown in Table 1. The oscillator energy, Eo , is an average energy gap (Wemple–DiDomenico, gap) [16,18] to a good approximation. From Table 1, the relationship Eo ≈ 2Egopt , it varies in proportion to the optical band gap, as was (rst found by Tanaka [19]. On the other hand, the dispersion energy, Ed , serves as a measure of the strength of interband transitions. The value of, Ed , decreases with increasing Sb content, owing to the increase in the eCective coordination of the cation as shown in Table 1. A small change of value of Ed , it may be that the cation coordination is the factor, which is weak aCected by the addition of Sb to the amorphous structure. An important achievement of the Wemple– DiDomenico model is that it relates the dispersion energy to other physical parameters of the material through an empirical formula [16,17]: Ed = Nc Za Ne (eV);

(5)

where  is a two-values constant with either an “ionic” or a “covalent” value (i =0:26±0:03 eV and c =0:37±0:04 eV, respectively), Nc is the coordination of the cation neighbor nearest to the anion, Za is the formal chemical valency of the anion and Nc is the total number of valence electrons per anion. The incorporation of Sb into the structure of the present Sb–Se (lms has the eCect, as mentioned in Table 1, of decreasing the oscillator strength. Therefore, addition of Sb into chalcogenide matrix decreases one or other of the quantities on the right-hand side of Eq. (5). Even if Sb photodoping were to change the bonding towards less ionic, this cannot be the major factor, since this factor would increase . On the other hand, in less ionic materials, the smaller S–P splitting increases the parameter Ne [9]. Therefore, in our case Sb–Se (lm samples, we assume that the average cation coordination is the factor that is most aCected by the addition of Sb to the glassy structure. We must take into account the chemical valency of the anion (Za ), does not change. The decrease of Eo and Ed might be explained by the increase in the number of scattering centers due to dissolving Sb atoms in the parent glassy (lm matrix. 4. Conclusions The results of measurement of the optical properties of amorphous Se100−x Sbx (lms (X = 5, 10 and 20 at%) show

qualitatively the same behavior as corresponding measurements performed on other amorphous materials. We have analyzed the optical-dispersion data using the Wemple– DiDomenico single eCective-oscillator model. The oscillator energy, Eo , varied in proportion to Egopt , in accordance with the (nding of Tanaka [16]. The increase of Sb content results in increase of the metallic behavior and hence decreases the value of optical energy gap (Egopt ). On the other hand, we attributed the decrease of the oscillator strength, Ed , with increasing Sb content, to the decrease of the eCective cation coordination number, Nc . Acknowledgements The author wishes to thank Prof. Dr. M. M.El-Ocker, Professor of Solid State Physics and Head of Physics Department, Faculty of Science, Al-Azhar University, Cairo, Egypt, for his help in the transmission and re;ection measurements at his laboratory. References [1] Tan CZ. J Non-Cryst Solids 1999;249:51–4. [2] Yayama H, Fujino S, Morinaga K, Takebe H, Hewak DW, Payne DN. J Non-Cryst Solids 1998;239:187–91. [3] Fagen EA, Fritzsche H. J Non-cryst Solids 1970;2:180–91. [4] Street RA, Nemanich RJ, Connell GAN. Phys Rev B 1978; 18(12):6915–9. [5] Kaminska E, Subramanya SG, Weber ER. Appl Phys Lett 2000;76(10):1279–81. [6] Kolobov VA, Elliott RS. J Non-cryst Solids 1995;189:297–300. [7] Tonchev D, Kasap SO. J Non-cryst Solids 1999;248:28–36. [8] Feltz A. Amorphous inorganic materials and glasses. Weinheim: VCH, 1993. [9] Marquez E, Wagner T, Gonzalez-Leal JM, Bernal-oliva AM, Prieto-Alcon R, Jimenez-Garay R, Ewen PJS. J Non-cryst Solids 2000;274:62–8. [10] El-Sayed M Farag, Ammar AH, Soliman HS. Chin J Lumin 2002;23(3):137–44. [11] Mott NF, Davis EA. Electronic processes in non-crystalline materials, 2nd ed. Oxford: Clarendon Press, 1979. [12] Kadouri El-H, Maurice T, Gratens X, Charar S, Benet S, Me;eh A, Tedenac JC, Liautard B. Phys Stat Sol A 1999;176:1071. [13] Swanepoel R. J Phys E 1983;16:1214. [14] Manifacier JC, Gasiot J, Fillard JP. J Phys E 1976;9:1002. [15] Chopra KL. Thin (lm phenomena (1977) p. 750. [16] Wemple SH, DiDomenico M. Phys Rev B 1971;3:1338. [17] Wemple SH. Phys Rev B 1973;7:3767. [18] El-Nahass MM, El-Den MB. Opt Laser Technol 2001;33:31–5. [19] Tanaka K. Thin Solid Films 1980;66:271.