Influence of annealing on the structure and optical properties of Zn40Se60 thin films

Optics & Laser Technology 44 (2012) 1116–1121

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Influence of annealing on the structure and optical properties of Zn40Se60 thin films M.A. Abdel-Rahim a,n, M.M. Hafiz a, A. Elwhab. B. Alwany b a b

Physics Department, Faculty of science Assuit University. Assuit, Egypt Ibb University, Ibb, Yemen

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 July 2011 Received in revised form 9 October 2011 Accepted 10 October 2011 Available online 29 October 2011

Thin films of Zn40Se60 were prepared by the vacuum thermal evaporation technique. The influence of annealed temperature on the structural and optical properties was investigated using the X-ray diffraction (XRD), scanning electron microscopy (SEM) and optical transmission. The XRD studies show that the as-deposited film is amorphous in nature, but the crystallinity improved with increasing the annealing temperature. Furthermore the particle size and crystallinity increased whereas the dislocation and strains decreased with increasing the annealing temperature. SEM studies showed that the annealing temperature induced changes in the morphology of the as-deposited sample. Various optical constant have been calculated for as-deposited and annealed films. The mechanism of the optical absorption follows the rule of direct transition. It was found that, the optical energy gap (Eg) decreased with increasing the annealing temperature. These results can be interpreted by the Davis and Motte model. On the other hand the maximum value of the refractive index (n) is shifted toward the long wavelength by increasing the annealing temperature. & 2011 Elsevier Ltd. All rights reserved.

Keywords: Zn40Se60 semiconductors Structure Optical properties

1. Introduction The growth of the groups II–VI semiconductors has attracted considerable attention because of their novel physical properties and many applications in optoelectronic devices. One of these compounds is Zinc Selenide (ZnSe) with cubic Zinc-blend structure and with a direct band gap of about 2.7 eV at room temperature. Zinc selenide (ZnSe) has unique physical properties such as wide optical energy band gap, high refractive index and low optical absorption in the visible and infrared spectral region [1]. On the other hand, the zinc selenide has several potential applications in devices such as blue light emitting diodes [2,3] photodiodes [4], thin film transistors [5] and Cr doped ZnSe laser [6]. The zinc selenide is also used as buffer layer materials instead of CdS in Cu based solar cells [7,8]. The advantages of using ZnSe over CdS include its non-toxicity, its wider energy band gap than CdS. Zinc selenide thin films have been prepared by various techniques such as thermal evaporation under vacuum, molecular beam epitaxy, organo-metallic chemical vapor deposition, solution growth spray pyrolysis etc. [9–12]. Among the various techniques, the vacuum thermal evaporation is very common due to its simplicity, low cost reproducibility and scalability

n

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deposit onto large area substrates. Moreover, the films produced with this method are highly adherent and uniform. On the other hand, the structural, electrical and optical properties of thin films are very sensitive to deposition conditions and post-deposition heat treatments [13–16]. The present work deals with some experimental observation on the effect of heat treatment on structure and optical properties of Zn40Se60 films. Scanning electron microscopy (SEM) and X-ray diffraction were used to determine the structural changes for Zn40Se60 films under different conditions. The effect of thermal annealing on optical properties is interpreted according to the density of states model in amorphous materials proposed by Mott and Davis [17]. In this paper, we report the study of the dependence of the optical properties of the Zn40Se60 thin films on annealing temperature.

2. Experimental details. Bulk Zn40Se60 was prepared by the melt-quench technique. Appropriate amounts of high purity (99.999%) Zn and Se (from Aldrich, UK) were weighted (5 g total weight) according to their atomic percentage. The weighted elements were placed into a quartz glass ampoule and sealed under vacuum of 10  4 Torr. The sealed ampoule was heated in Heraus programmable tube furnace (type R 07115), the heating rate was approximately 3.5 K/min. The temperature was kept at 1073 K for 24 h. The

M.A. Abdel-Rahim et al. / Optics & Laser Technology 44 (2012) 1116–1121

ampoule was manually stirred for realizing the homogeneity of the composition. After that, the ampoule was quenched into icecooled water. Thin films were prepared by thermal evaporation under vacuum of 10  5 Torr using the Edwards E-306 coating system. A constant evaporation rate (3 nm/sec) was used to deposit the films. The evaporation rates as well as the films thickness were controlled using a quartz crystal monitor (FTM5). The film composition was checked using the energy-dispersive spectroscopy (EDAX) technique. Zn40Se60 films were annealed at different temperature (373 r Tann r423 K) for half hour under gas Nitrogen. The morphology for as-deposited and annealed films were investigated using (SEM) type JEOL JSM-T200. The crystalline phases for as-prepared and annealed films were identified using a Philips diffractometer type 1710. The optical transmittance (T) and reflectance (R) of the as-deposited and annealed Zn40Se60 films were measured at room temperature using a double-beam spectrophotometer (SHIMADZU UV-2101 combined with a PC) in the wavelength 300–900 nm.

3. Results and discussion 3.1. Structure studies The composition of Zn40Se60 thin films was investigated using energy dispersive X-ray analysis (EDAX). The atomic percentage

ratio of Zn and Se were found to be 38.25 and 61.75, respectively, as shown in Fig. 1. The morphology of the annealed Zn40Se60 thin films was examined using SEM. The scanning micrograph of the annealed Zn40Se60 films are shown in Fig. 2(a, b). The microstructure for the annealed sample at 373 K for 0.5 h are shown in Fig. 2(a), the photomicrograph shows the existence of polycrystalline structures consisting of crystallites embedded in amorphous phases. The crystalline morphology is not distinct due to the fact that the crystallization is only in its initial stages. On the other hand, the scanning electron micrograph for the annealed Zn40Se60 thin films at 423 K for 0.5 h is shown in Fig. 2(b). A polycrystalline structure consists of different phases with different sizes appears. In general the crystallites are dispersed homogenously and occupy most of the film structure. In order to determine the crystalline phases that appeared in SEM for the annealed Zn40Se60 thin films, the X-ray diffraction pattern of films were analyzed. Fig. 3(a–c) shows XRD pattern of the as-deposited and annealed Zn40Se60 films. The XRD studies show that the as-deposited film is amorphous in nature as shown in Fig. 3(a) but the film annealed at 373 K shown in Fig. 3(b) did not show significant improvement in crystallinity. On the other hand the XRD peaks of the film annealed at 423 K, shows increase in intensity of the diffraction peaks that corresponds to the cubic phase along with appearance of new peaks that corresponds to Se phase formed as shown in Fig. 3(c). The average of crystalline size D were calculated using Scherer’s formula [18]. D ¼ 0:94l=b cos y,

ð1Þ

1500

Total

Atomic % 38.25 61.75 100.00

♦ Se Zn Ο ZnSe

311 ♦

1250 Intensity (a. u)

Elmt Type Zn K Se L

1117

1000 750 331 Ο

002 220 Ο

500 C

250

a

b 0 10

Fig. 1. Typical EDAX of as-deposited Zn40Se60 thin films.

20

30

40 50 2θ (Deg.)

60

70

80

90

Fig. 3. The X-ray patterns of Zn40Se60 thin films, (a) as-deposited (b) annealed at 373 K and (c) annealed at 423 K.

Fig. 2. The SEM photograph for Zn40Se60 thin films (a) annealed at 373 K and b annealed at 423 K.

1118

M.A. Abdel-Rahim et al. / Optics & Laser Technology 44 (2012) 1116–1121

where D is the particle size, l is the X-ray wavelength used, b is the angular line width of half maximum intensity (FWHM) and y is the angle between incident and the scattered X-ray. The dislocation density (d) is defined as the length of dislocation lines per unit volume of the crystal and is given by, d ¼1/D2. The strain value (e) is calculated from the following relation,   l 1 e¼ b , ð2Þ tan y D cos y

The intercept with x-axis gives the values of the direct optical energy gap Eg. The optical energy gap Eg decreases with increasing the annealing temperature as shown in Fig. 9. As the annealing temperature was increased the crystallite size increase and the strain value decrease, this leads to decreasing the optical energy gap Eg [20,21]. In the exponential edge region, the absorption coefficient is governed by Urbach’s relation [25].

The deduced D, d and e are listed in Table (1). It is observed that the strain and dislocation density decrease whereas particle size increases with increasing the annealing temperature, which indicates the improvement in crystallinity of the films. This result is in good agreement with the previous work [19–21].

a ¼ ao expðhn=Ee Þ,

where ao is a constant and Ee is interpreted as the width of the tails due to localized state in the forbidden gap. Therefore,

0.90

3.2. Absorption coefficient

where d is the film thickness. Fig. 6 shows the dependence of the absorption coefficient (a) on the incident photon energy (hn) for the as-deposited and annealed Zn40Se60 films. It is observed that the values of the absorption coefficient increase with increasing both the photon energy and annealing temperature. In chalcogenide glasses, a typical absorption edge can be broadly ascribed to either the three processes (i) residual below-gap absorption, which originates from defects and impurity (ii) Urbach tail, which is strongly related to the structure randomness of the system and (iii) interband absorption, which determined the optical energy gap. In the high absorption region (a 4104 cm  1) the parabolic relation can be applied [23,24].

a ¼ CðhnEg Þr =hn

0.75

T

0.60 0.45

0.15 0.00 400

500

600

700

800

900

λ (nm) Fig. 4. The spectral dependence of transmittance for as-prepared and annealed for Zn40Se60 films.

0.64 0.56 0.48

As.prepared. T Ann.= 343 K T Ann.= 363 K T Ann.= 393 K T Ann.= 423 K

0.40 0.32

ð4Þ

where C is a constant depending on the transition probability, Eg is the optical energy gap of the materials and r is a number characterizing the transition process, having a value 1/2 for the direct allowed transition and value of 2 for the indirect transition. Fig. 7 shows that the variation of (ahn)2 versus hn for as-prepared and annealed films is linear at the absorption edge, which confirmed direct band gap transition in Zn40Se60 films; these results are in good agreement with the previous works [18,20,22].

As.prepared. T Ann.= 343 K T Ann.= 363 K T Ann.= 393 K T Ann.= 423 K

0.30

R

The spectral distribution of transmittance and reflectance for as-prepared and annealed Zn40Se60 films are shown in Figs. 4 and 5. It could be noted that the transmittance decreases with increasing the annealing temperature but reflectance shows opposite behavior to this in the transmission spectrum. The optical absorption coefficient (a) was computed from the experimentally measured values of transmittance T(l) and reflectance R(l) according to the following relation [22]. " # 1 ð1RÞ2 a ¼ ln , ð3Þ d T

ð5Þ

0.24 0.16 0.08 400

500

600

700

800

900

λ (nm) Fig. 5. The spectral dependence of reflectance for as-prepared and annealed for Zn40Se60 films.

Table 1 Structure parameters of Zn40Se60 films at different annealing temperatures. Zn40Se60

dexp.

dstand

hkl

Kind of Phase

particle size D (nm)

Average of particle size for ZnSe phase

Average of strain values (lin-2.m-4)for ZnSe phase

Average of dislocation density d  1016 (lines/m2) for ZnSe phase

Annel. at 373 K

3.562 2.48 2.097 1.31

3.561 2.4730 2.0046 1.3006

3 0 2 3

1 0 2 3

1 2 0 1

Se Zn ZnSe ZnSe

5.71092 18.5648 18.2608 8.71663

13.488

0.00159

0.808

Annel. at 423 K

3.551 2.484 2.098 1.31

3.561 2.4730 2.0046 1.3006

3 0 2 3

1 0 2 3

1 2 0 1

Se Zn ZnSe ZnSe

6.06079 18.50466 19.66392 10.94494

15.304

0.00136

0.547

M.A. Abdel-Rahim et al. / Optics & Laser Technology 44 (2012) 1116–1121

9.80

1.80x105

As.prepared. T Ann.= 343 K T Ann.= 363 K T Ann.= 393 K T Ann.= 423 K

1.35x105

9.45 9.10

9.00x104

ln (α)

8.75 As.prepared. T Ann.= 343 K

4.50x105 8.40

T Ann.= 363 K T Ann..= 393 K

0.00 1.8

2.0

2.2 hν (eV)

2.4

2.6

1.8

Fig. 6. The spectral dependence of absorption coefficient for as-prepared and annealed for Zn40Se60 films.

1.9

T Ann.= 363 K

Eg Ee

Eg (eV)

T Ann.= 423 K

2.3

0.052 0.048

2.4

T Ann.= 393 K

9.0x1010

2.2

Fig. 8. Plots of ln(a) versus hn for the as-prepared and annealed Zn40Se60 films.

As.prepared. T Ann.= 343 K

1.2x1011

2.1 hν (eV)

2.6

1.5x1011

(αhν)2 (cm-1eV)2

T Ann.= 423 K

8.05

0.044 2.2

Ee (eV)

α (cm)-1

1119

0.040

6.0x1010 2.0

0.036

3.0x1010

300

0.0 1.8

2.0

2.2

2.4 2.6 hν (eV)

2.8

2

ð6Þ

10 As.prepared. T Ann.= 343 K T Ann.= 363 K

8

T Ann.= 393 K T Ann.= 423 K

6 n

plotting the dependence of lna versus hn gives straight line as shown in Fig. 8. The inverse of the slope gives the band tail width (Ee) of the localized states. The effect of annealing temperature on Ee for Zn40Se60 thin films are shown in Fig. 9. It’s observed that Ee increases with increasing the annealing temperature. The decrease in the optical energy gap Eg and the increase of localized states tail Ee with the annealing temperature can be interpreted by assuming the production of surface dangling bonds around the crystallites [26] during the process of crystallization. It has been suggested by many authors [26,27] that nearly ideal amorphous solids crystallize under heat treatment and that in the process of crystallization dangling bonds are produced around the surface of the crystallites. Further heat treatment causes the crystallites to break down [27] into smaller crystals thereby increasing the number of surface dangling bonds. These dangling bonds are responsible for the formation of some types of defects in highly polycrystalline solids. As the number of dangling bonds and defects increase with the increase in annealing temperature, the concentration of localized states in the band structure also increases gradually. Hence the heat treatment of the films causes an increase in the energy width of localized states thereby reducing the optical energy gap. On the other hand, the values of refractive index (n) and extinction coefficient (kex.) have been calculated using the following relation [28,29]. 2

425

Fig. 9. The variation of the optical energy gap and the width of tail states versus annealing temperature for Zn40Se60 films.

3.0

Fig. 7. The plots of (ahn)2 versus photon energy (hn) for the as-prepared and annealed for Zn40Se60 films.

R ¼ ½ðn1Þ2 þ k =½ðn þ1Þ2 þ k 

325 350 375 400 Annealing Temperature (K)

4

2 560

630

700

770

840

910

λ (nm) Fig. 10. Refractive index (n) versus wavelength (l) for Zn40Se60 films.

kex: ¼

al 4p

,

ð7Þ

where R is reflectance or reflectivity. The spectral dependence of refractive index (n) and extinction coefficient (kex.) on the wavelength for as-prepared and annealed Zn40Se60 thin films are shown in Figs. 10 and 11. The values of (n) have a maximum value (nmax.) at wavelength lc, which is shifted towards longer wavelength as the annealing temperature increased as shown in Fig. 10. On the other hand, the value of

1120

M.A. Abdel-Rahim et al. / Optics & Laser Technology 44 (2012) 1116–1121

0.8

0.6

As.prepared. T Ann.= 343 K T Ann.= 363 K

4

T Ann.= 393 K T Ann.= 423 K

3 0.4 ε2

Kex.

5

As.prepared. T Ann.= 343 K T Ann.= 363 K T Ann.= 393 K T Ann.= 423 K

2 0.2 1 0.0 450

500

550

600

650

700

0

λ (nm)

500

550

Fig. 11. Extinction coefficient (kex) versus wavelength (l) for Zn40Se60 films.

60 50

650

700

Fig. 13. Imaginary part (e2) of dielectric constant versus wavelength (l) at different annealing temperatures for Zn40Se60 films.

As.prepared. T Ann.= 343 K T Ann.= 363 K T Ann.= 393 K T Ann.= 423 K

70

600 λ (nm)

0.175

As.prepared. T Ann.= 343 K T Ann.= 363 K T Ann.= 393 K T Ann.= 423 K

0.140

ε1

40 (n2-1)-1

30 20

0.105 0.070

10 0.035

0 500

600

700 λ (nm)

800

900

0.000 2.6

2.7

e1 ¼ n2 k2 , and e2 ¼ 2nkex :

ð8Þ

The variation of these two parameters with the wavelength is shown in Figs. 12 and 13. On the other hand, the imaginary part of the dielectric constant for Zn40Se60 films increases with increasing the annealing temperature. The dispersion of the refractive index, n was analyzed using the concept of the single oscillator and can be expressed by Wimple-Didomenico relationship [34]. n2 1 ¼

Ed Eo Eo 2 E2

,

2.9

3.0

3.1

3.2

Fig. 14. Plots of (1/n2  1) versus the photon energy (hn) for Zn40Se60 films at different annealing temperatures.

35 As.prepared. T Ann.= 343 K

28

T Ann.= 363 K T Ann.= 393 K T Ann.= 423 K

21 n2

the extinction coefficient (kex.) decreases with increasing the wavelength. In general, the values of (n) and (kex.) increased with increasing the annealing temperature as shown in Figs. 10 and 11. This type of trend has also been observed for the thin films of various other semiconductors [12,30,31]. The increase of n and k with increasing annealing is caused by an increase of the particle size with annealing [32]. For a better understanding of the optical properties of the asprepared and annealed films, it is necessary to determine some optical constants such as real (e1) and imaginary (e2) parts of dielectric constant for the as-prepared and annealed Zn40Se60 films, which have been calculated using the relations [33].

2.8

(hν)2 (eV)2

Fig. 12. Real part (e1) of dielectric constant versus wavelength (l) at different annealing temperatures for Zn40Se60 films.

14

7

0 6.46x105

6.65x105

6.84x105 λ2

7.03x105

7.22x105

(nm)2

Fig. 15. Relative permittivity (er) versus wavelength (l2) at different temperatures for Zn40Se60 films.

ð9Þ

where Eo is the energy of the effective dispersion oscillator, E the photon energy, and Ed is the so-called dispersion energy, which measures the average strength of the interband optical transitions. Fig. 14 shows the relation between 1/(n2  1) and the

photon energy (hn)2 for as-prepared and annealed films. The values of Ed and Eo have been determined from the slope and the intersection of the straight lines with 1/(n2 1) axis. The obtained values of Eo and Ed for the typical Zn40Se60 thin films are listed in Table 2. It is observed that the values of Eo decrease

M.A. Abdel-Rahim et al. / Optics & Laser Technology 44 (2012) 1116–1121

1121

Table 2 Effect of annealing temperature on the values of the n, k, Eo, Ed, N/m*, and eL of Zn40Se60 thin films. T. ann. (K)

n at l ¼ 570 nm

k at l ¼570 nm

Eo (eV)

Ed (eV)

N/mn (m  3/Kg)  1058

eL

As-prepared 343 K 363 K 393 K 423 K

1.966 2.028 2.125 2.637 3.577

0.071 0.091 0.244 0.385 0.477

2.71604 2.62535 2.35273 2.29204 2.22198

34.865 25.598 35.777 23.035 26.165

5.607 15.814 16.120 16.425 16.683

43.853 44.365 50.73 54.099 56.290

56

N/m* εL 16

52

12

48

8

44

4 280

300

320 340 360 380 400 Annealing Temperature (K)

420

εL

(N/m*) ×1058 (m-3/kg)

20

40 440

Fig. 16. The ratio (N/m*)and eL versus annealing temperature for Zn40Se60 films.

with increasing the annealing temperature but the values of Ed increased with increasing the annealing temperature. The decreasing Eo and increasing Ed with the annealing temperature could be attributed to increase the rate of diffusion of atoms of the films with increasing the annealing temperature. The increase in the diffusion rate with increasing temperature gives more number of atoms at interstitial sites, thereby leading to impurity type scattering centers [35,36]. The high frequency dielectric constant (eL) can be deduced from the following relation [37,38].    e2 N e ¼ n2 ¼ eL  l2 ð10Þ mn 4p2 c2 eo where eL is the optical dielectric constant, e is the electronic charge, (N/mn) is the ratio of carrier concentration to the electron effective mass mn, eo the free space dielectric constant and c is the speed of light. From the linear plot of n2 versus l2 as shown in Fig. 15, the lattice dielectric constant eL and the ratio N/mn of the asdeposited and annealed films can be determined. The values of these two parameters with annealing temperature are given in Table 2 and are shown in Fig. 16. It can be seen that both the lattice dielectric eL and the ratio N/mn increases with increasing the annealing temperature. This behavior was observed in many chalcogenide glasses. In general, it can be concluded that both the high frequency dielectric constant eL and the ratio N/mn are related to the internal microstructure.

4. Conclusion Zn40Se60 thin films were deposited onto glass substrates under a vacuum of 10  5 Torr using a vacuum evaporation technique. X-ray analyses showed that the as-deposited films have an amorphous nature. Annealing the as-deposited Zn40Se60 films under Nitrogen gas showed amorphous to crystalline transition.

The average particle size increased with the increase in annealing temperature. The values of optical band gap (Eg) were found to decrease from (2.564 to 1.993 eV) with an increase of annealing temperature, which could be attributed to an increase in particle size and decrease in strain values. The variations of refractive index (n) and the extinction coefficient (kex.) have been studied as a function of annealing temperature. It was found that the dispersion of refractive index obeyed the single oscillator model. On the other hand the high frequency dielectric constant (eL) and the ratio (N/mn) increase with increasing the annealing temperature.

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