Influence of γ-irradiation on the optical properties of AgSbSe2 thin films

Nuclear Instruments and Methods in Physics Research B 305 (2013) 22–28

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Influence of c-irradiation on the optical properties of AgSbSe2 thin films A.M. Abdul-Kader ⇑, Y.A. El-Gendy Physics Department, Faculty of Science, Helwan University, Ain Helwan, Cairo, Egypt Physics Department, University College – Umm Al-Qura University, Saudi Arabia

a r t i c l e

i n f o

Article history: Received 7 January 2013 Received in revised form 11 April 2013 Available online 22 April 2013 Keywords: Thin films Chalcogenide Radiation effect Optical properties

a b s t r a c t Amorphous AgSbSe2 thin films were deposited onto glass substrates using electron beam evaporation. The effect of c-irradiation on the optical properties of the deposited AgSbSe2 films was studied in the wavelength range 550–2500 nm. A red shift in the transmission spectra was observed with increasing c-irradiation dose. The refractive index of the deposited films was determined as a function of c-dose. It was established that exposure of the deposited films to c-radiation leads to increased refractive index in the whole spectral region. The refractive index dispersion of the deposited films is adequately described by the single oscillator model, whereby, the values of the oscillator parameters were determined as a function of c-dose. Analysis of the optical absorption coefficient revealed the presence of an indirect optical transition with band gap value decreases with increasing c-dose. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Radiation effects induced by high (i.e. ion beam) and low (i.e. electron beam and gamma ray irradiations) linear energy transfer radiations have been used widely to modify materials properties. These modifications are related to several mechanisms, including the interaction of the radiation with matter [1]. It has been reported that the charge density on the Ge-atom in the low water peak single-mode optical fiber increases with c-dose, which reflects an increases in the Rayleigh scattering coefficient; however, with further increases in c-dose the molecular structure bonds will be broken up and a new defect centers are formed [2]. The influence of the c-radiation on the Ge–O bonds of the Ge-doped silicon dioxide network structure has been also investigated [3]. Amorphous chalcogenide thin film materials are interesting because of their technological applications and commercial importance [4]. They are recognized as promising materials for infrared optical elements, infrared optical fibers, xerography, switching and memory devices, photolithography and in the fabrication of inexpensive solar cells and recently for reversible phase change optical records [5–8]. Amorphous chalcogenide materials are characterized by their sensitivity to the influence of external factors especially ionizing radiation [9–11]. Irradiation of these materials with c-rays are usually connected with bond breaking or switching within the amorphous network, hence reflects change of the local structure of these materials [12,13]. In general, irradiation of chalcogenide thin film materials includes subtle effects such as shifts in ⇑ Corresponding author at: Physics Department, Faculty of Science, Helwan University, Ain Helwan, Cairo, Egypt. Fax: +20 225552468. E-mail address: [email protected] (A.M. Abdul-Kader). 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.04.043

the absorption edge (photo-bleaching and photo-darkening), and more substantial atomic and molecular reconfiguration [8]. In general, these phenomena are associated with changes in the optical constants and absorption edge shifts [14] allowing the use of these materials in the fabrication of a large number of optical devices. The ternary AgSbSe2 compound belongs to the chalcogenide materials. Wang et al. reported that film material of this compound possesses a desired phase change properties including a large resistance change, a single crystalline structure and smaller volume change upon crystallization [15]. The effects of the thermal annealing process on the structure and optical properties of the deposited AgSbSe2 films have been also reported [16]. The principal objective of this work is to investigate the effect of gamma rays irradiation on the optical properties of the deposited AgSbSe2 thin films. The optical parameters such as refractive index, Urbach’s energy and optical band gap, have been determined for the as-deposited and those irradiated AgSbSe2 films. 2. Experimental details Polycrystalline AgSbSe2 crystallizing in a cubic structure has been previously prepared according to the method described in Ref. [16]. An e-beam evaporation system (Leybold–Heraeus Combitron CM-30, Germany) was used to prepare the AgSbSe2 thin films. A vacuum pressure of 8  104 Pa was maintained during the evaporation process. The evaporation rate (6–8 nm s1) and film thickness were controlled during evaporation using the quartz crystal thickness monitor (Edward model FTM5). Four thin film samples deposited onto glass substrates of dimensions 1  1.5 cm2 were prepared in a single run. The deposition was carried out at room temperature (300 K).

A.M. Abdul-Kader, Y.A. El-Gendy / Nuclear Instruments and Methods in Physics Research B 305 (2013) 22–28

X-ray diffraction patterns of as-deposited and c-ray irradiated AgSbSe2 thin films deposited onto glass substrates were investigated using a X-ray diffractometer (Type Philips model X’-Pert) with monochromatic CuKa radiation operating at 35 kV and 100 mA. The structure and chemical composition of the deposited films was determined using transmission electron microscope (TEM) (Type JEOL JEM-1230) operating at 120 kV, interfaced with energy dispersive X-ray spectrometry unit (EDX) (Type Oxford). Full quantative analysis results were obtained from the EDX spectra by processing the data through ZAF correction program. A double beam spectrophotometer, with automatic computer data acquisition (Type Jasco, V-570, Rerll-00, and UV–VIS–NIR), was employed at normal light incident to record the optical transmission and reflection spectra of the deposited films over the wavelength range 550–2500 nm. AgSbSe2 thin film samples were exposed to c-radiation from a 60 Co source providing a dose rate of 2 kGy/h using a gamma cell available in NCRRT, AEA, Cairo, Egypt. The temperature of the irradiation cavity was lower than 320 K. The gamma irradiation was conducted at different c-doses of 2.5, 5, and 7.5 kGy, respectively.

3. Results and discussion 3.1. Structure analysis of the deposited film

Intensity [a.u]

Fig. 1 shows XRD patterns corresponding to as-deposited and irradiated AgSbSe2 films. The X-ray diffraction patterns depict the presence of two humps, without any of sharp diffraction peaks within the working 2h range. The absence of sharp diffraction peak in the X-ray patterns indicates the amorphous nature of the investigated films. The observed two humps in the X-ray diffraction patterns may be related to: the glass substrate (first hump) usually appears in the 2h range 20–30o while, the other in the 2h range 40–50o may attribute to the deposited thin film material. Similar two humps in the X-ray diffraction pattern have been also observed for chalcogenide Se90xTe5Sn5Inx thin films [17]. Fig. 2a shows the TEM micrograph and the corresponding electron diffraction pattern of as-deposited AgSbSe2 film (of thickness 70 nm suitable for TEM investigation). It is clear from the figure that the diffraction pattern of the deposited films consist of diffused rings, beside no discernible structure observed in the corresponding transmission micrograph confirms the amorphous nature of the

7.5 kGy

2.5 kGy as-deposited

0

20

40

60

80

2θ [Degrees] Fig. 1. X-ray diffraction patterns of AgSbSe2 thin films at different c-doses.

23

deposited films as revealed by X-ray diffraction. Fig. 2b shows the EDX spectrum of the film shown in Fig. 2a. The chemical composition revealed that the atomic percentage of Ag:Sb:Se was found to be 26.97:23.55:49.48. This result indicates that the exact chemical composition of the deposited film is Ag1.08Sb0.94Se1.98 which is nearly stoichiometric. Inset of Fig. 2a shows the obtained values of the atomic and weight percent of the Ag, Sb and Se. It should be pointed out that that the signals appear at 8.05 and 8.9 eV, respectively, in the EDX spectrum pattern (see Fig. 2b) corresponding to Cu from the copper grid. 3.2. Optical properties of the deposited films Fig. 3 shows the spectral variation of the transmission, T(k) and reflection, R(k) of as-deposited and irradiated AgSbSe2 films at different c-doses. It has been observed from the transmission spectra that in the weak absorption region (k > 800 nm), the deposited films are highly transparent, and characterized by interference effects, which indicated that the interfaces, air/film, and film/glass were flat and parallel. Strange absorption was observed at wavelengths lower than 800 nm, where interference effects suppressed almost completely due to a well-defined band edge. Moreover, the observed maxima and minima positions of the interference fringes in the reflectance spectra at the same wavelength positions of the corresponding minima and maxima in the transmittance spectra indicates the optical homogeneity of the deposited films. At long wavelengths R + T = 1, indicating that, no scattering or absorption occurs. It was also observed from the transmission spectra that the fundamental absorption edge shifts towards lower energies (red shift) with the increase of c-doses. Due to the presence of such interference patterns; the refractive index and thickness of the studied films have been determined from the recorded transmission spectra using the well-known Swanepoel’s method [18], which is based on that given by Manifacier et al. [19]. This method has been widely used to calculate interesting optical parameters of different chalcogenide films [20–22]). The dispersion of the refractive index, n can be adequately described by the two-term Cauchy dispersion relationship of the form [18]:

nðkÞ ¼ a þ b=k2

ð1Þ

Fig. 4 shows the refractive index dispersion of as-deposited and irradiated AgSbSe2 films at different c-doses. It is clearly seen that the refractive index increases with increasing of c-doses. The increase of the refractive index with c-doses could be attributed to the interaction of c-dose with the amorphous network as it is supposed in the models of photo-induced optical changes [23,24]. Photo-induced changes are usually connected to the effect of the atomic motions and/or bond changes during irradiation process; consequently reflects a redistribution of the chemical bonds in the network structure. These photo-structural changes are favored in the amorphous chalcogenides thin film materials because of the rapid localization of photo-excited carriers, and the freedom of low-coordination atoms to change their positions and bond configurations [12]. The refractive index data in the transmission and low absorption region can be describes by the Wemple–DiDomenico single oscillator model [25]:

n2  1 ¼

Ed Eo E2o

 ðhxÞ2

ð2Þ

where ð hxÞ is the photon energy, Eo is the energy of the effective dispersion oscillator, is an ‘‘average’’ energy gap and can be related to the optical band gap Eg in close approximation by Eo  2 Eg [26], and Ed is the oscillator strength. Therefore, plotting (n21)1 against

24

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Fig. 2a. TEM micrograph and the corresponding electron diffraction pattern of AgSbSe2 thin film.

Fig. 2b. Energy dispersive X-ray spectrum of film.

ð hxÞ2 as shown in Fig.5 allows us to determine the oscillator parameters by fitting a straight line to the points. The determined values of Eo, and Ed as well as the static refractive index ns (0), (where ns (0) = (1 + Ed /Eo)0.5) are listed in Table 1. The observed decreases in the film thickness with the increase of c-doses (see Table1) are correlated to the increase in the refractive index. This finding may be attributed to the increase in the density of the deposited films (photo-contraction) as exposed to c-rays. Similar result was also observed for chalcogenide Agx (Sb0.33S0.67) [27] and laser-irradiated Sb40S40Se20 films [28]. It worth mention that the determined Wemple–DiDomenico optical band gap Eg is quite different from the information obtained from the value of the optical gap, Eg, which probes the optical properties near the band edge of the material (as will be mentioned below). An important achievement of the Wemple–DiDomenico model is that it relates the dispersion energy Ed to other physical parameters of the material through an empirical formula [25]: Ed ¼ bN c Z a N e , where b has two constant values corresponding to either ‘‘ionic’’ or ‘‘covalent’’ (bi = 0.26 ± 0.03 eV and bc = 0.37 ± 0.04 eV, respectively), Nc is the coordination of the cation nearest neighbor to the anion, Za is the formal chemical valency of the anion and Ne is the total number of valence electrons (cores excluded) per anion. Since, the ternary AgSbSe2 compound has

NaCl type structure, one can taking Nc = 6, Za = 1 [25]) and Ne = 8.92, calculated for the exact chemical composition of the deposited film i.e. Ag1.08Sb0.94Se1.98 (N e ¼ ð1  1:08 þ 5 0:94 þ 6  1:98Þ=1:98, where 1, 5, 6 are the valence electrons for Ag, Sb and Se, respectively). Assuming that the film composition dose not changes with radiation, one can predict the magnitude of the refractive index corresponding to the exact composition of the deposited film using the relation [25]:

n2  1 ¼

Ed bNc za Ne ¼ Eo Eo

ð3Þ

i.e.

n2 ¼ 1 þ

bNc za Ne Eo

One applying to our case, we obtained the values of the refractive index n corresponding to the exact film composition. The determined values for different c-doses listed in Table 1, explain the importance of coordination number, valency, and ionicity, as deduced by the Wemple–DiDomenico model. In the case of the amorphous chalcogenide glasses, the optical absorption coefficient, a, changes rapidly for the photon energies comparable to that of the band gap giving rise to an absorption

A.M. Abdul-Kader, Y.A. El-Gendy / Nuclear Instruments and Methods in Physics Research B 305 (2013) 22–28

25

Reflectance, R

0.6

0.4

as-deposited 2.5kGy 5.0 kGy 7.5 kGy

0.2

1.0

Transmittance, T

0.8

0.6

0.4 as-deposited 2.5 kGy

0.2

5.0 kGy 7.5 kGy

0.0 1000

1500

2000

2500

Wavelength, λ [nm] Fig. 3. Transmission and reflection spectra of AgSbSe2 thin films with different c-doses.

edge. At the highest values of the absorption coefficient, a, when the condition ad > 1 takes place, a can be calculated from the relation [29]:

T ¼ ð1  RÞ2 expðadÞ;

ð4Þ

4.0

Refractive index, n

as-deposited 2.5 kGy 5.0 kGy 7.5 kGy

3.5

where T is the transmittance, R is the reflectance and d is the film thickness. The spectral variation of the calculated absorption coefficient, a vs. photon energy, ð hxÞ as a function of radiated dose is shown in Fig. 6. The figure depict that the films exhibit high absorption coefficient in the fundamental absorption region (104 < a < 105 cm1); where the absorption coefficient increases with increasing the irradiation doses. The spectral dependence of the absorption coefficient on the photon energy can be divided into two main regions: (i) A weak absorption tail at low absorption region where the optical absorption coefficient a < 104 cm1 , at which the absorption coefficient usually follows the Urbach’s rule [30] according to the equation:

ðah  xÞ ¼ ao expðhx=Ee Þ

3.0

1000

1500

2000

2500

Wavelength, λ [nm] Fig. 4. Variation of refractive index of AgSbSe2 thin films with different c-doses.

ð5Þ

where ao is a constant and Ee is the Urbach’s energy (the width of the band tail of the localized states in the band gap, associated with the amorphous state). In this region, transition between (defect) states in the gap and the bands take place. Plotting the dependence of log (a) vs. photon energy, h  x as shown in Fig. 7 will give a straight line. The energy width of the tail Ee could be obtained by extrapolating the linear portions of these curves. It was found that the determined Ee values increases with the increasing the radiation c-dose. The Ee values for different c-doses are listed in Table 2.

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as-deposited 0.17

2.5 kGy 5.0 kGy 7.5 kGy

2

(n -1)

-1

0.16

0.15

0.14

0.13 0.0

0.1

0.2

0.3

0.4 2

0.5

0.6

2

Photon energy squared ( ω) [eV ] Fig. 5. Plots of (n21)1 vs. photon energy squared, ( hx2 ). Table 1 Effect of c-irradiation on the values of the Wemple-DiDomenico refractive index dispersion parameters of AgSbSe2 thin films. Dose (kGy)

Film thickness [nm]

Ed [eV]

Eo [eV]

Eg = Eo/2 [eV]

n (0) = (1 + Ed/Eo)

As-deposited 2.5 5.0 7.5

594.4 562.6 535.2 527.3

15.75 16.03 16.24 16.28

2.68 2.56 2.47 2.41

1.34 1.28 1.24 1.21

2.623 2.696 2.751 2.786

(ii) A high absorption region (a > 104 cm1) where the absorption coefficient can be describe by the parabolic relation [31]: / ðahxÞ ¼ Bðhx  Eopt g Þ

ð6Þ

where h  x, Eopt and B are the photon energy, the optical gap and g band tailing parameter respectively. The exponent / is an index that characterizes the optical absorption process and is theoretically equal to 2, 1/2, 3 or 3/2 for indirect allowed, direct allowed, indirect forbidden and direct forbidden transitions, respectively. The usual method for determining the type of optical transition and the

as-deposited 2.5 kGy 5.0 kGy 7.5 kGy

10

Tauc Model

4

Urbach Tail

10

3

1.4

1.6

n ¼ 1 þ ðbN c N e ze Þ=Eo 2.724 2.783 2.829 2.868

5

-1

Absorption coefficient, α [α in cm ]

10

0.5

1.8

2.0

Photon energy ( ω) [eV] Fig. 6. Plots of absorption coefficient, (a) vs. photon energy, ( hx2 ).

2.2

A.M. Abdul-Kader, Y.A. El-Gendy / Nuclear Instruments and Methods in Physics Research B 305 (2013) 22–28

as-deposited 2.5 kGy 5.0 kGy 7.5 kGy

4.0

-1

Log(α) [α in cm ]

27

3.9

3.8

3.7

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

2.1

Photon energy ( ω) [eV] Fig. 7. Plots of log (a) vs. photon energy, ( hx).

the value of Eopt will be given by the intercept with the photon eng ergy axis. The best plot in the present work that covers the widest range of data was obtained for the (ah  x)1/2 vs. photon energy;ð hxÞ as shown in Fig. 8. The determined values of the optical energy gap corresponding to the indirect optical transition for different irradiated c-doses are listed in Table 2. Fig. 9 shows the variation of Eopt and Ee vs. the irradiated c-doses. It is evident from Fig .9 that g the increase of radiation doses leads to a decrease of the band gap value in a reverse manner to Ee. Similar results have been reported for c-irradiated Se76Te15Sb9 thin films prepared by thermal evaporation technique [32]. The variation of Eopt and Ee with c-irradiation doses can be exg plained according to the density of states model proposed by Mott

Table 2 Effect of c-irradiation on the optical band gap and Urbach’s energy values of AgSbSe2 thin films. Dose (kGy)

Eopt [eV] g

Ee [eV]

As-deposited 2.5 5.0 7.5

1.63 1.62 1.54 1.38

0.16 0.23 0.30 0.33

corresponding band gap value, Eopt g , involves plotting a graph of (ah  x1 =uÞ) vs. photon energy, photon energy;ð hxÞ, in accordance to Eq. (2). If an appropriate value of u is used to obtain linear plot,

400 as-deposited 2.5 kGy 5.0 kGy 7.5 kGy

200

(α ω )

1/2

-1

[cm .eV ]

-1 1/2

300

100

0 1.4

1.6

1.8

2.0

Photon energy, ( ω) [eV] Fig. 8. Plots of (a hx1=2 ) vs. a vs. photon energy, ( hx).

2.2

28

A.M. Abdul-Kader, Y.A. El-Gendy / Nuclear Instruments and Methods in Physics Research B 305 (2013) 22–28

0.35

1.6

Eg

Ee [eV]

0.25

1.5

ind.

[eV]

0.30

0.20 1.4

0

3 γ-dose [kGy]

6

9

0.15

Fig. 9. Variation of Eopt g and Ee with c-irradiation dose.

and Davis [33]: According to this model, the width of the localized states near the mobility edge depends on the defects and degree of disorder present in the amorphous network structure. When the crays interact with the amorphous material, induced defects will be formed, consequently, increases the density of the localized states, which leads to increase the energy width of the band tails of localized states, Ee. This effect is supposed to be connected with a redistribution of the chemical bonds in the network structure [34,35]. The high concentration of these localized states is responsible for the low value of optical band gap Eopt [36,37]. g 4. Conclusion AgSbSe2 thin films have been deposited onto glass substrates using electron beam evaporation technique. The amorphous nature of the as-deposited films has been confirmed using X-rays and transmission electron microscope. Exposure of the deposited films to different c-doses in the range 2.5–7.5 kGy, did not change the amorphous nature of the deposited films. Elemental chemical analysis of the deposited film indicates that the deposited film has the exact chemical form of Ag1.08Sb0.94Se1.98. A red shift in the fundamental absorption edge is observed upon the film exposed to crays in the c-dose range 2.5–7.5 kGy. The refractive indices of the investigated films were found to follow the two-term Cauchy dispersion relation. The refractive index increases notably with the increase of c-dose. The refractive index dispersion in the transmission and low absorption region is adequately described by the Wemple–DiDomenico single effective oscillator model, whereby, the values of the oscillator parameters have been calculated as a function of c-dose. The analysis of the optical absorption coefficient reveals the presence of non-direct optical transition. The energy values corresponding to this transition were found to be decrease with increasing c-dose, in a reverse manner to the Urbach’s energy. This finding was discussed in the basis of Mott and Davis model. References [1] A. Harisha, V. Ravindrachary, R.F. Bhajantri, Ismayil, G. Sanjeev, B. Poojary, D. Dutta, P.K. Pujari, Polymer Degrad. Stab. 93 (2008) 1554. [2] J.X. Wen, G.D. Peng, W.Y. Luo, Z.Y. Xiao, Z.Y. Chen, T.Y. Wang, Opt. Exp. 11 (2011) 23271.

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