Thermal annealing effect on the structural and the

Optik 126 (2015) 1352–1357

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Optik journal homepage: www.elsevier.de/ijleo

Thermal annealing effect on the structural and the optical properties of Nano CdTe films M. Dongol a , A. El-Denglawey a,b,∗ , M.S. Abd El Sadek c , I.S. Yahia d,e a

Nano and Thin Film Laboratory, Physics Department, Faculty of Science, South Valley University, Qena 83523, Egypt Physics Department, Faculty of Applied Medical Sciences, Taif University, Turabah 21995, Saudi Arabia Nanomaterials Laboratory, Physics Department, Faculty of Science, South Valley University, Qena 83523, Egypt d Nano-Science & Semiconductor Lab, Physics Department, Faculty of Education, Ain Shams University, Cairo, Egypt e Physics Department, Faculty of Science, King Khalid University, P.O. Box 9004, Abha, Saudi Arabia b c

a r t i c l e

i n f o

Article history: Received 10 March 2014 Accepted 24 April 2015 PACS: 78.20.Ci 78.65 78.66.Jg Keywords: X-ray diffraction Optical properties Chalcogenide glasses CdTe alloys

a b s t r a c t Structural and optical properties of as-prepared and annealed thermal evaporated CdTe films were investigated. The annealing temperature (373–523 K) affects both structural and optical parameters. Polycrystalline nature with preferred (1 1 1), (2 0 0) and (2 2 0) orientations was released, the corresponding two theta is 23.15◦ , 27.42◦ and 40.26◦ respectively. Structural parameters as dislocation density, interplaner distance, the number of crystallites per unit surface area, lattice constant, crystallite size and strain could be determined as a function of annealing. The results confirm the films nanostructure property. The optical gap, refractive index, dielectric constant and optical conductivity increase as a function of annealing. These effects were attributed to the nano structural property and the partial reduction of the defect density. © 2015 Elsevier GmbH. All rights reserved.

1. Introduction CdTe attracted the attention of many researchers for device applications as in gamma ray detection, electro-optical modulator and solar cell fabrication [1] due to its direct-band gap, high optical absorption coefficient in the visible range of the solar spectrum, band gap is close to the optimum value for efficient solar energy conversion, p- and n-type character, zincblende and wurtzite structures [2–4]. Due to its basic optical, electronic and chemical properties CdTe can become the base material for high efficiency, low cost thin film solar cell [5,6]. The properties of CdTe films are very sensitive to preparation conditions where many preparation techniques such as vacuum deposition [7–9], electro deposition [10], molecular beam epitaxial [11], metal–organic chemical vapor deposition [12], closed-space sublimation [13] and screen-printing [14,15] were employed to prepare CdTe films. Many articles studied the different properties of CdTe films [15–26] but many others are

∗ Corresponding author at: Nano and Thin Film Laboratory, Physics Department, Faculty of Science, South Valley University, Qena 83523, Egypt. Tel.: +20 1091922044/965216663. E-mail address: [email protected] (A. El-Denglawey). http://dx.doi.org/10.1016/j.ijleo.2015.04.048 0030-4026/© 2015 Elsevier GmbH. All rights reserved.

needed for more understanding. In this study, the effect of annealing on the structural and optical properties of CdTe films deposited on glass substrate have been studied and the obtained results are discussed.

2. Experimental technique 99.999% CdTe pure grains provided by Aldrich was used as a starting material without any further purifications. CdTe films of thickness 150 nm were prepared by thermal evaporation technique on a cleaned glass substrate using a high vacuum coating unit (Edwards type E306A) in a vacuum of 10−6 Torr. X-ray diffraction (XRD) was used to study the structural properties of CdTe films by Philips diffractometer (type1710) model using CuK␣ radiation ˚ Optical properties of CdTe films were investigated ( = 1.5406 A). by measuring transmittance, T and reflectance, R spectra at normal incidence and room temperature using a double-beam UV–vis scanning spectrophotometer (Shimadzu UV-2100 combined with PC). The measurements were carried out in the wavelength range 200–1100 nm with 10 nm steps. All the optical measurements were carried out after the samples were slowly cooled to room temperature. The evaporation rate as well as the film thickness of the

1353

β

θ

M. Dongol et al. / Optik 126 (2015) 1352–1357

θ

θ

Fig. 1. XRD of the as-deposited and annealed CdTe films.

Fig. 2. Annealing effect on crystallite size of CdTe films.

evaporated films was controlled using a quartz crystal monitor FTM5. Film thickness was checked by interferometry method [27]. Extinction coefficient, Q and refractive index, n were calculated using T, R and thickness, z taking into account the experimental error of the film thickness to be ±2% and of T and R to be ±1%. The error in the calculated values of n and k are estimated to be ±3% and ±2.5%, respectively. 3. Results and discussion

Table 1 Crystallite size of CdTe films calculated by Sherrer and Cauchy. T (K)

300 373 423 473 523

Sherrer

Cauchy

111

200

220

26 27 32 33 38

23 26 29 30 36

20 23 24 28 30

19 22 29 32 39

3.1. Structural properties The as deposited CdTe films were annealed at different temperatures within range 373–523 K for 1 h under vacuum of 10−3 Torr. The XRD pattern shows that the as prepared films have polycrystalline nature as depicted in Fig. 1. The observed preferred orientations are (1 1 1), (2 0 0) and (2 2 0) at two theta equal to 23.15◦ , 27.42◦ and 40.26◦ respectively. As the CdTe films annealed at different elevated temperatures the intensity of the mentioned peaks were increased and the full width half maximum (FWHM), decreased which reflect the enhancement of crystallite size. According to the preferred orientation, CdTe films have a cubic zinc-blend structure and the most preferred orientation is (1 1 1) plane which is perpendicular to the substrate, and both (2 0 0) and (2 2 0) have weak intensity [28,29]. The obtained XRD features allow calculating both grain diameter or crystal size and material strain. The X-ray diffraction line profile broadening obtained in a diffractometer is due to the instrumental and physical (crystallite size and lattice strains) factors. Both crystallite size and lattice strain could be calculated by the determination of the “pure” diffraction line profile, whose breadth depends solely on the physical factors. This “pure” line profile is extracted by subtracting the instrumental broadening factor from the experimental line profile. After correction the obtained “pure” line profile can be used to calculate the crystallite size and lattice strain according to [30]:

ˇ = ı(2) = B

1−

b2 B2

(rad)

(1)

2 is the diffraction angle. B and b are the breadths of the same Bragg peak from the XRD scans of the experimental and reference powder [31,32] respectively, in our case used as the full width half maximum (FWHM). Crystal size or average diameter of the crystallites could be calculated using Sherrer equation [30] L=

D ˇ cos

(2)

Fig. 3. Annealing effect on both crystallite size (L) and strain of CdTe films.

is the Bragg angle, is the wavelength of X-ray used (CuK␣ radi˚ L is the crystal size, and D is the shape factor ation) = 1:54,056 A, which is approximately unity. The crytallite size and strain of CdTe films were calculated according to [30]. This method is based on the assumption that the crystallite size and strain line profiles are both presumed to be Cauchy and the appropriate equation for the separation of crystallite size and strain takes the following form [33]: ˇ cos =

+ 4e sin L

(3)

where as mentioned before, and L, e are the crystallite size and strain respectively. The values of crytallite size and strain are calculated from ordinate-intersection and slope of the graph, respectively, see Fig. 2. The crystallite size calculated by Sherrer and Cauchy is tabulated in Table 1. The values of the crystallite size are included within the nano scale and confirm the nano structure property of the as prepared and annealed films. The obtained values of crystallite size and strain at different annealing temperatures are illustrated in Fig. 3. One can observe that there is a contradiction behavior of crystallite size and strain with annealing temperature. This behavior was released


δ

1354

Fig. 4. ı and N as a function of annealing temperature for CdTe films. Fig. 5. Lattice parameter of CdTe films annealed at 523 K.

by the decreasing of FWHM which reflects the decreasing in the concentration of lattice imperfections (structural defects) and consequently to the increasing of the crystallite size and film quality [15,28,32,34]. The dislocation density, ı of annealed CdTe films is defined as the length of dislocation lines per unit volume and evaluated from the relation [35,36]: ı=

1 L2

(4)

Table 2 Inter-planer distance (d) and the lattice constant (a) of CdTe films. Temperature

300 373 423 473 523

d(F())

6.467 6.487 6.461 6.474 6.489

d (calculated)

a (calculated)

(1 1 1)

(2 0 0)

(2 2 0)

3.8726 3.8721 3.8719 3.8799 3.8781

3.2370 3.2408 3.2490 3.2392 3.2454

2.2313 2.2334 2.2318 2.2361 2.2334

6.467 6.488 6.461 6.474 6.489

The number of crystallites per unit surface area (N) could be determined according to [37]: N=

d

(5)

(L)3

Fig. 4 shows both ı and N as a function of annealing temperature. It is noticed that N has the same trend of ı; both decreasing with the increasing of annealing temperature. More details will be considered through optical section. XRD profile could be used to calculate the inter-planer distance (d) and the lattice constant (a) according to next equations: Bragg’s law known as [35]: d=

m 2 sin

(6)

is the wavelength of the X-ray used, (d) is the lattice spacing, (m) is the order number and is the Bragg’s angle. The plane-spacing equation for cubic crystal is given by 1 = d2

(h2

+ k2

+ l2 )

(7)

a2

(h k l) are the Miller indices of the planes [38]. By substituting about (d) value according to Eq. (6) in Eq. (7), we get sin2 =

2 2 (h + k2 + l2 ) 4a2

[(h2 + k2 + l2 ) a= 2 sin

1/2

(8)

]

(9)

To obtain the most accurate value of (a) that is calculated from Eq. (9), it is preferable to determine it again for each diffraction line and plotting it against an angular function of cos2 () which is known as the Nelson Riley plots and is given by [39]: 1 f () = 2

cos2 cos2 + sin

(10)

Fig. 5 shows the obtained values of (a) for CdTe films annealed at 523 K.

The obtained values of (d) and the values of (a) calculated according to Eq. (10) are summarized in Table 2. The calculated values are augmented by [40–42]. It is noticed that (a) is characterized by slight changes, this changes confirms that the film crystallites are strained. This may be due to change of defects concentration, similar behavior was observed elsewhere [43]. The thermal annealing greatly reduces, but does not completely eliminate, the defect density. In addition, higher the annealing temperatures, higher the size of crystallization of CdTe films [44]. 3.2. Optical properties 3.2.1. Optical band gap calculation Both of T, R and z were used to determine the absorption coefficient, ˛ according to [45]: T = (1 − R)2 e−˛d

(11)

Optical absorption coefficient (˛) is given by: ˛=

1 (1 − R)2 ln z T

(12)

Extinction coefficient of CdTe film was calculated by using: Q =

˛ 4

(13)

An absorption edge of semiconductors corresponds to the threshold of charge transition between the highest nearly filled band and the lowest nearly empty band. According to inter-band absorption theory, the films optical band can be calculated using the following relation [46]. opt r

(˛h) = A(h − Eg )

(14)

where A is the probability parameter for the transition, which measure the disorder of the material [47,48] and related to the localized-state tail width E through: A=

4min n(0)cE

(15)

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α υ


υ

Fig. 6. (˛h)1/2 vs (h) for the as deposited and annealed CdTe films. Fig. 8. Costant A as a function of annealing temperature for CdTe films.

λ

opt

Fig. 7. A Egi for the as deposited and annealed CdTe films.

(

)

Fig. 9. n vs for the as deposited and annealed CdTe films.

where min is the minimum electrical conductivity, n(0) is the static refractive index, c is the light-velocity, and E = Ec − Ev represents the band tailing [49,50]. For crystalline semiconductors, E is null and according to Eq. (15), the smaller the A value, the higher the structural disorder. opt Egi is the optical band gap of the material, h is the incident photon energy and r is the transition coefficient. The value of r is 1/2, 3/2, 2 and 3, depending on the nature of electronic transition responsible for the absorption.r = 1/2 for allowed direct transition, r = 3/2 for forbidden direct transition, r = 2 for allowed indirect transition and r = 3 for forbidden indirect transition. The optical gap of CdTe was found, indirect optical band gap, and evaluated by extrapolating the straight line part of the curves (˛h)1/2 with energy axes (h ) i.e. (˛h )1/2 = 0 according to: (˛h)

1/2

opt

= A(h − Egi )

(16)

Fig. 6 shows the relation between (˛h)1/2 and (h ) for asdeposited and annealed CdTe films. The values of Eg opt and the constant A could be determined and depicted in Figs. 7 and 8. The obtained values of Eg opt are augmented by [26]. According to Eq. (15) and Fig. 8, values of the constant (A) and the crystallite size (L) opt confirm the behavior of Egi . Result obtained at 523 K was excluded due to its unusual behavior. 3.2.2. Refractive index and dielectric constant Both R, and Q at different were used to calculate refractive index, n according to [51]: 1+R n= + 1−R

4R (1 − R)2

− Q2

(17)

Τ

Fig. 10. Values of n at wavelength 700 nm for the as deposited and annealed CdTe films.

The spectral dependences of n() is plotted in Fig. 9 for different annealing temperatures, Ta of CdTe films, the anomalous dispersion is dominated through this wavelength. It could be noticed that n is practically dependent of Ta . The values of n at = 700 nm is shown in Fig. 10, the general trend of n increases as the annealing temperature is increased. The frequency dispersion of the dielectric constant, ε characterizes completely the propagation, reflection, and loss of light in multilayer structures. It provides information about the electronic structure of the material. Therefore, it is an important quantity for


δ

1356

ν (

)

Fig. 11. ε1 vs (h) for the as deposited and annealed CdTe films. Fig. 13. tan of CdTe films as a function of annealing temperature.

Fig. 14. 1 and 2 for the as deposited and annealed CdTe films. Fig. 12. ε2 vs (h) for the as deposited and annealed CdTe films.

the design of highly efficient optoelectronic devices. ε is described as [52,53]: ε = ε1 − iε2 tan ı =

ε2 ε1

(18) (19)

where ε1 is the real part of the dielectric constant, ε2 is the imaginary part of the dielectric constant and tan ı is the loss factor which determines how well a material can absorb the electromagnetic field [54,55]. Both ε1 and ε2 of ε are related to n and k according to equations [53,37]: ε1 = n2 − Q 2

(20)

ε2 = 2nQ

(21)

The spectrum of real and imaginary parts of the dielectric constant is shown in Figs. 11 and 12, the values of ε1 and ε2 were determined at = 700 nm is shown in same figures inset, the general trend increases as the annealing temperature increase. In addition, the loss factor, tan ı, of the as-deposited and annealed CdTe film has been calculated using Eq. (19). The variation of tan ı with photon energy for the as-deposited and the annealed films is shown in Fig. 13. Since tan ı displays a maximum starting around 1.46 eV which opt corresponds to indirect energy gap Egi . 3.2.3. Optical conductivity The optical conductivity is one of the powerful tools for studying the electronic states in materials [56,57]. If a system is subjected to an external electric field, in general, a redistribution of charges occurs and currents are induced. For small enough fields, the

induced polarization and the induced currents are proportional to the inducing field. The complex optical conductivity, is related to the complex dielectric constant, ε by [58]:

⎧ ⎫ = 1 + i2 ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎨ 1 = ωε2 ε0 ⎪ ⎬ ⎪ 2 = ωε1 ε0 ⎪ ⎪ ⎪ ⎩

ε = ε1 − iε2

⎪ ⎪ ⎪ ⎪ ⎭

(22)

ω is the angular frequency, ε0 is the free space dielectric constant. The real, 1 and imaginary, 2 parts of the optical conductivity dependence of energy as a function of annealing temperature are shown in Fig. 14. One can observe that the two vertical arrows show that both 1 and 2 suffered a peak near 1.46 eV which corresponds opt to indirect optical gap Egi . It is seen that the optical conductivity, 1 increases with increasing photon energy and also increases as a function of annealing temperature. This suggests that the increase in optical conductivity is due to electrons excited by photon energy. The origin of this increase may be attributed to some changes in the structure due to the charge ordering effect [59]. 4. Conclusions Thermal evaporation technique was used to prepare CdTe films on glass substrates. Structural and optical properties were investigated for the as-prepared and annealed films. XRD investigation proved that as deposited and annealed have polycrystalline nature with preferred (1 1 1), (2 0 0) and (2 2 0) orientations at two theta equal to 23.15◦ , 27.42◦ and 40.26◦ respectively. The dislocation density (ı), inter-planer distance (d), the number of crystallites per unit surface area (N), lattice constant (a), crystallite size (L) and strain (e) could be determined as a function of annealing temperature. The annealing temperature increases L and


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