Microstructure and crystal imperfections of nanosized ...

Superlattices and Microstructures 85 (2015) 67–81

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Microstructure and crystal imperfections of nanosized CdSxSe1x thermally evaporated thin films Alaa A. Akl a, A. S. Hassanien b,⇑,1 a b

Physics Department, Faculty of Science in Ad-Dawadmi, Shaqra University, 11911, Saudi Arabia Engineering Math. and Physics Dept., Faculty of Engineering (Shoubra), Benha University, Egypt

a r t i c l e

i n f o

Article history: Received 29 January 2015 Received in revised form 7 May 2015 Accepted 9 May 2015 Available online 19 May 2015 Keywords: Thin films Physical vapor deposition (PVD) Energy dispersive analysis of X-rays (EDAX) Defects Microstructure

a b s t r a c t Cadmium sulfoselenide CdSxSe1x thin films were thermally evaporated onto preheated glass substrates (523 K). The evaporation rate and film thickness were kept constant at 2.5 nm/s and 375 ± 5 nm, respectively. Microstructure and crystal imperfections of deposit CdSxSe1x thin films were studied using X-ray diffraction (XRD) and energy dispersive analysis by X-ray (EDAX). XRD analysis reveals the formation of films have the semi-crystalline nature and the hexagonal structure with preferential h0 0 2i direction. The microstructural parameters such as, lattice parameters, the crystallite size (D), microstrain hei, residual internal stress (S), dislocation density (d) and number of crystallite per unit volume (N) were calculated and found to be dependent upon the composition. The presence percentage of Cd, S and Se elements in the chalcogenide CdSxSe1x thin films were estimated by EDAX and a comparative study with other similar samples of the previous literature was discussed. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Chalcogenide CdSxSe1x thin films are most promising materials due to the possibility of adjusting and tailoring their electrical properties, especially in the optoelectronics, nonlinear optics and photo ⇑ Corresponding author. 1

E-mail address: [email protected] (A.S. Hassanien). On leave to: Physics Department, Faculty of Science in Ad-Dawadmi, Shaqra University, 11911, Saudi Arabia.

http://dx.doi.org/10.1016/j.spmi.2015.05.011 0749-6036/Ó 2015 Elsevier Ltd. All rights reserved.

68

A.A. Akl, A.S. Hassanien / Superlattices and Microstructures 85 (2015) 67–81

electrochemical solar cell applications [1–3]. Microstructure of thin films differs from the structure of the bulk material of similar compositions and can vary depending on the growth conditions. The thermal evaporation processes of obtaining epitaxial films are related to the interaction of the deposited atoms of compound to the glass substrate surface. At first complex processes occur, that affect the mechanical deposition film, thermal accommodation, the staking coefficient with the substrate surface, the nucleation rate and the continued growth of the film. All these factors cause a change in the microstructure properties and morphology of thin films [4,5]. Moreover, nanosized materials have anomalous different properties such as extreme ductility without strain hardening, higher strength and hardness, number of crystallite/cm3 and density of dislocation. It has also been found that microstructure and the stoichiometry of CdSSe thin films were extremely sensitive to its element component ratios where selenium does as donor impurities [6–10]. Thin films of the ternary CdSxSe1x compositions can be synthesized by different techniques such as thermal evaporation technique [11–13], chemical bath deposition (CBD) technique [14–16], laser ablation technique [17] and chemical spray pyrolysis [1,18]. The thermal evaporation process is widely used because it is an accurate method, where the preparative conditions (evaporation rate, film thickness, surface morphology and the structural state) can be controlled [4,5]. This technique has already been used for preparation of the thin films of semiconducting compounds and alloys including CdSSe ternary systems [11,12]. In a previous work, thin films of the ternary chalcogenide compound of CdS0.1Se0.9 have been prepared by thermal evaporation where the physical properties were studied as a function of the film thickness [12]. We report here the synthesis of cadmium sulfoselenide thin films using by physical evaporation technique. CdSxSe1x (0.0 6 x 6 0.4) thin films were characterized by XRD and EDAX. Microstructure and crystal imperfections were studied by using the line profile analysis method (LPA) and obtained results were investigated and discussed. A comparative study was carried out among the present thin films and similar samples of the previous literature.

2. Experimental details 2.1. Bulk preparation High-purity starting materials of CdS and Se (99.999%) were obtained to synthesis the chalcogenide CdSxSe1x compounds. Semicrystalline samples of CdSxSe1x were prepared by the solid state method using CdS and Se according to their atomic mass ratios in stoichiometric quantities. The mixture of the ingots has been milled together in an electric mill for 5 h. The outputs were pressed using a high pressure compressor to get pellets in order to be used in the thermal evaporation process and consequently to avoid the scattering of the component of the compositions. The density of the homogeneous formed pellet samples was measured at room temperature using the Archimedes principle. Non solvent buoyant liquid (Toluene) is used for measuring the weight of immersing pellet. According to the following equation:

q¼

W air q DW Liquid

ð1Þ

where Wair is the weight of the sample in air, DW = Wair Wsol is the weight difference and qLiquid is the density of toluene. To insure the accuracy of the density measurements, the weights have been repeated several times and then taking the average value. The molar volume (Vmol) of the ternary CdSxSe1x compounds was also calculated using the following relation:

V mol ¼

Mw

q

ð2Þ

where MW is the sum of individual atomic weights multiplied by their existent percentage in ternary samples [18,19]. Some parameters that concerned with the input materials of Cd, S and Se, which obtained from the previous literature and are used in the present study were reported in Table 1.

69


Table 1 The crystal structure and the values of: atomic mass, mass density, molar volume, ionic radius and electro-negativity of Cd, S and Se used in the present study. Element

Crystal structure

Atomic mass (g/mol)

Mass density (g/cm3)

Molar volume (cm3/mol)

Ionic radius (1012 m)

Electro-negativity (Pauli units)

Cd S Se

Hexagonal Orthorhombic Hexagonal or monoclinic

112.411 32.065 78.960

8.650 2.070 4.820

13.000 15.530 16.420

124 170 184

1.978 2.957 3.014

2.2. Thin films preparation In the present investigation, CdSxSe1x thin films were deposited onto preheated glass substrates (523 K). The glass substrates were cleaned successively using ethanol, acetone and finally ultrasonic cleaning. The thermal evaporation process was carried out by using Edward’s coating unit (E 306A) at a vacuum of the order 8.2 104 Pa. Molybdenum boat sources were used for the film growth process. Film thickness and deposition rate were monitored during deposition by a quartz crystal oscillator (Edwards model FTM3). The deposition rate during the evaporation process was kept constant at 2.5 nm/s. Prepared chalcogenide CdSxSe1x thin films were controlled to get films have the same thickness. The accuracy of the film thicknesses was checked after the evaporation process using multiple-beam Fizeau fringes in reflection. The obtained thin films have thicknesses 375 ± 5 nm. 2.3. Thin film characterizations A JEOL X-ray diffractometer (Model JSDX-60PA) operated at 40 kV and 35 mA was used to characterize the obtained samples. The used X-ray source was of Cu Ka-radiation and has a wavelength equal to 0.154184 nm. Continuous scanning was applied with a slow scanning rate (1°/min) and a small time constant (1 s). Diffraction angle in the range of 4–100° was scanned to be able to identify any possible phases of composition. The crystal imperfection parameters such as; crystallite size, microstrain, residual internal stress, number of crystallite/cm2 and dislocation density were estimated by using the line profile analysis technique (LPA) [20]. The observed broadening (B) of the full-width at half-maximum (FWHM) was investigated using step scanning mode of 2h = 0.02° where the counting time was kept constant at 4 s per step. X-ray profile of standard highly crystalline silicon was used to make instrumental broadening corrections (b). These broadening corrections were carried out to get the broadening of pure samples in order to determine the crystal imperfections. Energy dispersive analysis of X-rays (EDAX) is used to determine the composition elements of CdSxSe1x thin films. For EDAX measurements, smooth crystal surfaces were examined with a Kevex micro analyst 7500 system and ordinary ZAF corrections were applied. 3. Results and discussion 3.1. X-ray diffraction analysis Fig. 1 shows the XRD of as deposited thin film samples prepared from the compound CdSxSe1x (0.0 6 x 6 0.4). It can be seen that, a semi-crystalline phase is obtained for all selenium concentrations. The diffraction peaks of this phase were only three, where the diffraction lines located at the angles 2h = 23.05°, 25.55° and 34.20°. Miller indices of the calculated d-values of the diffraction peaks were (1 0 0), (0 0 2) and (1 0 1). They were compared to the standard d-values (PDF numbers 77-2306 and 77-2307). X-ray identification confirms the formation of the chalcogenide CdSxSe1x phase of a hexagonal structure, although the growth of films was on the surface of glass [1,14,21–23]. The lattice parame0

0

ters of the ternary compound CdSSe were estimated and found to be equal to 4.157 Å A and 6.980 Å A. This indicates that the crystal growth process of films has been occurring in the perpendicular direction to the substrate, i.e. in the direction of the c-axis of the unit cell [24]. Moreover, all diffraction lines lightly


Intensity of XRD (arb.u.)

(100)

(101)

(002)

70

CdS0.4Se0.6 CdS0.3Se0.7 CdS0.2Se0.8 CdS0.1Se0.9 CdS

10

20

30

40

50

60

70

80

90

100

2 θ (degree) Fig. 1. X-ray diffractograms of CdSxSe1x thin films at different Se-concentration.

deviate from its position with the addition of selenium as well as an increase in the peak broadening. This indicates that selenium atoms inserted in the host lattice of CdS [25]. In addition, the increasing of selenium content reduces the degree of the crystallization of the formed CdSSe thin films. Effect of selenium concentration on the d-values of the prepared compositions was recorded in the Table 2 and illustrated in Fig. 2a. We can observe that, the shift of the d-values increases linearly with the addition of more selenium. This result indicates that, the sulfur atoms have been replaced by selenium atoms, where the atomic radius of selenium is greater than the atomic radius of sulfur, therefore there is a variation occurs in the d-value [26]. On the other hand, the dependence of the corrected broadening, b as functions of Se-concentration is tabulated in Table 2 and graphically represented in Fig. 2b. One can see also that, there is a linear relation between the corrected peak breadth, b and the concentration of selenium in the CdSSe matrix, which is attributed to the decrease in the crystallite size with adding more Se [27]. The predicted chemical reactions that were occurring for synthesis the ternary CdSSe phase are as follows:

2CdS þ 2Se ! CdS þ CdSe þ SSe

ð3Þ

CdS þ CdSe þ SSe ! 2CdSSe

ð4Þ

The chemical reaction mechanism of the ternary CdSSe system can be explained as follows; according to electrochemical activity series of elements Cd is the electropositive, while the two elements S and Se are electronegative but Se has a negativity more than S. Therefore, sulfur can behave as positive with respect to selenium. Eqs. (3) and (4) describe the probability of the reaction procedures. Taking into considerations, the crystal structure of both CdS and CdSe is hexagonal structures, where the value of the lattice parameters for CdS is ‘‘a’’ = 4.124 Å and ‘‘c’’ = 6.686 Å (PDF number 80-0006), while Table 2 X-ray diffraction data analysis and the lattice parameters of ternary chalcogenide CdSxSe1x thin films. Cd:S:Se

1.0:1.0:0.0 1.0:0.4:0.6 1.0:0.3:0.7 1.0:0.2:0.8 1.0:0.1:0.9 1.0:0.0:1.0

0

2h (°)

Corrected broadening, b (radian)

dhkl (Å A)

(1 0 0)

(0 0 2)

(1 0 0)

(0 0 2)

24.90 24.42 24.37 24.19 24.13 23.95

25.55 25.90 25.84 25.65 25.60 25.40

3.576 3.645 3.653 3.680 3.689 3.716

3.486 3.440 3.448 3.473 3.480 3.508

Lattice parameters 0

2.699 103 3.378 103 4.018 103 4.511 103 5.632 103 6.094 103

0

a (Å A)

c (Å A)

4.129 4.209 4.218 4.249 4.257 4.291

6.750 6.880 6.896 6.946 6.959 7.015


71

5.6 4.8

β x 10

-3

(radian)

6.4

4.0

(b) 3.2

d (002) (A)

3.55

3.50

3.45

(a) 3.40 0.6

0.7

0.8

0.9

1.0

Se Concentration Fig. 2. Representation of both: (a) the shift in d-values, and (b) the corrected broadening as functions of Se-content. Table 3 Values of mass density and molar volume of the ternary chalcogenide CdSxSe1x systems, where 0.0 6 x 6 1.0. Cd:S:Se

1.0:1.0:0.0 1.0:0.4:0.6 1.0:0.3:0.7 1.0:0.2:0.8 1.0:0.1:0.9 1.0:0.0:1.0



Measured

Calculated

4.833 5.465 5.548 5.582 5.656 5.724

4.826 5.394 5.528 5.670 5.754 5.674

14.947 15.791 15.979 16.302 16.503 16.717

for CdSe is ‘‘a’’ = 4.291 Å and ‘‘c’’ = 7.015 Å (PDF number 77-2307). Consequently, the increasing of the selenium content leads to increase the unit cell volume of the formed CdSSe compounds. XRD data confirm that, no other phases formed consequently, the formed ternary CdSSe phase is a stable composition and it does not depend on the concentration of selenium. This result indicates that the stoichiometry of CdSSe thin films is verified.

3.2. Mass density and molar volume determinations Measured values of the mass density, q and molar volume, Vmol were recorded in Table 3 and illustrated in Fig. 3. It can be observed that, the density and molar volume of the CdSxSe1x compositions increases with the addition of more selenium. This is due to the density and atomic mass of the selenium atom is greater than those of the sulfur atom, as well as the difference in the atomic radii of each (the radius of Se atom is larger than that of S). This can be ascribed to the replacement of Se atoms instead of S atoms [18,19]. The accuracy of measured density and consequently the molar volume values with respect to the theoretical values was better than ±0.15%.

3.3. Effect of Se content on the lattice parameters The crystal structure of the ternary CdSxSe1x alloys was found to be hexagonal for all samples in the studied composition range of (0 6 x 6 0.4). The lattice constants of hexagonal structure ‘‘a’’ and ‘‘c’’ are determined by using the relation [20]:

72



18 17 16 15

(b)

14 13


6.0

5.5

5.0

(a) 4.5 0.0

0.2

0.4

0.6

0.8

1.0

Se concentration Fig. 3. The dependence of both; (a) mass density (g/cm3) and (b) molar volume (cm3/mol) on the selenium content in the bulk of ternary CdSSe.

1 2

d

2

¼

2

2

4 ðh þ hk þ k Þ l þ 2 3 a2 c

ð5Þ

The lattice parameters of ternary compound CdSxSe1x as functions of the selenium concentration have been determined using the two diffraction lines (1 0 0) and (0 0 2). The estimated values of ‘‘a’’ and ‘‘c’’ have been listed in the Table 2 and illustrated in Fig. 4.

"c/a" ratio

1.6355 1.6350

0.0

0.2

0.4

0.6

0.8

1.0

(c)

1.6345 -4

c/a = 1.6347 + 1.5932x10 x

"c" (nm)

1.6340 0.70

(b)

0.69 0.68 c (nm) =0.6739 + 0.0252 x

"a" (nm)

0.67 0.430

(a)

0.425 0.420 0.415

a (nm) = 0.4122 + 0.0153 x

0.410 0.0

0.2

0.4

0.6

0.8

1.0

Se concentration Fig. 4. Plotting of: (a) the lattice parameter ‘‘a’’, (b) the lattice parameter ‘‘c’’ and (c) the ratio ‘‘c/a’’, all are represented as a function of Se contents.

73


It is evident from the figure that, the lattice parameters ‘‘a’’ and ‘‘c’’ as functions of the selenium concentration are almost linear in accordance with the following fitting equations, respectively:

a ðnmÞ ¼ 0:4122 þ 0:0153x

ð6Þ

c ðnmÞ ¼ 0:6739 þ 0:0252x

ð7Þ

The ratio ‘‘c/a’’ was calculated for all CdSSe thin films as a function of selenium content and the results were plotting in Fig. 4. It was found that, there are also a linear relationship between ‘‘c/a’’ and Se-content in the CdSxSe1x matrix according to the following fitting equation:

c=a ¼ 1:6347 þ 1:5932 104 x

ð8Þ

It can be concluded that, the lattice parameters of CdSSe thin films are compatible with the Vegard’s law [28,29]. The lattice parameters ‘‘a’’ and ‘‘c’’ and also the ratio ‘‘c/a’’ of the CdSxSe1x compounds for any selenium concentration can be obtained using Eqs. (6)–(8), respectively. From Eqs. (6) and (7), and at x = 0, for pure CdS, the lattice parameters equal to 0.4122 nm and 0.6739 nm and agree completely with the PDF number 6-314, while at x = 1, for pure CdSe, the lattice parameters equal to 0.4275 and c = 0.6991 nm which is also in a good agreement with the PDF number 80-0006. In the other direction, Fig. 4c reveals that the ratio ‘‘c/a’’ almost constant where the slope of the obtained straight line is very small and equals about 1.6 104. This indicates that the expansions in the unit cell are occurring in the three dimensions with the same value. These results can be interpreted as follows, since the selenium atom is inserted within the host unit cell of CdS instead of sulfur atom and the size of selenium atom is greater than that of the sulfur atom. So the dimensions of the host unit cell will be enlarged in the three directions by the same extension. In addition, the lattice parameters strains Da/ao and Dc/co as functions of selenium contents were listed in Table 4 and illustrated in Fig. 5a and b. The represented measured data were fitted to obtain straight lines satisfying the following equations [28,29]:

Da=ao ¼ 2:1068 104 þ 0:03735x and Dc=co ¼ 7:1852 105 þ 0:03743x

ð9Þ

where (ao, co) are the lattice parameters of the standard CdS system, Da = a ao, and Dc = c co, (a, c) are the measured lattice parameters and x is the Se ratio of the composition. This is a result of the partial replacement of S2 ions by Se2 ions where the ionic radius of Se (0.184 nm) is greater than that of S (0.170 nm). These results are in good agreement with previous data [27,30–32]. 3.4. Crystallite size The crystallite size is one of the most important parameters of the micro-structure of nanocrystalline materials. It has a fundamental importance because of its macroscopic properties and technological applications. The volume weighted column length LV, contributes to the line broadening of XRD peaks evaluated by using Debye Scherrer’s formula [20]:

LV ¼

kk bC cos h

ð10Þ

Table 4 Both the lattice constants and the lattice parameters strain as functions of Se-concentration. Cd:S:Se

Lattice parameters ‘‘a’’

1.0:1.0:0.0 1.0:0.4:0.6 1.0:0.3:0.7 1.0:0.2:0.8 1.0:0.1:0.9 1.0:0.0:1.0

Lattice parameters ‘‘c’’

a (Å A)

D a = a ao

Da/ao 103

c (Å A)

D a = a ao

Dc/co 103

4.129 4.209 4.218 4.249 4.257 4.291

0.028 0.013 0.061 0.092 0.100 0.134

6.736 3.009 14.670 22.131 24.056 32.235

6.750 6.880 6.896 6.946 6.959 7.015

0.230 0.099 0.084 0.034 0.021 0.035

33.008 14.298 12.063 04.872 02.951 05.014

0

0

74


0.04

(b)

Δc/co)

0.03 0.02

0.00

(a)

0.03 0.02 0.01 -4

Δ a/ao) =2.1068x10 +0.03735x

)

)

Δ a/ao)

0.04

-5

Δ c/co) =7.1852x10 +0.03743x

)

)

0.01

0.00 0.0

0.2

0.4

0.6

0.8

1.0

Se Concentration Fig. 5. The lattice parameters strain as functions of the Se contents.

where k is the constant (0.94), k is the wavelength of the X-ray (0.154184 nm), bC is the corrected integral breadth of the Lorentzian (Cauchy) component in radiant of (0 0 2) line of XRD pattern; {bC = B b and bG = (B2 b2)1/2, where B is the broadening of the observed sample and b is the instrumental broadening correction} and h is Bragg’s diffraction angle. The crystallite size, D can be calculated from the volume-weighted column length LV (the crystallite is assumed to be a spherical shape). Hence, LV = 3D/4, where if not all spherical crystallite are of the same size, the value of D still depends on the size distribution. Crystallite size, D of the ternary CdSSe as a function of Se content was estimated from the diffraction line (0 0 2). The obtained results were recorded in Table 5 and plotted in Fig. 6a. It can be seen that, crystallite size decreases linearly with increasing selenium content of the CdSxSe1x network and satisfying the following fitted relationship:

D ðnmÞ ¼ 57:396 31:15x

ð11Þ

This result is due to that, the addition of more selenium contributes to the increment of the amorphous state of CdSxSe1x compositions. These results are in excellent agreement with previous published data [1,21]. On the other hand, the presence of any defects in the nanocrystalline materials such as crystallite size, vacancies, residual internal stress, micro strain, vacancy clusters and dislocations lead to the excess free volume at the grain boundaries. This free volume and surface/interface stress generates more stresses. These stresses cause the lattice parameters strains (Da and Dc). They given by the following equations [30,31]:

Da 4 Sa ¼ ao 3 KD Dc 4 Sc ¼ co 3 KD

ð12Þ ð13Þ

Table 5 Microstructure parameters of CdSxSe1x thin films as a function of Se contents. Cd:S:Se

1.0:1.0:0.0 1.0:0.4:0.6 1.0:0.3:0.7 1.0:0.2:0.8 1.0:0.1:0.9 1.0:0.0:1.0

Volume weighted column length Lv (nm)

Crystallite size, D (nm)

Microstrain e 103

36.710 29.350 24.640 21.970 17.590 16.250

55.060 44.020 36.960 32.950 26.390 24.380

2.976 3.675 4.379 4.954 6.197 6.760

Residual stress (dy/cm2) S1 109

S2 109

3.404 1.522 7.416 11.19 12.16 16.29

16.62 7.226 6.096 2.462 1.492 2.534

Number of crystallite/ cm2 1011

Dislocation density, d 1010 (line/cm2)

02.235 04.408 07.368 10.566 20.349 25.533

03.299 05.161 07.320 09.211 14.359 16.824

Crystallite size (nm)

Microstrain, ε x10-3


75

7 6 5 4

ε = 2.42x10-3 + 3.59x10-3 x

3

(b)

2

60

(a)

50 40 30

D (nm) = 57.396 - 31.15 x

20 0.0

0.2

0.4

0.6

0.8

1.0

Se concentration Fig. 6. Crystallite size and microstrain as functions of Se content.

where Sa and Sc are the interfacial energies per unit area along ‘‘a’’ and ‘‘c’’-directions, respectively, K is the bulk modulus and D is the crystallite size (the diameter of the spherical crystallite). The variation of lattice parameter strain with the reciprocal of the crystallite size of the CdSxSe1x matrix was calculated and recorded in table Tables 4 and 5 and then represented in Fig. 7a and b. This figure shows that, there is linear behavior between the lattice parameters strain and 1/D according to the following fitting equations:

Da=ao ¼ 0:01708 þ 1:4285ð1=DÞ and Dc=co ¼ 0:01728 þ 1:4326ð1=DÞ

ð14Þ

Since the addition of selenium atom to cadmium sulfide causes an increase in the dimensions of the unit cell, but reduce the amount and the number of crystallites which consisting of the crystal agglomerates and consequently the addition of more Se decreases the crystallite size. This is also evidence that, the selenium element does not support the occurrence of the crystallization process of the CdSxSe1x thin films.

36

(b) Δ c /co

24 12 0

Δ c /c o = -0.01728 + 1.4326 (1/D)

-12 10

(a)

Δ a /ao

0 -10 -20 -30

Δ a/a o = - 0.01708 + 1.4285 (1/D)

-40 0.015

0.020

0.025

0.030

0.035

0.040

0.045

1/D (nm-1 ) Fig. 7. The relationship between the lattice parameters strain and the reciprocal of the crystallite size.

76


3.5. Microstrain Microstrain causes a broadening of the XRD lines without affecting the peak position. Microstrain of any sample is expressed as the root mean square value, hei and hence its value is always positive. Generally, the source of microstrain is ascribed to crystal imperfections, such as excess volume of grain boundaries, vacancies and vacancy clusters and dislocations. Therefore, nanostructure materials which have a small crystallite size and microstrain contribute to the broadening of the XRD lines [33]. The broadening due to microstrain is caused by the non-uniform displacements of the atoms with respect to their reference-lattice positions and the microstrain within the domain is considered as lattice defects [34]. Since the microstrain is equivalent to variations in the d-spacing within domains by an amount depending on the elastic constants of the material and the nature of internal stresses. The microstrain, e of the CdSxSe1x thin films were calculated by the following relations [20]:

e¼

bG cot h 4

ð15Þ

The estimated values of microstrain of CdSxSe1x thin films were listed in Table 5 and represented graphically in Fig. 6b. It is observed that, microstrain is increased by increasing the Se content according to the following linear fitted equation:

e ¼ 2:42 103 þ 3:59 103 x

ð16Þ

This can be explained as follows, selenium atoms were considered as impurities with respect to CdS, so they lead to displace the plane position which indicates to the formation of the hexagonal structure. This shift in the diffracted plane position or the microstrain is produced as a result of the growth process of samples and the preparation condition [35]. 3.6. Dependence of microstrain on crystallite size To understand the imperfections of nanocrystalline materials, the effect of the following must be taken into account; (i) the intracrystalline pressure, (ii) interfacial stresses, (iii) grain-boundary enthalpy, (iv) excess grain-boundary volume and (v) supersaturating of vacancies. At the same time, it cannot be ignored the lattice distortion owing to the atomistic structure of these materials. Therefore, the effect of microstrain on the crystallite size, which produced from the interfacial stresses was investigated. Microstrain broadening is ubiquitous at infinitesimal crystallite size. It is most prominent in nanocrystalline materials with D, of around 30 nm or below [36]. The behavior of an interfacial microstrain with crystallite size was represented in Fig. 8. It can be seen that, this

7

Microrostrain ε x10-3

6

5

4

3

2 20

25

30

35

40

45

50

55

Crystallite size (nm) Fig. 8. The variation of the microstrain versus the crystallite size.

60


77

microstrain is decreased with increasing the crystallite size. This result is due to the decreasing of the volume occupied by the arranged atoms inside of the agglomerate crystalline; so the total surface area was increased. Therefore, this change in the surface will decrease the shift of plane position; i.e. it reduces the microstrain [37].

3.7. Average residual stress Residual internal stress is another important factor that depends on the synthesis and affects the microstructure properties of crystalline materials. This parameter describes the microstrain in the crystal lattice that is evident in the variation of the lattice parameter [35]. The change in the distance between atoms is important to the residual internal stresses, where it is important for preparing– property relationships. The total residual internal stress in thin films is composed of a thermal stress and an intrinsic stress. The thermal stress is due to the difference in the thermal expansion coefficients of the film and substrate material. The intrinsic stress is due to the accumulating effect of the crystallographic flaws that are built in the film during growth. It is difficult to avoid thermal stress for depositing films at any temperature [37]. There are several ways to characterize the microstructure and residual stress of materials. One of the most common is a line profile analysis of X-ray diffraction (LPAXRD). This technique has some advantages over others. In the present case lattice parameters ‘‘a’’ and ‘‘c’’ of the hexagonal crystal system are calculated for the two lines (1 0 0) and (0 0 2) by using the following equations:

E ao a 2c ao E co c S2 ¼ 2c co

S1 ¼

ð17Þ ð18Þ

(b)

18 12 6 0

9

S2 = 17.3623 - 18.2026x10 x

-6

9

Stress, S1 x10 dy/cm

2

2

9

Stress, S x10 dy/cm

2

Residual internal stress is a convolution of both S1 and S2, where ‘‘a’’, ‘‘c’’ and ‘‘ao’’, ‘‘co’’ are the lattice parameters of the thin film samples and bulk samples, respectively, while E and c are the Young’s modulus and Poisson’s ratio of the bulk sample, respectively. For CdSSe, the value of E is 34.77 GPa and c is 0.334 [38]. The estimated residual stresses S1 and S2 of the hexagonal CdSxSe1x at different Se concentrations were tabulated in Table 5 and graphically represented in Fig. 9. It was observed that, the residual

5

(a)

0 -5 -10 -15 -20

9 S1 = 5.2863 - 19.2212x10 x

0.0

0.2

0.4

0.6

0.8

1.0

Se-concentration Fig. 9. Dependence of residual stresses S1 and S2 along hai and hci directions, respectively on Se content.

78


internal stresses were decrement with an addition of more Se as a fitted linear relationship according to the equations:

S1 ¼ 17:3623 18:2026 109 x and S2 ¼ 5:2863 19:2212 109 x

ð19Þ

The residual internal stresses are due to both of the different preparative parameters and the size of inserted selenium atom in hexagonal unit cell. As revealed from XRD analysis, there is a reduction in the crystallization process of CdSxSe1x by adding more selenium. Therefore, the effect of preparation parameters is neglected and hence the remaining one is the effect of the size of the inserted selenium atom only. Thus, the resultant of the overall internal stresses decrease with increasing selenium content [32,39]. 3.8. Dislocation density The line imperfection acting as a boundary between the slipped and un-slipped region, lies in the slip plane and is called a dislocation. Dislocations are generated and move when a stress is applied. The strength and ductility of materials are controlled by dislocations. The dislocation density (d), is defined as the length of dislocation lines per unit volume. In the present study, the dislocation density is estimated from the following relation [40,41]:

d¼

1

ð20Þ

D2

It is known that, the calculated value of d is attributed to the amount of defects in a crystal growth of CdSxSe1x thin films. Dislocation density as a function of Se content were calculated and recorded in Table 4 and represented in Fig. 10a. This figure reveals that, the d values were depending on the number of Se atoms of CdSxSe1x compound. This result is a logic where XRD showed that, the formed phase is partially crystalline so the line defects are easily occurred. These results are in good agreement with the data obtained by others [13,40,42] for CdS and like phase thin films. 3.9. Number of crystallite per unit volume

δ x 1010 (line/cm2 )

11

N x10 Cryst./cm

2

The crystallite number per unit volume depends on the following parameters; crystallite size, crystallite shape, equidimensions crystallites and the degree of agglomeration of thin films. It can determine the best size and shape of crystallites by the density and the type of atoms which present in

30

(b) 20 10 0 16

(a)

12 8 4 0.0

0.2

0.4

0.6

0.8

1.0

Se concentration Fig. 10. Dependence of both the dislocation density and number of crystallite per unit area upon the Se-concentration.

79


crystalline clusters. If the current atoms are light, the crystallite size will be medium and contains the largest volume of crystalline clusters or agglomerate while in the case of high-density atoms (in the range of less than 100 nm), the crystallite size or agglomerate may be less. The number of crystallites per unit volume (N) of CdSxSe1x thin films was calculated from the estimated values of crystallite size (D), using the following formula [43,44]:

N¼

t

ð21Þ

D3

where t is the film thickness. These calculated values of (N) as a function of Se content were listed in Table 5 and plotted in Fig. 10b. It was observed that, for all investigated thin films of different Se content, the number of crystallite size per unit volume was increased by adding of more Se. This is due to that, the reduction in the crystallite size of CdSSe matrix when someone adds more Se to the matrix [12,35]. 3.10. EDAX analysis of CdSxSe1x thin films The energy dispersive analysis of X-rays (EDAX) was used to study the quantitative elemental analysis of the undoped and Se doped CdS thin film as well as the study of the stoichiometry of CdSxSe1x Table 6 Atomic mass percentage of ternary composition CdSxSe1x thin films, calculated atomic ratios, previous published data and our measured data by using electron dispersion X-ray spectroscopy (EDAX). Sample

CdSe

CdS0.1Se0.9 CdS0.2Se0.8

Calculated values of the atomic mass percentage

Previous published data

Cd

S

Se

Cd

S

Se

Preparation method

Ref. No.

Cd

S

Se

58.7398

00.00

41.26017

63.00

0.00

37.00

[24]

57.89

00.00

42.11

40.05

0.00

59.95

38.00

0.00

62.00

38.0675 34.7092

47.12 – 34.47

0.00 – 12.95

52.88 – 52.58

[16] – [19]

49.85 49.61

07.43 09.67

42.72 40.72

08.03 09.78 – 13.42

44.73 40.75 – 20.13

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[16] [5] – [19]

49.28 49.09

12.46 18.12

38.26 32.79

[16] [17]

–

–

–

[19]

–

–

–

[16] [24]

–

–

–

77.81

22.19

00.00

60.2154 61.7670

1.7176 3.5238

CdS0.3Se0.7 CdS0.4Se0.6

63.4007 65.1231

5.4255 7.4395

31.1738 27.4464

47.24 49.47 – 66.45

CdS0.5Se0.5

66.9418

9.5475

23.5107

50.31 32.40

19.62 20.18

30.07 47.42

CdS0.6Se0.4

68.8649

11.7861

19.3489

68.62

18.83

12.55

CdS0.8Se0.2

73.0629

16.6728

10.2642

49.53 70.00

28.73 22.00

21.74 8.00

71.48

22.98

5.54

47.62 82.00

39.41 18.00

12.97 0.00

72.48

27.52

0.00

50.53

49.17

0.00

48.86

51.14

0.00

CdS

77.8060

22.1940

00.00

Present EDAX data of the atomic mass percentage

[19] [17]

[19] [16] [24] [19] [17] [16]

80


thin films. The elemental analysis was carried out for Cd, S and Se elements of the ternary CdSSe network to get the average atomic percentage ratio of Cd:S:Se. Comparative study of the atomic ratios of CdSxSe1x systems among the theoretical data, published previous data and our present data were recorded in Table 6. It can be seen that for all prepared thin films of CdSSe systems, the selenium content has been completely incorporated to the cadmium sulfide. The relative analysis of EDAX of the present investigated CdSxSe1x thin films reveals that, it cannot obtain the same theoretical stoichiometry for all films. This is due to the experimental errors and the thermal evaporation process. The obtained results were found to be in good agreement with the previous literature [1,14]. 4. Conclusions (i) Chalcogenide Ternary thin films of CdSxSe1x compositions can be deposited by thermal evaporation technique by doping selenium element to cadmium sulfide with different Se concentrations. (ii) X-ray diffraction analysis revealed that, the thin films of the ternary CdSxSe1x matrix are deposited in the hexagonal crystal structure with preferential orientation along h0 0 2i direction. (iii) The microstructural parameters and crystal imperfection were calculated using line profile analysis method (LPA) and their dependency upon selenium doping concentration are investigated. (iv) Comparative study of the quantitative elemental analysis of our obtained data and the previously published data is carried out.

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