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Effect of CdCl2 heat treatment in (Ar + O2) atmosphere on structural and optical properties of CdTe thin films
Optik 160 (2018) 307–321
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Original research article
Effect of CdCl2 heat treatment in (Ar + O2 ) atmosphere on structural and optical properties of CdTe thin films M.F. Hasaneen a,b,∗∗ , W.S. Mohamed a,∗ a b
Physics Department, Faculty of Science, Sohag University, 82524 Sohag, Egypt Physics Department, Faculty of Science, Aljouf University, Aljouf, Saudi Arabia
a r t i c l e
i n f o
Article history: Received 11 December 2017 Received in revised form 26 January 2018 Accepted 28 January 2018 Keywords: CdTe thin films CdCl2 heat treatment X-ray diffraction Field emission scanning electron microscope Optical energy gap
a b s t r a c t Cadmium telluride (CdTe) thin films were deposited by thermal evaporation method under base pressure 2 × 10−5 mbar on corning glass substrate. In this work, we study the effect of cadmium chloride (CdCl2 ) heat treatment in argon and oxygen (Ar + O2 ) atmosphere on the structural, morphological and optical properties of CdTe thin films to achieve high quality thin film absorber layer for solar cells applications. The structural characteristics were studied by X-ray diffraction (XRD). The microstructure parameters of the films such as average grain size (Davg ), average lattice strain (εavg ) and dislocation density (D ) have been calculated via XRD line broadening analysis. XRD investigation revealed that, the samples show polycrystalline nature pronounced with cubic zinc blende structure with a strong preferentially (1 1 1) texture orientation. It has been found that the crystallinity of the films enhanced during (Ar + O2 ) annealing and CdCl2 heat treatment. The surface morphology of CdTe thin films was investigated by field emission scanning electron microscope (FE-SEM). XRD and FE-SEM results of CdCl2 treated films showed recrystallization and progressive increase in grain size. The optical properties of all samples were estimated using UV–vis–near-infrared (NIR) spectrophotometer. The Swanepoel envelope method has been employed to evaluate the various optical parameters of CdTe films such as refractive index and film thickness. The as-deposited films have a relatively high value of refractive index of 2.54, in contrast the CdCl2 treated films have a relatively low value of 2.1 in the whole wavelength range. The optical energy gap values slightly decrease from 1.56 eV for as-deposited films up to 1.54 eV for CdCl2 heat treated films. CdCl2 heat treatment in (Ar + O2 ) atmosphere is found to be a promising method to improve the physical properties of CdTe thin film solar cells. © 2018 Elsevier GmbH. All rights reserved.
1. Introduction The II–VI semiconductor compounds, i.e., cadmium telluride (CdTe) has been in the focus of comprehensive research due to their possible utilization in many applications such as Opto-electronic devices [1,2], photovoltaic cells [3], laser windows material [4,5], gamma rays and X-ray detectors [6,7], photoconductive devices [8], and light emitting diodes [9]. The main fundamental characteristics of CdTe are: perfect direct band gap around 1.45 eV at room temperature (RT), which is the optimum value for solar cell devices [10], it is cheap and inordinately steady during the application processing, resulting
∗ Corresponding author. ∗∗ Corresponding author at: Physics Department, Faculty of Science, Aljouf University, Aljouf, Saudi Arabia. E-mail addresses: [email protected] (M.F. Hasaneen), [email protected] (W.S. Mohamed). https://doi.org/10.1016/j.ijleo.2018.01.112 0030-4026/© 2018 Elsevier GmbH. All rights reserved.
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in powerful interests and quick evolution of CdTe-based solar cells devices in recent years [11], large optical absorption coefficient (>104 cm−1 ) which means that around 90% of incident sunlight in the visible range can be absorbed by only a few micrometers of CdTe thin films. CdTe is one of the most suitable candidates as absorber layer for CdTe/CdS solar cells devices [12,13]. Solar cells which formed from CdTe/CdS systems are one of the expected candidates for prevalent commercial success in solar energy transformation. On the other hand, CdTe can be formed in both n-type and p-type conductivity, therefore, CdTe is useful for homojunction and heterojunction organization. CdTe thin film has been prepared by many different techniques like, thermal evaporation [14], electron beam evaporation [15], RF sputtering [16], sol–gel spin coating [17], close spaced sublimation (CSS) [18,19], pulsed laser deposition (PLD) [20], electrodeposition [21,22], metalorganic chemical vapor deposition (MOCVD) [23], spray pyrolysis deposition (SPD) [24], screen printing [25], molecular beam epitaxy (MBE) [26] and chemicals deposition techniques including chemical bath deposition (CBD) [27]. The thermal evaporation is the most widely method for preparation of thin films because of its easy to use, scalability and reproducibility [28,29]. Furthermore, the deposition rate is high which can be controlled easily and damage to the substrate during deposition can be minimized. Several studies have reported on the physical properties of CdTe thin films. However, the effect of CdCl2 heat treatment on the optical and structural properties of CdTe thin films has not been fully studied yet. Generally, the as-deposited films would have many undesirable characteristics that affect the competence of the device execution, such as small grains, low crystallinity and many grain boundaries which lead to high electrical resistivity. The existence of Cl atoms could diffuse into the grain boundary of CdTe thin films during crystal growth which causes improve the recrystallization, the grain growth, and electronic properties of CdTe thin films [30,31]. In fact, the main key for enhancing the efficiency of the thin film solar cell is heat treatment by CdCl2 on the top of the CdTe thin films (300–400 nm) with annealing temperature in the range 350–450 ◦ C for 10–60 in air or inert gas under atmospheric conditions. Several researchers have been studied this treatment to improve the efficiency of solar cell thin film; however, a full comprehension of that treatment has not been completed yet and remain a subject of discussion. The major objective of this study was to investigate the effects of CdCl2 heat treatment in (Ar + O2 ) atmosphere on physical properties of thermally evaporated CdTe thin films and compared with (Ar + O2 ) annealing process. This study has been carried out by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and UV–vis–near-infrared (NIR) spectrophotometer. The structural, morphological and optical properties on these films are reported in this paper.
2. Preparation and examination of CdTe thin films 2.1. Deposition technique CdTe thin films were prepared by thermal vacuum evaporation technique (Edward’s high vacuum unit model AUTO 306) using a stoichiometric CdTe powder (99.999%) from Sigma Aldrich as a source material. The CdTe thin films were deposited onto corning glass substrates (1 cm × 1 cm × 0.1 cm). The glass substrates were cleaned in an ultrasonic bath in deionized water, acetone and ethanol and dried using an argon gas flow. The CdTe films were deposited with a thickness between 1100 and1200 nm at room temperature with a base pressure of 2 × 10−5 mbar. The deposition rate and film thickness during the deposition process can be controlled by the digital film thickness monitor (INFICON SQM-160). The deposition rate was varied in the range of (2–3) Å/s. The chamber pressure was approximately constant at 5 × 10−5 mbar and 2 × 10−5 mbar at room temperature (RT) in the beginning and end of film deposition respectively.
2.2. CdCl2 heat treatment and (Ar + O2 ) annealing procedure The heat treatment in (Ar + O2 ) atmosphere with the presence of CdCl2 was carried out inside a furnace tube model Kejia to improve the optical and structural properties of the CdTe thin films. On the other hand, annealing in (Ar + O2 ) atmosphere is preferred to air ambient when preparing efficient CdTe solar cell to avoid the impurities from air which can affect the film structure. Fig. 1 summarized the heat treatment procedure. The heat treatment process can be summarized as:
(a) CdCl2 layer of thickness about 150 nm has been deposited on the top of the as-deposited CdTe thin films by thermal evaporation technique. (b) The achieve of thermal annealing was carried out by the temperature sequence at 400 ◦ C for 15 min, 450◦ for 15 min and finally at 480 ◦ C for 10 min. (c) During heat treatment process the gasses of Ar and O2 were flowing in the furnace tube by rate of 8 sccm. (d) The samples were kept in the furnace tube until their gradual cooling to room temperature is achieved, which takes around 3 h. (e) The (Ar + O2 ) annealing experimental was done under the same conditions (i.e., time, temperature, gas flow) without the presence of CdCl2 .
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Alumina tube
F
F
309
Gas outlet Samples P
P
Thermocouple Temperature controller
O2
Ar
Bubble
Bubbler
Fig. 1. Schematic diagram of the equipment used to perform the heat treatment procedure.
2.3. Investigation techniques The crystallographic structure of the CdTe thin films was studied by X-ray diffractograms ((Philips X’pert MRD) with Cu-Ka radiation (i.e., = 1.5418 Å), incident angle of 0.75◦ and over a 2 scan range between 10◦ and 80◦ . The field emission scanning electron microscope (FE-SEM, Model: Quanta 250 FEG, with accelerating voltage 30 K.V and magnification up to 1,000,000×) was used to study the surface morphology and composition. A double beam UV–vis–near-infrared (NIR) spectrophotometer (Model: JASCO, V-570 in a wavelength range between 200–2500 nm at normal incidence) was used to evaluation the optical properties of CdTe thin films. 3. Results and discussion 3.1. Structural analyses by XRD The X-ray diffraction studies were carried out on as-deposited, (Ar + O2 ) and CdCl2 heat treated CdTe films. The average grain size (Davg ) was evaluated using the Debye-Scherer formula as the following relation [32]: Davg =
L ˇ2 cos
(1)
where L is a Scherrer constant equal to 0.94, is the wavelength of source radiation (i.e. = 1.5418 Å), ˇ2 is the full width at half maximum (FWHM) in radians and is the Bragg angle. The average lattice strain (avg ) which depending on the growing conditions was evaluated by the following relation [32,33]: εavg =
ˇ2 cos 4
(2)
The dislocation density (D ) is known as whole length dislocation lines per unit volume of the crystals and was determined using Williamson-Smallman relation [32,33]: D =
1 2 Davg
(3)
Fig. 2 illustrates the XRD patterns of thermally evaporated CdTe thin films at each process (i.e., as-deposition, (Ar + O2 ) annealing and CdCl2 heat treatment). It has been found that, all films are polycrystalline representing a cubic zinc blende structure with a strong preferentially (1 1 1) texture orientation. From Fig. 2 it can be deduced that the intensity of the peaks increases with both (Ar + O2 ) annealing and CdCl2 heat treatment indicates an improved crystalline quality that occurs after heat treatment process. It is well known that, CdCl2 heat treatment usually improves the crystallinity of CdTe up to certain temperature. This may be related to the increase in surface mobility of adatoms on the substrate surface due to sufficient thermal energy because of higher annealing temperature [33]. The sites of diffraction peaks of as- deposited CdTe films are observed at 2 = 23.38◦ , 38.98◦ , 46.18◦ , 72.04◦ , and 75.94◦ corresponding to C (111), C (220), C (311), C (422) and C (511) planes, respectively. These XRD peaks agree with the Joint Committee on Power Diffraction and Standards (JCPDS) reference
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Fig. 2. The XRD patterns of as-deposited, (Ar + O2 ) annealed and CdCl2 treated CdTe thin films.
Table 1 The structural parameters of as-deposited, (Ar + O2 ) annealed and CdCl2 treated CdTe thin films. Process 2 (◦ )
Structure (hkl)
As-deposited film 23.38 38.98 46.18 72.04 75.94
FWHM (radians)
Davg (nm)
C (111) C (220) C (311) C (422) C (511)
0.478 0.892 0.666 0.847 0.721
17.7 9.87 13.6 12.1 14.6
(Ar +O2 ) annealed film C (111) 23.38 38.98 C (220) C (311) 46.06 C (422) 72.10 C (511) 75.94
0.369 0.314 0.321 0.480 0.329
23.0 28.0 28.0 21.4 31.8
CdCl2 heat treated film 23.38 C (111) 39.04 C (220) C (311) 46.12 C (422) 72.04 C (511) 75.94
0.363 0.325 0.310 0.393 0.362
23.33 26.48 29.20 26.11 29.00
Mean value of Davg (nm)
εls (10−3 )
13.6
10.1 11.0 6.80 5.10 4.00
26.4
7.8 3.9 3.3 2.9 1.9
26.8
7.7 4.0 3.2 2.4 2.0
Mean value of εls (10−3 )
D (1011 cm−2 )
7.4
3.10 10.2 5.40 6.83 4.69
6
3.96
1.89 1.27 1.27 2.18 0.99
1.52
3.84
1.83 1.42 1.17 1.46 1.18
1.41
Mean value of D (1011 cm−2 )
Note: C, Cubic; FWHM, full width at half maximum; Davg , average of grain size; εls , lattice strain; D , dislocation density.
file No: 15-0770 Sys, Cubic (C). Some new phases are observed for CdCl2 heat treated and (Ar + O2 ) annealed CdTe films like CdTeO3 phase with positions at 2 = 10.06◦ , 20.5◦ , 27.16◦ , 31.18◦ , 34.3◦ and 40.36◦ corresponding to JCPDS file No: 22-0128 and Cd3 TeO6 phase with a position at 2 = 70.9◦ corresponding to JCPDS file No: 82-0125 as shown in Fig. 2. In fact, when heating cadmium telluride in the oxygen atmosphere, this is accompanied by losses in weight, which is referred to the sublimation of the highly volatile Te. This result more Cd than Te are present on the film surface and the resulting films produced are Cd-rich CdTe material. The sublimation of Te is creating the vacancies in the crystal lattice which gives the chance for oxygen to diffuse into the crystal lattice. Incorporate the oxygen into the crystal lattice may allow to modification of the band gap structure which is followed up by a change in physical properties, as mentioned in previous studies [34]. The structural parameters like the average grain size (Davg ), the average lattice strain (εavg ) and dislocation density (D ) for as-deposited, (Ar + O2 ) annealed and CdCl2 heat treated CdTe films were estimated by using standard relations [1–3] and presented in Table 1. The average grain sizes are 13.6 nm, 26.4 and 26.8 nm for as-deposited, (Ar + O2 ) annealed and CdCl2 heat treated CdTe films respectively, this means the average grain size increase with both (Ar + O2 ) annealing and CdCl2 heat treatment process. These results might be due to many reasons: coalesce of small grains into bigger ones, the relaxation of excessive internal lattice strains [35], and Cl and O take position in crystal lattice of CdTe layers through CdCl2 heat treatment process [33]. It was found that, the average lattice strain (εavg ) and dislocation density (D ) of CdTe films are decreased with the (Ar + O2 ) annealing and CdCl2 heat treatment process as tabulated in Table 1. This indicates that the CdCl2 heat treatment in (Ar + O2 ) atmosphere could improve the crystalline quality and reduce the lattice imperfection of CdTe films by decreasing the amount of lattice strain and dislocation density.
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3.2. Surface morphology and microstructure analysis by FE-SEM The surface morphologies and microstructures investigation of CdTe thin films grown on glass substrates were undertaken employing a field emission scanning electron microscope (FE-SEM). FE-SEM is one of the most important methods to analyze the surface morphology and microstructure of thin films that provide high definition images at low accelerating voltages and small working distances. Fig. 3 illustrates the surface morphologies and microstructures images for all samples. The surface morphology image of the as-deposited CdTe thin films as shown in Fig. 3(A) referring that the CdTe thin film consist of small grains with varying values around 7–62 nm. The grain size of CdTe films after (Ar + O2 ) annealing show a partially larger than the as-deposited with varying values between 45 and 155 nm i.e., Fig. 3(B). After CdCl2 heat treatment a great change in the shape and size of the CdTe grains in the range of 75–155 nm is observed as shown in Fig. 3(C). It’s clear that the CdCl2 heat treatment leads to grain boundary segregation which is called grain growth. This may be attributed to the crystallization seed for CdTe during heat treatment process which reduce the atomic diffusion barrier at the CdTe grain boundaries. On comparing the average grain size of the CdTe samples obtained by the SEM microstructure analysis software with the value obtained through XRD analysis, we notice that the size obtained from the SEM analysis is over-estimated. Such a difference can be explained by the fact that, each grain is formed by aggregation of many nanocrystals [33]. 3.3. Optical analysis by UV–vis–near-infrared spectroscopy 3.3.1. Optical transmittance and estimation of the optical energy gap The optical behavior of CdTe thin films which determined by both transmittance (T ) and reflectance (T ) spectra have been estimated by a double beam UV–vis–near-infrared (NIR) spectrophotometer in the wavelength range of 200–2500 nm at normal incidence. The transmittance and reflectance spectra of all samples are illustrated in Fig. 4. It has been found that, the maximum intensity of transmittance up to 89%, 76% and 69 for as-deposited, (Ar + O2 ) annealed and CdCl2 heat treated CdTe films, respectively. Moreover, the interference fringes are observed in the transmittance spectra at a higher wavelength (i.e., 800–2100 nm) which refer to the homogeneity and harmony of the deposited films. The optical energy gap (Eg ) of all samples were calculated by the Tauc equation which describe the correlation between the optical absorption coefficient (␣) and the optical energy gap (Eg ) as the follows: n
(˛h) = K (hv − Eg ) where ˛ =
(1 − R)n 1 ln[ ] T d
(4)
here h is the phonon energy, K is a constant, which related to electronic transition probability, d is the thickness of the deposited film and n is an exponent, which called the power factor of the electronic transition mode. The values of n for direct allowed, indirect allowed, direct forbidden and indirect forbidden electronic transitions are 1/2, 2, 3/2 and 3, respectively by plotting (˛h) as a function of (h) according to Eq. (4) for all n values (1/2, 2, 3/2 and 3) for all samples, it was found that the best fit of our experimental results is obtained for n = 1/2 (i.e., direct allowed transition). Moreover, the direct allowed transition of CdTe thin films have been commonly observed by several authors [36–38]. The linear part of the plot of (˛h)2 vs. (h) to the axis of energy at ␣ = 0 is shown in Fig. 5. It can be observed that the optical energy gap slightly decreases from 1.56 eV for as-deposited film to 1.55 eV and 1.54 eV for (Ar + O2 ) annealed and CdCl2 heat treated CdTe films respectively. Such decreases in the optical energy gap may be attributed to the increase in the crystallinity and grain size of CdTe thin films upon heat treatment, which in turn leads to a deficiency of the role of defect states or decrease in the density of localized state [15]. These results are consistent with the earlier reports on CdTe films obtained by many authors. For example, Nima E. Gorji [39] was reported the decrease in optical energy gap as a result of CdCl2 treatment due to the increase in grain size. The CdCl2 heat treatment in (Ar + O2 ) atmosphere results in a smaller optical energy gap, which in turn leads to higher absorption. This feature highlights the advantage of CdCl2 heat treated CdTe film as an absorber layer in heterostructure thin film solar cells. 3.3.2. Estimation of the band tail width or Urbach energy The absorption coefficient (␣) of the band tail energy or Urbach energy (Eu ) is a quantity specifies the degree of structural disorder in thin films. In fact, this quantity characterizes the local defect or localized states near the optical energy gap. The Urbach empirical relation can be calculated by using the following relations [40], ˛ = ˛0 exp(
hv − Ep0 ) Eu
In (˛) = In(˛0 ) + exp(
(5) hv − Epo ) Eu
(6)
where ␣0 and Ep0 are the Urbach absorption edge parameters and Eu is the band tail width or Urbach energy. The Eu values can be deduced from the slope of linear fit to the logarithmic plot of the absorption coefficient to the photon energy as shown in Fig. 6. The estimated values of the Eu for all samples were given in Table 2. It is observed that the Eu value is found to increase with decreasing optical energy gap. It was found that the obtained values of Eu for as-deposited, (Ar + O2 ) annealed and CdCl2 heat treated CdTe films are 81, 84 and 86 meV, respectively. As a result, simultaneous decreasing of optical energy gap and broadening of band tail width have taken place after heat treatment process. This may be attributed to appearance
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Fig. 3. FE-SEM images of CdTe thin films at different processes: (A) as-deposition, (B) (Ar + O2 ) annealing and (C) CdCl2 heat treatment.
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Fig. 4. The spectral distribution of transmittance and reflectance of as-deposited, (Ar + O2 ) annealed and CdCl2 treated CdTe thin films deposited on glass substrate. Table 2 The refractive index (n), static refractive index (n0 ), high frequency dielectric constant (ε∞ ), dispersion energy (Ed ), single oscillator energy (E0 ), static dielectric constant (εs ), free carrier concentration (N), plasma frequency (ωp ), relaxation time (), oscillator wavelength (0 ), oscillator strength (S0 ) and electronic polarizability (˛p ) of as-deposited, (Ar + O2 ) annealed and CdCl2 treated CdTe thin films deposited on glass substrate. Optical parameter
As-deposited film
(Ar + O2 ) annealed film
CdCl2 heat treated film
Energy band gap, Eg (eV) Urbach energy Eu (meV) Approximate value of the refractive index, n Film thickness, d (nm) Electronic Polarziability, ˛p × 10−24 Relative density p Single oscillator energy, E0 (eV) Dispersion energy, Ed (eV) Static refractive index, n0 Lattice dielectric constant, ∞1 High frequency dielectric constant ∞2 free carrier concentration, N × 1019 Plasma frequency, ωp (1014 Hz) Average oscillator strength, S0 (10−4 nm−2 ) Inherent absorption wavelength, 0 (nm)
1.56 81 2.54 1118 7.66 0.85 6.00 27.5 2.3 6.0 6.78 26.2 42.2 0.93 228
1.55 84 2.29 1000 6.95 0.66 6.10 30.8 2.44 6.08 6.00 0.18 0.29 28 42
1.54 86 2.10 1282 6.31 0.53 7.6 35.3 2.36 4.73 4.57 9 14.7 1.28 161
of defects, which cause redistribution of the localized states from band to tail and then facilitates more possible transitions from band to tail and tail to tail, which leads to the simultaneous shrinkage and extending of optical energy gap and Urbach tail, respectively. 3.3.3. Estimated of the refractive index and film thickness The values of refractive index and the film thickness can be calculated by using the Swanepoel’s envelope method, which is mainly based on a creating the envelopes of interference maxima and minima suggested by Mainficier et al. [41,42]. Fig. 7 gives an example of the typical transmittance T() and reflectance R() spectra for as-deposited CdTe thin film which is used for optical constant calculation. The first approximate value of the refractive index (n) in the spectral regions of the medium and weak absorption can be estimated by the following relation [41],
n2 = [N1 + N12 − s2
12
]
(7)
Where N1 = 2s
T − T s2 + 1 m M TM Tm
+
2
(8)
here TM and Tm are the maximum value of the transmittance and the corresponding minimum value at a certain wavelength, respectively. One of these values taken from an experimental interference extreme and the other one taken from the
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Fig. 5. Direct optical energy gap determination by Tauc relation of as-deposited, (Ar + O2 ) annealed and CdCl2 treated CdTe thin films deposited on glass substrate.
corresponding envelope. The refractive index of the glass substrates has value which calculated according to the following relations, s=
1 + Ts
1 TS
12
−1
(9)
where Ts is the maximum value of the transmittance spectra of glass substrate as noted in Fig. 4. The fundamental relation which describe the interference fringes can be written as follows, 2nd = m
(10)
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Fig. 6. Linear fit of the absorption coefficient ln(␣) vs. incident photon energy (h) of as-deposited, (Ar + O2 ) annealed and CdCl2 treated CdTe thin films.
Fig. 7. Typical transmittance T() and reflectance R() spectra for as-deposited CdTe thin film estimated by Swanepole’s envelops method.
where m is an integer for maxima and half integer for minima, n is refractive index, d is the film thickness. Where the order numbers m is integer for maxima and half integer for minima. Furthermore, if n1 and n2 are the refractive indices of the thin film at two corresponding adjacent maxima or minima at wavelengths 1 and 2 , respectively, then the film thickness d can be calculated from the basic interference waves equation as the following relation, d=
1 2 2(1 n1 − 2 n2 )
(11)
The values of refractive indices n and the film thicknesses d for different samples were calculated by using relation (7) and (11) respectively and tabulated in Table 2. Using the approximate values of the refractive index listed in Table 2 one can obtain the values of the refractive index over the whole spectral range of measurement i.e., 250–2500 nm, by using the two-term Cauchy empirical formula as given below. n () = a +
b 2
(12)
where a and b are Cauchy’s constants, which depending on the intercept and slope of n vs. −2 linear plot. The dispersion of refractive index n is fitted by Eq. (12). The least squares fit of the two sets of values of n are n = 2.53 + 2.51 × 105 /2 , n = 2.21 + 2 × 105 /2 and n = 1.94 + 1.96 × 105 /2 for as deposited, (Ar+O2 ) annealed and CdCl2 heat treated CdTe films, respectively. As shown in Fig. 8, the refractive index for all samples progressively decreases up to 1000 nm and above that range, stability occurs without any remarkable change with increase the wavelength. It is also observed that the refractive index changes between 2.1 and 2.54 for all samples. K. Punitha [43] have reported that the refractive index changes between 2.46 and 2.51 for vacuum evaporated CdTe thin films deposited on glass substrates, which matches well with our finding. It is worth to indicate that, the refractive index of a semiconductor material usually has a close relationship with an optical energy gap [33], which is agreement with our results as shown in Table 2.
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Fig. 8. Dispersion of refractive index as a function of wavelengths of as-deposited, (Ar + O2 ) annealed and CdCl2 treated CdTe thin films deposited on glass substrate.
Fig. 9. Plot of the electronic polarizability vs. incident photon energy (h) of as-deposited, (Ar + O2 ) annealed and CdCl2 treated CdTe thin film deposited on glass substrate.
3.3.4. Electronic polarizability The essential character of optical properties can be determined by electronic polarizability (␣p ), which provides an information about the form of bound system and the internal structure of the molecule. The electronic polarizability depends on the refractive index using Classius-Mossotti of dielectric constant as shown in the following relation [44],
␣p =
n2 − 1 n2 + 2
3M 4Navo D
(13)
where ˛p is the electronic polarizability, n is the refractive index, M is the molecular weight, D is density and Navo is the Avogadro’s number. Fig. 9 illustrates the graph of photon energy with (n2 −1)/(n2 + 2). The electronic polarizability values of all samples can be calculated from the extrapolation of the linear zone to the y-axis. As seen in Table 2 the values of electronic polarizability (˛p ) decrease from 7.66 × 10−24 for as-deposited film to 6.95 × 10−24 and 6.31 × 10−24 for (Ar + O2 ) annealed and CdCl2 treated CdTe films, respectively. The decreasing of ˛p can be explained by spread of the Cl and O inside the CdTe
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Fig. 10. Linear graph of the refractive index related quantity (n2 −1)−1 vs. square of photon energy (h)2 according to WD model of as-deposited, (Ar + O2 ) annealed and CdCl2 treated CdTe thin films deposited on glass substrate.
layer which take new sites during CdCl2 heat treatment and (Ar + O2 ) annealing, therefore the volume of the electronic space decreased, as well as decreasing the volume occupied by the electrons. 3.3.5. Relative density Relative density can be defined as the relationship between the refractive index of the deposited film nf and the refractive index of the bulk nb (i.e., nb for CdTe is 2.72). The relative density can be calculated by Lorentz-Lorentz relation through the following relationship [43],
p=
pf pb
=
n2f − 1
n2b + 1
n2b + 1 n2b − 1)
(14)
The relative density of the film decreases with both (Ar + O2 ) annealing and CdCl2 heat treatment respectively, as reported in Table 2, which may be due to the crystallization enhancing and increase in crystallite size. 3.3.6. Dispersion of the refractive index The refractive index dispersion can be analyzed using Wemole-DiDomenico (WDD) single oscillator model which identifies the dispersion of optical constant. Below the absorption region, the relation between refractive index and oscillator strength can be obtained by the following relation [33],
n2 − 1 =
Ed E0
E02 − (h)
2
(15)
where E0 and Ed are the single oscillator energy and dispersion energy respectively. By drawing the refractive index which takes a quantity 1/(n2 −1) versus the square of photon energy (h)2 and fitting a straight line as shown in Fig. 10, the values of both single oscillator energy (Ed ) and dispersion energy (E0 ) directly estimated from the intercept (E0 /Ed ), and from the slope (1/E0 Ed ) on the vertical axis, respectively. Table 2 shows that the values of E0 and Ed are increasing with both (Ar + O2 ) annealing and CdCl2 heat treatment. This may be attributed to an increase in the optical energy gap, which has a relationship with E0 according to WDD modal (E0 ≈ 2Eg ). These results mean that the optical energy gap and dispersion constant change with both (Ar + O2 ) annealing and CdCl2 heat treatment. Furthermore, the static refractive index n0 can be calculated from the oscillator parameters E0 and Ed , which supply an important information on the structure and density of the investigated material such as the lattice dielectric constant ε∞1 = n20 by the following relation, Ed 1/2 ) E0
n0 = (1 +
(16)
As shown in Table 2 the static refractive index increases with both (Ar + O2 ) annealing and CdCl2 heat treatment. This may be attributed to the enhancement of the crystallinity. 3.3.7. Dielectric constants The real (ε )and imaginary (ε ) parts of complex dielectric constant of CdTe thin films for each process can be calculated by using refractive index (n) and extinction coefficient (k) (i.e., k = ␣/4) data, according to the following relations,
ε (h) = ε (h) − iε (h)
(17)
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Fig. 11. Linear graph of the dispersion of the real part of dielectric constant as a function of 2 of as-deposited, (Ar + O2 ) annealed and CdCl2 treated CdTe thin films deposited on glass substrate.
Fig. 12. Linear graph of the dispersion of the imaginary part of dielectric constant as a function of 3 of as-deposited, (Ar + O2 ) annealed and CdCl2 treated CdTe thin films.
Both first and second parts of the previous relation (i.e., real and imaginary part, respectively) of complex dielectric constant are expressed by the following [45–47],
ε (h) = n2 (h) − k2 (h)
(18)
ε (h) = 2n(h)k(h)
(19)
In the real part ( ), the lattice dielectric constant at high frequencies (ε∞2 ) which explained the participation of the free carrier electric susceptibility and lattice vibrational modes of dispersion, can be estimated as the following [44],
= n2 − k2 = ∞2 −
e2 42 c2 0
N m*
2
(20)
where c is the light velocity, e is the electron charge, m* is the effective mass of the charge carrier, ε0 is the permittivity of free space and N is the free carrier concentration in conduction band. According to Drude model the high frequency dielectric constant ∞2 and the free carrier concentration N can be calculated from the intercept and negative slop of the = f(2 ) linear plot as illustrated in Fig. 11. From the same line, the electrons pair with oscillating electric field which called the electron plasma frequency (ωp ) is given by [48], ωp2 =
N m*
(
e2 ) ε0 ε∞1
(21)
The estimated values of the n0 , ε∞ , N and ωp for all samples were tabulated in Table 2. In the imaginary part, the variation of ε as a function of wavelength 3 (nm)3 are shown in Fig. 12. The value of the optical relaxation time can be calculated from the positive slope of the obtained straight line in imaginary part, which controlled by a linear relationship with 3 according to the following relation,
ε = 2nk =
1 e2 N 1 3 ( )( )( ) 4 3 ε0 c 3 m*
(22)
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319
Fig. 13. Linear graph of (n2 −1)−1 as a function of the −2 of as-deposited, (Ar + O2 ) annealed and CdCl2 treated CdTe thin films deposited on glass substrate.
3.3.8. Inherent absorption wavelength (interbond oscillator wavelength) studies In the ultraviolet range, many semiconductors material have an inherent absorption wavelength also usually called interbond oscillator wavelength. The Inherent absorption wavelength (0 ) and the average oscillator strength (S0 ) can be calculated using Drude-voigt dispersion relation [43]:
n2 − 1 =
e2 mc 2
( 12
Nm S0
−
0
(23)
1 ) 2
where Nm is the number of molecular per unit volume of thin film. From the previous equation we only take a single term Sellemeier oscillator as a function of refractive index [43], (n20 − 1) (n2
− 1)
2
=1−(
0 )
(24)
where n0 is the static refractive index. Relation (24) can be modified as the following, (n2 − 1)
−1
=
1 S0 20
−
1 S0 2
(25)
Fig. 13 shows the graph plotted between (n2 −1)−1 versus −2 , so the values of S0 and 0 can be calculated from the negative slope (1/S0 ), and the intercept ( 1 2 ) on the vertical axis as given in Table 2. S0
4. Summary and conclusions Based on the previous results, the main finding can be summarized in the following points: – The influence of (Ar + O2 ) annealing and CdCl2 heat treatment on the structural, optical and morphological properties of thermal evaporated CdTe thin films has been investigated with an effort to promote recrystallization and progressive increase in grain size upon heat treatment process. – Various techniques have been used for investigated the feature and properties of films, such as X-ray diffraction, field emission scanning electron microscope and UV–vis–near-infrared (NIR) spectroscopy. – The structural analysis via XRD indicates that, the films show polycrystalline nature pronounced with cubic zinc blende structure with a strong preferentially (1 1 1) texture orientation. – The influence of (Ar + O2 ) annealing and CdCl2 heat treatment are observed on enhancing the crystalline quality and reducing the lattice imperfection of CdTe films by decreasing the amount of lattice strain and dislocation density as well as shrinking optical energy gap. – Significant increase of grain size after CdCl2 heat treatment in (Ar + O2 ) atmosphere has been confirmed by XRD analysis and FE-SEM investigation, which has a great benefit in solar cell fabrications. – The impact of heat treatment process on the optical parameters such as optical energy gap, refractive index, dielectric constant, electronic polarizability and dispersion energy parameters have been discussed in detail. – The optical energy gap decreases from 1.56 eV for as-deposited film to 1.55 eV and 1.54 eV for (Ar + O2 ) annealed and CdCl2 treated CdTe films respectively, which highlights the advantage of CdCl2 treated films as an absorber layer in thin films solar cells. – The results show that the CdCl2 heat treatment in (Ar + O2 ) atmosphere can be used to improve the optical and morphological properties of CdTe absorber layer.
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– Generally, the whole experimental results in this work reveals that the CdCl2 heat treatment in (Ar + O2 ) atmosphere produced high quality CdTe thin film absorber layer solar cell.
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Optik journal homepage: www.elsevier.de/ijleo
Original research article
Effect of CdCl2 heat treatment in (Ar + O2 ) atmosphere on structural and optical properties of CdTe thin films M.F. Hasaneen a,b,∗∗ , W.S. Mohamed a,∗ a b
Physics Department, Faculty of Science, Sohag University, 82524 Sohag, Egypt Physics Department, Faculty of Science, Aljouf University, Aljouf, Saudi Arabia
a r t i c l e
i n f o
Article history: Received 11 December 2017 Received in revised form 26 January 2018 Accepted 28 January 2018 Keywords: CdTe thin films CdCl2 heat treatment X-ray diffraction Field emission scanning electron microscope Optical energy gap
a b s t r a c t Cadmium telluride (CdTe) thin films were deposited by thermal evaporation method under base pressure 2 × 10−5 mbar on corning glass substrate. In this work, we study the effect of cadmium chloride (CdCl2 ) heat treatment in argon and oxygen (Ar + O2 ) atmosphere on the structural, morphological and optical properties of CdTe thin films to achieve high quality thin film absorber layer for solar cells applications. The structural characteristics were studied by X-ray diffraction (XRD). The microstructure parameters of the films such as average grain size (Davg ), average lattice strain (εavg ) and dislocation density (D ) have been calculated via XRD line broadening analysis. XRD investigation revealed that, the samples show polycrystalline nature pronounced with cubic zinc blende structure with a strong preferentially (1 1 1) texture orientation. It has been found that the crystallinity of the films enhanced during (Ar + O2 ) annealing and CdCl2 heat treatment. The surface morphology of CdTe thin films was investigated by field emission scanning electron microscope (FE-SEM). XRD and FE-SEM results of CdCl2 treated films showed recrystallization and progressive increase in grain size. The optical properties of all samples were estimated using UV–vis–near-infrared (NIR) spectrophotometer. The Swanepoel envelope method has been employed to evaluate the various optical parameters of CdTe films such as refractive index and film thickness. The as-deposited films have a relatively high value of refractive index of 2.54, in contrast the CdCl2 treated films have a relatively low value of 2.1 in the whole wavelength range. The optical energy gap values slightly decrease from 1.56 eV for as-deposited films up to 1.54 eV for CdCl2 heat treated films. CdCl2 heat treatment in (Ar + O2 ) atmosphere is found to be a promising method to improve the physical properties of CdTe thin film solar cells. © 2018 Elsevier GmbH. All rights reserved.
1. Introduction The II–VI semiconductor compounds, i.e., cadmium telluride (CdTe) has been in the focus of comprehensive research due to their possible utilization in many applications such as Opto-electronic devices [1,2], photovoltaic cells [3], laser windows material [4,5], gamma rays and X-ray detectors [6,7], photoconductive devices [8], and light emitting diodes [9]. The main fundamental characteristics of CdTe are: perfect direct band gap around 1.45 eV at room temperature (RT), which is the optimum value for solar cell devices [10], it is cheap and inordinately steady during the application processing, resulting
∗ Corresponding author. ∗∗ Corresponding author at: Physics Department, Faculty of Science, Aljouf University, Aljouf, Saudi Arabia. E-mail addresses: [email protected] (M.F. Hasaneen), [email protected] (W.S. Mohamed). https://doi.org/10.1016/j.ijleo.2018.01.112 0030-4026/© 2018 Elsevier GmbH. All rights reserved.
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in powerful interests and quick evolution of CdTe-based solar cells devices in recent years [11], large optical absorption coefficient (>104 cm−1 ) which means that around 90% of incident sunlight in the visible range can be absorbed by only a few micrometers of CdTe thin films. CdTe is one of the most suitable candidates as absorber layer for CdTe/CdS solar cells devices [12,13]. Solar cells which formed from CdTe/CdS systems are one of the expected candidates for prevalent commercial success in solar energy transformation. On the other hand, CdTe can be formed in both n-type and p-type conductivity, therefore, CdTe is useful for homojunction and heterojunction organization. CdTe thin film has been prepared by many different techniques like, thermal evaporation [14], electron beam evaporation [15], RF sputtering [16], sol–gel spin coating [17], close spaced sublimation (CSS) [18,19], pulsed laser deposition (PLD) [20], electrodeposition [21,22], metalorganic chemical vapor deposition (MOCVD) [23], spray pyrolysis deposition (SPD) [24], screen printing [25], molecular beam epitaxy (MBE) [26] and chemicals deposition techniques including chemical bath deposition (CBD) [27]. The thermal evaporation is the most widely method for preparation of thin films because of its easy to use, scalability and reproducibility [28,29]. Furthermore, the deposition rate is high which can be controlled easily and damage to the substrate during deposition can be minimized. Several studies have reported on the physical properties of CdTe thin films. However, the effect of CdCl2 heat treatment on the optical and structural properties of CdTe thin films has not been fully studied yet. Generally, the as-deposited films would have many undesirable characteristics that affect the competence of the device execution, such as small grains, low crystallinity and many grain boundaries which lead to high electrical resistivity. The existence of Cl atoms could diffuse into the grain boundary of CdTe thin films during crystal growth which causes improve the recrystallization, the grain growth, and electronic properties of CdTe thin films [30,31]. In fact, the main key for enhancing the efficiency of the thin film solar cell is heat treatment by CdCl2 on the top of the CdTe thin films (300–400 nm) with annealing temperature in the range 350–450 ◦ C for 10–60 in air or inert gas under atmospheric conditions. Several researchers have been studied this treatment to improve the efficiency of solar cell thin film; however, a full comprehension of that treatment has not been completed yet and remain a subject of discussion. The major objective of this study was to investigate the effects of CdCl2 heat treatment in (Ar + O2 ) atmosphere on physical properties of thermally evaporated CdTe thin films and compared with (Ar + O2 ) annealing process. This study has been carried out by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and UV–vis–near-infrared (NIR) spectrophotometer. The structural, morphological and optical properties on these films are reported in this paper.
2. Preparation and examination of CdTe thin films 2.1. Deposition technique CdTe thin films were prepared by thermal vacuum evaporation technique (Edward’s high vacuum unit model AUTO 306) using a stoichiometric CdTe powder (99.999%) from Sigma Aldrich as a source material. The CdTe thin films were deposited onto corning glass substrates (1 cm × 1 cm × 0.1 cm). The glass substrates were cleaned in an ultrasonic bath in deionized water, acetone and ethanol and dried using an argon gas flow. The CdTe films were deposited with a thickness between 1100 and1200 nm at room temperature with a base pressure of 2 × 10−5 mbar. The deposition rate and film thickness during the deposition process can be controlled by the digital film thickness monitor (INFICON SQM-160). The deposition rate was varied in the range of (2–3) Å/s. The chamber pressure was approximately constant at 5 × 10−5 mbar and 2 × 10−5 mbar at room temperature (RT) in the beginning and end of film deposition respectively.
2.2. CdCl2 heat treatment and (Ar + O2 ) annealing procedure The heat treatment in (Ar + O2 ) atmosphere with the presence of CdCl2 was carried out inside a furnace tube model Kejia to improve the optical and structural properties of the CdTe thin films. On the other hand, annealing in (Ar + O2 ) atmosphere is preferred to air ambient when preparing efficient CdTe solar cell to avoid the impurities from air which can affect the film structure. Fig. 1 summarized the heat treatment procedure. The heat treatment process can be summarized as:
(a) CdCl2 layer of thickness about 150 nm has been deposited on the top of the as-deposited CdTe thin films by thermal evaporation technique. (b) The achieve of thermal annealing was carried out by the temperature sequence at 400 ◦ C for 15 min, 450◦ for 15 min and finally at 480 ◦ C for 10 min. (c) During heat treatment process the gasses of Ar and O2 were flowing in the furnace tube by rate of 8 sccm. (d) The samples were kept in the furnace tube until their gradual cooling to room temperature is achieved, which takes around 3 h. (e) The (Ar + O2 ) annealing experimental was done under the same conditions (i.e., time, temperature, gas flow) without the presence of CdCl2 .
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Alumina tube
F
F
309
Gas outlet Samples P
P
Thermocouple Temperature controller
O2
Ar
Bubble
Bubbler
Fig. 1. Schematic diagram of the equipment used to perform the heat treatment procedure.
2.3. Investigation techniques The crystallographic structure of the CdTe thin films was studied by X-ray diffractograms ((Philips X’pert MRD) with Cu-Ka radiation (i.e., = 1.5418 Å), incident angle of 0.75◦ and over a 2 scan range between 10◦ and 80◦ . The field emission scanning electron microscope (FE-SEM, Model: Quanta 250 FEG, with accelerating voltage 30 K.V and magnification up to 1,000,000×) was used to study the surface morphology and composition. A double beam UV–vis–near-infrared (NIR) spectrophotometer (Model: JASCO, V-570 in a wavelength range between 200–2500 nm at normal incidence) was used to evaluation the optical properties of CdTe thin films. 3. Results and discussion 3.1. Structural analyses by XRD The X-ray diffraction studies were carried out on as-deposited, (Ar + O2 ) and CdCl2 heat treated CdTe films. The average grain size (Davg ) was evaluated using the Debye-Scherer formula as the following relation [32]: Davg =
L ˇ2 cos
(1)
where L is a Scherrer constant equal to 0.94, is the wavelength of source radiation (i.e. = 1.5418 Å), ˇ2 is the full width at half maximum (FWHM) in radians and is the Bragg angle. The average lattice strain (avg ) which depending on the growing conditions was evaluated by the following relation [32,33]: εavg =
ˇ2 cos 4
(2)
The dislocation density (D ) is known as whole length dislocation lines per unit volume of the crystals and was determined using Williamson-Smallman relation [32,33]: D =
1 2 Davg
(3)
Fig. 2 illustrates the XRD patterns of thermally evaporated CdTe thin films at each process (i.e., as-deposition, (Ar + O2 ) annealing and CdCl2 heat treatment). It has been found that, all films are polycrystalline representing a cubic zinc blende structure with a strong preferentially (1 1 1) texture orientation. From Fig. 2 it can be deduced that the intensity of the peaks increases with both (Ar + O2 ) annealing and CdCl2 heat treatment indicates an improved crystalline quality that occurs after heat treatment process. It is well known that, CdCl2 heat treatment usually improves the crystallinity of CdTe up to certain temperature. This may be related to the increase in surface mobility of adatoms on the substrate surface due to sufficient thermal energy because of higher annealing temperature [33]. The sites of diffraction peaks of as- deposited CdTe films are observed at 2 = 23.38◦ , 38.98◦ , 46.18◦ , 72.04◦ , and 75.94◦ corresponding to C (111), C (220), C (311), C (422) and C (511) planes, respectively. These XRD peaks agree with the Joint Committee on Power Diffraction and Standards (JCPDS) reference
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Fig. 2. The XRD patterns of as-deposited, (Ar + O2 ) annealed and CdCl2 treated CdTe thin films.
Table 1 The structural parameters of as-deposited, (Ar + O2 ) annealed and CdCl2 treated CdTe thin films. Process 2 (◦ )
Structure (hkl)
As-deposited film 23.38 38.98 46.18 72.04 75.94
FWHM (radians)
Davg (nm)
C (111) C (220) C (311) C (422) C (511)
0.478 0.892 0.666 0.847 0.721
17.7 9.87 13.6 12.1 14.6
(Ar +O2 ) annealed film C (111) 23.38 38.98 C (220) C (311) 46.06 C (422) 72.10 C (511) 75.94
0.369 0.314 0.321 0.480 0.329
23.0 28.0 28.0 21.4 31.8
CdCl2 heat treated film 23.38 C (111) 39.04 C (220) C (311) 46.12 C (422) 72.04 C (511) 75.94
0.363 0.325 0.310 0.393 0.362
23.33 26.48 29.20 26.11 29.00
Mean value of Davg (nm)
εls (10−3 )
13.6
10.1 11.0 6.80 5.10 4.00
26.4
7.8 3.9 3.3 2.9 1.9
26.8
7.7 4.0 3.2 2.4 2.0
Mean value of εls (10−3 )
D (1011 cm−2 )
7.4
3.10 10.2 5.40 6.83 4.69
6
3.96
1.89 1.27 1.27 2.18 0.99
1.52
3.84
1.83 1.42 1.17 1.46 1.18
1.41
Mean value of D (1011 cm−2 )
Note: C, Cubic; FWHM, full width at half maximum; Davg , average of grain size; εls , lattice strain; D , dislocation density.
file No: 15-0770 Sys, Cubic (C). Some new phases are observed for CdCl2 heat treated and (Ar + O2 ) annealed CdTe films like CdTeO3 phase with positions at 2 = 10.06◦ , 20.5◦ , 27.16◦ , 31.18◦ , 34.3◦ and 40.36◦ corresponding to JCPDS file No: 22-0128 and Cd3 TeO6 phase with a position at 2 = 70.9◦ corresponding to JCPDS file No: 82-0125 as shown in Fig. 2. In fact, when heating cadmium telluride in the oxygen atmosphere, this is accompanied by losses in weight, which is referred to the sublimation of the highly volatile Te. This result more Cd than Te are present on the film surface and the resulting films produced are Cd-rich CdTe material. The sublimation of Te is creating the vacancies in the crystal lattice which gives the chance for oxygen to diffuse into the crystal lattice. Incorporate the oxygen into the crystal lattice may allow to modification of the band gap structure which is followed up by a change in physical properties, as mentioned in previous studies [34]. The structural parameters like the average grain size (Davg ), the average lattice strain (εavg ) and dislocation density (D ) for as-deposited, (Ar + O2 ) annealed and CdCl2 heat treated CdTe films were estimated by using standard relations [1–3] and presented in Table 1. The average grain sizes are 13.6 nm, 26.4 and 26.8 nm for as-deposited, (Ar + O2 ) annealed and CdCl2 heat treated CdTe films respectively, this means the average grain size increase with both (Ar + O2 ) annealing and CdCl2 heat treatment process. These results might be due to many reasons: coalesce of small grains into bigger ones, the relaxation of excessive internal lattice strains [35], and Cl and O take position in crystal lattice of CdTe layers through CdCl2 heat treatment process [33]. It was found that, the average lattice strain (εavg ) and dislocation density (D ) of CdTe films are decreased with the (Ar + O2 ) annealing and CdCl2 heat treatment process as tabulated in Table 1. This indicates that the CdCl2 heat treatment in (Ar + O2 ) atmosphere could improve the crystalline quality and reduce the lattice imperfection of CdTe films by decreasing the amount of lattice strain and dislocation density.
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3.2. Surface morphology and microstructure analysis by FE-SEM The surface morphologies and microstructures investigation of CdTe thin films grown on glass substrates were undertaken employing a field emission scanning electron microscope (FE-SEM). FE-SEM is one of the most important methods to analyze the surface morphology and microstructure of thin films that provide high definition images at low accelerating voltages and small working distances. Fig. 3 illustrates the surface morphologies and microstructures images for all samples. The surface morphology image of the as-deposited CdTe thin films as shown in Fig. 3(A) referring that the CdTe thin film consist of small grains with varying values around 7–62 nm. The grain size of CdTe films after (Ar + O2 ) annealing show a partially larger than the as-deposited with varying values between 45 and 155 nm i.e., Fig. 3(B). After CdCl2 heat treatment a great change in the shape and size of the CdTe grains in the range of 75–155 nm is observed as shown in Fig. 3(C). It’s clear that the CdCl2 heat treatment leads to grain boundary segregation which is called grain growth. This may be attributed to the crystallization seed for CdTe during heat treatment process which reduce the atomic diffusion barrier at the CdTe grain boundaries. On comparing the average grain size of the CdTe samples obtained by the SEM microstructure analysis software with the value obtained through XRD analysis, we notice that the size obtained from the SEM analysis is over-estimated. Such a difference can be explained by the fact that, each grain is formed by aggregation of many nanocrystals [33]. 3.3. Optical analysis by UV–vis–near-infrared spectroscopy 3.3.1. Optical transmittance and estimation of the optical energy gap The optical behavior of CdTe thin films which determined by both transmittance (T ) and reflectance (T ) spectra have been estimated by a double beam UV–vis–near-infrared (NIR) spectrophotometer in the wavelength range of 200–2500 nm at normal incidence. The transmittance and reflectance spectra of all samples are illustrated in Fig. 4. It has been found that, the maximum intensity of transmittance up to 89%, 76% and 69 for as-deposited, (Ar + O2 ) annealed and CdCl2 heat treated CdTe films, respectively. Moreover, the interference fringes are observed in the transmittance spectra at a higher wavelength (i.e., 800–2100 nm) which refer to the homogeneity and harmony of the deposited films. The optical energy gap (Eg ) of all samples were calculated by the Tauc equation which describe the correlation between the optical absorption coefficient (␣) and the optical energy gap (Eg ) as the follows: n
(˛h) = K (hv − Eg ) where ˛ =
(1 − R)n 1 ln[ ] T d
(4)
here h is the phonon energy, K is a constant, which related to electronic transition probability, d is the thickness of the deposited film and n is an exponent, which called the power factor of the electronic transition mode. The values of n for direct allowed, indirect allowed, direct forbidden and indirect forbidden electronic transitions are 1/2, 2, 3/2 and 3, respectively by plotting (˛h) as a function of (h) according to Eq. (4) for all n values (1/2, 2, 3/2 and 3) for all samples, it was found that the best fit of our experimental results is obtained for n = 1/2 (i.e., direct allowed transition). Moreover, the direct allowed transition of CdTe thin films have been commonly observed by several authors [36–38]. The linear part of the plot of (˛h)2 vs. (h) to the axis of energy at ␣ = 0 is shown in Fig. 5. It can be observed that the optical energy gap slightly decreases from 1.56 eV for as-deposited film to 1.55 eV and 1.54 eV for (Ar + O2 ) annealed and CdCl2 heat treated CdTe films respectively. Such decreases in the optical energy gap may be attributed to the increase in the crystallinity and grain size of CdTe thin films upon heat treatment, which in turn leads to a deficiency of the role of defect states or decrease in the density of localized state [15]. These results are consistent with the earlier reports on CdTe films obtained by many authors. For example, Nima E. Gorji [39] was reported the decrease in optical energy gap as a result of CdCl2 treatment due to the increase in grain size. The CdCl2 heat treatment in (Ar + O2 ) atmosphere results in a smaller optical energy gap, which in turn leads to higher absorption. This feature highlights the advantage of CdCl2 heat treated CdTe film as an absorber layer in heterostructure thin film solar cells. 3.3.2. Estimation of the band tail width or Urbach energy The absorption coefficient (␣) of the band tail energy or Urbach energy (Eu ) is a quantity specifies the degree of structural disorder in thin films. In fact, this quantity characterizes the local defect or localized states near the optical energy gap. The Urbach empirical relation can be calculated by using the following relations [40], ˛ = ˛0 exp(
hv − Ep0 ) Eu
In (˛) = In(˛0 ) + exp(
(5) hv − Epo ) Eu
(6)
where ␣0 and Ep0 are the Urbach absorption edge parameters and Eu is the band tail width or Urbach energy. The Eu values can be deduced from the slope of linear fit to the logarithmic plot of the absorption coefficient to the photon energy as shown in Fig. 6. The estimated values of the Eu for all samples were given in Table 2. It is observed that the Eu value is found to increase with decreasing optical energy gap. It was found that the obtained values of Eu for as-deposited, (Ar + O2 ) annealed and CdCl2 heat treated CdTe films are 81, 84 and 86 meV, respectively. As a result, simultaneous decreasing of optical energy gap and broadening of band tail width have taken place after heat treatment process. This may be attributed to appearance
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Fig. 3. FE-SEM images of CdTe thin films at different processes: (A) as-deposition, (B) (Ar + O2 ) annealing and (C) CdCl2 heat treatment.
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Fig. 4. The spectral distribution of transmittance and reflectance of as-deposited, (Ar + O2 ) annealed and CdCl2 treated CdTe thin films deposited on glass substrate. Table 2 The refractive index (n), static refractive index (n0 ), high frequency dielectric constant (ε∞ ), dispersion energy (Ed ), single oscillator energy (E0 ), static dielectric constant (εs ), free carrier concentration (N), plasma frequency (ωp ), relaxation time (), oscillator wavelength (0 ), oscillator strength (S0 ) and electronic polarizability (˛p ) of as-deposited, (Ar + O2 ) annealed and CdCl2 treated CdTe thin films deposited on glass substrate. Optical parameter
As-deposited film
(Ar + O2 ) annealed film
CdCl2 heat treated film
Energy band gap, Eg (eV) Urbach energy Eu (meV) Approximate value of the refractive index, n Film thickness, d (nm) Electronic Polarziability, ˛p × 10−24 Relative density p Single oscillator energy, E0 (eV) Dispersion energy, Ed (eV) Static refractive index, n0 Lattice dielectric constant, ∞1 High frequency dielectric constant ∞2 free carrier concentration, N × 1019 Plasma frequency, ωp (1014 Hz) Average oscillator strength, S0 (10−4 nm−2 ) Inherent absorption wavelength, 0 (nm)
1.56 81 2.54 1118 7.66 0.85 6.00 27.5 2.3 6.0 6.78 26.2 42.2 0.93 228
1.55 84 2.29 1000 6.95 0.66 6.10 30.8 2.44 6.08 6.00 0.18 0.29 28 42
1.54 86 2.10 1282 6.31 0.53 7.6 35.3 2.36 4.73 4.57 9 14.7 1.28 161
of defects, which cause redistribution of the localized states from band to tail and then facilitates more possible transitions from band to tail and tail to tail, which leads to the simultaneous shrinkage and extending of optical energy gap and Urbach tail, respectively. 3.3.3. Estimated of the refractive index and film thickness The values of refractive index and the film thickness can be calculated by using the Swanepoel’s envelope method, which is mainly based on a creating the envelopes of interference maxima and minima suggested by Mainficier et al. [41,42]. Fig. 7 gives an example of the typical transmittance T() and reflectance R() spectra for as-deposited CdTe thin film which is used for optical constant calculation. The first approximate value of the refractive index (n) in the spectral regions of the medium and weak absorption can be estimated by the following relation [41],
n2 = [N1 + N12 − s2
12
]
(7)
Where N1 = 2s
T − T s2 + 1 m M TM Tm
+
2
(8)
here TM and Tm are the maximum value of the transmittance and the corresponding minimum value at a certain wavelength, respectively. One of these values taken from an experimental interference extreme and the other one taken from the
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Fig. 5. Direct optical energy gap determination by Tauc relation of as-deposited, (Ar + O2 ) annealed and CdCl2 treated CdTe thin films deposited on glass substrate.
corresponding envelope. The refractive index of the glass substrates has value which calculated according to the following relations, s=
1 + Ts
1 TS
12
−1
(9)
where Ts is the maximum value of the transmittance spectra of glass substrate as noted in Fig. 4. The fundamental relation which describe the interference fringes can be written as follows, 2nd = m
(10)
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Fig. 6. Linear fit of the absorption coefficient ln(␣) vs. incident photon energy (h) of as-deposited, (Ar + O2 ) annealed and CdCl2 treated CdTe thin films.
Fig. 7. Typical transmittance T() and reflectance R() spectra for as-deposited CdTe thin film estimated by Swanepole’s envelops method.
where m is an integer for maxima and half integer for minima, n is refractive index, d is the film thickness. Where the order numbers m is integer for maxima and half integer for minima. Furthermore, if n1 and n2 are the refractive indices of the thin film at two corresponding adjacent maxima or minima at wavelengths 1 and 2 , respectively, then the film thickness d can be calculated from the basic interference waves equation as the following relation, d=
1 2 2(1 n1 − 2 n2 )
(11)
The values of refractive indices n and the film thicknesses d for different samples were calculated by using relation (7) and (11) respectively and tabulated in Table 2. Using the approximate values of the refractive index listed in Table 2 one can obtain the values of the refractive index over the whole spectral range of measurement i.e., 250–2500 nm, by using the two-term Cauchy empirical formula as given below. n () = a +
b 2
(12)
where a and b are Cauchy’s constants, which depending on the intercept and slope of n vs. −2 linear plot. The dispersion of refractive index n is fitted by Eq. (12). The least squares fit of the two sets of values of n are n = 2.53 + 2.51 × 105 /2 , n = 2.21 + 2 × 105 /2 and n = 1.94 + 1.96 × 105 /2 for as deposited, (Ar+O2 ) annealed and CdCl2 heat treated CdTe films, respectively. As shown in Fig. 8, the refractive index for all samples progressively decreases up to 1000 nm and above that range, stability occurs without any remarkable change with increase the wavelength. It is also observed that the refractive index changes between 2.1 and 2.54 for all samples. K. Punitha [43] have reported that the refractive index changes between 2.46 and 2.51 for vacuum evaporated CdTe thin films deposited on glass substrates, which matches well with our finding. It is worth to indicate that, the refractive index of a semiconductor material usually has a close relationship with an optical energy gap [33], which is agreement with our results as shown in Table 2.
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Fig. 8. Dispersion of refractive index as a function of wavelengths of as-deposited, (Ar + O2 ) annealed and CdCl2 treated CdTe thin films deposited on glass substrate.
Fig. 9. Plot of the electronic polarizability vs. incident photon energy (h) of as-deposited, (Ar + O2 ) annealed and CdCl2 treated CdTe thin film deposited on glass substrate.
3.3.4. Electronic polarizability The essential character of optical properties can be determined by electronic polarizability (␣p ), which provides an information about the form of bound system and the internal structure of the molecule. The electronic polarizability depends on the refractive index using Classius-Mossotti of dielectric constant as shown in the following relation [44],
␣p =
n2 − 1 n2 + 2
3M 4Navo D
(13)
where ˛p is the electronic polarizability, n is the refractive index, M is the molecular weight, D is density and Navo is the Avogadro’s number. Fig. 9 illustrates the graph of photon energy with (n2 −1)/(n2 + 2). The electronic polarizability values of all samples can be calculated from the extrapolation of the linear zone to the y-axis. As seen in Table 2 the values of electronic polarizability (˛p ) decrease from 7.66 × 10−24 for as-deposited film to 6.95 × 10−24 and 6.31 × 10−24 for (Ar + O2 ) annealed and CdCl2 treated CdTe films, respectively. The decreasing of ˛p can be explained by spread of the Cl and O inside the CdTe
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Fig. 10. Linear graph of the refractive index related quantity (n2 −1)−1 vs. square of photon energy (h)2 according to WD model of as-deposited, (Ar + O2 ) annealed and CdCl2 treated CdTe thin films deposited on glass substrate.
layer which take new sites during CdCl2 heat treatment and (Ar + O2 ) annealing, therefore the volume of the electronic space decreased, as well as decreasing the volume occupied by the electrons. 3.3.5. Relative density Relative density can be defined as the relationship between the refractive index of the deposited film nf and the refractive index of the bulk nb (i.e., nb for CdTe is 2.72). The relative density can be calculated by Lorentz-Lorentz relation through the following relationship [43],
p=
pf pb
=
n2f − 1
n2b + 1
n2b + 1 n2b − 1)
(14)
The relative density of the film decreases with both (Ar + O2 ) annealing and CdCl2 heat treatment respectively, as reported in Table 2, which may be due to the crystallization enhancing and increase in crystallite size. 3.3.6. Dispersion of the refractive index The refractive index dispersion can be analyzed using Wemole-DiDomenico (WDD) single oscillator model which identifies the dispersion of optical constant. Below the absorption region, the relation between refractive index and oscillator strength can be obtained by the following relation [33],
n2 − 1 =
Ed E0
E02 − (h)
2
(15)
where E0 and Ed are the single oscillator energy and dispersion energy respectively. By drawing the refractive index which takes a quantity 1/(n2 −1) versus the square of photon energy (h)2 and fitting a straight line as shown in Fig. 10, the values of both single oscillator energy (Ed ) and dispersion energy (E0 ) directly estimated from the intercept (E0 /Ed ), and from the slope (1/E0 Ed ) on the vertical axis, respectively. Table 2 shows that the values of E0 and Ed are increasing with both (Ar + O2 ) annealing and CdCl2 heat treatment. This may be attributed to an increase in the optical energy gap, which has a relationship with E0 according to WDD modal (E0 ≈ 2Eg ). These results mean that the optical energy gap and dispersion constant change with both (Ar + O2 ) annealing and CdCl2 heat treatment. Furthermore, the static refractive index n0 can be calculated from the oscillator parameters E0 and Ed , which supply an important information on the structure and density of the investigated material such as the lattice dielectric constant ε∞1 = n20 by the following relation, Ed 1/2 ) E0
n0 = (1 +
(16)
As shown in Table 2 the static refractive index increases with both (Ar + O2 ) annealing and CdCl2 heat treatment. This may be attributed to the enhancement of the crystallinity. 3.3.7. Dielectric constants The real (ε )and imaginary (ε ) parts of complex dielectric constant of CdTe thin films for each process can be calculated by using refractive index (n) and extinction coefficient (k) (i.e., k = ␣/4) data, according to the following relations,
ε (h) = ε (h) − iε (h)
(17)
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Fig. 11. Linear graph of the dispersion of the real part of dielectric constant as a function of 2 of as-deposited, (Ar + O2 ) annealed and CdCl2 treated CdTe thin films deposited on glass substrate.
Fig. 12. Linear graph of the dispersion of the imaginary part of dielectric constant as a function of 3 of as-deposited, (Ar + O2 ) annealed and CdCl2 treated CdTe thin films.
Both first and second parts of the previous relation (i.e., real and imaginary part, respectively) of complex dielectric constant are expressed by the following [45–47],
ε (h) = n2 (h) − k2 (h)
(18)
ε (h) = 2n(h)k(h)
(19)
In the real part ( ), the lattice dielectric constant at high frequencies (ε∞2 ) which explained the participation of the free carrier electric susceptibility and lattice vibrational modes of dispersion, can be estimated as the following [44],
= n2 − k2 = ∞2 −
e2 42 c2 0
N m*
2
(20)
where c is the light velocity, e is the electron charge, m* is the effective mass of the charge carrier, ε0 is the permittivity of free space and N is the free carrier concentration in conduction band. According to Drude model the high frequency dielectric constant ∞2 and the free carrier concentration N can be calculated from the intercept and negative slop of the = f(2 ) linear plot as illustrated in Fig. 11. From the same line, the electrons pair with oscillating electric field which called the electron plasma frequency (ωp ) is given by [48], ωp2 =
N m*
(
e2 ) ε0 ε∞1
(21)
The estimated values of the n0 , ε∞ , N and ωp for all samples were tabulated in Table 2. In the imaginary part, the variation of ε as a function of wavelength 3 (nm)3 are shown in Fig. 12. The value of the optical relaxation time can be calculated from the positive slope of the obtained straight line in imaginary part, which controlled by a linear relationship with 3 according to the following relation,
ε = 2nk =
1 e2 N 1 3 ( )( )( ) 4 3 ε0 c 3 m*
(22)
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Fig. 13. Linear graph of (n2 −1)−1 as a function of the −2 of as-deposited, (Ar + O2 ) annealed and CdCl2 treated CdTe thin films deposited on glass substrate.
3.3.8. Inherent absorption wavelength (interbond oscillator wavelength) studies In the ultraviolet range, many semiconductors material have an inherent absorption wavelength also usually called interbond oscillator wavelength. The Inherent absorption wavelength (0 ) and the average oscillator strength (S0 ) can be calculated using Drude-voigt dispersion relation [43]:
n2 − 1 =
e2 mc 2
( 12
Nm S0
−
0
(23)
1 ) 2
where Nm is the number of molecular per unit volume of thin film. From the previous equation we only take a single term Sellemeier oscillator as a function of refractive index [43], (n20 − 1) (n2
− 1)
2
=1−(
0 )
(24)
where n0 is the static refractive index. Relation (24) can be modified as the following, (n2 − 1)
−1
=
1 S0 20
−
1 S0 2
(25)
Fig. 13 shows the graph plotted between (n2 −1)−1 versus −2 , so the values of S0 and 0 can be calculated from the negative slope (1/S0 ), and the intercept ( 1 2 ) on the vertical axis as given in Table 2. S0
4. Summary and conclusions Based on the previous results, the main finding can be summarized in the following points: – The influence of (Ar + O2 ) annealing and CdCl2 heat treatment on the structural, optical and morphological properties of thermal evaporated CdTe thin films has been investigated with an effort to promote recrystallization and progressive increase in grain size upon heat treatment process. – Various techniques have been used for investigated the feature and properties of films, such as X-ray diffraction, field emission scanning electron microscope and UV–vis–near-infrared (NIR) spectroscopy. – The structural analysis via XRD indicates that, the films show polycrystalline nature pronounced with cubic zinc blende structure with a strong preferentially (1 1 1) texture orientation. – The influence of (Ar + O2 ) annealing and CdCl2 heat treatment are observed on enhancing the crystalline quality and reducing the lattice imperfection of CdTe films by decreasing the amount of lattice strain and dislocation density as well as shrinking optical energy gap. – Significant increase of grain size after CdCl2 heat treatment in (Ar + O2 ) atmosphere has been confirmed by XRD analysis and FE-SEM investigation, which has a great benefit in solar cell fabrications. – The impact of heat treatment process on the optical parameters such as optical energy gap, refractive index, dielectric constant, electronic polarizability and dispersion energy parameters have been discussed in detail. – The optical energy gap decreases from 1.56 eV for as-deposited film to 1.55 eV and 1.54 eV for (Ar + O2 ) annealed and CdCl2 treated CdTe films respectively, which highlights the advantage of CdCl2 treated films as an absorber layer in thin films solar cells. – The results show that the CdCl2 heat treatment in (Ar + O2 ) atmosphere can be used to improve the optical and morphological properties of CdTe absorber layer.
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– Generally, the whole experimental results in this work reveals that the CdCl2 heat treatment in (Ar + O2 ) atmosphere produced high quality CdTe thin film absorber layer solar cell.
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