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Article

An In-Depth Analysis of CdTe Thin-Film Deposition on Ultra-Thin Glass Substrates via Close-Spaced Sublimation (CSS)

by
Nowshad Amin
1,2,*,
Mohammad Rezaul Karim
3,4 and
Zeid Abdullah ALOthman
5,*
1
College of Engineering, Universiti Tenaga Nasional, Jalan IKRAM-UNITEN, Kajang 43000, Selangor, Malaysia
2
Institute of Sustainable Energy, Universiti Tenaga Nasional, Jalan IKRAM-UNITEN, Kajang 43000, Selangor, Malaysia
3
Center of Excellence for Research in Engineering Materials (CEREM), Deanship of Scientific Research (DSR), College of Engineering, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia
4
K.A.CARE Energy Research and Innovation Center, Riyadh 11451, Saudi Arabia
5
Chemistry Department, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia
*
Authors to whom correspondence should be addressed.
Coatings 2022, 12(5), 589; https://doi.org/10.3390/coatings12050589
Submission received: 28 February 2022 / Revised: 17 April 2022 / Accepted: 20 April 2022 / Published: 25 April 2022
(This article belongs to the Topic Inorganic Thin Film Materials)
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Versions Notes

Abstract

:
This study evaluated the impact of the deposition pressure on the formation of cadmium telluride (CdTe) thin films on ultra-thin (100 µm) Schott glass substrate at high temperature (T > 450 °C) by Close-Spaced Sublimation (CSS) technique. CdTe thin films were grown under the pressure range of 1 Torr to 200 Torr to explore the impact of deposition pressure on CdTe thin-film properties. The microstructural, compositional and optoelectrical characteristics were examined. X-ray Diffraction (XRD) analysis revealed the cubic phase crystallite CdTe films with (111) preferential orientation. Scanning Electron Microscopy (SEM) demonstrated that the CdTe morphology and grain size could be regulated via the deposition pressure, whereby maximum grain growth was detected at low pressure (1–5 Torr). The thickness of CdTe films was reduced from 6 µm to 1.5 µm with the rise in deposition pressure. Moreover, the optical direct energy gap was derived in the range of 1.65–1.69 eV for the pressure value of 200 Torr to 1 Torr. Carrier density and resistivity were found to be in the order of 1013 cm−3 and 104 Ω cm, respectively. The experimental results suggest that the pressure range of 1–5 Torr may be ideal for CSS-grown CdTe films on flexible ultra-thin glass (UTG) substrates.

1. Introduction

CdTe is a promising absorber material for thin-film solar cells with an absorption coefficient over 104 cm−1 and an energy bandgap of about 1.5 eV that possesses the possibility to be deposited in both substrate and superstrate configuration [1]. Up till now, superstrate CdTe thin film, which is fabricated via close-spaced sublimation (CSS) at high temperature ambient of 500–600 °C, has resulted in the most successful results [2]. CSS is a simple and relatively inexpensive technique among several developed methods to deposit CdTe that offers a high deposition rate in a short period and high efficiency. In a CSS system, deposition parameters including pressure and applied temperature are known to have a dominant impact on film specifications [3]. However, there is an extremum, namely, the glass transformation temperature (Tg = 560 °C) to be considered during the thin-film deposition on the flexible glass substrate [4]. As a result, regulating the deposition pressure during the sublimation process is an effective way to further improve thin-film specifications [5]. Related earlier studies demonstrated the effect of sublimation pressure on the mechanism of the CSS deposition system on normal borosilicate (1.1 to 3 mm thick) glass. In the study conducted by Alamri, high-rate sublimation is derived from the CSS process using low argon pressure [6]. Furthermore, CSS CdTe deposition can be explained by the diffusion transport model, in which Cd and Te atoms travel to the substrate while diffusing through the ambient gas and colliding with gas molecules before condensing on the substrate. The deposition rate is an inverse function of gas pressure, and the mean free path (MFP) is the shortest distance between two subsequent collisions, according to this theory [7]. As a result, the influence of deposition pressure and MFP on deposition rate rises as deposition pressure increases from 1 to 20 Torr. Major et al. reported that the adatom arrival rate (grain growth) on the substrate is controllable by altering the chamber pressure and low free sublimation and transport are anticipated due to the extended MFP at low pressure [8]. Falcão et al. showed the limited deposition rate at high pressure [9]. Zelaya et al. stated the enhanced preferred crystallinity at (111) orientation for low pressure deposition [10]. However, the effect of deposition pressure on flexible CdTe samples prepared on UTG glass substrate via CSS has not been well investigated. Lightweight and flexible thin-film devices are highly desirable for specific power applications such as building integrated photovoltaics, unmanned aerial vehicles, and space. Although flexible metallic and polyimide foils are often utilized, ultra-thin glass (UTG) can be employed as an alternative substrate with appealing features [11]. In addition, ultra-thin glass offers both bendability and transparency with a smooth surface and it is highly resistant to air and moisture, which could be the best possible candidate as the substrate [12]. Schott ultra-thin glass has been effectively established in the lightweight X-ray optics for space-borne application [13] and photodetectors [14]. This flexible substrate has the thermal expansion coefficient (CTE) of 7.2 × 10−6 K−1, very near to CdS (6.26 × 10−6 K−1) and CdTe (5.9 × 10−6 K−1) compared to 1.1 mm borosilicate glass (3.3 × 10−6 K−1). CTE equilibrium helps in minimizing the development of any tensile or compressive stress during high temperature/pressure deposition and/or cracks or shrinkage during the cooling period [15]. Therefore, the microstructural and optoelectrical characteristics of CdTe films grown on ultra-thin (100 µm) glass by CSS are explored in this study, with an emphasis on the influence of deposition pressure.

2. Experimental Procedure

2.1. Deposition of CdTe Thin film

Initially, UTG substrates with a dimension of 3 cm × 3 cm were rinsed with deionized (DI) water and then ultrasonically cleaned by methanol, acetone (10 min for each step) and a final 20 min of DI water. After cleaning, the jet stream drying process was applied using industrial nitrogen gas. Dried samples were placed on a holder with ~14 cm distance to the target in the (Kurt. J. Lesker, Allegheny, PA, USA, 4-gun, 4-halogen heater mode) sputtering chamber, as shown in Figure 1a. In this work, CdS thin film is employed as the most promising n-type buffer material with the capacity to improve the conversion efficiency of CdTe thin-film solar cells due to its broad transmittance range and optimal bandgap (Eg = 2.4 eV) [3]. To generate uniform films, all substrates were plasma-cleaned at 40 W for 10 min in 5 sccm Ar before CdS deposition, and the substrate holder was rotated three times per minute. In order to deposit the window layer, highly pure (99.99%) CdS target with a diameter of 50.6 mm was fixed and sputtered in the 2 × 10−5 Torr vacuum condition by a Radio Frequency (RF) sputtering system. Deposition time of 20 min, deposition pressure of 3 × 10−2 Torr and RF power of 40 Watt yielded 120 nm thick sputtered CdS (Figure 1b).
CdTe thin films were deposited on 120 nm CdS-coated UTG. CSS growth was performed using a sintered 2 cm × 2 cm × 1 cm CdTe brick source, 1 mm separation between source and substrate, and 1 Torr Argon in a CSS chamber, followed by a 10-min deposition period at 600 °C and 500 °C, source and substrate temperature, respectively. The distance of source and substrate in the CSS method was fixed at 1 mm (Figure 1c) to reduce the loss possibility or undesired mass scattering during the sublimation process [16]. Other reports are available presenting the detailed strategies used in the CSS method [5]. The deposition rate R is an inverse function of source temperature TSO and Boltzmann constant k, as shown in Equation (1) [6].
R = 1.9 (exp (−Ea/kTso))
This equation represents a direct proportion to the exponential value of the activation energy Ea, which is mainly regulated by the deposition pressure. Accordingly, in this research, ambient pressure was varied during the sublimation process. The resultant CdTe thin film grown by CSS is presented in Figure 1d and the deposition parameters are denoted in Table 1.

2.2. Thin-Film Characterization

Crystallography study was conducted by X-ray diffraction (XRD) (2theta mode, Shimadzu XRD-6000, Tokyo, Japan) by scanning process with a step size of 0.02° in the 2theta range of 20–60°. Surface morphology, thickness and grain size were determined by scanning electron microscope (SEM) (Hitachi SU1510, Tokyo, Japan). Perkin-Elmer-Lambda-35 UV–vis spectrophotometer (Waltham, MA, USA) was used to study the optical properties. Elemental compositions were analysed by the energy-dispersive X-ray (EDX) system (Tokyo, Japan). The electrical parameters were measured by the ECOPIA HMS 3000 Hall Effect measurement system (Anyang, Korea).

3. Results and Discussion

CdTe grain growth analysis is critical for achieving the best CdTe growth parameters, as it relates to deposition pressure, and ultra-thin glass specifications [17]. The optoelectronic property of CdTe thin film is heavily dependent on CdTe absorber parameters, namely, the diffusion length of minority carriers (Ln) and the recombination rate of photogenerated carriers in the space charge region (SCR), as well as the thickness of the SCR. The diffusion length of a carrier type in a material is defined as the average distance travelled by an excited carrier before recombining [18]. The diffusion length in CdTe film grains is known to be 1 to 5 µm and the active layer thickness is supposed to follow the limits of 1–5 µm too [19]. Ambient pressure is reported to be effective on grain growth, surface properties, crystal orientation and carrier density. Moreover, tensile stress, current leakage, shunting, and high recombination may be avoided by controlling the deposition pressure [20]. Using XRD spectrum data, Bragg’s low is integrated to quantify the crystallinity of produced films in relation to their lattice constant, phase, strain, and defect densities. By using Brag and Vegard’s law in cubic structure, shown in Equations (2) and (3) [21]:
[d(hkl) = (n λ/2 sin θ)]
[acubic = dhkl (h2 + k2 + l2)1/2]
where n, d, θ and (h,k,l) denote positive integers, interplanar spacing, the angle between the neighbouring crystal planes and the miller indices, respectively. Following that, crystallite (Dhkl) size is calculated using the Scherrer’s formula, Equation (4) [22]:
[Dhkl = 0.9 λ/(β cosθ)]
In addition, the microstrain (ε) and dislocation density are derived using Equations (5) and (6) [22].
[ε = β/4tanθ]
[δ = n/D2]
The D parameter relates to the crystallite size or the intermediate diameter of single-crystal orientation in polycrystalline, and decreasing crystallite size increases the lattice mismatch. Table 3 traces X-ray data of 2θ, the crystal structure of the deposited CdTe thin films, analysed by the advanced XRD-6000 system. Here, XRD patterns shown in Figure 2 demonstrate the intense desired diffraction peak at 2θ = 23.9°, confirming the pure cubic structure of (111) orientation [2]. As a matter of fact, X-rays penetrate to the thickness of around 2 μm and, clearly, XRD results are focused on the bulk of CdTe layer [23]. However, Figure 2 shows the angular positions of preferred orientation (111) and other low intense peaks are slightly shifted for the sample deposited at 200 Torr. It has been found that the peak shift has resulted from the Te residue at the surface of the films under compressive stress as verified by a previous study [24]. Also, hydrostatic pressure analysis on Te validates the downward shift of Te atoms with increasing pressure [12]. The shift toward lower angles might also be due to the existence of both uniform strain (tension) that displaced the peaks and non-uniform strain (compression) that causes the peaks to expand. This phenomenon may be a sign of structural change caused by a change in mechanical response (adhesivity) in the samples [25].
However, the tensile or compressive stress is not recorded to be effective until 100 Torr of deposition pressure. Moreover, there is no obvious deformation found in samples, probably due to the coherence in the coefficient of thermal expansion (CTE) between 100 µm glass and deposited CdS/CdTe material [23]. Besides, mechanical properties of the substrate material are highly effective for the prepared samples at different deposition conditions [26]. Table 2 illustrates the mechanical properties comparison between the standard borosilicate glass that is commonly used as the substrate for thin-film deposition and the ultra-thin (100 μm) glass.
As compared to a standard borosilicate substrate, the same amount of deposition pressure has a reduced impact on the ultra-thin glass substrate. Also, the hardness stability (Knoop), stiffness (Young’s modulus), and material expansion or contraction are perpendicular to the direction of loading (Poisson’s ratio). Consequently, the material’s smoothness and response to the stress (shear modulus) are expected to improve compared to borosilicate glass. This validates the compatibility of UTG as a potential substrate for CdTe thin-film deposition. In this study, analyses of preferred crystal orientation, dislocation density, strain, micro-structural specification, besides the coherence of CTE between the UTG and the film layers, are inspected to optimize the film properties and adhesion.
XRD analysis confirms the gradual deterioration in crystallinity and the preferred crystal orientation with the increase in pressure [27]. The (111) crystalline plane is dominant and the intensities of other CdTe peaks are comparatively weak, conferring the higher crystallinity of samples grown at the range of 1–5 Torr ambient. Generally, deviations in peak magnitudes can be attributed to scattering pattern differences of crystal mechanisms, shifts in the lattice orientation or to the difference in crystallite size. Likewise, the heights of main peak for high deposition pressure altered following the plots reported in other works [28]. XRD diffraction peaks at 2θ = 23.9°, 39.5°, 46.62° and 56.9° are well-indexed with the JCPDS Card No. 15-0770 [29]. By using Brag and Vegard’s law, lattice constant a is calculated [30]. The average crystallite size (Dhkl), microstrain (ε) and dislocation factors are estimated as reported in a related study [2]. Reduction of crystallite size enhances the lattice mismatch, as also observed elsewhere that used the standard borosilicate substrate [31]. Structural factors and values are shown in Table 3.
The average crystallite size is about 21 nm to 38 nm, which increases with the rise in pressure until 100 Torr and decreases thereafter. While depositing the CdTe film at 200 Torr, the collision among species is extremely high due to very low MFP. Hence, the compression stress increases and causes alteration from the initial orientation to a reduced crystalline phase. Also, lower adhesion due to week atomic bond structure is resulted [32]. Accordingly, the preferred orientation is lost upon high-pressure deposition and the growth rate also decreased. The reduction of the XRD intensity in non-stoichiometric films may be explained by the poor crystallinity in high pressure [33]. High deposition pressure reduces the MFP enhancing the collision probability [31] and lattice constant changes, respectively. Here, the lattice constant is unchanged at ~ 0.64 nm. However, due to the change in crystal orientation at high pressure, a small alteration is detected for the sample deposited at 200 Torr. Figure 3a illustrates the FWHM and crystallite size changes with deposition pressure. Moreover, microstrain and dislocation density variation with growing pressures are shown in Figure 3b. The displayed error bars indicate the identical validity of the provided data for all of the intervals.
When using a flexible holder, deposition and cooling tension can cause structural defects. Microstrains can also impair film adhesion unless the substrate is stiff. Figure 3b shows that, despite the flexibility of the UTG, microstrain and dislocation density values in the range of 10−3 and 1011, respectively, are evidence and support for a reduction in the mismatch rate between the CdTe film and ultra-thin glass compared to the results on standard borosilicate glass in the literature. When compared to the findings published in the literature for CdTe deposition on the standard glass substrate, the strain rate using UTG is in the same order as using the conventional rigid borosilicate holder, and the CdTe film properties are retained in the optimal range onto the flexible glass [34,35]. The maximum microstrain of 8.33 × 10−3 and dislocation density of 2.42 × 1011 are obtained for the sample grown at 200 Torr. This result demonstrates that the strain and dislocation density rise with the pressure higher than 100 Torr. Stress during deposition may result in the lattice misalignments and the fluctuations among diffraction patterns, as obvious in XRD analysis. As a result, the dislocation density is detected to be almost stable for the sample grown in the deposition pressure range of 1 to 100 Torr and increased a bit for 200 Torr due to changes in crystallite orientation. This leads to the formation of lower-quality films with nonuniform crystallinity. The obtained microstructural results are consistent with other published literature [31].
SEM micrographs on morphology are shown in Figure 4 for CdTe film deposited at the pressure range of 1 to 200 Torr. CdTe films are known to form by island nucleation and expansion, followed by coalescence, channel creation and secondary nucleation [33]. The process pressure and kinetic energy of the species involved in the process have a direct impact on the coating’s quality. Increasing the pressure above a specific threshold may cause the film’s stress to shift from compressive to tensile stress [15,36]. For 200 Torr of deposition pressure, the XRD graph shows a compressive shift in the peak as well as a higher FWHM or tensile stress in the CdTe thin films. However, for the high deposition pressure, e.g., 200 Torr, grain size reduces to 1.5 μm, which might be the result of incomplete nucleation under high pressure as well as stress on the ultra-thin glass. Incomplete nucleation also induces structural misalignment and phase alteration in samples deposited at 200 Torr, as seen in Figure 2. In addition, large columnar grains and narrow grain boundaries, which are favourable for the efficient CdTe thin-film device [37], are seen in Figure 4 from SEM data for samples deposited at a low deposition pressure <20 Torr. When samples are deposited at 200 Torr, more grain boundaries can be recognised, and areas around some of the grain boundaries appear brighter, resulting in a brighter overall figure. These boundaries facilitate the separation of photo-generated charge carriers and offer a low resistance channel for electrons to move to the front contact, preventing photocurrent from crossing grain boundaries and resulting in increased carrier concentration in this sample. The format of grain boundaries within a very thin, about 1.5 μm CdTe, layer may result in high dopant migration, current leakage [38], recombination at the surface [20], blocking the current transport [39]. Thus, tuning the deposition pressure is vital to achieve high-quality films and large grain size. It is detected from the SEM images that high deposition pressure decreases the mass transfer energy that results in poor crystallinity and small grain size [31].
Meanwhile, the degree of texturing decreases with the increasing pressure [8]. Several methods, like the Scherer formula, Williamson–Hall plot, etc., are available to calculate the diffraction peak profile. However, comparison of final grain size from different methods is complex, due to the difference in results, the order of magnitude and basis in each system and only estimation of grain size can be provided [40]. XRD presents the size of the crystallite, while SEM calculates the physical grains. Each grain can entail multiple spheres with dissimilar orientations. Accordingly, the size resulting from XRD analysis is smaller or with perfect grains, equal to the SEM calculated values. Grain size results from SEM measurement possibly are the accumulation of small crystallites into a large grain [24]. Consequently, grain size analysed by SEM is an average value, whereas XRD shows the crystallite size using the diffracted beam [41]. Ultimately, XRD analysis shows the measurements related to crystallite size within the grain and SEM presents the average grain size.
The variations in grain specification with the ambient pressure are shown in Figure 5a. The average grain size of the films deposited at 200 Torr is smaller (<1 μm) than the other samples (~2 to 5 μm). The smaller grain size of the film is mostly attributed to a reduction in the mean free path of molecules as pressure increases, as well as a decrease in nucleation and island formation number [42]. The reducing pattern of average thickness plotted in Figure 5b affirms the distorted nucleation of CdTe at high deposition pressure. EDX compositional analysis is shown in Figure 6, which confirms that the UTG specifications are not affecting the film growth. The EDX spectra show balanced Cd and Te dominant composition and atomic percentage irrespective of deposition pressure or substrate configuration. This ensures the thermal and chemical stability of CdTe composition on ultra-thin glass. The obtained results also comply with the XRD analysis. An additional rise in pressure leads to reduced growth rate, pinhole formation as well as incomplete nucleation, as expected [9].
Solar spectrum and semiconductor studies reveal that the optimum bandgap of the absorber layer requires to be in the range of 1.4 to 1.7 eV for the AM1.5 solar spectrum [20]. The absorption graph and the Tauc plot in Figure 7a,b show the changes in the absorption profile and direct energy gap (Eg) of CdTe films upon the change of deposition pressure. The direct bandgap is assessed by an extrapolated straight-line across the curve from the plot.
The energy bandgaps (Eg) of the CdTe films are found in the range of 1.65 to 1.69 eV, which changes with respect to the surface morphology and high stress experienced by the substrate and deposited material at high ambient pressure [43]. The optical bandgap increases slightly for thick samples due to the rise in crystallinity, less defect or localized state formation risk and the possibility of adding more atoms to arrange a stable lattice structure, as also shown here in Figure 7c [44]. It is noted that the bandgap of the thin-film absorbers has the greatest impact on the JSC. Chander et al. reported that the CdTe film before treatment had Eg in the range of 1.6–1.77 eV, owing to the deviation phenomenon located in interatomic scales [45]. Another reason for bandgap variation could be due to the compositional impurity or Te abundance in CdTe films leading to the generation of shallow acceptors. Moreover, the bandgap shift could be the result of strong interaction between substrate and vapor atoms or the changes in sublimation rate that directly affect the film carrier density and mobility [46]. The energy bandgap may shift to the average value of 1.5 eV after post-deposition treatments [41]. Previous studies demonstrated that the moderated bandgaps were found after the annealing and CdCl2 treatment, which may be the individual or collective effect of dislocation density revival, realignment in orientation and grain size increase [47]. A research study by Kokate et al. [48] shows that the direct optical Eg of deposited CdTe thin film were 1.65 eV and 1.5 eV after treatment. Similarly, Shaaban et al. also indicated that the bandgap of the treated CdTe samples were found around 1.45 to 1.55 eV [49]. Reduced bandgap range minimizes the interface recombination losses, repelling electrons, and/or passivating defects that function as recombination sites at the interface [50]. Moreover, the refractive index, which is defined by the opto-electrical polarizability of ions or regional fields within the material, is in the range of 2.85–2.87, as presented in Table 4. The data of refractive index and bandgap values are in accord with the study conducted by Rahman et al. [2]. As plotted in Figure 7c, the application of high deposition pressure causes the interatomic distance reduction and higher lattice strain, which probably causes a slightly lower bandgap, as also explained in other literature [51].
Structural disorder and local defects in the samples could be quantified by the Urbach energy (EU) value [20]. Urbach energy was estimated by adding a fitting line to the logarithmic plot of the absorption coefficient to study the electrical transitions between localized states. Here, the Urbach energy values are presented in Table 4 as well as in Figure 8a as the derivation method. The optical bandgap is inversely proportional to EU, as shown in Figure 8b. An increase in the Urbach energy may be attributed to the presence of defects, which leads to the restructuring of the localized states in thinner samples. The greater the value of EU, the more the phonon state disorder, which is detrimental for doping and/or compositional disparity in the films [2]. Urbach energy has a close relationship with VOC, where lowering the EU lowers the VOC, indicating that the EU may be used as a measure of absorber quality [52]. Ultimately, thin-film absorbers with EU values of less than 30 meV, which is less than thermal energy at ambient temperature, are critical for achieving good solar performance.
The electrical properties shown in Table 5 confirm the p-type CdTe conductivity of the films with the charge carrier concentration of 1 × 1013 cm−3 to 1 × 1014 cm−3. The increase in the carrier for intrinsic materials commonly is the result of the bandgap reduction [53]. The conductivity for different deposition pressure changes by the structural defects and surface chemical reactions [14]. The relation among carrier concentration, mobility, and resistivity are presented as the σ = qnμ and σ = 1/ρ, where σ is the electrical conductivity, n is the carrier concentration, μ is the mobility of the majority carrier and ρ is the resistivity of semiconductor [54].
The resistivity demonstrated in Table 5 is in the order of 104 Ω-cm, which changes slightly with the deposition pressure in accordance with an earlier study [55]. With the grain size and thickness reduction in high pressure deposition, the resistivity is expected to reduce too. Different scattering processes, such as ionized impurity and grain boundary scattering, restrict mobility. At high charge carrier concentrations, ionized impurities in the ionization mechanism can form clusters, causing increased scattering. The smaller absorber thickness and narrower depletion width may account for a one-order-of-magnitude increase in carrier concentration for samples grown at 200 Torr. Localized depletion has two effects: (1) it reduces the hole density at the interface, lowering the rate of recombination; and (2) it narrows the barrier to electron collection to a tunnelling thickness [56].
Hence, tunnelling through the barriers is achievable at high charge carrier concentrations due to the narrow depletion zone around the grain boundaries. This tunnelling increases mobility even further at high carrier concentrations. Lower carrier concentrations, however, may prevent the formation of ionized impurities and reduce the scattering effect caused by defective nucleation and poor crystalline structure. As a result, despite increased mobility, a reduced crystalline structure may minimize carrier dispersion at grain boundaries that results in the increment of the carrier mobility [57], as shown in Figure 9. Furthermore, when the absorber thickness declines, so does the full device current density [58]. Error bars here demonstrate the highest data deviation rate in carrier concentration results, which might be attributed to the coating’s limited electrical capacity prior to CdCl2 treatment. The overall results from thicker CdTe samples show poor electrical response, which might be due to the high roughness of the samples [8]. The carrier concentration in all of the deposited films was ≤1014 cm3, which would be predicted to result in poor device performance [56]. Using the CdCl2 treatment technique, more optoelectrical improvement may take place.
A negative impact in the electrical properties of the CdTe films at the lower deposition pressure can be justified by the grain boundary scattering relationship, as presented in Equation (7) [59].
α = (λ/d) × [r/(1 − r)]
In the above equation, α denotes the boundary scattering rate, whereas r, d and λ indicate the reflection coefficient, grain size and the mean free path (MFP), respectively. Only electrons within a distance of less than the mean free path from the surface can be scattered from the surface. Also, because the MFP is highly affected by the deposition ambient, surface scattering tends to impact the surface part of the thin-film resistivity [7]. Considering the reflection to be similar to identical grain boundaries, resistivity is directly related to grain size as smaller grain size results in lower resistivity [60]. Moreover, lower carrier scattering results in an increase of mobility. Consequently, the resistivity for the CdTe thin film deposited at 100 Torr demonstrates the lowest among all, probably due to the least scattering and uniform surface morphology. It is critical to consider the impact of grain boundaries on the electrical properties and clarify the functional quality of a polycrystalline thin film. However, it is obvious that the post-deposition treatment is needed to achieve better electrical properties for CdTe thin films, even deposited on ultra-thin glass substrates for flexible solar cell applications.

4. Conclusions

The influence of deposition pressure variation on CdTe thin films deposited on 100 µm thick glass substrate during the close-spaced sublimation (CSS) method was investigated thoroughly in this study. Analysis results verify the pressure dependence on film growth rate as well as grain growth. XRD results confirmed polycrystalline CdTe thin-film deposition that is oriented along (111) preferential plane with a cubic zinc blende structure, whereby the crystallinity is affected by the increase in deposition pressure. The surface morphology of the CdTe films showed a densely packed uniform pattern, which is free from crystal defects with the grain size ranging from 0.9 to 6 µm. The optical direct bandgap was found in the range of 1.65–1.69 eV. The film deposited at 200 Torr demonstrated the highest carrier concentration and mobility. Morphology and crystal orientation characterizations suggest that the properties of CdTe thin film were not affected much for ultra-thin glass as a substrate. More importantly, this study demonstrated that the 100 µm thick glass survived high deposition temperature and pressure as no substrate deformation or structural changes occurred during CdTe deposition by CSS. All in all, the results recommend that deposition at low pressure of about 1 to 5 Torr may be the optimum deposition condition for CdTe absorber layer on UTG substrates. Here, these samples attain larger grain size, high crystallinity, adequate electrical properties as well as near optimal band gap, which are good to be utilized for CdTe thin-film solar cell application. Micromechanical tests and analysis of interface and coating strength can further enhance the choice of proper substrate for flexible CdTe thin-film solar cells.

Author Contributions

Conceptualization, N.A.; data curation, N.A.; formal analysis, N.A.; funding acquisition, Z.A.A.; methodology, N.A.; investigation, N.A.; project administration, N.A., M.R.K. and Z.A.A.; writing—original draft preparation, N.A.; writing—review and editing, N.A.; visualization, M.R.K. and Z.A.A.; validation, N.A.; supervision, N.A., M.R.K. and Z.A.A. All authors have read and agreed to the published version of the manuscript.

Funding

This project was funded by the National Plan for Science, Technology, and Innovation (MAARIFAH), King Abdul Aziz City for Science and Technology, Kingdom of Saudi Arabia, award No. 13-ENE2229-02.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The authors acknowledge the National Plan for Science, Technology, and Innovation (MAARIFAH), King Abdul Aziz City for Science and Technology, Kingdom of Saudi Arabia for its grant with award number 13-ENE2229-02. The authors also acknowledge substantial support from IRMC as well as the Institute of Sustainable Energy of Universiti Tenaga Nasional (UNITEN).

Conflicts of Interest

The authors declare no conflict of interest.

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Coatings 12 00589 g001 550
Figure 1. (a) Sputtering chamber, (b) CdS thin film, (c) source - substrate distance in CSS and (d) CdTe thin film.
Figure 1. (a) Sputtering chamber, (b) CdS thin film, (c) source - substrate distance in CSS and (d) CdTe thin film.
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Figure 2. XRD patterns of CdTe thin films deposited at different deposition pressure.
Figure 2. XRD patterns of CdTe thin films deposited at different deposition pressure.
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Figure 3. (a) Variation of FWHM and crystallite size, (b) microstrain and dislocation density of CdTe thin film grown at different deposition pressures.
Figure 3. (a) Variation of FWHM and crystallite size, (b) microstrain and dislocation density of CdTe thin film grown at different deposition pressures.
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Coatings 12 00589 g004 550
Figure 4. SEM surface morphology of CdTe thin films.
Figure 4. SEM surface morphology of CdTe thin films.
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Coatings 12 00589 g005 550
Figure 5. (a) Grain size and (b) thickness of CSS deposited films at different pressures.
Figure 5. (a) Grain size and (b) thickness of CSS deposited films at different pressures.
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Figure 6. EDX spectra of CdTe thin films.
Figure 6. EDX spectra of CdTe thin films.
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Figure 7. (a) Absorption graph, (b) Tauc plot for optical bandgap calculation and (c) variation of lattice constant and bandgap with the deposition pressure.
Figure 7. (a) Absorption graph, (b) Tauc plot for optical bandgap calculation and (c) variation of lattice constant and bandgap with the deposition pressure.
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Figure 8. (a) Urbach energy derivation and (b) bandgap versus Urbach energy of CdTe.
Figure 8. (a) Urbach energy derivation and (b) bandgap versus Urbach energy of CdTe.
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Coatings 12 00589 g009 550
Figure 9. Electrical properties of CdTe films grown by various pressures.
Figure 9. Electrical properties of CdTe films grown by various pressures.
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Table
Table 1. CSS condition for CdTe thin-film sublimation.
Table 1. CSS condition for CdTe thin-film sublimation.
ParameterState
Source-Substrate Temperature (°C) 600/500
Source-Substrate Distance (mm) 1
Deposition Time (min) 10
Ambient Gas Argon
Deposition Pressure (Torr) 1
5
20
100
200
Table
Table 2. Mechanical properties of CdTe, borosilicate and ultra-thin Schott glass [21,22,23].
Table 2. Mechanical properties of CdTe, borosilicate and ultra-thin Schott glass [21,22,23].
Mechanical PropertiesCdTeBorosilicate GlassD263T Schott UTG
Young’s modulus (E) 49 GPa64 GPa73 MPa
Poisson’s ratio (μ) 0.40.20.21
Hardness, Knoop (GPa) 3.664.15.8
Surface roughness (nm) –1.5> 1
Shear modulus (GPa) 7.5326.530.1
Table
Table 3. Calculated XRD parameters of CdTe thin films.
Table 3. Calculated XRD parameters of CdTe thin films.
Sample ID1 Torr5 Torr20 Torr100 Torr200 Torr
Parameters
Miller indices (hkl)(111)(111)(111)(111)(111)
Angle of incidence (θ)11.9511.9311.9111.9011.83
FWHM (β) (deg)0.300.260.230.220.40
Interplanar spacing dhkl (Å)0.3720.3720.3730.37340.376
Lattice a (Å)6.4446.4546.4616.4686.511
Crystallite size D (nm)28.2932.6436.9038.5721.21
Microstrain ε (/103)6.185.364.754.558.33
Dislocation density δ (×1011)1.361.020.800.732.42
Table
Table 4. Optical properties of CdTe film under various deposition pressures.
Table 4. Optical properties of CdTe film under various deposition pressures.
Deposition Pressure (Torr)Bandgap,
Eg (eV)		Urbach Energy,
EU (meV)		Refractive Index (n)
11.69282.851
51.68742.855
201.68752.857
1001.67672.859
2001.65882.870
Table
Table 5. Electrical parameters of CdTe thin films deposited under various pressure.
Table 5. Electrical parameters of CdTe thin films deposited under various pressure.
Deposition Pressure (Torr)Carrier Concentration [×1013] (/cm3)Mobility (cm2/Vs)Resistivity [×104] (Ω-cm)
11.0787.30
51.3895.03
201.55133.10
1003.47151.20
20010.5153.97
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Amin, N.; Karim, M.R.; ALOthman, Z.A. An In-Depth Analysis of CdTe Thin-Film Deposition on Ultra-Thin Glass Substrates via Close-Spaced Sublimation (CSS). Coatings 2022, 12, 589. https://doi.org/10.3390/coatings12050589

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Amin N, Karim MR, ALOthman ZA. An In-Depth Analysis of CdTe Thin-Film Deposition on Ultra-Thin Glass Substrates via Close-Spaced Sublimation (CSS). Coatings. 2022; 12(5):589. https://doi.org/10.3390/coatings12050589

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Amin, Nowshad, Mohammad Rezaul Karim, and Zeid Abdullah ALOthman. 2022. "An In-Depth Analysis of CdTe Thin-Film Deposition on Ultra-Thin Glass Substrates via Close-Spaced Sublimation (CSS)" Coatings 12, no. 5: 589. https://doi.org/10.3390/coatings12050589

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