Characterization of chemically-deposited aluminum-doped CdS thin films with post-deposition thermal annealing

    Characterization of chemically-deposited aluminum-doped CdS thin films with post-deposition thermal annealing A. Fern´andez-P´erez, C. Navarrete, P. Valenzuela, W. Gacit´ua, E. Mosquera, H. Fern´andez PII: DOI: Reference:

S0040-6090(16)30848-3 doi: 10.1016/j.tsf.2016.12.036 TSF 35693

To appear in:

Thin Solid Films

Received date: Revised date: Accepted date:

26 May 2016 22 November 2016 19 December 2016

Please cite this article as: A. Fern´andez-P´erez, C. Navarrete, P. Valenzuela, W. Gacit´ ua, E. Mosquera, H. Fern´andez, Characterization of chemically-deposited aluminum-doped CdS thin films with post-deposition thermal annealing, Thin Solid Films (2016), doi: 10.1016/j.tsf.2016.12.036

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Characterization of chemically-deposited aluminum-doped CdS thin films with post-deposition thermal annealing

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A. Fern´andez-P´ereza,∗, C. Navarretea , P. Valenzuelab , W. Gacit´ uab,c , E. Mosquerad , H. e Fern´andez a

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Departamento de F´ısica, Facultad de Ciencias, Universidad del B´ıo-B´ıo, Collao 1202, Concepci´ on, Chile. b Centro de Biomateriales y Nanotecnolog´ıa, Universidad del B´ıo-B´ıo, Collao 1202, Concepci´ on, Chile. c Departamento de Ingenier´ıa en Maderas, Facultad de Ingenier´ıa, Universidad del B´ıo-B´ıo, Collao 1202, Concepci´ on, Chile. d Laboratorio de Materiales Funcionales a Nanoescala, Departamento de Ciencia de los Materiales, Universidad de Chile, Beauchef 851, Santiago, Chile. e Department of Physics and Astronomy, University of Exeter, Stocker Road, Exeter, EX4 4QL, United Kingdom.

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Abstract

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Aluminium-doped CdS thin films were grown, using chemical bath deposition, on glass substrates in an ammonia-free system, with post-deposition thermal annealing at 300◦ C in air

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atmosphere. Their structural, morphological, mechanical, electrical and optical properties were studied by X-ray diffraction (XRD), atomic force microscope (AFM), nanoindentation, four-point probes method and UV-Vis spectrophotometer, respectively. XRD patterns show that doped CdS films have an hexagonal structure, with preferred orientation along the (0 0 2) plane, and their average crystallite size start to decrease when Al content reaches a certain value. The AFM studies reveal that surface roughness decreases with thermal annealing. Additionally, we found that the Young’s modulus and hardness of the films decreases with increasing Al doping, and the electrical resistivity decreases with thermal an1

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nealing. The band gap was found to be in the range 2.39-2.49 eV for as-deposited films

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and 2.33-2.39 eV for annealed films. Current-voltage (I − V ) measurements were also carried out to the films, which showed rectifying behavior with Ag contacts for

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some doping levels.

Keywords: CdS thin film, Chemical bath deposition, Structural properties, Optical

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properties, Nanoindentation

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1. Introduction

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Cadmium sulphide (CdS) is one of the most promising materials for technological appli-

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cations such as opto-electronic devices [1] and solar cells [2, 3, 4, 5], and over the years have been the subject of intense research. For example, in thin film solar cells based on CdTe and Cu(InGa)Se2 (CIGS) absorber layers, CdS is the commonly used optical window material due to its high optical transparency, wide band gap (2.42 eV) and n-type conductivity [3, 4, 5]. Currently, chemical bath deposition (CBD) is a simple, inexpensive and scalable method to prepare CdS thin films, grown by precipitation in a controlled chemical reaction. Several ∗

Corresponding author. Tel.: +56 41 3111103 Email address: [email protected] (A. Fern´ andez-P´erez)

Preprint submitted to Elsevier

December 22, 2016

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studies on CdS deposition with this technique employ ammonia (NH3 ) in the chemical bath

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as the complexing agent and hydroxide source [5, 6, 7]. However, other authors show that good-quality CdS thin films can be prepared by CBD without ammonia. In these cases, the

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use of other complexing agents such as sodium citrate [8], potassium nitrilotriacetate [9],

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and nitrilotriacetic acid [10] are reported.

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Recent work in the field focused primarily on the structural, electrical and optical properties of CdS thin films [7, 8, 11, 12]. However, there is an emerging requirement to study

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the mechanical properties of this material at nanometer scale, mainly related to the response

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of the surface layer of these films to external mechanical effects including wear, damage and

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thermal degradation [13]. Furthermore, the adhesion of the coatings to the substrates is an important factor with which to evaluate the mechanical properties of the coated parts. In terms of the introduction of impurities in the films, the effect of doping agents on CdS thin films synthesized by CBD has been investigated. Doping with Ag [14], Ga [15], B [16, 17], Na and K [18] and Al [19, 20] has different effects on the physical properties of CdS thin films. For example, in Refs. [15, 16, 17, 18, 19] authors show a decrease in both the band gap and the crystallite size of doped CdS films, for some specific ratios between doping agents and Cd. Also, a decrease in the dark resistivity of the doped CdS films have

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been produced, for different concentrations of B and Al in the chemical bath solution [17, 19]

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and with thermal annealing for un-doped CdS films [7, 21]. In this regard, the dark resistivity of CdS films needs to be further reduced, in order to enhance their application

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in solar cells and other optoelectronic devices [5]. It is important to note that in all these

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works, ammonia is one of main components of the chemical bath, except in [14] where

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ammonia-free chemical bath deposition was performed to synthesize Ag-doped CdS thin films, which showed a decrease in the band gap and transmittance

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spectrum with increasing Ag content.

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The aim of this work is to investigate the structural, mechanical, electrical and optical

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properties of Al-doped CdS thin films, that were grown by CBD in an ammonia-free system with different Al doping levels. Additionally, the films were exposed to post-deposition thermal annealing in air atmosphere at a temperature of 300◦ C. The surface morphology and structural properties of the films were characterized by atomic force microscope and Xray diffraction; their mechanical properties were studied using the nanoindentation technique with a cube corner indenter tip; the electrical resistivity of the films was measured by four-point probes, and d.c. I − V measurements of a CdS/Ag junction were carried out. Optical analysis was performed with a thin film spectrophotometer in the

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wavelength range of 380-1050 nm.

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2. Experimental details

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2.1. Thin film deposition

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Several sets of CdS films were grown by chemical bath deposition (CBD) on soda-lime

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glass slide substrates (25 mm × 75 mm × 1 mm). The recipe to prepare doped CdS films is described in [8] and is based on a mixture of cadmium chloride (CdCl2 ), sodium citrate

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(C6 H5 O7 Na3 ), potassium hydroxide (KOH), pH 10 borate buffer, thiourea (CS(NH2 )2 ), and

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de-ionized water; all reactants are provided by Sigma-Aldrich. In situ doping was performed

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by adding aluminum chloride (AlCl3 ) to the mixture, with different molar ratios in the solution, R = [Al]/[Cd] (0.00, 0.01, 0.03, 0.05, 0.07, 0.10, 0.15, 0.20 and 0.25), where the

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initial concentration of Cd at 0.01 M remains constant. Prior to deposition, substrates were washed with water and liquid soap, then cleaned ultrasonically in de-ionized water for 30 min, and finally dried at room temperature. All the films were grown at a temperature of 70◦ C for 120 min. After deposition, all the films were annealed at 300◦ C in standard atmospheric pressure for 2 h. In order to study the effect of Al-doping on the electron transport properties across a CdS/Metal junction, Ag thin films were deposited on the annealed films by sputtering technique.

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2.2. Structural characterization

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In order to determine the structural properties of the films, X-ray diffraction measurements (XRD) were carried out using a Bruker Endeavor D8 Advance unit (with 40 kV,

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30 mA Cu-Kα radiation, λ = 0.15406 nm). The surface morphology was examined by an

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2.3. Nanomechanical characterization

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atomic force microscope, Nanosurf model NaioAFM, in contact mode.

The mechanical properties were determined by the nanoindentation technique using a

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Hysitron Triboindenter model TI-900 with a cube corner diamond tip. All these tests was

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performed at room temperature and standard atmospheric pressure. 2.4. Electrical properties

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Measurements of electrical resistivity were carried out with a four-point configuration under illumination and at room temperature, using a Jandel RM3-AR unit. The d.c. I − V measurements of CdS/Ag junction were performed by a Keithley 2400 electrical sourcemeter. 2.5. Optical properties Transmittance, reflectance and absorbance spectra were obtained by using a thin film UV-Vis spectrophotometer, Filmetrics model F10-RT, in the wavelength range of 380-1050 nm at normal incidence. 6

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3. Results and discussion

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3.1. Structural and surface morphological analysis The CdS:Al films obtained were yellowish, homogeneous, with good adherence to the

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substrate. In Fig. 1 we show the XRD spectra for as-grown CdS:Al films, with different Al

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content. The main peak position is located around 2θ = 26.78◦ and corresponds to the (0 0

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2) plane of the hexagonal crystalline phase of CdS (wurtzite). Although this diffraction line could be produced by the (1 1 1) plane of the cubic crystalline phase of CdS (zinc-blende),

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the secondary peaks observed at about 2θ = 48.25◦ and 2θ = 55.07◦ , in all our samples,

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correspond to the (1 0 3) and (0 0 4) crystalline planes of the hexagonal phase of CdS, according to PDF No. 80-0006. Besides, as we show in Fig. 1, no peaks of Al, AlS or Al2 S3

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were observed, which indicates that Al3+ ions do not change the crystal structure of the CdS film. Similar results were reported in [19] for Al-doped CBD-CdS films, but prepared by a different recipe.

XRD patterns of annealed CdS:Al films are shown in Fig. 2. We observe a very similar spectrum to the case of as-grown films, which implies thermal annealing does not change their crystalline phase and therefore no phase transition was found. However, the sharpness of (0 0 2) peak in the case of the films with R = 0.07 and R = 0.15 is increased with the annealing treatment, but decreases for the R = 0.20 sample. 7

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Using the XRD spectra, we calculate crystallite size and interplanar distance dependance

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on the Al/Cd molar ratio in solution. The interplanar distance d was calculated with Bragg formula, λ = 2d sin θ, where λ = 0.15406 nm and θ is the angle of the diffraction position.

0.9λ β cos θ

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D=

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The crystallite size D was estimated using Scherrer’s formula [19],

(1)

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And the dislocation density (δ) and the strain (ε) were calculated using the following rela-

δ =

1 D2

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ε =

β cos θ 4

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tions [22, 23],

where β is the FWHM of the diffraction peak, in radians. The crystallographic parameters of the CdS:Al films are summarized in Tables 1 and 2 for the (0 0 2) crystalline planes. ˚ In the case of as-deposited CdS:Al films, considering that the ionic radius of Al3+ , 0.53 A, ˚ [24], then the difference in the interplanar is smaller than the ionic radius of Cd2+ (0.95 A) distance may be due to Al3+ ions substitutionally replacing the Cd2+ ions in the lattice, causing the interplane distance to decrease, something which is observed when doping concentration increases [19]. For annealed films this tendency is less clear, probably because, beyond R = 0.05, Al3+ ions both substitutionally and interstitially replace Cd2+ in the 8

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lattice, which causes increased d values again. Exceptions are the R = 0.20 and R = 0.25

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films, where there is a reduction in the interplanar distance of the (0 0 2) crystalline planes in comparison with as-deposited films.

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On the other hand, average crystallite size decreases with increasing Al content, showing

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a sharp reduction when R ≥ 0.10 for both as-deposited and annealed films. Furthermore,

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in the case of R = 0.20 and R = 0.25 the annealing causes both the crystallinity and the average crystallite size to decrease strongly, with an increase in strain and dislocation

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density.

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Next, the surface morphology of the films was analyzed by AFM in contact mode. In

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Figs. 3 and 4 we show the images obtained in an area of 1 x 1 µm2 for as-grown and annealed CdS films, respectively. We observe, for different Al doping levels, a similar surface morphology and small differences in the surface roughness. The average surface roughness, for R = 0.01, R = 0.05 and R = 0.07 films, is found to be 4.012 nm, 3.415 nm and 4.136 nm, respectively, for as-deposited samples. For thermally annealed samples, the corresponding average surface roughness were 3.736 nm, 2.881 nm and 3.095 nm. The annealing treatment makes an uniform surface film, thus reducing the roughness.

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3.2. Nanomechanical properties

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Several indentations were performed on the films, with a cube corner indenter tip. Previous works suggest that indentation should not exceed 10% of the total film thickness in

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order to avoid the influence of the glass substrate [25]. For this reason, the load used was 40

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µN with a maximum depth penetration of 10 nm. We performed several indentation cycles

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on each film, and obtain the Young’s modulus (E) and the hardness (H) using the methods described in [26], which are based on the following equations, 1 − νs2 [1/Er − (1 − νi2 )/Ei ]

(4)

H =

Smax A

(5)

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where νs and νi are the Poisson ratios of the sample and the indenter tip, respectively, and Er and Ei are the reduced and Young’s modulus of the indenter, respectively. Smax is the peak indentation load and A is the projected area of the indentation. In this case, we calculate for a cube corner indenter tip with Ei = 1140 GPa and νi = 0.07 and for the films we use νs = 0.37 [27]; the results are presented in Table 3. We observe a decrease in the hardness and Young’s modulus with increased Al doping. Annealed films have values that are slightly lower than as-deposited films. The values found are similar to those previously reported mechanical values for hexagonal bulk CdS [27, 28]. 10

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3.3. Electrical properties

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Electrical resistivity of the films, under illumination at room temperature, was examined by using the four-point probes method, with collinear electrodes

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on the film surface. The results are shown in Table 4, for as-deposited and

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annealed films. We found resistivity values in the range 3.56 × 107 - 2.14 × 108

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Ω-cm for as-deposited films. In addition, we found resistivity values between 6.29 × 105 and 4.05 × 106 Ω-cm for annealed films. The highest value is founded

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for R = 0.10 and the lowest for R = 0.25. The increasing of conductivity is

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explained by the replacement of Cd2+ ions in Al3+ ions into the lattice of CdS.

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The substitution of Al atom in a Cd site as a donor can provide one charge carrier for conduction through CdS lattice. Thermal annealing also produces a decrease in the resistivity, in about two orders of magnitude with respect to as-deposited films. Annealed films have an uniform surface and lower roughness than as-deposited films. This produces fewer voids in the internal structure of the films, and this more compact morphology prevents carrier recombination centers. Also, as we show in the next section, the formation of CdO in the doped films after the annealing process produces a decrease in its electrical resistivity,

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as was shown in previous works related to thermal annealing of CdS thin films

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[29, 30].

In order to study an application of these films, in relation to how annealing

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and doping processes modify the electrical features of these films, we synthesize

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a metal-semiconductor junction. For this purpose, we deposit an Ag film with

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a thickness of 100 nm and 1 cm2 of area on the annealed films (R=0.00, R=0.10 and R=0.25) by using sputtering technique. Then, d.c. I − V measurements

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within the range of -1.0 V to +1.0 V was carried out on the junction. The

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results are shown in Fig. 5. It is observed that Schottky contacts is formed by

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Ag and the doped films, and an ohmic contact is formed by Ag and the un-doped film. When Al is introduced as doping agent, the forward voltage decreases with increased doping, due to the lower resistivity of the films. The forward voltage of these Schottky junctions are about +0.24 V for R=0.10 sample and +0.15 V for R=0.25. 3.4. Optical analysis In Figs. 6 and 7 we show the absorbance, transmittance and reflectance spectra for as-deposited and annealed films. It is clear that the as-deposited films showed better optical transmittance than annealed films, in the visible region, except for Al-doped films with 12

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R = 0.00 and R = 0.05. The absorption edge is found to be shifted towards the longer

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wavelength for thermally annealed films. The values of optical transmission lie between 45% and 85% in the region spectra above the absorption edge, but, in the wavelength

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range between 500 and 600 nm, the optical transmission decreases by 10 % on average for

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thermally annealed films. For annealed films, the absorbance increases in all samples, and

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remains almost constant in the wavelength range between 380 and 460 nm. Also, for the samples with R ≥ 0.10, transmittance is higher in the near-infrared region

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than in the UV-Vis region. If we calculate the integral of each transmittance curve we find

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the highest value for doping films with R ≥ 0.15. In this sense, an application of these

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films as a window layer in a solar cell could increase its efficiency; the absorber layer will be iluminated with more photons that have energies higher than its band gap, which in the case of CdTe and CIGS is around 1.3 eV [3, 5]. Due to the doped CdS window layer having a transmittance higher that than undoped CdS, a greater number of photons could increase the electron-hole pair generation in the absorber layer. In order to calculate the band gap for each sample, we need to obtain its absorption coefficient α, which depends on the incident photon energy ~ω as:

α = A(~ω − Eg )n/2 13

(6)

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where A is a constant, Eg is the optical band gap, and n is equal to 1 for direct band gap

transmittance T by the equation, T (1 − R)2



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(7)

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1 α = − ln t

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materials such as CdS. The absorption coefficient α is related to the reflectance R and

where t is the film thickness [31, 32]. Then, the band gap was determined for each film

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by plotting α2 vs ~ω and extrapolating the straight-line portion to the energy axis. As an

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example, we show this procedure in Fig. 8 for as-deposited films.

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Film thickness was estimated using the software Filmeasure which fits the transmittance and reflectance spectra to a layer model, considering the samples to be constituted by an

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air/CdS/glass/air system. In Table 5 we summarize the band gap and the thicknesses of as-deposited and thermally annealed thin films. In the case of as-deposited films, we observe an increase in the band gap when R ≥ 0.10 which implies that Al produces an increase in the transparency of the films, as seen in the transmittance results, and this also produces an increment in the band gap value. For thermally annealed films, a decrease in the band gap value is observed if we compare with as-deposited films, but the trend is similar. Regardless, we believe that oxidation takes place

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Cd(OH)2 → CdO + H2 O

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in the annealing process, with the following reaction,

(8)

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The measured optical band gap is slighty reduced because the CdO is present in the film

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and this affects the transmittance, reflectance and the absorption coefficient. This small difference is also present in the band gap calculation, as the authors in [7] report for thermally-

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undoped CdS films. We also related the difference in the band gap value for different doped

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films with the values of crystallite size and the strain, detailed in Tables 1 and 2. As authors

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report in [10, 33], a decrease in the average crystallite size, in general, increases the strain in the films, and this is related to an increase in their average band gap. In our case, we

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found both an increase in the average band gap value for doped as-deposited films, and in the strain of the films. 4. Conclusion

Aluminium-doped CdS thin films were synthesized using the chemical bath deposition in an ammonia-free system. Film thicknesses were between 85 nm and 220 nm, dependent on the Al content. XRD analysis reveals that the films have an hexagonal crystalline structure with preferred orientation along (0 0 2) planes. No significant change in the crystalline structure was founded after the annealing process. Crystallite sizes decreases with Al content and 15

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with the annealing process, and were found to be in the 8-48 nm range. The strain and the

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dislocation density increases with Al content. AFM studies reveal that the surface roughness is found to decreased with thermal annealing. Nanomechanical properties were calculated

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from nanoindentation measurements, and a decrease in the hardness and Young’s modu-

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lus is observed with Al doping and thermal annealing. Electrical resistivity decreases

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with thermal annealing and Schottky junctions are formed by Ag and annealed doped films. Finally, an increase in the average band gap value was found with increasing

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Al content both for as-deposited and thermally annealed films. The annealed films have a

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lower band gap value because CdO appears in the films after the annealing process, which

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has a lower band gap value than CdS (2.18 eV). Acknowledgments This work was supported by CONICYT FONDECYT Grant No. 11130369; and DIUBB GI Grant 152007/VC.

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technique, Mater. Res. Bull. 46 (2011) 6-11. [26] W.C. Oliver, G.M. Pharr, An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments, J. Mater. Res. 7 (1992) 1564. [27] S. Adachi, Handbook on physical properties of semiconductors, Kluwer Academic Publishers, Norwel, 2004. [28] D.I. Bolef, N.T. Melamed, M. Menes, Elastic constants of hexagonal cadmium sulfide, J. Phys. Chem. Solids 17 (1960) 143-148. [29] P.J. George, A. S´ anchez, P.K. Nair, M.T.S. Nair, Doping of chemically deposited intrinsic CdS thin films to n type by thermal diffusion of indium, Appl. Phys. Lett. 66 (26) (1995) 3624-3626. [30] A. Rmili, F. Ouachtari, A. Bouaoud, A. Louardi, T. Chtouki, B. Elidrissi, H. Erguig, Structural, optical and electrical properties of Ni-doped CdS thin films prepared by spray pyrolysis, J. Alloys Compounds 557 (2013) 53-59. [31] J.I. Pankove, Optical Processes in Semiconductors, Dover Publications Inc., New York, 1975. [32] E. Rosencher, B. Vinter, Optoelectronics, Cambridge University Press, Cambridge, 2004. [33] A. Rakhshani, A. Al-Azab, Characterization of CdS films prepared by chemical-bath deposition, J. Phys., Condens. Matter 12 (2000) 8745.

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5. List of Figures

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• Figure 1: X-ray diffraction pattern of as-grown CdS:Al films grown at different molar

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ratio in solution R=[Al]/[Cd].

• Figure 2: X-ray diffraction pattern of CdS:Al films grown at different molar ratio in solution R=[Al]/[Cd] annealed in air at 300 ◦ C for 2 h.

• Figure 3: Two dimensional AFM images of as-deposited CdS:Al thin films grown at different molar ratios in solution. (a) R=0.01, (b) R=0.05 and (c) R=0.07.

• Figure 4: Two dimensional AFM images of annealed CdS:Al thin films grown at different molar ratios in solution. (a) R=0.01, (b) R=0.05 and (c) R=0.07.

• Figure 5: Current-voltage characteristics of CdS:Al/Ag junction prepared 18

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with different molar ratios in solution for annealed CdS:Al films.

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• Figure 6: The absorbance (left), transmittance (center) and reflectance (right) spectra

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of as-deposited CdS:Al thin films.

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• Figure 7: The absorbance (left), transmittance (center) and reflectance (right) spectra

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of annealed CdS:Al thin films.

• Figure 8: Plot of α2 vs hν for as-deposited CdS:Al thin films with R=0.01 (Black),

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6. List of Tables

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portion of the curves.

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R=0.05 (Red) and R=0.07 (Blue). Dashed lines are the linear fit of the straight-line

• Table 1: Crystallographic parameters of as-deposited CdS:Al thin films.

• Table 2: Crystallographic parameters of annealed CdS:Al thin films.

• Table 3: Young’s modulus and hardness of CdS:Al thin films.

• Table 4: Electrical resistivity of CdS:Al thin films.

• Table 5: Band gap and thickness of CdS:Al thin films.

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Table 1: Crystallographic parameters of as-deposited CdS:Al thin films.

δ (1014 lines/m2 ) 5.888 5.203 4.434 4.550 4.249 11.023 19.407 24.171 30.592

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D (nm) 41.21 43.84 47.49 46.88 48.51 30.12 22.70 20.34 18.08

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d (˚ A) 3,334 3.332 3.331 3.332 3.325 3.321 3.323 3.317 3.324

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Peak angle (2θ) 26.72 26.73 26.74 26.73 26.79 26.82 26.81 26.86 26.80

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Sample R=0.00 R=0.01 R=0.03 R=0.05 R=0.07 R=0.10 R=0.15 R=0.20 R=0.25

ε (× 10−4 ) 8.412 7.906 7.299 7.395 7.145 11.509 15.271 17.046 19.176

d (˚ A) 3,341 3.336 3.330 3.313 3.336 3.329 3.329 3.306 3.308

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Peak angle (2θ) 26.66 26.70 26.75 26.89 26.70 26.76 26.76 26.95 26.93

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Sample R=0.00 R=0.01 R=0.03 R=0.05 R=0.07 R=0.10 R=0.15 R=0.20 R=0.25

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Table 2: Crystallographic parameters of annealed CdS:Al thin films.

D (nm) 31.31 32.83 28.10 37.73 40.02 25.02 21.46 8.77 8.26

δ (1014 lines/m2 ) 10.201 9.278 12.664 7.025 6.244 15.974 21.714 130.017 146.568

ε (× 10−4 ) 11.070 10.558 12.335 9.186 8.661 13.856 16.150 39.523 41.957

Table 3: Young’s modulus and hardness of CdS:Al thin films

Sample R=0.00 R=0.01 R=0.03 R=0.05 R=0.07 R=0.10 R=0.15 R=0.20 R=0.25

Young’s modulus (GPa) as-deposited thermally annealed 64.8222 ± 10.2398 53.0609 ± 8.6067 51.3508 ± 11.9731 53.9122 ± 7.3464 53.0358 ± 7.6338 53.2212 ± 10.2513 63.1763 ± 10.8333 49.7646 ± 7.5493 58.7246 ± 12.7056 48.7601 ± 12.6407 58.0229 ± 16.6768 49.6390 ± 11.7173 55.4888 ± 11.2160 46.7050 ± 7.6927 58.8144 ± 12.0206 45.0935 ± 13.0239 56.7948 ± 10.8969 47.7305 ± 5.6642

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Hardness (GPa) as-deposited thermally annealed 1.6875 ± 0.3488 1.6256 ± 0.3326 1.4870 ± 0.2318 1.5712 ± 0.3059 1.4358 ± 0.2915 1.5425 ± 0.2807 1.3812 ± 0.4059 1.6444 ± 0.4272 1.4572 ± 0.2782 1.4411 ± 0.2078 1.4061 ± 0.2484 1.6198 ± 0.3281 1.5758 ± 0.3500 1.4452 ± 0.3756 1.6228 ± 0.2141 1.4562 ± 0.2125 1.5140 ± 0.2158 1.3831 ± 0.3100

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Table 4: Electrical resistivities of CdS:Al thin films

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R=0.05 R=0.07 R=0.10 R=0.15 R=0.20 R=0.25

Resistivity (Ω-cm) As-deposited Thermally annealed – 1,6692 × 106 8 1,3160 × 10 1,6929 × 106 2,1362 × 108 4,0473 × 106 1,1688 × 108 3,0290 × 106 8 1,7067 × 10 1,3223 × 106 3,5618 × 107 6,2949 × 105

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Table 5: Band gap and thicknesses of CdS:Al thin films

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R=0.00 R=0.01 R=0.03 R=0.05 R=0.07 R=0.10 R=0.15 R=0.20 R=0.25

Band gap (eV) as-deposited thermally annealed 2.3932 2.3357 2.3944 2.3627 2.3663 2.3549 2.3700 2.3455 2.3505 2.3468 2.3548 2.3357 2.3942 2.3172 2.4449 2.3367 2.4910 2.3948

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Thickness (nm) as-deposited thermally annealed 131.00 124.95 85.25 88.37 100.87 82.16 109.62 101.18 155.85 150.92 190.83 201.50 228.40 211.70 234.88 210.80 220.95 201.67

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Highlights

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Al-doped CdS thin films were synthetized by chemical bath deposition technique in an ammonia-free system. Surface roughness, Young’s modulus and hardness decreases with increasing Al-doping. Thermal annealing modifies crystallite size and interplanar distance of the films. Electrical resistivity decreases with thermal annealing. Band gap increases with increasing Al-doping, but decreases with thermal annealing.

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