Conference-ICSFS-16-Structural Study of CdS Films Annealed in Oxidizing Atmosphere

Cadmium sulphide thin films were deposited onto glass substrates by chemical bath deposition (CBD) from a bath containing cadmium acetate, ammonium acetate, thiourea and ammonium hydroxide. The CdS films were annealed in air at various temperatures and time in order to investigate the influence of post-deposition annealing and CdCl2 treatments on the structure. The various structural parameters such as grain size, lattice constant, dislocation density and strain, were investigated using XRD. The as-deposited films have the mixed (cubic and hexagonal) phase. The grain size increases due to annealing and grain growth is a function of annealing temperature and time duration. In general the results showed that the strain, grain growth and the re-crystallization of CdS depends on the post-deposition annealing duration, temperatures and the presence of CdCl2. [DOI: 10.1380/ejssnt.2012.542]

Low series resistance, high transmittance and optimum band gap are very important requirements for the buffer material [26].So the CdS film should be less resistive, thin to allow high transmission and uniform to avoid short circuit effects [26].However, generally as deposited CdS thin films show high resistivity and optical transmittance, which can be improved.Post-deposition heat treatment is one of fundamental steps to improve the electrical and optical properties of CdS thin films.During the post-deposition heat treatments, fundamental structural changes happen for the film, which influence the optical and electrical properties of the semiconductor film and hence the device.Knowledge of the principal structural changes of the film during post-deposition treatments is very important in understanding and optimizing the performance of the photovoltaic device [27].
There are studies about the influence of heat treatments in different atmospheres on the physical properties of CdS thin films [21,24,[28][29][30][31][32][33][34][35][36][37].Metin et al. observed that annealing of CBD grown CdS films in nitrogen atmosphere influence the sharpness of the optical absorption edge [21].Self-oxidation and sulfur evaporation were considered as responsible for this behavior.Annealing over 300 • C was seen to degrade the optical properties of the film.Han et al. [30] annealed the CBD prepared CdS under oxidizing (oxygen) and reducing (argon-hydrogen) atmospheres and reported that the optical band gap decrease with annealing treatments and the Fermi level is shifting closer to the conduction band when annealed in oxidizing atmosphere while the shift is towards the valence band when annealed in reducing atmosphere.H. Kim and D. Kim [31] investigated the influence of the heat treatment on the microstructure of CdS and the performance of CdS/CdTe junctions.It was found that the annealing created pores in the chemically deposited CdS films; the performance of cells was improved after annealing CdS films; the CdS annealing time of 10 min was enough to obtain the optimum solar cell performance.No noticeable change in the optical transmittance and X-ray diffraction (XRD) patterns was observed after CdS annealing at temperature of 400, 450 and 530 It was observed that the as-deposited and the film annealed at 423 and 523 K showed mixed phases dominated by cubic and hexagonal, respectively.The phase transformation from mix to pure stable hexagonal phase started at 623 K.The grain size of the films decreases from 45.67 to 28.30 nm with increasing annealing temperature, after that it increases from 41.00 to 141.00 nm when annealing temperature reaches to higher values of 623 K.The defects like dislocation density and strain in the films do not change regularly with increasing annealing temperature.
The film annealed at 523 K shows the highest dislocation density (12.5×10 14 lines/m 2 ).Wan et al. [35] annealed CdS films in air with or without a CdCl 2 coated layer.They concluded that: Air annealing without the CdCl 2 coating resulted in oxidation of the CdS films.The oxides of CdO and CdSO 4 presented on the CdS grain surface and at the grain boundaries.Lose of sulphur during annealing in air led to a significant sulphur deficiency.These two annealing effects hindered the coalescence and grain growth of CdS films when annealed in air without CdCl 2 pre-coating.The CdCl 2 coating protected the CdS films from oxidation during air annealing even at a temperature as high as 500 • C. The anti-oxidation was attributed to the incorporation of a significant amount of Cl and the formation of ClS, which prevented the oxygen in-diffusion and therefore the chemical incorporation; The CdCl 2 annealing promoted the formation of V Cd in CdS thin films.The native defects and their complexes provided lattice sites for the atom rearrangement during the phase transformation from cubic to hexagonal structures.The transformation began to occur at a relatively low temperature of ∼300 • C. The anti-oxidation and homogeneous hexagonal phase formation led to easy and significant grain growth of CdS films.
Desale et al. [36] deposited CdS films by SILAR method and annealed at 250 • C in air for 30 minutes.The crystallographic structure and the crystallite size were studied from XRD patterns.The crystallite sizes were found to increase with annealing, and it shows the hexagonal structure.Park [37] deposited CdS by CBD on glass substrates and analyzed the influence of annealing on structural and optical properties at annealing temperature of 100 to 500 • C in N 2 atmosphere for 60 minutes and found that the crystal structure changed from cubic to hexagonal phase when annealed at 200 • C, the crystallite size increased with increasing annealing temperature at temperatures over 200 • C and the dislocation density decreased.
Despite the fact that CdS has been investigated extensively from the point of view as a hetero-junction partner to the absorber layer in solar cells, and studied to understand the influence of thermal annealing in oxidizing atmospheres, there are many issues remains that still need to be understood.In this paper we discuss the influence of the annealing in air on the structural properties of chemically deposited CdS thin films.

II. EXPERIMENTAL
The CdS thin films were deposited on glass slides by the CBD technique described in a previous work [4].
The chemical bath contained 0.033 M cadmium acetate (Cd(OOCCH 3 ) 2 •2H 2 O), 1 M ammonium acetate (CH 3 CO 2 NH 4 ), 0.067 M thiourea (H 2 NCSNH 2 ) and 28-30% ammonium hydroxide (NH 4 OH).The temperature of the bath was maintained constant at 90 • C. All the films used in the present study were deposited simultaneously from the same bath.The films were cut into equal portions (1 cm 2 ) to perform the heat treatments, and fresh samples were used for each annealing temperatures.The films annealed in air at 350 • C, 400 • C and 450 • C for 60 minutes were used in this investigation.In order to study the effect of annealing duration, 400 • C was selected as the temperature and annealed at different time intervals between 5 and 60 minutes.The thickness of the films was measured using an Alpha Step 100 surface profiler.The XRD measurements were performed using a Rigaku Xray difractometer with CuKα source with λ = 1.5418Å.The various structural parameters such as lattice constant, grain size, dislocation density and strain have been evaluated.The data were collected over a 2θ range of 20-60 degrees, with a grazing angle equal to 1.5 • .

A. X-ray diffraction
The XRD patterns of CdS thin films, both as deposited (virgin) and annealed in air at different temperatures and time durations are shown in Fig. 1. Figure 1 It can be seen from the diffraction patterns that the peak intensity changes with the annealing temperature and annealing duration for both the CdCl 2 treated and untreated samples indicating structural changes of material.The identification and assignments of the observed diffraction patterns were carried out using the JPDS 41-1049 Hexagonal (H) and JCPDS 10-0454 Cubic (C) reference patterns.For all samples we have observed that an intense peak appears at approximately 26.507 • , which corresponds to the CdS hexagonal plane (002), or cubic plane (111).But, the presence of the peaks (100) and (101) at 24.807 • and 28.182 • indicate that the phase is hexagonal or at least a mixture of hexagonal and cubic.Nevertheless the presence of doublets approximately at 26.5 • , 43.9 • and 52 • indicate that the films contain a mixture of hexagonal (wurtzite) and cubic (zinc-blende) structures.
The relative intensity of the peaks (100) and (101) increase with the annealing time and temperature, however, the intensity of (002) decreases, this effect is more pronounced for the films treated with CdCl 2 .The above features indicate recrystallization and a phase transformation from mixed phase to stable hexagonal phase.The presence of mixed phases of cubic and hexagonal in CdS  films and phase transformation were also observed before [19,21,24,30,31,35,37].

B. Lattice parameters
The lattice parameters a and c were estimated from the XRD line positions according to the relation [38]: where d hkl is the interplanar spacing, and (h, k, l) are the Miller indices.The calculated values of the lattice parameters were later refined using the method developed by Taylor and Nelson [39,40].In this method the lattice parameters a i and c i (where i corresponds to a particular XRD line) calculated for different peaks were plotted against cos 2 θ(sin −1 θ + θ −1 ).This graph yields a straight line and the intercept of the graph at cos 2 θ(sin −1 θ + θ −1 ) = 0 gives the lattice constant of the sample.The variation of lattice parameters a and c as a function of temperature for CdS thin films deposited by CBD is shown in Fig. 2. It can be seen that the lattice parameters have a dependence on temperature, annealing time and CdCl 2 treatments.For the as-deposited samples, the value of a (a as−deposited = 4.1225 Å) and c (c as−deposited = 6.702Å) are less than the powder sample (a powder = 4.1408 Å and c powder = 6.7198Å).The lattice parameter a of CdS film without with CdCl 2 treatment increase with temperature and reaches a maximum value for T = 400 • C and then decrease for T > 400 • C, while c, decreases, and when T > 350 • C gradually increase as the temperature increases.In the case of CdS treated with CdCl 2 both a and c increase at lower annealing temperatures and at higher temperatures the value decreases.
Figure 3 shows the variation of lattice parameters a and c of CdS thin films as a function of annealing time for T = 400 • C. In the case of CdS films annealed without CdCl 2 treatment, at the initial stages of annealing both a and c decrease as the annealing time increases and reaches a minimum in 5 minutes for c, and in 30 minutes for a, then both increase with annealing time.Contrary to the above observation, in the case of CdCl  maximum in 15 minutes, then decrease.In Fig. 3, the zero time signifies that of the as-deposited film.These results, indicates that the structural changes are not proportional to the annealing temperature and time.

C. The average diameter of grains and the strain
The average diameter of the CdS grains and the residual strain were calculated from the full-width at halfmaximum (FWHM) using the relation [41].where θ is the Bragg angle and; FWHM is the full width at half maximum of the peaks, λ = 1.54056Å is the wavelength of CuKα radiation, D is the average diameter of the grains, and K is the shape factor which is approximately unity and ε is the residual strain of the films.The integral breadth b, was obtained from a powder sample of polycrystalline silicon.A plot of β cos θ versus sin θ will give a straight line and the grain size D and the strain ε can be calculated from the intercept and slope, respectively.The variation of the average grain size D (nm) with respect to the annealing temperature is presented in Fig. 4. When the annealing temperature was increased to 450 • C, the grain size increases from 18 nm of the as-deposited film to 30 nm approximately of the CdS film without  (nm) FIG. 6: The fit of the generated data to the parabolic grain growth law (Eq.( 3)).The data was generated by applying a polynomial curve fitting for the first phase in Fig. 5 where the grain size increases with the annealing time.CdCl 2 treatment, and from 18 to 24 nm approximately in the case of CdCl 2 treated film.Figure 5 shows the variation of grain size with the annealing time.As one can see from Fig. 5 there are three phases for the recrystallization, in the first phase the grain growth occurs in the first 5 minutes in the case of CdS film treated with CdCl 2 and in 15 minutes for untreated film.In this interval the grain size reaches a maximum.The second phase is between 5 and 30 minutes of annealing duration where the grain size decreases.Beyond 30 minutes the grain size remains more or less constant with a tendency to slightly increase in size.
This increase and decrease of the grain size with temperature and annealing duration can be associated in part by two processes that occur during the post-deposition treatment, the process of recrystallization and the loss of material during the annealing.The annealing at lower temperature promotes recrystallization; nevertheless, at high temperatures and long annealing durations the loss of material becomes a very important factor.The time dependent grain growth was studied by applying the parabolic grain growth law [42], where D 0 and D are the average grain sizes before and after annealing, t is the annealing time, A is a constant and n is the grain growth exponent (the ideal value of n above the half melting temperature is 0.5).We have estimated the values of n in the case of untreated and CdCl 2 treated films for the annealing temperature of 400 • C using the data of the early stages of annealing when the grain growth occurs (0 to 15 minutes in Fig. 5) and after 30 minutes.In order to facilitate this calculation we have made a polynomial curve fitting using the data of the first phase in Fig. 5 where the grain size increases with annealing time and various points were taken from the polynomial fit.A plot of Ln((D 2 − D 2 0 ) 1/2 ) against Ln(t) is shown in Fig. 6 and the slope of the graph corresponds to the value of n for the annealing temperature.From figure it is clear that the grain growth follows two exponents, one in the first 5 minutes for CdS treated with CdCl 2 and a second one in the 15 minutes interval for the untreated samples.Those values of n are expected to be about 0.5, but from Fig. 6 it is clear that it deviates significantly from the ideal case.
Variation of strain in CdS thin films with the annealing temperature is shown in Fig. 7, and in Fig. 8 the dependence of strain on the annealing (T = 400 • C) duration is demonstrated.The minus sign indicates compressive strain and plus sign indicates tensile strain.
As we can see from Fig. 7, the as-deposited film is under compressive strain and in the case of untreated film the strain increases to a maximum value when annealed at 350 • C, then decrease with annealing temperature.However, in the case of CdS film treated with CdCl 2 the strain decrease attaining the minimum value at T = 350 nealing duration when annealed at T = 400 • C is shown in Fig. 8.The compressive strain increases with annealing time and reaches a maximum value and then starts to decrease in the case of CdS film annealed without CdCl 2 treatment.Contrary to this case, for CdS film treated with CdCl 2 the strain decrease with the annealing time, attains zero strain and reaches the maximum of tensile strain at 15 minutes of annealing, and then decreases, once again attains the zero-strain state and enters the compressive strain region.The change in strain indicates a variation in the concentration of lattice imperfections with the annealing temperature and time.

D. Dislocation density
The dislocation density (δ), defined as the length of dislocation lines per unit volume of the crystal, was evaluated from the formula suggested by Williamson and Smallman [43].
The dependence of the dislocation density on temperature and annealing time is shown in Fig. 9.The inset is the dependence of the dislocation density on the annealing time.As expected the dislocation density decreases as the grain size increase.The above observations imply that as the grain size increase with annealing temperature, defects like dislocation density decrease and the crystalline quality of the film improves.

IV.
We have investigated the influence of post-deposition annealing on the structural properties of CBD prepared CdS thin films.The as-deposited CdS film has a mixture of hexagonal (wurtzite) and cubic (zinc-blende) phases.The recrystallization and phase transformation from mixed phase to stable hexagonal phase after the annealing in air is more pronounced for the films treated with CdCl 2 .The lattice parameters of as-deposited CdS thin films is lower than that of the powder sample indicating that the as-deposited film is under strain.The grain growth has three stages during the re-crystallization, in the first stage the grains grow in size and attains a maximum, in the second stage the grain size decreases and in the third stage the grain size remains more or less constant with a tendency to increase.The grain growth exponent does not obey the ideal parabolic law.In general the lattice parameters, grain growth, strain and the dislocation density of CdS depends on the post-deposition annealing time, temperatures and CdCl 2 treatment.This can be due to the combination of two effects, the process of recrystallization and the loss of material during the annealing.The annealing at lower temperature promotes recrystallization; however, at higher temperatures or for longer annealing durations the loss of material becomes a very important factor.
(a) shows the XRD patterns of CdS films annealed without CdCl 2 treatment at temperatures of 350, 400 and 450 • C. Figure 1(b) corresponds to the XRD patterns of CdS films treated with CdCl 2 and annealed at temperatures of 350, 400 and 450 • C. Figure 1(c) is the XRD patterns of films annealed in air without CdCl 2 treatment at 400 • C for 5, 15, 30, 45 and 60 minutes, and Fig. 1(d) shows the XRD patterns of the films treated with CdCl 2 and annealed in air at 400 • C for 5, 15, 30, 45 and 60 minutes.For comparison XRD patterns of as-deposited film is included in all figures.

FIG. 1 :
FIG. 1: XRD patterns of as-deposited and annealed CdS thin films at different temperature and time duration; (a) as-deposited and annealed films without CdCl2 treatment at different temperatures, (b) as-deposited and annealed films with CdCl2 treatment at different temperatures, (c) XRD patterns of films annealed in air without CdCl2 treatment at 400 • C for 5, 15, 30, 45 and 60 minutes, and (d) XRD patterns of the virgin and the annealed CdS thin films in air at 400 • C for 5, 15, 30, 45 and 60 minutes with CdCl2 treatment.

2 FIG. 3 :
FIG. 3: The variation of lattice parameters a and c of CdS thin films as a function of annealing time for T =400 • C. The legend CdCl2 stands for the samples treated with CdCl2.The dotted straight line corresponds to the stress free value of the lattice constant.

2 FIG. 4 : 2 FIG. 5 :
FIG.4: Variation of the average grain size D ( Å) with respect to the annealing temperature.The legend CdCl2 stands for the samples treated with CdCl2.

FIG. 7 :
FIG. 7:The dependence of residual strain of CdS films on the annealing temperature.The legend CdCl2 stands for the samples treated with CdCl2.

FIG. 8 :
FIG.8:The dependence of residual strain of CdS films on the annealing time for T =400 • C. The legend CdCl2 stands for the samples treated with CdCl2.
• C and then starts to increase.The dependence of strain on anhttp://www.sssj.org/ejssnt(J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) e-Journal of Surface Science and Nanotechnology Dependence of the dislocation density on annealing temperature and time.The inset is the dependence of the dislocation density on the annealing time for T =400 • C. The legend CdCl2 stands for the samples treated with CdCl2.