Appl. Phys. A DOI 10.1007/s00339-014-8488-y

Effect of annealing on photo accelerated chemically deposited Indium sulfide thin films with various cationic precursors A. C. Dhanya • K. Deepa • T. L. Remadevi

Received: 6 March 2014 / Accepted: 1 May 2014 Ó Springer-Verlag Berlin Heidelberg 2014

Abstract The present work reports the deposition of Indium sulfide thin films by a recently established novel method called photo-assisted chemical deposition technique. It is a very low cost method for the deposition of thin films, and can be easily scaled up for industrial production. Indium sulfide thin films are deposited on glass substrates through various cationic precursors and the effect of annealing on structural, optical and morphological properties was investigated. Films have been characterized with respect to their structural, optical and morphological properties by means of X-ray diffraction, UV–VIS-NIR Spectrophotometer, SEM and AFM techniques. As-deposited films on glass substrates were amorphous and became crystalline after annealing. The grain size of all the annealed films are larger than the as-prepared films and attained a maximum value for the film prepared with sulfate precursor. The calculated strain was compressive in nature. Surface roughness was estimated from the AFM measurements and found to be decreased in annealed samples. The film deposited with chloride precursor showed a higher visible transmittance of around 80 % and became 90 % on annealing. The variation of packing density follows the variation of the refractive index. The optical band gap of the samples was estimated and found to be within the range of 2.45–2.71 eV, which is in quite agreement with the literature.

A. C. Dhanya (&)  K. Deepa  T. L. Remadevi School of Pure and Applied Physics, Kannur University, Kannur, Kerala, India e-mail: [email protected] K. Deepa  T. L. Remadevi Department of Physics, Pazhassi Raja N.S.S.College, Mattannur, Kerala, India

1 Introduction The actual technology for solar cells based on a single crystal Si has reached in a matured state. However in some cases, the involved costs for getting higher efficiencies are too high to use the devices for terrestrial applications. Thus, it is commonly ascertained that a high-efficient thinfilm solar cell, prepared via a simple technology in large scale could be an alternative to reduce the involved costs in the fabrication process. These ideas led to the fact that thin film research is crucial in photovoltaic development. Some of the metal chalcogenides have optoelectronic properties that propose its utilization in photovoltaic structures. Efforts are concentrated on replacing CdS layer by Indium sulfide thin films to avoid toxic cadmium and obtain more environmentally friendly photovoltaic technology. Indium sulfide thin films have attracted much research interest due to their potential use in the manufacture of optoelectronic devices. These films have wide band gap of 2.0–3.7 eV depending on the preparation conditions [1] and high transmittance in the visible spectrum [2, 3]. Therefore, they have been used as buffer layer in photovoltaic structures [4–6]. They also have been used as absorber layers in nanostructured solar cells [7]. Indium sulfide films are relatively non-toxic and can be prepared by numerous dry and wet methods such as thermal evaporation (PVD) [8], chemical bath deposition (CBD) [9], atomic layer deposition (ALD) [10], spin coating [11], successive ionic layer adsorption and reaction (SILAR) [12] and chemical spray pyrolysis (CSP) [2], photochemical deposition [13], etc. Dinesh pathak et al. [14] deposited AgInSe2 thin films onto Si substrate by thermal evaporation technique at two different temperatures and structural, optical properties of the films were investigated. The obtained films were highly crystalline and the optical band

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gap was found to be larger with higher temperature. The structural and optical properties of AgInSe2 films grown by hot wall technique were reoriented in the (1 1 2) direction. Highest transparency of 94 % at 890 nm wavelength was observed for the films [15]. Highly stoichiometric Indium selenide films were prepared by laser ablation. The structural, optical and electrical properties have been investigated as functions of the substrate temperature. An increase in substrate temperature resulted in a more ordered structure. The roughness of the film was found to increase at higher deposition temperatures [16]. The present work establishes the deposition of Indium sulfide thin film by a recently introduced novel technique called photo-assisted chemical deposition [17]. It is a very low cost technique for the deposition of thin films, and can be easily scaled up for industrial production. It is more advantageous than all the conventional solution deposition techniques. The objective of this work is to explore the effect of annealing on the growth mechanism and properties of photo-assisted chemically deposited Indium sulfide thin films prepared with various cationic precursors. The structural, optical and morphological properties of the asprepared and annealed Indium sulfide thin films were characterized and compared.

2 Experimental details 2.1 Photo accelerated deposition mechanism (PCD) The experimental set up used to deposit the samples by UV irradiation is shown in Fig. 1. UV lamp of 125 W emitting UV radiation of 355 nm was used to irradiate the solution. Since the substrate was kept vertically very near to the UV source, energy diffused both into the solution and the substrate. The energy from the UV source is utilized to initiate the molecules of the reaction bath. In the case of irradiation of a substrate, there may occur photo-induced effect, which causes an enhancement of surface migration and rearrangement of adsorbing atoms. Quartz beaker was used as the container. The absorption of UV radiation by quartz is less, and hence there is no much filtration by the container wall. All the chemicals used were of analytical grade and the chemical bath contained 0.025-M Indium chloride, 0.3-M thioacetamide and one drop of glacial acetic acid. Glacial acetic acid acts as a complexing agent to hold the metal ions. The final solution was taken in a 50-ml beaker and stirred well using a magnetic stirrer, then placed under UV light. The quality of the surface of the substrate is an important factor for the deposition, where the film–substrate interaction occurs. The nature of the condensed film depends on the structure of the substrate, its temperature and cleanliness. Prior to use as a substrate, it was kept in detergent solution

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Fig. 1 Schematic diagram of PCD set up

for many hours and boiled in chromic acid solution for hours. Well-cleaned soda lime glass substrates were used to deposit the film. The substrate has been taken after 2 h from the reaction bath and dried in air. Different samples were prepared with various cationic precursors. Films were annealed to 100 °C for 20 min. Here, we have made an attempt to compare the different properties of as-prepared and annealed films with respect to each other. The sample prepared with Indium chloride as cationic precursor is named as CL. The same procedure has been repeated with different cationic precursors, namely Indium nitrate and Indium sulfate and the samples were named as NO and SO. The annealed samples were also named similar by prefixing the letter ‘A’ in all of them. 2.2 Characterization techniques The thickness of the samples was determined by the gravimetric method using a microbalance of sensitivity 0.0001 gm/div. The samples were characterized with respect to their structural, morphological and optical behavior. The structural analysis has been carried out using a Bruker AXS D8 X-ray diffractometer with Cu-Ka radi˚ as the source. The optical ation of wavelength 1.5405 A studies were carried out using a Hitachi-U-3410 UV–VIS– NIR spectrophotometer in the range 200–1,000 nm. The morphological studies have been carried out by a JEOL Model JSM6490 and an atomic force microscope Model 204C operated in contact mode employing sharpened nibs with gold coated tips, respectively.

3 Results and discussion 3.1 Appearance and film thickness The films are highly adhered onto the substrate and its appearance changes from lemon yellow to bright yellow

Effect of annealing on Indium sulfide thin films Table 1 Thickness of as-prepared and annealed samples Sample code CL

Thickness (nm) As-prepared 432

Sample code

Thickness (nm) Annealed

ACL

300

NO

481

ANO

365

SO

1,400

ASO

605

Fig. 3 X-ray diffractograms of annealed films with various cationic precursors

Fig. 2 X-ray diffractograms of as-prepared films with various cationic precursors

with the deposition time. The thickness of the film (t) is determined by gravimetric method using Eq. (1) using a microbalance by knowing the bulk density of the material. t ¼ m  qA

ð1Þ

where ‘m’ is the mass of the film deposited on area ‘A’ and ‘q’ is the bulk density of the material. The estimated average value of thickness of both as-prepared and annealed films is given in Table 1. The thickness of annealed films reduces with respect to as-prepared films. The thickness is more for the films prepared from Indium sulfate as the cationic precursor. 3.2 Structural properties The structural analysis of the films is carried out with the help of diffraction peaks obtained from X-ray diffractometer. The XRD patterns of the as-prepared films are shown in Fig. 2. The films are less crystalline until they get annealed to 100 °C. The result of amorphous nature of the as-deposited InS film is consistent with the literature report [13, 18].The as-deposited film with Indium chloride as cationic precursor is crystalline compared to other two, and the crystallinity becomes worse as moving from CL to SO. The film deposited using Indium sulfate is strictly amorphous at room temperatures. Thus, among the as-prepared

films, Indium chloride is the best to prepare crystalline Indium sulfide thin films. Films became crystalline upon annealing and the X-ray diffractograms of annealed films with various cationic precursors are shown in Fig. 3. All the films are polycrystalline in nature. Various factors that influence the stable poly crystalline state of a material include the lowest surface energy, the grain boundary energy and the diffusion of surface atoms [17]. Lee et al. reported that the strain energy minimization and surface energy minimization would compete with one another to determine the preferred orientation of grain growth and the final texture of thin films. The extent of these energy states depends on the thickness of the films. The minimization of the strain energy promotes one type of texture, while the minimization of surface energy promotes the other. Moreover at lower film thickness surface energy dominates, whereas at higher film thickness strain energy will be significant [19]. The films ACL and ANO showed cubic structure with two common peaks at (3 1 1) and (4 4 0) orientations corresponding to a phase of In3S4 (JCPDS data card no 88-2495). The other peaks coincide with the orientations of (2 2 2), (4 0 0) and (3 3 1) planes with similar structure. But the polycrystalline nature of ASO matches with the In6S7 phase consists of orthorhombic crystallites according to JCPDS data card no. 80-0126. The major and minor peaks correspond to the orientations of the crystallites are entirely different for different cationic solutions. The films prepared from sulfate solution have more thickness and the sample with higher thickness exhibits a different phase. The results indicate that the formation of the films mainly depends on the film thickness, whereas the structural changes occur by the surface diffusion and migration of grain boundaries during the coalescence of

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3.3 Morphological properties

Fig. 4 The variation of grain size and strain in the films

two differently oriented nuclei. In such cases, smaller nuclei may easily rotate on coalescence that induces the structural changes [20]. The mean crystallite sizes of all the fabricated films are determined using the FWHM of the various peaks using Debye–Scherrer’s equation given by, D ¼ 0:9k=bcosh

ð2Þ

where 0.94 is the value of the shape factor, ‘k’ is the ˚ , ‘b’ is the FWHM of wavelength of X-rays which is 1.54 A diffraction peak measured in radians and ‘h’ is the Bragg’s angle. Dinesh Pathak et al. obtained a grain size of 47 nm in the case of hot wall vacuum-evaporated AgIn S2 thin films. The values are given in Fig. 4. There is an increase in the average value of crystallite size for all annealed films and is higher in ASO. The improvement in the crystallinity of annealed films indicated the reorientation of the grains along particular directions [21]. Strain may be present in the film irrespective of the method of preparation, may be due to presence of defects, stoichiometric deviations, mismatch of thermal expansion coefficients of substrate and film, etc. The distorted lattice in strained film can affect the microstructural properties and hence optoelectronic behavior of the layers. The strain in the as-grown films is evaluated using the relation: a  a0 e¼  100% ð3Þ a0 where ‘a’ is the lattice constant of the ZnS film, and ‘a0’ is the unstrained bulk lattice parameter. Films crystallized in hexagonal structure can exhibit variation in both the lattice constants, and c. In the present case it is found that the variation in is marginal and, hence, variation in c axis length is considered to determine the strain. The strain is observed to be compressional for all the films. This variation of strain with grain size is shown in Fig. 4. It is found that in all the films, the compressional strain varies directly with grain size.

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The surface morphology and topography of the films are studied using SEM and AFM techniques. Figure 5 shows the SEM images of the annealed samples with various cationic precursors. Surface morphology of the films prepared from Indium chloride appears to be of pebble-like structure. The surface nature of sample ANO is compact but not clear, and the surface of ASO is completely covered with crystallites of almost similar size and shape. ASO is found to be denser than other films and appeared to be more uniform than other samples. This is due agglomeration of crystallites at higher thickness. Figure 6 shows the typical AFM images of the surface of InS layers prepared with various cationic precursors measured over an area of 5 9 5 lm. The analysis showed that the surface roughness is highly influenced by the cationic precursor and is more with as-prepared films. The average surface roughness is shown in Table 2 and it varied from 21.1 to 42.9 nm in annealed films, which shows that annealed films are smoother than as-prepared films. The film deposited with sulfate precursor showed an average grain size of 46 nm with the surface roughness of 43 nm. The surface roughness of the film mainly depends on the mobility and the diffusion of surface-adsorbing atoms [22]. If the diffusion of adsorbing atoms and coalescence is completed as soon as the nucleation of the second layer starts, surface of the layers become smooth and hence layer by layer growth would follow. In ACL and ANO, roughness is comparatively small so that the same growth mechanism might be followed in it. The reaction is comparatively speedy with Indium sulfate precursor. Thus, the surface of the adsorbing atoms may not have enough time to occupy their equilibrium positions of an ordered state before it interacts with another adsorbing atom, and hence the growth could not follow the layer by layer mechanism. Thus in this case, the growth of the second layer starts well before the completion of the first layer. Hence, the surface structure of the films formed at higher rates of deposition seems to be rough when compared with those formed at lower deposition rates [23]. The grain size is small in as-prepared films and since the deposition is through wet chemical procedures, there might be the formation of hydroxide layer at anywhere within the film. Both these reasons may lead to the lowering of surface roughness in the as-prepared films. A plot showing the variation of grain size and surface roughness with thickness in annealed and as-prepared films is shown in Fig. 7. In all the films, surface roughness increases with grain size and all the annealed films are smoother than the as-prepared ones. This decreased surface roughness in the annealed films makes it highly suitable for optoelectronic and photovoltaic applications.

Effect of annealing on Indium sulfide thin films

Fig. 5 SEM images of the annealed films with various cationic precursors

3.4 Optical properties The variation of optical transmission of the films with the wavelength in the visible region is shown in Fig. 8. It can be observed that significant changes are observed in the visible transmittance of the films with various cationic precursors. The optical transmittance of the films is mainly influenced by the surface roughness and grain size, where it varies directly with the grain size and inversely with the surface roughness [24]. As prepared films, except CL is less transmitting as compared to the annealed one. Observed lower transmittance of the as-deposited films could be due to the poor crystallinity as well as the higher surface roughness [25]. The film deposited with chloride precursor showed a higher visible transmittance of around 80 % for CL and 90 % for ACL. All annealed films are more transmitting as compared with NO and SO, and the increase in transmittance in annealed films can be explained in terms of enhanced grain size and lowered surface roughness. The films deposited via chloride as cationic precursor is highly suitable for window layer applications due to its higher uniform transmittance in the visible range.

Figure 9 shows the reflection spectra of the films. It is seen that the reflectivity of all the films lies within the range 15–30 %. The refractive index of the films, shown in Fig. 10 is a direct measure of film density, which could give information regarding voids present in the films. Both as-prepared and annealed films are highly reflecting in the visible-infrared region, and can be used in infrared cameras. The values of refractive index are lower than the theoretical value. The change of lattice constant could also influence the refractive index of the films. Mehan et al. [26] reported that elongation of lattice constant would lead to an increase of refractive index in rf-sputtered ZnO films, which supports our present result. The increase in the volume fraction of the voids could effectively reduce the refractive index due to poor packing density of the grains. In order to verify the effects of porosity or voids present in the films on the refractive index of the layers, the packing density p of the films was calculated using the relation:   n2f ¼ ð1  pÞn4v þ ð1 þ pÞn2v n2s  ½ð1 þ pÞn2v þ ð1  pÞn2s  ð4Þ where nf is the refractive index of the films at a particular wavelength, nv is the index of the voids, ns is the bulk value

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Fig. 6 AFM images of as-prepared and annealed samples

Table 2 Optical band gap of the films

Sample code

Band gap energy (eV)

CL

2.47

NO

2.46

SO

2.45

ACL

2.70

ANO

2.71

ASO

2.47

if refractive index and p is the packing density. In our study, it can be concluded that the variation of packing density follows the variation of the refractive index and is

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depicted in Fig. 11. Thus, the monitoring of the refractive index is an indirect way of measuring the packing density of the grains in the films. The extinction coefficient represents the spectral dependence of absorption coefficient. In bulk material it depends only on absorption of light by atoms. But in thin films it is related to surface roughness, poly crystallinity, crystallite size, etc. These factors change the attenuation of light in the film and apparently modify the values from the bulk, and the values are calculated from [27] k ¼ ak=4p

ð5Þ

Figure 12 depicts the variation of k against the wavelength of the films. All the films show a gradual linear

Effect of annealing on Indium sulfide thin films

Fig. 9 Reflection spectra of as-prepared and annealed films Fig. 7 The variation of grain size and surface roughness in asprepared and annealed films for different cationic precursors

Fig. 10 films

Variation of refractive index in as-prepared and annealed

Fig. 8 The optical transmission characteristics of as-prepared and annealed films

decrease in ‘k’ with increase in wavelength in the visible region except ANO and ASO, which exhibit minor increase. The relatively high value of ‘k’ for annealed films may be due to the crystallographic defects such as grain boundaries and voids present in the layers or from the increased surface roughness as seen from the AFM images [28]. From the plots of (ahc)2 versus hc as shown in Fig. 13, the energy band gaps of the films are evaluated. The calculated optical band gaps of the films increased from 2.45 to 2.47 eV in as-prepared films and 2.47–2.71 eV in annealed films. Table 2 shows the values of the energy band gap with different cationic precursors. In this study, large variation in the optical band gap is observed with the annealed one than as-prepared films. In

Fig. 11 Refractive index and packing density of the films

annealed films, the grain size increased whereas the surface roughness decreased. The grain boundaries are the dominant sources for the presence of structural disorder in the

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the layers by decreasing the grain boundaries and these studies are under progress [29]. 4 Conclusion

Fig. 12 Variation of k against the wavelength of the films

Indium sulfide thin films were deposited successfully onto glass substrate through a new procedure called photo accelerated chemical deposition technique. Films were prepared using various cationic precursors and the asdeposited and annealed films were characterized with respect to their structural, optical and morphological behavior. Thickness of the annealed samples is less as compared to the as-prepared films and the corresponding values are 432 and 300 nm for CL and ACL, 481 and 365 nm for NO and ANO and 1400 and 605 nm for SO and AS0, respectively. The phase of the deposited film changed from amorphous or nanocrystalline nature to polycrystalline after annealing at 100 °C. It was observed that the film from sulfate precursor exhibits an entirely different structure. Films with chloride and nitrate precursors have cubic structure, whereas that with sulfate has orthorhombic structure. The grain size of the films varied from 25 to 50 nm, as we move from as-prepared to annealed and the variation of compressive strain for the same films are -0.012 and -0.017, -0.16 and -0.076 and -0.113 and -0.126, respectively. The surface of all the films was highly uniform, and the surface roughness decreased on annealing. The film deposited with chloride precursor exhibits high uniform transparency in the visible region, and the surface roughness was comparatively small and is highly suitable in solar cells as window layers. The variation of packing density follows the variation of the refractive index. Thus, the monitoring of the refractive index is an indirect way of measuring the packing density of the grains in the films. The relatively high value of extinction coefficient for annealed films may be due to the crystallographic defects such as grain boundaries and voids present in the layers or from the increase surface roughness as seen from the AFM images. The annealing of the film increased the band gap as it could decrease the disorder present in the layers by decreasing the grain boundaries and the values are 2.47, 2.46, 2.45, 2.70, 2.71 and 2.47 eV for CL, NO, SO, ACL, ANO and ASO, respectively. Acknowledgments The authors express their sincere gratitude to SAIF STIC, CUSAT for offering technical support and Mr. Girish Kunte and Mr.Vaishag, IISc Bangalore for AFM measurements. One of the authors (TLR) acknowledges to KSCSTE for giving financial support under the project 001/SRSPS/2008/CSTE.

Fig. 13 (ahc)2 versus hc plot of the films

layers leading to the compensation of the energy band gap. It is expected that the annealing of the film might increase the band gap, as it could decrease the disorder present in

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