J Mater Sci: Mater Electron (2013) 24:4782–4789 DOI 10.1007/s10854-013-1475-9

Effect of doping concentration on UV accelerated chemically deposited ZnS:Mn thin films A. C. Dhanya • K. Deepa • T. L. Remadevi

Received: 3 March 2013 / Accepted: 27 August 2013 / Published online: 11 September 2013 Ó Springer Science+Business Media New York 2013

Abstract Pure and Mn alloyed ZnS thin films have been prepared by UV accelerated chemical deposition technique which is simple, economic and easy to monitor. Influence of doping concentration on ZnS thin films was investigated through the structural, compositional, morphological, optical and luminescent studies. The XRD studies confirmed the formation of crystalline films with hexagonal structure. In doped samples the intensities of the prominent peaks increased up to 0.5 wt% Mn and then decreased. The optimum concentration means the amount required to get most suitable characteristics for photovoltaic application. The thickness of the films and the sizes of the crystallites varied in consistent with the structural results. Crystallites became larger in size on doping and appeared to be denser than undoped film. Various structural parameters like stress and micro strain were calculated. The observed strain is compressive in nature which rapidly increased with doping and then remained almost same with doping concentration. The SEM studies revealed the formation of films with almost similar morphology of spherical architectures. All the films exhibited uniform transmission in the high visible region, with a maximum of 80 % for the sample with optimum Mn concentration. Both direct and indirect band gap decreased due to the incorporation of Mn, but showed a blue shift in the fundamental absorption edge with doping concentration up to the optimum dopant content. Undoped and doped films exhibit five distinct luminescence peaks A. C. Dhanya (&)  K. Deepa  T. L. Remadevi Department of Physics, 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

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located around 391, 451, 458, 482 and 492 nm. The observed variation in the intensity of the luminescence in doped films clearly indicated the influence of thickness of the films which varied on doping.

1 Introduction Doped and undoped semiconductor thin films have been studied widely during the past few decades to travel around their size dependent optical properties and potential applications in various areas such as photocatalysis, solar cells etc. [1–6]. There has been increasing interest in the development of reliable luminescent materials for applications in flat panel displays. The researchers in this area concentrate on suitable film deposition techniques and diverse types of materials and their alloys which can be used as active components in optoelectronic devices. ZnS thin films have a prominent place in optoelectronics and thin film electroluminescent devices due to its wide band gap, high refractive index, high optical transmittance and luminescence. However the performance of ZnS thin films can be further improved by doping. Doped with transition metal element or rare earth element, thin films of ZnS can also be used as effective phosphor materials [7]. The preparation of Fe and Ni doped ZnS nano crystals by means of mercaptoethanol to yield the surface of the particles were reported earlier. The optical absorption spectra of ZnS nano crystals showed a blue shift in the absorption edge with the increase in Fe content [8]. Co2? doped ZnS samples enhanced visible light emission with emission intensities of 35 times larger than that of undoped samples [9]. Manganese doped ZnS thin films have been studied due to their radiative life time shortening and enhanced emission efficiencies etc. [10, 11]. The electro luminescence

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properties of ZnS:Mn has proven to be suitable for luminescence applications [12–14]. Manganese is generally incorporated as Mn2? ion in the substitution sites of the ZnS lattice. The excitation and decay of this ion produces a yellow luminescence at approximately 590 nm, which is associated with a transition between the 4T1?6A1 energy levels [15]. This emission intensity generally increases with increasing Mn2? concentration. However, the quenching of Mn2? emission has been observed at high Mn2? concentrations [16, 17]. The most common techniques used to deposit ZnS films include sputtering, sol–gel, spray pyrolysis, chemical vapor deposition (CVD), molecular beam epitaxy (MBE), atomic layer epitaxy (ALE), chemical bath deposition (CBD) and photo chemical deposition (PCD) [18–22]. CBD of ZnS thin films has been carried out usually in an alkaline medium with ammonia, tartaric acid or hydrazine hydrate as complexing agents in the cationic precursor solution by many researches [23]. In PCD, the aqueous acidic solution contains thiosulfate and metal ions. The thiosulfate ions releases solvated electrons and sulfur atoms by absorbing UV light and the compound is formed on the illuminated region by UV light above the substrate [24–26]. For the synthesis of certain metal chalcogenides CBD is inefficient to start the chemical reaction as such without the help of thermal or photo assistance. It is also incompetent to convert the preparatory materials effectively into the thin film form either due to the homogeneous reaction or due to sudden precipitation. Further the reaction walls are deposited with the material which is undesired. In the present work we have utilized the illumination of UV light for assisting the chemical deposition of ZnS thin films at room temperature. This method is cost effective and simple for synthesizing highly reproducible large area thin films. Here we have tried to explore the effect of Mn doping concentration on UV accelerated chemically deposited zinc sulfide thin films by analyzing the changes in structural, morphological, optical and photo luminescent behaviors.

2 Experimental All the chemicals used were of analytical grade (Merck). For the deposition of ZnS film, 0.5 M zinc acetate and 1 M thiourea were used as cationic and anionic precursors. The growth solution was prepared by dissolving 0.5 M zinc acetate followed by the sequential addition of 10 ml hydrazine hydrate, 10 ml ammonium acetate (0.5 M), 15 ml ammonia solution (4 M) and 1 M thiourea. Distilled water was used as solvent in each step and the reaction mixture was continuously stirred used a magnetic stirrer.The addition of required amount of hydrazine hydrate turns the cationic solution into milky white precipitation due to the formation

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of hydroxide content and the addition of ammonia results in a clear transparent solution. Hydrazine hydrate act as complexing agent to hold the metal ions. Ammonium acetate is used as a buffer to maintain the pH through out the reaction process and ammonia makes the pH of the solution to a particular value. The incorporation of Mn into ZnS was carried out using Manganese acetate. Doped Films were prepared by adding 0.1, 0.3, 0.5, and 0.7 wt% of manganese acetate in the cationic solution and it was continuously stirred with magnetic stirrer. The synthesis of these films were repeated a number of times to confirm the reproducibility. Commercially available soda lime microscopic glass slides with a dimension of 7.5 9 2.5 cm were degreased thoroughly in detergent solution and kept overnight in chromic acid. Then they were washed in distilled water and dried in hot air to use as the substrate. UV lamp of 125 W emits UV radiation at 355 nm was used to irradiate the solution. The substrate was kept vertically very near to the UV source in the chemical bath taken in quartz beaker. The aqueous chemical bath was placed under UV illumination and the deposition on the glass substrate was carried out for 5 h in order to obtain high quality transparent films with appropriate thickness. The films are named as UD, 0.1Mn, 0.3Mn, 0.5Mn and 0.7 Mn respectively for undoped and doped films with various doping concentrations. 2.1 Characterization techniques All the samples were characterized with respect to their structural, morphological, luminescent and optical behavior. The crystalline phase identification of the products was done through an X-ray diffractometer (Bruker AXS D8) ˚ as the with Cu-Ka radiation of wavelength 1.5405 A source. The morphological studies have been examined by JEOL Model JSM6490. The optical studies were carried out using a Hitachi-U-3410 UV–Vis–NIR spectrophotometer in the range 200–1,000 nm. The PL spectra were taken at room temperature by using F-2500 FL spectro fluorometer and the thickness of the samples was determined by gravimetric method using a micro balance of sensitivity 0.0001g/div.

3 Results and discussion 3.1 Deposition mechanism In CBD, higher temperature of about 70 °C is required to decompose thiourea. In this method the dissociation of thiourea is carried out through the photo excitation of the precursor solution. As the temperature of the precursor solution doesn’t exceed about 40–50 °C on illumination,

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we expect the photolytic dissociation of thiourea into sulfur ion. Here the photo assistance can ideally replace the heat supplied to the aqueous precursor solution in CBD. The energy from the light source has also been utilized to increase the kinetic energy and hence the inter diffusion of the adsorbing particles. The direct entry of UV rays on to the glass substrate also causes an amplified vibration among the molecules of the glass substrate. These electronic excitations support the adherence of the incoming atoms, and develop the crystalline behavior. The absorption of UV light by quartz vessel is neglected as the film formed on the curved portion of the vessel is very thin. Thus CBD is improved all the way through UV assistance. The thermally activated processes are more intense than the photo assistance. Hence the deposition is carried out very slowly unlike in conventional CBD. Since the average deposition rate is small (*1.5 lm/h), the aqueous solution remained clear even after the completion of deposition. In all wet chemical deposition techniques, complexing agent plays an important role in binding the metallic ions to avoid the homogeneous precipitation of the corresponding compound. In the present work, zinc acetate and thiourea furnish the role of cations and anions and both hydrazine hydrate and ammonia act as complexing agents. HH can react with metal ions to form the metal complex easily than that of ammonia due to the high value of the complex formation constant for the complex formation [27] Hence the role of ammonia is limited to maintain the pH of the alkaline bath. The metal complexes hydrolyze slowly and produce positive ions in the solution. The presence of hydrazine hydrate also improves the homogeneity and growth rate of the films.The various reactions involved in this growth process can be written as [28], ðCH3 COOÞ2 Zn2 H2 O ! Zn2þ þ 2CH3 COOH

ð1Þ

Zn2þ þ 4NH3 ! ZnðNH3 Þ2þ 4

ð2Þ

Zn2þ þ 3ðN2 H4 Þ ! ZnðN2 H4 Þ2þ 3

ð3Þ

CSðNH2 Þ2 þ 2OH ! S2 þ 2H2 O þ CH 2 N2

ð4Þ

Zn

2þ

2

þ S

! ZnS

XRD patterns with various intensities. It is seen that the preferential orientation of undoped films is along (1 0 10). But on Mn doping the intensity of the peak at (1 0 10) reflections lowered where as the intensity of reflection from (0 1 12) improved. Both orientations become prominent at 0.5 Mn and reduce on further increase in the Mn concentration. But the incorporation of Mn does not create any phase change. The grain size of the films is estimated from the Scherrer equation and it varies from 39 to 77 nm with a maximum for the sample 0.5 Mn. Both size and thicknesses of the films increase with doping concentration and found to be maximum for 0.5 Mn. It can be concluded that 0.5 wt% is the optimum amount required to get crystalline films. The lattice constant ‘a’ for the ZnS with hexagonal structure is calculated by the relation 1=d2 ¼ 4=3a2 ðh2 þ hk þ k2 Þ þ l2 =C2

All polycrystalline thin films are in a state of stress irrespective of their preparation technique. As the films are not epitaxial, there may not be structural or chemical compatibility. Since the deposition is carried out at room temperature the main cause for the stress may be the lattice mismatch between substrate and the deposited film [29– 31]. The residual stress may also depend on the doping concentration and the growth rate. Deviation of the calculated lattice parameter ‘a’ indicates that the films

ð5Þ

3.2 XRD studies The thickness of the as prepared sample varies from 6.53 to 8.71 lm, with a maximum of 11 lm for 0.5 Mn. The estimation of structure and particle size has been done using XRD results. The XRD diffraction profile of all the samples is shown in Fig. 1. The patterns show the polycrystalline behavior with three zinc sulfide peaks of hexagonal structures oriented along (1 0 10), (0 1 12) and (1 0 34) planes at 29.53, 30.34 and 49.51 degrees [JCPDS card no. 89–2349]. Both doped and undoped films show similar

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ð6Þ

Fig. 1 XRD profile of undoped and doped samples

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are under stress/strain, leading to either elongation or compression of the lattice constant. The average stress and micro strain are calculated using the relation   a  a0 S¼ Y=2r ð7Þ a0  ¼ ða  a0 Þ=a0

ð8Þ

where a and a0 are the lattice parameters and ‘Y’ and ‘r’ are the Young’s modulus and Poisson’s ratio of the bulk samples respectively. For ZnS the value of Y is 74.46 GPa and r is 0.29 [32]. The observed strain is compressive in all doped and undoped films and is maximum for 0.7 Mn. It is found that compressive strain rapidly increased on doping and remains more or less constant with increase in doping concentration. The increase in grain size is consistent with their compressive strain, which can be attributed to the slow deposition rate till the optimum content of Mn. For maximum doping concentration grain size is found to decrease with decrease in thickness. The thickness and various structural parameters of the films are tabulated in Table 1. 3.3 SEM studies The morphology of thin films can be understood in terms of nucleation and growth phenomena, which depend on the relative strength of interactions of metal ions with surface priming layer. The SEM photograph of both undoped and two doped samples are shown in Fig. 2. It depicts almost similar morphology of about spherical architectures. The crystal growth enhances when they fall on nucleation

centers already developed during the induction period. The presence of spherical shaped grains with a compact texture on an amorphous back ground indicates nucleation through multiple orientations [33]. The doped films are appeared to be denser than undoped one. The increase in density may result in an increase in stress between the crystal lattices and is found to be maximum for higher concentration. 3.4 Optical studies The optical properties of pure and doped films have been studied from the transmission measurements over 300–1,000 nm range. Percentage transmission versus wavelength for both films is shown in Fig. 3. The films exhibit uniform transmittance in the visible region around a wavelength of 600–800 nm, with a maximum of 80 % for the sample 0.5 Mn having maximum grain size. As the grains grow bigger, the grain boundaries become less defined due to the size allocation of the grains [34]. All other doped samples exhibit transmittance lower than that of the undoped film having lower thickness. The lowering of optical transmission in all these films may be due to the higher value of thickness which is a common phenomenon observed in all thin films. But the maximum transmittance of the films having optimum content of Mn is mainly attributed to its highly crystalline behavior. The optical band gap is calculated from the analysis of the spectral absorption among the fundamental absorption edge. The direct and indirect band gap of the samples is estimated using Tauc relation [35].

Table 1 Thickness and other structural parameters of the samples Sample code UD

0.1 Mn

0.3 Mn

0.5 Mn

0.7 Mn

2h (°)

˚) d (A

hkl

Thickness (lm)

˚) a (A

˚) a0 (A

Grain size (nm)

S (GPa)

Micro strain (e)

6.53

3.7304

3.823

39

-3.107

-0.024

6.65

3.7028

3.823

48

-4.031

-0.031

10.2

3.7087

3.823

59

-3.839

-0.030

11

3.6928

3.823

77

-4.378

-0.034

8.71

3.689

3.823

41

-4.493

-0.035

29.52

3.0222

(1 0 10)

30.36

2.9435

(0 1 12)

49.51

1.8395

(1 0 34)

29.57

3.0186

(1 0 10)

30.32

2.9457

(0 1 12)

49.54

1.8440

(1 0 34)

29.54 30.29

3.0217 2.9481

(1 0 10) (0 1 12)

49.47

1.8393

(1 0 34)

29.59

3.0170

(1 0 10)

30.31

2.9465

(0 1 12)

49.51

1.8364

(1 0 34)

29.58

3.0172

(1 0 10)

30.30

2.9478

(0 1 12)

49.48

1.8368

(1 0 34)

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Fig. 3 Transmission versus wavelength plot the samples

exponent ‘n’ depends on the type of transition. For direct allowed transition n = 1/2, indirect allowed transition n = 2. The direct and indirect band gap of the films are calculated from (aht)2 versus ht and (aht)1/2 versus ht plots respectively. The values of ht and aht are tabulated from the absorbance data.

Fig. 2 SEM images of both undoped and doped (0.1, 0.7 Mn) samples

ðahtÞ1=n ¼ Aðht  Eg Þ

ð9Þ

where ‘A’ is the parameter which depends on the transition probability Eg is the band gap of the material and the

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a ¼ ð2:303  absorbance=thicknessÞ

ð10Þ

hc ¼ ð1; 241=kÞ

ð11Þ

The direct and indirect band gap of the films are calculated from (aht)2 versus ht and (aht)1/2 versus ht plots respectively as in Fig. 4. For undoped film the direct and indirect band gap are obtained as 3.55 and 3.37 eV respectively. The band gaps of all doped films except 0.5 Mn, are less than undoped one. But both direct and indirect band gap go on increasing with the increase in doping concentration up to the optimum content. But for maximum Mn concentration the band gaps reduced. It is well known that the energy band gap of a semiconductor is affected by the residual strain, defects, charged impurities, disorder at the grain boundaries and also particle size confinement. In addition compressional strain increases the band gap due to the compressed lattice of the film and tensile strain will result in a decrease in band gap due to the elongated lattice [36]. Compressional strain in doped films increases gradually with increasing doping concentration. Usually the band gap decreases on doping due to introduction of new energy levels. Here also the band gap of doped films is less compared to undoped film except for 0.5 Mn. This could be due to the high compressional strain in it compared to other films. The reason for the dependence of Eg on compressive strain can be explained in terms of changes in the interaction between the electrons. Under compressive strain, the length of the bond between the adjacent ions is shortened, thereby widening the band gap. Though the compressional strain in 0.7 Mn is slightly

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Fig. 5 Variation of direct and indirect band gaps with Mn concentration

3.5 Photoluminescence studies

Fig. 4 Direct and indirect band gaps of undoped and doped samples

higher, the effect of high doping concentration leads to the decreased value of band gap [37]. Hence the presence of compressional strain also contributes to change in band gap from undoped to doped samples. Grain size of the films also plays an important role in determining the band gap of the films. The effect of crystallite size is found to be more prominent than the induced strain in all doped films. It is found that the grain size of all doped films is larger than that of the undoped film. It is maximum for the sample doped with 0.5 wt% of manganese acetate concentration. As grain size increase, film becomes more transmitting and is most suitable for window layer applications due to its wide band gap. The sudden lowering of grain size and thickness leads to the decrease in transmittance with highest doping concentration. The variation of both direct and indirect band gap of the films with doping concentration is plotted in Fig. 5.

When a semiconductor absorbs a photon, an electron may be excited to higher energy quantum state. When the excited electron relaxes to a lower energy quantum state we get a PL spectrum. The photo luminescence spectra of ZnS films mainly depend on the preparation conditions, defects present, crystallite size and shape [22]. The PL intensity depends on the measurement temperature and the energy of the exciting light. The room temperature photoluminescence spectra of doped and undoped ZnS films are recorded with an excitation wavelength of 320 nm under similar conditions. The luminescence spectra shown in Fig. 6 exhibited different luminescence intensities with a variety of emission peaks. All layers deposited with and without Mn show a broad UV emission peak at 391 nm. The other emission peaks occurred are at 451, 458, 482 and 492 nm respectively. The doped films show an increase in the intensity of all emission peaks. The luminescence peak located around 451and 458 nm are associated with sulphur vacancies in the lattice [38]. Two tiny peaks at 482 and 492 nm are due to the first harmonic of the excitation wavelength 500 nm and is not sample related [36]. The observed variation in the intensity of the luminescence in doped films clearly indicated the influence of thickness of the films on luminescence. It was reported that the luminescence intensity depends on the thickness [39], accordingly the films with smaller thickness show higher luminescence intensity and vice versa. It is found that the sample 0.5 Mn shows a maximum value of intensity, which in turn related to the surface roughness. As thickness increases surface roughness increases and increases the surface defect states, resulting in increased luminescence intensity [40].

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variation in the intensity of the luminescence in doped films clearly indicated the influence of thickness of the films on luminescence These luminescent films with very good emission properties could be utilized to fabricate emissive devices. UV assistance during the deposition greatly improved the properties that demand in various optoelectronic applications and provides a novel approach to pursue new research in the study of metal chalcogenide films. Acknowledgments The authors express our sincere gratitude to SAIF STIC, CUSAT and SAIF STIC, IIT Madras for offering technical support. One of the authors (TLR) acknowledges to KSCSTE for giving financial support under the project 001/SRSPS/2008/CSTE.

References Fig. 6 Photoluminescence spectra of undoped and doped ZnS films

4 Conclusion The UV assisted chemical deposition technique is simple and economic and requires less monitoring. Highly transparent Mn doped thin films were deposited successfully with this novel deposition procedure. The present study explored the effect of Mn alloying on UV accelerated chemically deposited zinc sulfide thin films by observing the changes in structural, morphological, optical and photo luminescent behavior with doping concentration. The XRD studies confirmed the formation of crystalline films with hexagonal structure and concluded that 0.5 % is the optimum content required to get crystalline films. Increasing in Mn content increased the thickness up to this particular concentration and then decreased. The diameter of the crystallites increased gradually from 39 to 77 nm up to 0.5 % of Mn concentration, beyond that it reduced to 41 nm. Various structural parameters like grain size, stress and micro strain were calculated. The observed strain was compressive in nature which rapidly increased on doping and then remained more or less same with doping concentration. The SEM studies revealed the formation of uniform and homogenous films with almost similar morphology of spherical grains. All the films exhibited uniform transmission in the high visible region, with a maximum of 80 % for the sample with optimum Mn concentration. These highly transparent films are beneficial in photovoltaic applications. Both direct and indirect band gap went on increasing with the increasing amount of doping concentration up to optimum value of the dopant, but less than undoped one. Five different emissions in violet and blue regions were visible in the room temperature luminescent spectrum of all the films. The presence of Mn resulted in an increase in intensity of all emission peaks. The observed

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