Chemo-synthesis and microstructural evolution of Zn1

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May 9, 2016 - the ZnSO4 and CoSO4 in appropriate volumes with the hydrazine hydrate (N2H4) as a reducing agent, triethanolamine (TEA) as a complexing ...

Materials Letters 179 (2016) 95–99

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Materials Letters journal homepage: www.elsevier.com/locate/matlet

Chemo-synthesis and microstructural evolution of Zn1  xCoxSe thin films S.T. Pawar a, S.S. Kamble a,b,n, S.M. Pawar c, A. Sikora d, G.T. Chavan a, V.M. Prakshale a, P.R. Deshmukh e, L.P. Deshmukh a,nn a

Thin Film & Solar Studies Research Laboratory, Solapur University, Solapur 413255, Maharashtra, India N.K. Orchid College of Engineering & Technology, Solapur 413002, Maharashtra, India c Division of Physics & Semiconductor Science, Dongguk University, Seoul 100-715, Korea d Electrotechnical Institute, Division of Electrotechnology & Materials Science, ul. M Skłodowskiej-Curie 55/61, 50-369 Wroclaw, Poland e Supercritical Fluids & Nano Processes Laboratory, Yeungnam University, 712-749 Korea b

art ic l e i nf o

a b s t r a c t

Article history: Received 22 March 2016 Received in revised form 6 May 2016 Accepted 8 May 2016 Available online 9 May 2016

We report on the synthesis of Zn1  xCoxSe (0 r x r 0.275) thin films by a modified solution deposition technique. Compositional and structural analysis confirmed the successful realization of Co(ZnSe) thin films. Addition of Co2 þ into ZnSe host lattice caused morphological modifications from globule like morphology (ZnSe) to the formation of leaf like appearance composing the disc-decked micro-flakes elongated in size. To support the observations, Sobel transformation imaging technique has been used. & 2016 Elsevier B.V. All rights reserved.

Keywords: Solution deposition Thin films Co(ZnSe) FESEM AFM Sobel transform

1. Introduction

2. Methods and measurements

Chemical methods for the preparation of inorganic chalcogenide thin films offer the advantages of ease of preparation, economy and industrial scalability [1]. However, the recipe of chalcogenide thin film deposition differs on a large scale. For instance, chemical deposition of ZnSe requires 0.3 mL TEA and deposition temperature of 80 °C [2]. Conversely, chemical deposition of CoSe requires 16 mL TEA and the deposition has to be carried at room temperature [3]. There has been no report owing to such extreme deposition conditions. In this paper we report on the synthesis and microstructural growth of Zn1  xCoxSe thin films synthesized via a solution deposition route.

Zn1  xCoxSe thin films were obtained onto the glass substrates by a solution deposition method at the pre-optimized deposition conditions (temperature ¼80 °C, deposition time ¼90 min, pH ¼10 7 0.1 and substrate rotation speed ¼60 7 2 rpm). The deposition was carried out by reacting the equi-molar solutions of the ZnSO4 and CoSO4 in appropriate volumes with the hydrazine hydrate (N2H4) as a reducing agent, triethanolamine (TEA) as a complexing agent and ammonia to maintain pH. Sodium selenosulphate was used as the source for Se2  ions and was prepared by refluxing selenium metal powder in the presence of Na2SO3 at 80 °C for 9 h [2]. Terminal layer thickness of the asobtained films was determined by the AMBIOS Make XP-1stylus profilometer. The film composition was determined using the EMAX electron dispersive spectroscope (20 kV). Structural assessment was done with a Rigaku X-Ray diffractometer in 2θ range from 20–80°. The surface morphologies were viewed through a field emission scanning electron microscope (S-4200, Hitachi, Japan). An atomic force microscope (Innova, Bruker) was used to obtain the surface topographical features in Tapping Mode at ambient conditions. Standard probes were used (frequency ¼290 kHz, k ¼ 40 N/m and tip curvature o 10 nm).

nn

Corresponding author. Corresponding author at: Thin Film & Solar Studies Research Laboratory, Solapur University, Solapur 413255, Maharashtra, India. E-mail addresses: [email protected] (S.S. Kamble), [email protected] (L.P. Deshmukh). n

http://dx.doi.org/10.1016/j.matlet.2016.05.058 0167-577X/& 2016 Elsevier B.V. All rights reserved.

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Table 1 Some physical properties of Zn1  xCoxSe thin films. Composition (X)

0 0.005 0.01 0.025 0.05 0.075 0.1 0.15 0.2 0.25 0.275 Standard

Film thickness (nm)

370.8 416.2 576.5 618 848.5 1236 1159 1025 854 645 293.7

Atomic %

ZnSe (Cubic)

CoSe (Hexagonal)

Zn

Co

Se

a (Å)

a (Å)

c (Å)

59.65 58.80 56.64 54.30 51.47 47.48 46.52 42.30 43.30 44.34 45.59

– 0.64 2.24 6.78 8.91 9.61 10.45 16.89 21.13 18.48 13.82

40.35 40.56 41.12 38.92 39.62 42.91 43.03 40.81 35.57 37.18 40.39

5.5889 5.5714 5.5596 5.5614 5.5332 5.5224 5.5574 5.5889 5.5692 5.5889 5.5772 5.56

– – 3.523 3.618 3.626 3.629 3.620 3.539 3.538 3.536 3.526 3.63

– – 5.298 5.299 5.301 5.302 5.300 5.298 5.296 5.295 5.991 5.30

D (nm)

RMS roughness Rq, (nm)

122.65 117.36 86.82 74.65 60.14 57.71 51.01 45.28 41.42 40.38 37.32

74.22 93.14 111.14 112.47 126.04 141.36 148.07 217.95 115.28 103.70 94.66

Fig. 1. (A) Typical EDS patterns for Zn1  xCoxSe thin films. (B) Representative X-ray diffractograms for Co(ZnSe) thin films.

The acquired data were processed using SPIP software from Image Metrology. 3. Results and discussion 3.1. Physical observations Both ZnSe and Zn1  xCoxSe (0 r x r 0.275) thin films were thin, uniform, smooth, diffusely reflecting and tightly adherent to the substrate support. As-grown films showed a change in colour from smoky white to dark-blue as a result of Co2 þ addition into

ZnSe host lattice [4]. The terminal layer thickness is found to be increased with increase in x upto 0.075 and then decreased for further higher values of x (Table 1). We attribute the variation in layer thickness to the enhanced rate of nucleation by the addition of Co2 þ (up to x ¼0.075) and a considerable scattering may cause the observed decrease in layer thickness at higher x values. 3.2. Compositional studies Fig. 1(A) depicts the typical EDS patterns for ZnSe and Zn1  xCoxSe samples. The as grown ZnSe film is non-

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Fig. 2. Morphological evolution in Zn1  xCoxSe (0 r x r 0.275) thin films.

stoichiometric. The analysis confirmed that Zn2 þ from the ZnSe lattice has been replaced by Co2 þ atoms. A marginal replacement of Se2  cannot be denied.

JCPD values [5] and addition of Co2 þ in ZnSe is found to form a solid solution upto x ¼0.075 and further separate phases are observed for the higher x values.

3.3. Structural analysis

3.4. Morphological studies

The X-ray diffractograms (Fig. 1(B)) were obtained for all the ZnSe and Zn1  xCoxSe deposits within 2θ range from 20° to 80°. It appeared that the deposits are poly-crystalline throughout the studied composition range and both ZnSe and Zn1  xCoxSe deposits have mixed cubic and hexagonal wrutzite type structures [5–7]. A new ZnSeO4 phase has been detected in the as-grown films. The growth orientation is along o111 4 direction. The d values and intensities of reflections have good match with the

3.4.1. FESEM Fig. 2 shows the morphological evolution in ZnSe and Zn1  xCoxSe (0 r x r0.275) films. Morphology transformation is from globule like appearance to leafy deck like structure, much elongated in size. At high concentration, effect of Co incorporation is much stimulus that morphology is transformed into a threadedweb like structure. At relatively high concentration of Co (x 40.1) formation of globules initiated, which of course are of ZnSe,

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Fig. 3. Typical 3-D AFM surface topographs of Zn1  xCoxSe thin films.

tending towards the formation of separate phases. The elongated threaded structure might be of CoSe. The size determination of these particles is not possible. 3.4.2. AFM analysis Surface topographic images (Fig. 3) showed the presence of hillocks and valleys suggesting the crystalline nature of the asdeposited films. For pure ZnSe(x ¼0) the topograph consists of smooth rounded hillocks and valleys supporting the FESEM observations that growth is globular crystalline and sluggish. With increased Co2 þ content in the film, the surface topography tends to become rough and crystallite roundness went on increasing showing collapsing of the hillocks thereby filling the valleys that decreases the surface roughness at excessively higher x values (x 40.2) (Table 1). Transformation based on Sobel differential operator was used to provide better edge detection. The fine structures and features were assessed by examining the region properties of the foreground of a binary image of the particles, created using a Sobel edge detection algorithm, to segment the particles from the background areas [8–10]. The angular spectra

indicated the presence of a privileged orientation of the morphological features. The surface roughness (Rq rms) is cited in Table 1.

4. Conclusions Synthesis of ZnSe and Zn1  xCoxSe (0 r x r0.275) ternary thin films was made feasible by a solution deposition technique. Effect of Co2 þ integration into ZnSe host lattice was assessed through the physical, compositional, structural and morphological studies. Zn2 þ replacement to accommodate Co2 þ into a host ZnSe lattice was observed in the compositional studies. The structural assessment showed a mixed phase structure consisting of cubic zinc blend and hexagonal wrutzite phases. A morphological evolution from globular crystallites to leaf like disc-decked micro-flakes is observed through FESEM studies. To support the microstructural observations we have also conducted AFM and the advanced imaging, Sobel transformation technique to provide a better visibility of the fine structures and features of the materials (Fig. 4).

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Fig. 4. Angular spectra revealing privileged orientation of the morphological features and Sobel transform indicating the edges of the morphological features.

Acknowledgments One of the authors (STP) would like to acknowledge Solapur University, Solapur for the grant of the Departmental Research Fellowship.

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