Effect of Substrate Temperature on Microstructural and Optical Properties of Nanostructured ZnTe

Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Effect of Substrate Temperature on Microstructural and Optical Properties of Nanostructured ZnTe Thin Films Using Electron Beam Evaporation Technique 1

M. Shobana1, N. Madhusudhana Rao1,a, S. Kaleemulla1, M. Rigana Begam2, M. Kuppan1 1 – Centre for Crystal Growth, Thin Films Research Laboratory, School of Advanced Sciences, VIT University, Vellore, Tamil Nadu, India 2 – Department of Science and Humanities, Indira Gandhi College of Engineering and Technology for Women, Chengalpattu, Kanchipuram, Tamil Nadu, India a – drnmrao@gmail.com DOI 10.2412/mmse.7.95.308 provided by Seo4U.link

Keywords: thin films, physical vapour deposition, II-VI semiconductors, Zinctelluride/ZnTe, substrate temperature, optoelectronic devices.

ABSTRACT. Single phase ZnTe nanostructured thin films were deposited on glass substrates using electron beam evaporation at various substrate temperatures (Ts like 423 K, 523 K and 623 K) under high vacuum of 2 x 10 -4 Pa. Effect of substrate temperature Ts on the structural, optical and morphological properties of prepared films have been investigated using powder X-ray diffractometer, UV-Vis-NIR spectroscopy, Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) analysis. Powder X-ray diffraction (PXRD) studies revealed that all the coated samples crystallized well in polycrystalline zinc blende structure along preferential <111> orientation. Optical transmission spectra exhibited an interference fringes, which confirms the formation of smooth and uniform films. SEM micrograph showed that the particles were spherical in shape with average size of 25nm. AFM images proved that densely packed columnar grain growth with evidence of nanostructure topography. Further the estimated lattice constant (a), average grain size (D) increased and average lattice strain (ε) decreased and also optical band gap energy (Eg) decreased with increase of Ts.

Introduction. Currently, there has been substantial interest among scientific research community in the field of semiconductor device thin film technology i.e. (i) to develop high-performance functional materials by different existed/novel techniques and (ii) Good quality/robust adhesion for a long time/uniform layer, defect free, homogeneous and stoichiometric with nanosized structures. One of the most important factor that in the growth of thin film is, optimisation of deposition parameters for attaining controlled size as well as shape of grains. ZnTe belongs to a class of AII-BVI inorganic chalcogenide semiconductor with direct transition wide optical band gap (2.26 eV at room temperature, λ≈547 nm) and native P-type electrical conduction due to Te excess and Zn vacancy nonstoichiometry, which is also called as ‘Self-Compensation’ effect [1], [2], [3]. Due to these unique properties, ZnTe is an exemplary candidate for optoelectronic device application, Also an imperative source for stable window and contacting material in CdTe, CdS, HgCdTe, etc. based multi-layer solar cells to achieve higher efficiency [4], [5], [6], [7]. Till date, numerous deposition processes: thermal evaporation [8], pulsed laser deposition [9], rf magnetron sputtering [10], closed space sublimation and electron beam evaporation [11], [12] etc. had been employed by many other researchers for the growth of ZnTe thin films but each of these techniques has its own merits and demerits. Electron beam evaporation (EBE) is a conventional, well known, most suitable and cost-effective comparatively with other physical vapour deposition (PVD) methods. To the best of authors knowledge very few studies have reported on thickness dependence [13]-[14] and substrate temperature dependence [15] ZnTe thin films by EBE method. In the present work, an attempt has been made to investigate the influence of Ts on major physical properties of E-beam evaporated thin 1

© 2017 The Authors. Published by Magnolithe GmbH. This is an open access article under the CC BY-NC-ND license http://creativecommons.org/licenses/by-nc-nd/4.0/

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

films under identical deposition conditions. The nucleation growth kinetics and surface morphology of films can be affected by thermodynamic conditions mainly substrate/growth temperature, annealing temperature, deposition rate and also the thickness of a film.

Experimental Synthesis. Device-grade ZnTe films have been grown on a microscopic glass substrates size of 75x 22x1.35mm3 using EBE method (Instrument: Hind Hivac 12A4D coating unit) by varying growth temperature (Ts) viz 423 K, 523 K and 623 K. Commercial ZnTe powder 150mg (≥99.99% purity, Sigma-Aldrich Chemicals Company, USA) weighed accurately, taken in a graphite crucible and used as a source material. Prior to deposition: (a) the substrates were chemically well-cleaned followed by ultrasonic technique and then dried in hot air oven at 100°C (b) The chamber (12 inches) thoroughly cleaned to attain high vacuum and avoid impurity (c) Substrates were fixed with a resistive heater assembly and were mounted over a vertical platform above the evaporation target. The Thickness of the films was found using a digital quartz crystal thickness monitor (Model: "Hind Hivac" DTM-101) attachment during the deposition. After deposition, the films were allowed cool slowly to room temperature. The films obtained were uniform, pinhole free and had good adhesion to the surface of the substrate. ZnTe films were grown by optimizing other deposition parameters.

Characterization. Structural analysis was carried out using P-XRD on a high-resolution X-ray Bruker D8 Advance diffractometer having CuKα1 (λ=1.5406Å) X-ray source at the scan rate of 4°C per min. Optical transmission spectra of the all films were recorded using UV-Vis-NIR JASCO V670 double beam spectrophotometer in the wavelength range of 200nm to 2500nm. Microstructural analysis were performed with HR-SEM (Model: FEI Quanta 200 FEG) and AFM (Model: Nanosurf Easyscan 2) in static force operating mode with silicon tip CONTOR cantilever type. All these measurements reported in our research were taken at room temperature.

Results and Discussion Structural study. From Fig. 1 (a) powder X-ray diffraction patterns, all the films were identified to be single-phase polycrystalline with zinc blende (space group F 43m No.216) structure. It can be seen that preferential diffraction peaks at 2θ values 25.27°, 41.86° and 49.48° which corresponds to <111>, <220> and <311> planes respectively are in good agreement with the standard JCPDS file no.# 150746. In the case of elevated temperature, slight shift towards higher angle in diffraction peaks, the intensity of predominant <111> orientation gradually decreased simultaneously <220> and <311> orientations increased (clear proof for <111> plane from Fig. 1 (b)). Scherer’s and Williamson- Hall formulae were employed to calculate the average grain size (D) and lattice strain (ε). From Table 1, it can be manifested that lattice constant (a) and average crystallite size (D) increased whereas lattice strain (ε) decreased with increase of substrate temperature Ts. Optical study. Fig.2 (a) shows optical transmission spectra of all the samples as a function of wavelength occur an interference fringes in the Vis-NIR range which suggested that homogeneity, uniform thickness and surface smoothness of thin films. It can be found that sharp fall of transmission at absorption band edge that consider the good crystallinity of deposited films and also transmittance percentage increaseses concurrently blue shift in absorption wavelength. Fig.2 (b) depicts that Tauc’s plot of (αhν)2 versus energy (Eg=hν) of incident photon radiation. The optical band gap energy can be evaluated by extrapolating linear portion of the respective Tauc’s plot curve to α=0 (x-axis). As Ts increases the band gap decreases which is attributed to well-known quantum confinement effect.

The absorption coefficient (α) can be calculated using the below equation:

α=-

1 ln(T) t

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(1)

Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

where t and T – film thickness (μm) and transmittance (a.u) respectively.

The value of the band gap (Eg) are given in Table 1.

a)

b) Fig. 1. (a) P-XRD patterns of ZnTe thin films deposited at various Ts, (b) Enlarged P-XRD patterns of ZnTe thin films in the 2θ range of 24.8-25.6° for <111> reflection.

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

a)

Fig. 2. a) Transmission spectra of ZnTe thin films deposited at different Ts, b) (αhν)2 Vs hν plots of ZnTe films deposited at different T. Table 1. Lattice parameter, grain size, micro-strain and energy band gap for ZnTe thin films deposited at different Ts. Ts (K)

Lattice Constant a (Å)

Average Grain Size D (nm)

Mean Lattice Strain

6.103

12

0.009

2.24

523 K

6.105

13

0.008

2.20

623 K

6.113

14

0.007

2.18

Standard 423 K

6.102

Calculated

(ε)

Band Gap Energy Eg (eV)

Microstructural study – Surface morphology/topography. Surface morphology of as-grown ZnTe thin film sample was shown in Fig. 3 (a) with magnification of 100000x. It can be obviously seen that uniform, spherical shape grains are distributed over 500nm scale scanning surface with mean particle size of 25.6nm. Fig.3 (b) & (c) illustrates two and three dimensional AFM images of ZnTe thin films deposited at 423 K. It is confirmed that compact, nearly porous-free and columnar growth pattern. These obtained results do agree the formation of nanocrystalline ZnTe thin films.

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

a)

b)

c)

Fig. 3. (a) SEM micrograph, (b) 2D AFM, (c) 3D AFM images of ZnTe thin films at Ts- 423 K. Summary. Good quality nonostructured ZnTe thin films were deposited by EBE method at different substrate temperature with fixed optimum growth environment. Above-mentioned results exactly stated that defect-free, high degree crystallinity with nanoscale size grains harvested in all the samples. Hence, our research emphasized as Ts is one of the major factors for thin film sample coating, which tailors microstructural and optical properties.

Acknowledgements. One of the authors (MS) gratefully acknowledges VIT University, Vellore for providing huge support, financial assistance and excellent research facilities. All authors thank SAIF, IIT Madras for SEM examination.

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