Electrical Properties of Thermally Evaporated CdSe and ZnCdSe Thin Films

Electrical Properties of Thermally Evaporated CdSe and ZnCdSe Thin Films

Available online at www.sciencedirect.com ScienceDirect Materials Today: Proceedings 3 (2016) 1487–1493 www.materialstoday.com/proceedings Recent A...

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ScienceDirect Materials Today: Proceedings 3 (2016) 1487–1493

www.materialstoday.com/proceedings

Recent Advances in Nano Science And Technology 2015 (RAINSAT2015)

Electrical Properties of Thermally Evaporated CdSe and ZnCdSe Thin Films S. Selva Priyaa, B. Lakshmi Shreea , P. Therasa Ranjania, P. Karthickb, K. Jeyadheepanb, M. Sridharana* a

Functional Nanomaterials & Devices Lab, Centre for Nanotechnology & Advanced Biomaterials and School of Electrical & Electronics Engineering, SASTRA University, Thanjavur-613 401, India b School of Electrical & Electronics Engineering, SASTRAUniversity, Thanjavur-613 401, India

Abstract Cadmium selenide (CdSe) and zinc (Zn) doped cadmium Selenide (ZnCdSe) thin films were deposited onto thoroughly cleaned glass substrates by thermally evaporating CdSe and Zn powders and then laterannealed at 200 ˚C for 2 h in vacuum. CdSe and ZnCdSe thin films were characterized using X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), energy dispersive analysis of X-rays (EDAX) and UV-Vis spectroscopy to study the structural, morphological, compositional and optical properties. The XRD patterns showed that the films were polycrystalline in nature. FE-SEM micrographs indicated that the films were smooth and homogeneously distributed. EDAX affirms the presence of Zn, Cd and Se. The optical band gap decreased with increasing thickness. The carrier concentration, current voltage (I-V) characteristics and resistivity of the films were studied using the Hall effect, electrometer and four point probe method. ZnCdSe films exhibited Ohmic behaviour in the range of -10 to +10 V. © 2015Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of [Conference Committee Members of Recent Advances In Nano Science and Technology 2015.]. Keywords:Thermal Evaporation, ohmic, Carrier Concentration

*Corresponding author. Tel.: +91-4362-304000 Ext 2277; fax: +91-4362-264120. E-mail address:[email protected]

2214-7853© 2015 Elsevier Ltd.All rights reserved. Selection and Peer-review under responsibility of [Conference Committee Members of Recent Advances In Nano Science and Technology 2015. ].

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1. Introduction Binary and ternary II-VI semiconductor compounds are widely explored in recent decades especially in the area of photovoltaics (PV). Out of these, ternary semiconducting compounds are gaining much interest in PV cells because of their peculiar properties especially their band gap [1]. Ternary alloys have attracted attention in many areas including nanoscale engineering because of their easily tunable properties in terms of both physical and optical by just modifying their compositions [2]. For example ternary alloy of CdZnTe are used in solar cells because of their tunable band gap [3,4]. Similarly, ternary compound Zinc cadmium selenide (ZnCdSe) thin film is extensively used in solar cells. They are mainly used as an alternative source of window layers in solar cells beside cadmium sulphide. They are mostly utilized as a window layer for Cadmium Telluride solar cells [1]. Cadmium zinc selenide materials are used in radiation detectors and utilized as a laser screen materials in projection color TV [5]. Cadmium zinc selenide materials are used in light emitting diodes because of their good optical properties [6]. Rajpure et. al., deposited CdZnSe thin films and studied their micro-structural and optical properties and showed that the films were polycrystalline. The band gap increased with increase in zinc concentration [5].Sridharan et. al., studied influence of thickness on the optical constants of CdZnTe thin films deposited by vacuum evaporation and showed that films are polycrystalline and exhibited good transparency at higher wavelength [7, 8]. Deepak et. al., deposited Te thin films using thermal evaporation technique and described the influence of film thickness on the structural and optical properties of the nanocrystalline Te thin films [9]. Darkowski et. al., prepared cadmium zinc selenide thin films on to titanium sheets by electrode deposition and showed that the introduction of zinc (Zn) to cadmium selenide (CdSe) improved the power conversion efficiency to 14% [10]. Al. Bassam et. al.,studied I-V, CV characteristics as a function of air annealing of ZnCdSe prepared crystals by vapor phase technique. They indicated that it exhibited Schottky behavior with resistivity of (1-100) Ωcm [11].In our previous work we have studied the influence of Zn concentrations on ZnCdSe thin films and showed that the optical constants of the films varied drastically with changes in Zn concentration [12].Various techniques including both physical and chemical vapor deposition methods have been employed to deposit thin films. ZnCdSe thin films are prepared by different techniques like thermal evaporation, electrode deposition [13], Molecular beam epitaxy [14], chemical bath method etc. Thermal evaporation method has been adopted to deposit ZnCdSe thin films because of its ease of operation and cost effectiveness. So far not many works have been reported about the I-V characteristics of ZnCdSe films. In the present study, ZnCdSe thin films are deposited by thermal evaporation technique by varying the composition of CdSe. The influence of CdSe concentrations on ZnCdSe thin films were then investigated for their micro-structural, optical and electrical properties using various characterization techniques. 2. Experimental Details CdSe and ZnCdSe thin films are coated onto throughly cleaned glass substrates by thermally evaporating Zn and CdSe powders of high purity (99.999%, Alfa Aesar). The source materials were placed in molybdenum boats and the glass substrates of 2 × 1 cm2 dimensions cleaned thoroughly with distilled water, acetone and ethanol was then placed on to the substrate holder [15, 16]. The source to substrate distance was then kept at 15 cm in order to obtain films of uniform thickness. CdSe and ZnCdSe thin films of different thickness were obtained by varying the concentration of CdSe against fixed concentration of Zn. All the depositions were done at a base pressure of 1.1 × 10-5 mbar with deposition time of 4 min. The as deposited ZnCdSe thin films were subjected to annealing at 200°C for 2 h in vacuum. The temperature was increased gradually by 5 °C/ min and after it has reached a stable state the films were annealed for 2 h and then allowed to cool down at a rate of 5 °C/min. Proportional integral derivative controller was deployed to control the temperature. The deposited thin films were then characterized using XRD (Rigaku Ultima III) in the scanning range of 2θ = 10 to 80° using CuKα radiation (λ = 1.5406 Å) to examine the micro-structural properties of the films depending on the thickness. FE-SEM (JEOL JSM 6701F)was taken to analyze the morphology of the films which operated at an accelerating voltage of 3 KV. EDAX (BRUKER Instrument) was carried out to find the chemical composition of the film which operated at energy of 12 KeV. UV –Vis Spectroscopy (Lambda 35, Perkin Elmer) was done to find out

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the optical properties of the thin films and to study how the optical constants changes with the thickness of CdSe. Current-Voltage characteristics of the films were carried out by using Keithley 2450 interactive source meter. 3. Results and Discussions 3.1 Structural Analysis Figs. 1(a), (b) shows the XRD patterns of CdSe and ZnCdSe thin films. CdSe and ZnCdSe thin films are denoted as A1, A2, A3 and S1, S2, S3 respectively with increasing thickness. XRD patterns revealed the polycrystalline nature of the films with cubic structure which matched well with JCPDS card no 01-088-2346. The peaks at 2θ angle of 25.365° corresponds to CdSe with preferential orientation along (111) plane. The crystallite size (D) of the films were evaluated using the Debye’s Scherrer formula, k ×λ (1) D= β × cosθ where, k, λ, β, θ are the shape factor (0.94), wavelength of CuKα used, full width at half maxima (FWHM) and half of the diffraction angle respectively. The size of the crystallite increased from 17 to 25 nm with increase in thickness. The film with lowest thickness was found to be amorphous while films with increasing thickness showed increase in crystallinity. The reason for increasing crystallinity is, at higher concentrations of CdSe the films were dense which in turn improved the crystallinity.

(a)

(b)

Fig. 1. XRD patterns of (a) CdSe and (b) ZnCdSe thin films

The diffraction pattern indicated that all the films were polycrystalline with zinc blende (cubic) crystal structure which fits well with JCPDS card no 01-089-4174. The preferential peak oriented along the plane (200) plane corresponds to ZnCdSe. The diffraction peak oriented along the plane (111) corresponds to CdSe. The grain size of the ZnCdSe thin films were 17, 25 and 26 nm respectively with increase in thickness. At lower concentrations of CdSe the film was amorphous in nature. Increase in concentration of CdSe in turn increased the crystallinity of the films. Because at higher concentrations of CdSe, annealing caused agglomeration of the particles which in turn initiated the reorientation of the particles leading to increased crystallanity. The lattice constant of the ZnCdSe thin films varies that it either increases or decreases with Zn and CdSe concentrations. The lattice constant of ZnCdSe ternary alloy is given as a linear combination of binary alloys which is basically determined by Vegard’s law [17].

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3.2 Morphological Analysis Figs. 2(a-c). shows FE-SEM micrographs of CdSe thin films. Micrographs showed that the films were smooth, uniform with globular structure. The films became denser at higher thickness. The size of the crystallites increased with thickness which was confirmed by XRD.

Fig. 2.FE-SEM micrographs of CdSe thin films

Fig. 3(a-c) Shows FE-SEM micrographs of ZnCdSe thin films. Micrographs showed that the films consist of fine particles which were uniformly distributed on the surface making the films to be dense. As a result of annealing the native defects in the films were reduced this on the other hand caused agglomeration of the particles thereby increasing the grain size which matched with XRD.

Fig. 3. FE-SEM micrographs of ZnCdSe thin films

Fig. 4. EDAX spectra of ZnCdSe thin films

EDAX spectra of ZnCdSe films are shown in Fig. 4(a-c). The spectra affirm presence of Zn, Cd and Se in the deposited ZnCdSe thin films. The concentration of Zn, Cd and Se varies with varying concentration of CdSe.

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3.3 Optical Analysis The transmittance spectra of Cdse and ZnCdse films are shown in Fig. 5(a), (b). Transmittance of CdSe and ZnCdSe decreased from 87 to 75% and from 93 to 50% respectively with increasing thickness.

Fig. 5. Transmittance spectra of (a) CdSe and (b) ZnCdSe thin films

This decrease in transmittance in turn enhanced the absorption of the films. This is because at higher thickness the films became dense which lead to decreased transmittance.The band gap energy values of the films are determined using the following formula (2) (αhυ ) n = A(hυ − E g ) where, Eg, α, hν and A areenergy band gap, coefficient of absorption, energy of photon and optical constant respectively. Plot of hν verses (αhν) 2 for CdSe and ZnCdSe thin films areshown in Fig. 6 and Figs. 7(a-c). 4 12 2 -1 2 (αhν) (x 10 ) (eVm )

A1

A3

A2

3

2

1

0

1.4

1.6

1.8 2.0 hν (eV)

2.2

2.4

2.6

Fig. 6.Plot of hν Vs (αhν) 2 for CdSe thin films

The energy band gap was estimated from Tauc’s plot by extrapolating a tangential line to hν axis. The band gap of CdSe thin films were in the range from 1.61 to 1.95 eV. The band gap of CdSe can be altered with Zn doping. The band gap of ZnCdSe thin films varied in the range from 1.92-2.35 eV on increase in thickness [1,5,12, 18]. This shows that Cdse band gap can be altered with Zn doping. The reason for increase in band gap may be attributed due to the exchange of Cd atoms with Zn atoms [7, 12, 19]. On varying the concentration of CdSe, the composition of the ZnCdSe films shifted which in turn altered the energy band gap of the films. This enhanced Eg makes them useful in application like solar cells.

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S2

(αhν)2 (x 10

13 )(eV.m-1)2

S1

S3

1.8

0.9

0.0

2

3

hν (eV) 2

Fig. 7. Plot of hν Vs (αhν) forZnCdSe thin films

3.4 I-V Analysis The type of conductivity of the films was measured using hot probe method. All the films showed n-type conductivity. Fig. 8. shows the current Vs voltage plot of ZnCdSe thin films. I-V measurements were taken in the range from -10 to +10 V. The current increased linearly with increase in corresponding voltage [11]. I-V plot revealed that the films were Ohmic in nature. Current (A)

Current (A) S1

S2

-10

-4

6.0x10

6.0x10

-10

-4

4.0x10

-10

Voltage (V)

Voltage (V)

4.0x10 2.0x10

-8

-6

-4

-2

2

4

6

8

-10

-2.0x10

-4

2.0x10

-8

-6

-4

-2

2 -4

-2.0x10

-10

-4.0x10

-4

-4.0x10

-10

-6.0x10

-4

-6.0x10

Current (A) S3

-3

2.0x10

-3

1.5x10

-3

Voltage (V)

1.0x10

-4

5.0x10 -8

-6

-4

-2 -4 -5.0x10 -3

-1.0x10

-3

-1.5x10

-3

-2.0x10

Fig. 8. Current Vs Voltage plot of ZnCdSe thin films

2

4

6

8

4

6

8

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4. Conclusion CdSe and ZnCdSe thin films of different thickness have been deposited by thermal evaporation. XRD results confirmed the polycrystalline nature of the films. FE-SEM micrographs revealed the uniform distribution of the films. EDAX spectra revealed the presence of Cd, Zn and Se in ZnCdSe thin films. Optical analysis indicated that the band gap of CdSe films varied upon addition of Zn. On doping with Zn the band gap of ZnCdSe thin films varied in the range between 1.9-2.35 eV. I-V characteristics showed that ZnCdSe films were ohmic in nature and it exhibited n-type conductivity.

Acknowledgements Authors sincerely thank SASTRA University for providing necessary infrastructural and experimental facilities.

References [1] C. Natarajan, G. Nogami, M. Sharon, Thin Solid Films 261 (1995) 44-51. [2] Qing Kang, Qingyun Lai, ShouZhuoYao, Craig A. Grimer, Jinhua Ye, J. Phys. Chem. C. 116 (2012) 16885-16892. [3] M. Sridharan, M. Mekaladevi, Sa. K. Narayandass, D. Mangalaraj, H. C. Lee, Cryst. Res. Technol. 39(4) (2004) 328-332. [4] M. G. Sridharan, Sa. K. Narayandass, D. Mangalaraj, H. C. Lee, J. Optoelectron. Adv. Mater. 7(3) (2005) 1483-1491. [5] K. Y. Rajpure, S. H. Bamane, C, D, Lokhande, C. H. Bhosale, Indian. J. Pure.Appl. Phys. 37 (1999) 413-420. [6] Hua-chiang Wen, Chu-Shou Yang, Wu-Ching Chou, Applied Surface Science 256 (2010) 2128-2131. [7] M. Sridharan,Sa. K. Narayandass, D. Mangalaraj, H. C. Lee,Journal of MaterialsScience: Materials in Electronics 13(8) (2002) 471-476. [8] M. Sridharan,Sa. K. Narayandass, D. Mangalaraj, H. C. Lee, J. AlloysCompd. 346(1-2) (2002) 100-106. [9] P. DeepakRaj, R. Venkatesan, P. Dhivya, S. Gayathri, M. Sridharan, J. Optoelectron. Adv. Mater 16 (2014) 782-787. [10] A. Darkowski, A. Grabowski, Solar Energy Materials 23 (1991) 75-82. [11] A. A. Al-bassam, Solar Energy Materials & Solar Cells 57 (1999) 323-329. [12] S. SelvaPriya, B. Lakshmi Shree, P. TherasaRanjani, M. Sridharan, Nanomaterials and Energy (2015) Accepted in Press DOI:10.1680/nme. 15. 00001. [13] R. Chandramohan, C. Sanjeeviraja, S. Rajendran, T. Mahalingam, M. Jayachandran, Mary Juliana Chokalingam, B. Electrochem. 14(11) (1998) 402-406. [14] L. Hernandez, Z. Rivera-Alvarez, L. M. Hernandez Ramirez, Hernandez- Calderon, Solid-State Electronics 47 (2003) 759-762. [15] P. Dhivya, M. Sridharan, J. Electron. Mater. 43(9) (2014) 3211-3216. [16] P. Dhivya, A. K. Prasad, M. Sridharan, International Journal of Hydrogen Energy 37(23) (2012) 18575-18578. [17] R. Chandramohan, C. Sanjeeviraja, S. Rajendran, T. Mahalingam, M. Jayachandran, Proceedings of the Solid State Physics Symposium (1998) 181-182. [18] Sunyoung Ham, Soyeon Jeon, Ungki Lee, Ki-Jung Paeng, Noseung Myung, Bull. Korean Chem. Soc. 29 (2008) 939-942. [19] Hao Wei, Yangjie Su, Ziyi Han, Tongtong Li, Xinglong Ren, Zhi Yang, Liangming Wei, Fensong Cong, Yafei Zhang, Nanotechology 24 (2013) 235706.