Deviation in tuning of optical properties of ...

Integrated Ferroelectrics An International Journal

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Deviation in tuning of optical properties of polycrystalline AgSeTe thin films Neeru Chaudhary, S. K. Tripathi & Navdeep Goyal To cite this article: Neeru Chaudhary, S. K. Tripathi & Navdeep Goyal (2018) Deviation in tuning of optical properties of polycrystalline AgSeTe thin films, Integrated Ferroelectrics, 186:1, 84-90, DOI: 10.1080/10584587.2017.1370320 To link to this article: https://doi.org/10.1080/10584587.2017.1370320

Published online: 04 Jan 2018.

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INTEGRATED FERROELECTRICS , VOL. , – https://doi.org/./..

Deviation in tuning of optical properties of polycrystalline AgSeTe thin films Neeru Chaudhary, S. K. Tripathi, and Navdeep Goyal Department of Physics, Panjab University, Chandigarh, India

ABSTRACT

ARTICLE HISTORY

The semiconductor alloys of Selenium, Tellurium doped with metals are extensively studied for their optical properties and electrical transport phenomena. This work reports the optical constants of Silver doped Selenium Telluride thermally evaporated two dimensional structures. The optical energy gap calculated from the Tauc plots follows drop to a certain doping level, and then follow the deviation in the trend. Similar deviation is observed in the refractive index, dielectric constants and absorption coefficient in the studied compositions. Conventional method of Swanepoel is used to find the optical coefficients. The trend is explained using the concepts of mean coordination number, electronegativity, chemical bond approach and cohesive energy approach.

Received  November  Accepted  April  KEYWORDS

Optical coefficients; semiconductors; coordination number; cohesive energy, electronegativity

1. Introduction From last few decades the focus of researchers is to synthesize, characterize and find applications from semiconducting nano materials [1–7]. The major contribution is of nano chalcogenides and the maximum research is concentrated to study the amorphous state in optoelectronics. Many studies have been done on amorphous Selenium. It is commercially an important material now. Recently, semiconductors based on Selenium, Tellurium with different third element are used in a number of solid-state devices. Its various applications in device fabrication like rectifiers, photocells, photocopying, light meters, solar cells, xerography, switching and memory, etc. have made it very useful material [8]. Amorphous Selenium is considered as mixture of long helical chain like structures and eight membered ring type structures with strong covalent bonds between atoms and weak forces between neighbouring units. In amorphous state its optical band gap is 2.03 [9]. Selenium, a highly sensitive material is replaced with Silver to make a ternary Agx Se85-x Te15 (x = 0, 2, 6, 10, 15) composite alloy. Tellurium will provide strength to the composite as commercially, Tellurium is used in alloys to improve machinability. This gray material resists oxidation by air and is non-volatile. Silver is a good conductor and also widely used in Electronics, hence became the third choice in ternary composites as SeTeAg. CONTACT Navdeep Goyal [email protected] Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/ginf. ©  Taylor & Francis Group, LLC

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2. Experimental studies 2.1. Preparation of material

Conventional technique of melting and then ice cooled quenching is used to synthesize Agx Se85-x Te15 (x = 0, 2, 6, 10, 15) material samples. For Agx Se85-x Te15 the pure elements {Selenium pellets 2N; Tellurium 2N and Silver powder 2N} were weighed in required atomic weight percentages, using WensarTM MAB220 model electronic weighing machine having resolution of 10−4 gm. In case of Agx Se85-x Te15 the highest melting point is of Silver (1234.93 K) hence, melted inside the furnace at 1250 K. The melted compositions are then ice cooled. This process can lead to the formation of either amorphous or crystalline samples. Crystallization is demonstrated by abrupt change in volume at the melting point while if there is gradual break in slope it will lead to formation of amorphous samples. Here a fast quenching has led to the formation of crystals but of nano size. The prepared ingots were crushed into fine powder. This bulk material was further used to prepare thermally evaporated thin films on Acetone cleaned glass substrates under vacuum of 10−5 Torr. To achieve the metastable equilibrium the films were taken out of the deposition chamber after 24 hours.

2.2. Characterization

The X-ray diffraction patterns of Agx Se85-x Te15 (x = 0, 2, 6, 10, 15) are shown in Fig. 1. X-Ray diffraction pattern shown confirms the amorphous nature of Se85 Te15 and nanocrystalline nature of Agx Se85_x Te15 (x = 2, 6, 10 and 15) alloys which has sharp and clearly resolved peaks. These measurements were done on pow˚ der sample of bulk compounds using Cu Kα line radiation of wavelength 1.54 A. The average size of the crystallite was determined using Debye-Scherrer’s formula: D = 0.9λ/β cos θ where β is the full width at half maximum (FWHM) of the peak, λ is X-ray wavelength and θ is the Bragg angle. The average crystallite size of prepared compounds are 37, 32, 37, 36 nm for x = 2, 6, 10, 15. Average crystallite size and symmetry with lattice constants of Agx Te15 Se85-x is given in Table 1.

Figure . XRD of Agx Se-x Te (x = , , , , ).

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Table . Average crystallite size and symmetry with lattice constants of Agx Te Se-x (x = , , , ). Sample name

Symmetry

a

b

c

Average crystallite size (nm)

Se Te Ag Se Te Ag Se Te Ag Se Te Ag

Tetragonal Tetragonal Tetragonal Tetragonal

. . . .

. . . .

. . . .

. . . .

3. Results and discussion The transmittance spectra of the thermally evaporated thin films on glass substrates are given in Fig. 2. A Double UV/VIS PerkinElmer UV WinLab is used for measuring optical transmission. The envelope method suggested by Swanepoel is used to calculate the refractive index (Fig. 3), Extinction Coefficient (Fig. 4) and Absorption coefficient (Fig. 5). The optical band gap (Eg ) of the films is calculated from Tauc’s plot. Figure 6 shows plot between (αhν)2 vs. photon energy (hν) which fits best to the experimental values and suggests direct transition.

Figure . Transmission spectra of Agx Se-x Te (x = , , ,  and ).

Figure . Wavelength v/s refractive index (n) w.r.t. Ag atomic weight percentage variation of 1/2 Agx Se-x Te (x = , , ,  and ). {Formula used: n = [ M + (M2 − s2 ) ]1/2 where M = 2s(TM −Tm ) [s2 + 1] [ TM Tm ] + 2 }.

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Figure . Wavelength v/s extinction coefficient (k) w.r.t. Ag atomic weight percentage variation of Agx Se-x Te (x = , , ,  and ). {k = αλ/(π)}.

The results can be best described in the light of electronegativity, chemical bond approach and coordination number. The values of homonuclear bond energy and electronegativity of pure elements used are tabulated in Table 4. The heteronuclear bond energies D(A–B) are related to homonuclear bonds D(A–A) and D(B–B) and electronegativities as follows [10]: D (A − B) = [D (A − A) ×D (B − B)]1/2 + 30(χ A − χ B)2

(1)

As per Chemical Bond Approach, the sequence of forming bonds depends on bond energy [11]. In Agx Se85-x Te15 (x = 0, 2, 6, 10, 15), Se bonds with Ag having the highest possible heteronuclear bond energy (52.42 Kcal./mol) are formed first, followed by the Se-Te bonding (44.18 Kcal/mol) and then Se-Se (44 Kcal/mol). Further, the cohesive energy of composite alloys can be calculated by equation 2 [12]  CE = C i Di (2)

Figure . hυ (E) v/s absorption coefficient w.r.t. Ag atomic weight percentage variation of Agx Se-x Te (x = , , ,  and ). {α = πk/λ}.

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Figure . Variation of optical band gap of Agx Se-x Te (x = , , ,  and ).

where Ci is the number of predictable bonds and Di is the energy of every expected bond (tabulated in Table 2). The cohesive energy of the investigated system increases with increasing Ag content. Secondly, electronegativity of the prepared compositions is decreasing which is calculated by Sanderson’s principle [13]. According to this, electronegativity of an alloy is the geometric mean of electronegativity of its constituent elements. Electronegativity of an atom is measure of the power to attract electron to itself in a molecule. The correlation of the electron affinity with the optical band gap was studied by various workers [14]. Mulliken [15], states that the electronegativity is the average of the ionization potential and the electron affinity. The average coordination number (CNave ) in ternary system, using the concept Table . Electronegativity, cohesive energy, experimental optical band gap and theoretical band gap of Agx Te Se-x (x = , , , , ). Bonds formed Sr. No. Composition

Se-Ag

Se-Te

Se-Se

Electro negativity

    

— . . . .

. . . . .

. . . . .

. . . . .

Se Te Se Te Ag Se Te Ag Se Te Ag Se Te Ag

Cohesive Expt. optical Theoretical energy band gap band gap Kcal/Mol (eV) (eV) . . . . .

. . . . .

. . . . .

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Table . Absorption coefficient, refractive index, real dielectric constant, imaginary dielectric constant and average coordination number of Agx Te Se-x (x = , , , , ).

X at %     

Absorption coefficient (α) at  nm. in (m-)

Refractive index (n)

Extinction coefficient(k)

Real dielectric constant (ϵ )

Imaginary dielectric constant (ϵ”)

Average coordination number

    

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

Table . Homonuclear bond energy, electronegativity and coordination number of the pure elements. Element

Homo-nuclear bond energy (Kcal/Mol)

Electronegativity

Coordination number

  

. . .

  

Se Te Ag

of nearest-neighbour coordination is defined by Equation 3 CNave =

Xa + Yb + Zc X+Y+Z

(3)

where, in our investigating system, X, Y and Z are the % at. wt. of Se, Te and Ag respectively and a, b and c are their respective coordination numbers (values in Table 3). The optical band gaps of Se, Te and Ag are 2.05, 0.5 and 0.046 eV [9, 16–17] then theoretical band gap calculation can be done as follows: Eg (XYZ) = aEg (X) + bEg (Y) + cEg (Z)

(4)

where a, b, and c are the volume fractions and Eg (X), Eg (Y) and Eg (Z) are the optical gaps of elements X, Y and Z respectively. The values of theoretical band gaps are exactly matching to the experimental values upto x = 6 in Agx Se85-x Te15 (x = 0, 2, 6, 10, 15) compositions but in case of x = 10 and 15 the results show deviation from the theoretical values. This may be attributed to increase in the fermi level and reversal in the properties due to increase in the coordination number. Moreover, due to increase in crystallinity as clearly depicted from Figure 1 band gap increases. Electronegativity decreases, cohesive energy and average coordination number increases in the studied compositions hence, led to deviation in optical band gap. 4. Conclusion The envelope method suggested by Swanepoel has been applied to calculate optical coefficients for various samples of Agx Se85-x Te15 using the transmittance Spectra. Energy gap (Eg), decreases with increasing Ag-content up to certain level. The decrease is explained on the fundamentals of the chemical bond approach and electronegativity. The Deviation from the expected results of decrease in band gap

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is explained on the basis of reaching a threshold value in the coordination number. The refractive index, dielectric constants and absorption coefficient increase with Ag-content in the wavelength range 550–850 nm. The increase of the refractive index is explained on the basis of the increase in the density of the material with Ag content and mean coordination number. Hence, points towards its application in fabricating optical devices. Moreover absorption coefficient increases with Ag content which makes it suitable for optical memory storage devices. References 1. Z. H. Khan, N. Salah, S. Habib, A. A. Al-Ghamdi, and S. A. Khan, Electrical and optical properties of a-Se x Te 100–x thin films. Optics & Laser Technology, 44(1), 6–11 (2012). 2. N. F. Mott, and E. A. Davis, Electronic processes in non-crystalline solids. Clarendon, Oxford, 465, 1979. 3. T. Z. Babeva, D. Dimitrov, S. Kitova, and I. Konstantinov, Optical properties of phase-change optical disks with Sbx Se100− x films. Vacuum 58(2), 496–501 (2000). 4. K. Tanaka, Ion-conducting amorphous semiconductor AgAsS. Journal of non-crystalline solids 164, 1179–1182 (1993). 5. Z. H. Khan, S. A. Khan, N. Salah, S. S. Habib, and A. A. Al-Ghamdi, Electrical transport properties of thin film of a-Se87 Te13 nanorods. Journal of Experimental Nanoscience 6(4), 337–348 (2011). 6. N. Salah, S. S. Habib, and Z. H. Khan, Direct bandgap materials based on the thin films of Sex Te100− x nanoparticles. Nanoscale research letters 7(1), 509 (2012). 7. Karunapati Tripathi, Adam A. Bahishti, M. A. Majeed Khan, M. Husain, and M. Zulfequar, Optical properties of selenium–tellurium nanostructured thin film grown by thermal evaporation. Physica B: Condensed Matter 404(16), 2134–2137 (2009). 8. E. A. Davis, N. F. Mott, Conduction in non-crystalline systems V. Conductivity, optical absorption and photoconductivity in amorphous semiconductors. Philosophical Magazine 22.179; 903–922 (1970). 9. A. K. Singh, Recent Advances in Amorphous Semiconductors—A Correlative Study on SeBased Metallic Chalcogenide Alloys. Reviews in Advanced Sciences and Engineering 1(4), 292–301 (2012). 10. L. Pauling, Nature of the Chemical Bonds, Cornell University Press, Ithaca, New York. 1960. 11. K. Sedeek, and M. Fadel, Electrical and optical studies in some Bi doped amorphous chalcogenide thin films. Thin Solid Films 229(2), 223–226 (1993). 12. B. Jozef, O. Stanford, S. Mahadevan, A. Gridhar, A. K. Singh, Chemical bond approach to the structures of chalcogenide glasses with reversible switching properties. J. Non-Cryst. Solids 74(1), 75–84, (1985). 13. R. T. Sanderson, Inorganic Chemistry, Third Edition, London New York. 1991. 14. A. A. Al-Ghamdi, Optical band gap and optical constants in amorphous Se96−x Te 4 Agx thin films. Vacuum 80(5), 400–405 (2006). 15. R. S. Mulliken, A new electroaffinity scale; together with data on valence states and on valence ionization potentials and electron affinities. The Journal of Chemical Physics 2(11), 782–793 (1934). 16. J. H. Saadee, Optical Properties of Tellurium Thin Film Prepared by Chemical Spray Pyrolysis Method. Journal of Kufa-Physics 3(2) (2011). 17. G. Alagumuthu, and R. Kirubha, Synthesis and characterization of silver nanoparticles in different medium. Open Journal of Synthesis Theory and Applications 1(2), 13 (2012).