Temperature dependence of the structural and optical properties of

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Temperature dependence of the structural and optical properties of

Chalcogenide Letters Vol. 6, No. 8, August 2009, p. 359 365





    Faculty of Science, Helwan University, Cairo, Egypt aElectron Microscope and Thin films Department, Physics division, National

    Research Centre, Cairo, Egypt

    Nearly stoichiometric thin films of the ternary AgSbSe compound have been deposited at 2room temperature by conventional thermal evaporation of the presynthesized material onto

    glass substrate. The X-ray and electron diffraction studies revealed that the as-deposited

    films are amorphous in nature, while an amorphous-to-crystalline phase transition could be

    obtained by thermal annealing at 373 K. The elemental chemical composition of as-

    deposited films was confirmed using the energy dispersive X-ray analysis. The

    transmission spectra of the as-deposited and annealed films were recorded at normal light

    incidence in the wavelength range 600-2500 nm. The refractive index and optical band gap

    have been calculated for the investigated films. The dispersion parameters, (E, E) static od

    refractive index n(0), static dielectric constant, ( and the carrier concentration to the sseffective mass ratio, N/m* have been calculated. An analysis of the optical absorption

    spectra revealed anon direct optical transition characterizing the as-deposited films and

    those annealed at 343 and 374 K while; direct and indirect optical transitions characterized

    the films annealed at 398 K

    (Received August 3, 2009; accepted September 15, 2009)

    Keywords: AgSbSe, Thin films, XRD, Electron diffraction, amorphous-crystalline 2


    1. Introduction

    Due to extensive applications in solid-state devices and future prospects, chalcogenide glasses have received much attention in recent years. A good device is one that is of low cost, fast, accurate and easy to use. Amorphous selenium has been emerged as promising material because of its potential technological importance. It is widely preferred in the fabrication of electrophotographic devices and, more recently, switching and memory devices [1, 2] have found

    selenium-based materials to offer attractive advantages. The use of chalcogenide films for reversible optical recording by the amorphous-to-crystalline phase change has recently been reported [3].

     Silver-containing chalcogenide glasses are considerable interest for applications in optical recording and as solid electrolytes. Therefore, the knowledge of optical, electrical, and structural properties of Ag-chalcogenide amorphous materials is of essential importance. The AgSbTe [4], AgSbS [5], and AgInSbSe [6] systems have been previously studied, however, 222

    very little work concerning the optical and electrical properties of AgSbSe have been presented 2

    [7-9]. In the present work, a systematic study of the structure and optical properties of thermally evaporated AgSbSe thin films annealed at different temperatures has been studied. The effect of 2

    (thermal annealing on the refractive index, high frequency dielectric constant (), and carrier ?*concentration to the effective mass ratio (N/m) were presented.


    2. Experimental details

    Polycrystalline ingot of the ternary AgSbSecompound was prepared by the direct fusion 2

    of a mixture of the constituent elements in stoichiometric ratio, and purity 99.999%, in vacuum-sealed silica tube. Thin films were deposited by conventional thermal evaporation of the -3presynthesized material onto precleaned glass substrates held at room temperature, in ~1.5×10Pa

    vacuum using a high vacuum coating unite (Type Edwards 306 A). The structural characteristics of the prepared ingot material as well as the as-deposited and annealed AgSbSe films were 2

    examined by means of an X-ray diffractometer (Type Philips X’pert) with Ni-filtered CuK α

    radiation operating at 35 kV and 100 mA. The chemical composition of the as-deposited films was identified using energy dispersive X-ray unit interfaced with a scanning electron microscope (Type JEOL-JSGM-T200). The microstructure of the as-deposited and annealed films was also examined using Transmission electron microscope (Type JEOL-JSGM-T1230). A double beam spectrophotometer, with automatic computer data acquisition (Type Jasco, V-570, Rerll-00, and UVVISNIR), photometric accuracy of ?0.0020.004 absorbance and ?0.3% transmittance,

    was employed at normal light incidence to record the optical transmission and reflection spectra of the as-deposited and annealed films over the wavelength range 6002500 nm. The thickness of the

    deposited films was from the interference fringes [10].

    3. Results and discussions

    3.1 Structural characterization

    The X-ray diffractgrams of the prepared AgSbSe bulk material as well as the films 2

    annealed at different annealing temperatures, T are shown in Fig. 1a, b. Comparing the reflection a

    planes of Fig.1-a with the standard XRD data (JCPDS cards no 12-0379), indicates that all the reflection planes can be indexed to the cubic phase of the ternary compound AgSbSe with a cell 2

    parameter a = 0.578 nm. No reflections corresponding to any of the free elements or binary alloys were observed.



    T = 398 Ka2Film 223AgSbSeAgSe Se22AgSe2AgSbSeAgSbSe T;,,,? = 373 Ka T = 343 Ka


    ;((!? (a);(!!?Intensity [a.u] ;(((?


     ;,(!?Powder ;,((?


     2? [Degrees]


    Fig.1 X-ray diffraction pattern of (a) the prepared AgSbSe powder and (b) annealed 2

    films (of thickness 780 nm).

    The XRD analysis, carried out on the as-deposited films (not given) and those annealed for 1 h in an Ar atmosphere at annealing temperatures T <343 K are amorphous in nature, while those a

    annealed at T? 373 K are crystalline. Analysis the XRD diffraction pattern of the films annealed a ooat T ? 373 K indicates that the film contains two peaks at and , respectively, 2??30.9744.39a

    corresponding to reflections from (200), (220) planes of AgSbSe single cubic phase. However, 2othe XRD pattern for the film annealed at 398 K shows small diffraction peaks at , 2??33.52

    oo and , respectively, corresponding to reflection from the (112), (122), and (514) 40.4155.05

    planes, which belongs to the binary AgSe, SbSe phases, beside AgSbSe as a major phase. 2232

    Transmission electron micrographs of as-deposited AgSbSe films, and those annealed at 2

    Ta?343 K showed no discernible structure (See Fig.2). The corresponding diffraction patterns

    exhibited diffuse rings confirming the amorphous nature of the films as revealed by X-ray diffraction. On the films beinge annealed at Ta ? 373 K, a distinct structure was observed in the

    transmission mode. The corresponding selected area diffraction shows crystallization of the films, as identified previously via X-ray diffraction analysis.

     Fig.2 TEM micrograph and the corresponding electron diffraction pattern of the annealed

    AgSbSe film. Film thickness 70 nm. 2

    Fig.3 shows the EDX spectra for a typical representative sample of AgSbSe films 2

    deposited onto glass substrate. The result indicates that the chemical composition of AgSbSe 2

    films had elemental composition of 24.08:23.86:52.05 corresponding to Ag: Sb: Se, which indicating a deficiency in Ag (~0.92at %) and Sb (~1.14at %) with an excess of Se (~2.05at %) hence, led to consider that the as-deposited film had a chemical formula AgSbSe, 0.9630.9542.082

    revealing a nearly stoichiometric composition. A comparison between the elemental chemical compositions of the prepared bulk material, as-deposited films, and the calculated values are shown in the inset of Fig.3.

     Fig. 3 EDX spectra of AgSbSe film deposited onto glass substrate. Film thickness 780 nm. 2


    3.2 Optical properties of AgSbSe films 2

    Fig.4 shows the transmission spectra of as-deposited AgSbSe film of thickness 780 nm 2

    and another samples of the same film thickness annealed in Ar atmosphere for 1h, at annealing temperatures 343, 373 and 398 K. It was found that the absorption edge shifts towards lower energies as the annealing temperatures increases. Furthermore, the transmission was found to decrease with the increasing in the annealing temperatures.





     as-depositedTransmission, T T=343 Ka0.2 T=373 Ka T=398 Ka0.01000150020002500

    Wavelength, ) [nm]

     Fig. 4 Transmission spectra of AgSbSe thin films annealed at different temperatures 2

    The refractive index, n, film thickness and the order of interference of the investigated films were computed from the transmission spectra using the well-known Swanepole method [10] with s=1.51 (substrate refractive index). The sets of values of refractive index calculated according to the above mentioned method can be fitted to a reasonable function such as the two-term Cauchy 2dispersion relationship; n()) = a+b/); ( where a and b are the Cauchy parameters) which can be

    used for extrapolation the refractive index to shorter wavelengths. The refractive indexes, n of

    AgSbSe films annealed at different temperatures are shown in Fig.5. 2


     as-deposited=343 K Ta4.8=373 K Ta=398 K Ta



    Refractive index, n



    Wavelength, ) [nm]

     Fig.5 spectral distribution of refractive index, n for as-deposited and annealed AgSbSe films. 2


    As could be seen from the figure, the refractive index decreases with increasing wavelength and increases on increasing the annealing temperature. Wemple and DiDomenico [11] have developed a model where the refractive index dispersion is studied in the region of transparency below the gap, using the single-effective oscillator approximation. Defining two parameters, the oscillator energy, E and the dispersion energy E this model concludes that: od

    EE2od1, n? (1) 22EEo

    Both Wemple parameters can be obtained from the slope and intercept of the plot 212(n1)?f(E) with the y-axis as shown in Fig.6.



     as-deposited T=343 K0.08a T=373 Ka T=398 Ka energy, E (eV)

     2-1Fig. 6 plot of (n-1) vs. photon energy.

    The values of Wemple-DiDomenico dispersion parameters, E, E static refractive index, od

    no (calculated extrapolating the Wemple-DiDomenico optical-dispersion equation to, E??; (n= o

    1+E/E) as well as static dielectric constant, ( for the AgSbSe films annealed at different dos2

    temperatures are listed in Table 1. The oscillator energy E is related by an empirical formula to o

    the optical gap value: E?2E [11]. The calculated values of the optical band gap are also presented og

    in Table1.

    Table 1 Refractive index dispersion parameters.

    Eind.E T E /2?Eaog od(eV) En( osg(eV) (eV) (K) (eV)

    303 13.673 2.204 2.684 7.202 1.102 1.160

    343 13.923 1.885 2.896 8.386 0.943 1.081

    373 14.632 1.766 3.047 9.285 0.882 0.966

    398 14.907 1.716 3.112 9.687 0.851 0.930

    The obtained refractive index data can be further analyzed to obtain the high frequency dielectric constant via a procedure that describes the contribution of the free carriers and lattice vibration modes of the dispersion. The optical dielectric constant of AgSbSe2 films was calculated using the relation [12].

    ?2212(?(i(?((() (2) 1212


where ( and ( are the real and imaginary parts of the dielectric constant. The values of ( and ( 1212

    for different incident photon energies can be obtained from the values of n and k () k??)4

    using the well-known relations:

    22(?nk(?2nk , (3) 12

    Since the reflectivity of a semiconductor in the NIR region shows anomalous dispersion as the incident photon energy approaches the corresponding value of plasma wavelength, ). When p22n>>k and . The real dielectric constant can be expressed as [13]: ,:~~1

    2?.eNm222 ; (4) (?([(()]?pLLp1(o

where ( is the lattice dielectric constant (or limiting value of the high frequency dielectric L

    constant), the plasma frequency and is the angular frequency (=2c/), c is the speed of light) p

    of the lattice atoms, e is the electronic charge, N/m* carrier concentration to the effective mass -12-2ratio, and ( is the dielectric permittivity 8.85×10 F/m. Therefore, plotting ( vs. in the NIR o1

    spectral region (not shown) allow us to determine the values of the plasma frequency, and p

    lattice dielectric constant, ( from the slope and intercept, respectively. These calculated values L

    are listed in Table 2. The observed disagreement between the values of static dielectric constant obtained according to Wemple and DiDomenico single-effective oscillator model and lattice dielectric constant obtained according to Eq.3 may be attributed to the contribution of the free carriers to the refractive index [14].

    Table 2. values of ( n , and N/m* L,p

    ?T a( Nm n?( pLl[K] 22--15 -[×[×10s31] ]

    303 7.983 2.825 4.39 1.04

    373 9.694 3.157 7.51 1.48

    398 11.009 3.318 9.19 1.66

    423 11.464 3.386 9.46 1.73

    The analysis of the absorption coefficient, ? at the fundament absorption edge was found

    to follow the relation;

    p (5) ;??;??A?(;?E)g

    where A is constant and the exponent p characterize the type of the optical transition. A plot of 1/2(?h?) for as-deposited film and for those annealed at 343 and 378 K (shown in Fig.8-a) indicates a non direct optical transition with energy values 1.16, 1.08 and 0.97 eV, respectively. However, the analysis of the absorption coefficient for the film annealed at 398 K (Fig.8-b) indicated the presence of both direct and indirect optical transition with values of 0.96 and 0.93 eV, respectively.



    250 T=343 Ka??;;:;??; T=373 K300100a2] as-deposited10-12001.0x101/21/2.eV]]80-1-1-11/2]-1.eV.eV(b)-1150-1200 [cm.eV2a)-160 [cm?;1/2h) [cm??9( [cmh1001/25.0x10?)1/2(?)indirect40h?=0.93 eVEh?g(100?(direct50E=0.966 eVg20 (h?) [eV] (h?) (eV)

     1/2Fig. 7 a, b (a) plot of (?h?) for the as-deposited film and those annealed at 343 and 373 K vs. photon 21/2energy; and (b) plot of (?h?) and (?h?) for the film annealed at 398 K vs photon energy.

    4. Conclusion

    Nearly stoichiometric AgSbSe thin films were deposited at room temperature by thermal 2

    evaporation onto glass substrates. The X-ray and electron diffraction studied revealed that the as-deposited films and those annealed at temperatures < 373 are amorphous in nature, while an amorphous-to-crystalline phase transtion could be obtained for the films annealed at temperatures ?373 K. Onset of mainor peaks corresponding to AgSe and SbSebinary phases, beside the 223

    ternary AgSbSe as a major phase were obtained when the film being annealed at 398 K. The 2

    (effect of annealing temperature on the refractive index, high frequency dielectric constant (), ?*and carrier concentration to the effective mass ratio (N/m) were presented. The refractive index

    and consquently the high frequency dielectric constant were found to increase with the increase in the annealing temperatures. The analysis of the optical absorption coefficient for the deposited films reveald the presence of a non direct optical transition for as-deposited and those annealed at 343 and 373 K, while a direct and non direc optical transition for the film annealed at 398 K.


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