THE OPTICAL PROPERTIES OF ZnS x SE 1-x THIN FILMS DEPOSITED BY THE QUASI-CLOSED VOLUME TECHNIQUE

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1 1 THE OPTICAL PROPERTIES OF ZnS x SE 1-x THIN FILMS DEPOSITED BY THE QUASI-CLOSED VOLUME TECHNIQUE MIHAIL POPA Department of Physical Sciences and Engineering, Alecu Russo Balty State University, MD-31, R.Moldova Abstract: ZnS x Se 1-x thin films were deposited by the quasi-closed volume technique under vacuum. The film structure was studied by X-ray diffraction technique. The studies shown the films are polycrystalline and have a zinc blende structure. The spectral dependences of transmission, reflection, absorption and refraction index were studied. Values of band gap energy determined from the reflectance and absorption spectra are in good agreement with the theoretical values E g.. Key words: thin films, transmission, absorption and emission spectra, refractive index 1. INTRODUCTION Recent investigations on physical properties of ZnS x Se 1-x thin films were stimulated by the fact that this compound is applied in various modern technologies of solid state devices (lasers or laser diodes functioning in the blue field, heterojunctions of solar cell, photodetectors, etc) [1-3]. It is well established that the electronic transport and optical properties of ZnS x Se 1-x thin films, strongly depend on the preparation method, deposition conditions (substrate nature, evaporation rate, substrate temperature, etc.) and also post-deposition heat-treatment. The information on the influence of these factors on the electronic transport and optical properties of ZnS x Se 1-x films are rather few [4, ]. In this paper some optical properties of ZnS x Se 1-x thin films prepared by quasi-closed volume technique under vacuum are investigated.. EXPERIMENTAL DETAILS ZnS x Se 1-x thin films with x =,.,.4,.,.6,.8, 1. were prepared by physical vapour deposition of powders of ZnSe (with 99,999% purity) and ZnS (with 99,999% purity) ( p 1 Torr) onto unheated glass substrates. The quasiclosed volume technique was used. The experimental arrangement and deposition technique used for preparation of ZnS x Se 1-x films were similar to these described in [4]. The deposition of films was performed in a pyrex cylinder chamber (placed vertical under vacuum) closed

2 at the top by the substrate holder. The source-substrate distance was about 7 cm. 1 The source temperature was T ev = 11K, and deposited rate was r d = As. The film thickness was determined by an interpherence microscope MII-4 and for the investigated samples ranged between, µm and 1, µm. The crystalline structure of the ZnS x Se 1-x thin films was studied by X-ray diffraction using CuK α radiation (λ = 1.418Å). It was found that the layers are polycrystalline and have a cubic structure of zinc blende type, with a strong orientation of the crystallites of the planes (111) parallel to the support surface. Surface structure was studied by means of scanning electron microscopy. The transmission spectra (in the spectral range of nm) and reflection spectra (in the spectral range of nm) of the ZnS x Se 1-x thin films have been recorded, using a spectrophotometer of Hitachi U-34 type. 3. RESULTS AND DISCUSSION The transmission spectra for two ZnS, Se, thin films with different thicknesses are shown in Fig. 1. The transmission of samples D4 and D8 increases sharply to a peak value and then decreases slowly in the range of -6 nm, after which the evolution of spectra depends on the thickness of samples. Three interference maxima appear in the sample D4 with thickness d =. µm (Fig. 1), while in the sample D8 with thickness d =.9 µm there are five maxima. The transmission spectra for the samples with thicknesses greater than 1 µm are formed of a combination of maxima and minima, and the difference between the maximum and minimum transmission decreases with the increase of thin films thickness.

3 3 1 8 T (%) 6 4 Sample D4, d =,µm Sample D8, d =,9µm ZnS, Se, λ (nm) Fig. 1. Evolution depending on the thickness of the transmission spectra of ZnS, Se, thin films

4 4 T (%) x = x =, x =,4 x =, x =,6 x =,8 x = λ (nm) Fig.. Evolution depending on x of transmission spectra of ZnS x Se 1-x thin films The presence in the transmission spectra of maxima and minima due to the interference of beams reflected at the layer surfaces is an indication that the samples are uniform from the point of view thickness and the layer surfaces are flat. This was also highlighted by the scanning electron microscopy studies [6, 7], which showed that the roughness of the free surface of the layer is small. A high roughness or irregularity of thickness would lead to the disappearance of interference, namely to the disappearance of minima and maxima in the transmission spectra. Another analysis was based on the increased concentration of S and the decreased concentration of Se (Fig. ). It is observed that edge transmission spectrum is moving in the lower wavelength region from 4 nm to 3 nm with increasing the x value.

5 The reflection spectrum of ZnS, Se, thin films is presented in Fig. 3. One can observe that the reflection does not exceed,% in the wavelength range of nm, while the transmission is between 7-97% (Fig. 1 and 3). This tells us that the layers are transparent and the absorption of electromagnetic waves in these layers is also quite small. It is also found that the transmission maxima coincide with the reflection minima, which demonstrates the presence of the interference in the respective films. 1,,8 Sample D4, ZnS, Se,, d =, µm R (%),6,4,, λ (nm) Fig. 3. The reflectance spectra of ZnS, Se, thin films The absorption coefficient of the ZnS x Se 1-x thin film was calculated from the transmission spectrum (considering the very low reflection of layers) using the formula: 1 1 ƒ = ln. (1) d T Figure 4 presents the absorption spectra of seven polycrystalline ZnS x Se 1-x thin films. It is observed that the absorption edge moves to higher energies from. ev up to 3.3 ev with increasing of x value.

6 6 In the range of low photon energies, the absorption coefficient has a value different from zero, which can be attributed to the light absorption at the boundaries between crystallites [8]. The high values of the absorption coefficient α at photon energies below the fundamental absorption edge may be due to structural defects, which determine the appearance of localized levels in the band gap, leading to electronic transitions as a result of the absorption of incident radiation. The authors of the paper [8] have explained the form of the absorption spectra by small roughness of the thin film, which leads to the idea of small crystallite size and that of a high degree of crystallization. 8 7 α (cm -1 ) x= x=, x=,4 x=, x=,6 x=,8 x=1 1,, 1, 1,,, 3, 3, 4, Fig. 4. The absorption spectra of ZnS x Se 1-x.thin films For materials with direct band gap we can write Tauc relationship [8]: where A is a constant which is different for different transitions. The presentation of data in coordinates (αhν) = f(hν) gives a linear dependence. By extrapolating this dependence to (αhν) one can determine the width of the band gap (Eg) at the given temperature. ()

7 7 The dependencies α (hν) = f(hν) for seven ZnS x Se 1-x thin films are shown in Fig.. The increase of the thickness of samples leads to increasing the optical absorption and, respectively, the optical band gap deduced from the linear dependence α (hν) = f(hν). The found values of E g =,68 3, ev are in good agreement with those established for ZnSe and ZnS crystals [9-11]. 1 9 α (hν) x1 1 (cm - ev - ) x=, E g =,68 ev x=,, E g =,77 ev x=,4, E g =,88 ev x=,, E g = 3, ev x=,6, E g = 3,14 ev x=,8, E g = 3,3 ev x=1, E g = 3, ev,,3,6,9 1, 1, 1,8,1,4,7 3, 3,3 3,6 3,9 4, 4, Fig.. The dependencies a = f(hν) of ZnS x Se 1-x thin films Similarly, one can plot the graph where and are the maximum and minimum of the reflectance spectrum. For ordinate null value the intersection of extrapolated straight line with the ordinate gives the band gap width E g. The dependencies for six ZnS x Se 1-x thin films are presented in Fig. 6 a, b, c, d, e, f. The values of the band gap between.6 ev and 3. ev were obtained by using the method described above.

8 8 [hν*ln((r max -R min )/(R-R min ))] Sample A4-1, ZnSe d =, mm, Eg =,6 ev,6,8 1, 1, 1,4 1,6 1,8,,,4,6,8 3, 3, 3,4 a [hν*ln((r max -R min )/(R-R min ))] Sample B4-, ZnS, Se,8, d =,67 mm, Eg =,78 ev 1, 1, 1,4 1,6 1,8,,,4,6,8 3, 3, b [hν*ln((r max -R min )/(R-R min ))] [hν*ln((r max -R min )/(R-R min ))] Sample C4-, ZnS,4 Se,6, d =,46 mm, Eg =,89 ev,6,8 1, 1, 1,4 1,6 1,8,,,4,6,8 3, 3, 3,4 d Sample F4-1, ZnS,8Se, d =,7 mm, Eg = 3,34 ev,6,8 1, 1, 1,4 1,6 1,8,,,4,6,8 3, 3, 3,4 3,6 3,8 4, e c [hν*ln((r max -R min )/(R-R min ))] [hν*ln((r m ax -R m in )/(R -R m in ))] Sample E4-1, ZnS,6 Se,4, d =,8 mm, Eg = 3,16 ev,6,9 1, 1, 1,8,1,4,7 3, 3,3 3,6 3, Sample G3-1, ZnS d =,8 mm, Eg = 3, ev, 1, 1,,, 3, 3, 4, Fig. 6. The evolution of width of band gap Eg depending on x for ZnS x Se 1-x thin films f

9 9 For comparison, in Table 1 are shown the E g values determined from the reflectance spectra, from absorption spectra and the theoretical values. We note that these values are in good agreement. Table 1. Comparison of theoretical and experimental values of Eg Type of Energy band gap, E g semiconductor theoretical determined from absorption spectra determined from reflection spectra ZnSe ZnS. Se ZnS.4 Se ZnS. Se ZnS.6 Se ZnS.8 Se ZnS The refractive index of the thin film was determined from the transmission spectra using the "envelope" method [1-13] proposed by R. Swanepoel. The steps of calculating the index of refraction for ZnS x Se 1-x thin films were the following: a) Calculation of the refractive index of the substrate, n s, from its transmission spectrum, T sub = f(λ) using the relations [1, 13] 1 1 ns = 1 ; (4) T sup T sup b) Plotting windings interference minima and maxima of the transmission spectrum of a thin film T = f(λ) and determining the wavelength for each of a pair of values T M and T m ; c) Calculation of coefficient N using the relationship: T T n 1 N= n M m S + S +. () TM Tm d) Calculation of n values, using the formula: n = [N + ( N n s ) 1/ ] 1/, (6) 1/

10 ,9 ZnS x Se 1-x 8 n,9 x =,8 6,8 x=,6 4,8 x=,4,7 x=, λ [nm] Fig. 7. The dispersion of refractive index for ZnS x Se 1-x thin films The dispersion of refractive index of ZnS x Se 1-x thin films is represented in Fig. 7. Depending on the increase of the concentration of S and decreasing the concentration of Se, the refractive index of the respective layers increases from.8 (for the sample x =.) to.9 (for the sample x =.8). These values are in good agreement with those reported in the literature for ZnSe and ZnS crystals [1, 11]. In the frame of a single oscillator model [14] dispersion of refractive index in the region of transparency (for photon energies lower than the width of the band gap) can be described by the equation EEd n -1 = E - ( hƒë ), (7) where E is a parameter which value is equal to about double the width band gap (E E g ) and E d is a dispersion parameter.

11 11, (n -1-1) -1,13,1,1,11 ZnS,4 Se,6 d =.46µm T sub = 3K r d = 1.nm/s E d = 47.3eV E =.8eV 8 6 4,11, (hν) [ev] Fig. 8. The dependence type 1/(n -1 1) = f(hν) for ZnS,4 Se,6 sample Figure 8 presents the dependence 1/(n -1 1) = f(hν) for ZnS, Se, thin films into the glass. The respective values of E d and E are obtained from the slope of the linear dependence. The values for other ZnS x Se 1-x thin layers are summarized in Table. ZnS x Se 1-x Table. Parameters of single oscillator model Thickness of films d (µm) Substrate temperature T sub (K) Deposition rate r d (nm/s) E d (ev) E (ev). 3 1, 43.47,4, ,9,6, ,8, ,3,8,1 3 1.,83 6,7 1, 3 1.,8 6,93

12 1 4. CONCLUSIONS We have studied the transmission spectra, reflection and absorption of polycrystalline ZnS x Se 1-x thin films. It has been found that in the wavelength range of nm the reflectance does not exceed.%, and the transmission coefficient are in the range of 7-97%. This tells us that the layers are transparent and the absorption of electromagnetic waves in these layers is also quite small. The band gap energy values determined from the reflection and the absorption spectrums dates (E g =.6-3.eV) are in good agreement with the respective values for the massive crystals. From the transmission spectra using the envelope method Swanepoel proposed has been determined the refractive index of the polycrystalline ZnS x Se 1-x thin films. It decreases with increasing thickness and increase after the heat treatment. For an explanation of the normal dispersion of the refractive index has been used a single oscillator model. REFERENCES 1. Patil D.S., Gautam D.K., Analysis of effect of temperature on ZnSSe based blue laser diode characteristics at 7 nm wavelength, Physica B, 344, , (4).. Venkata Subbaiah Y.P., Prathap P., Ramakrishna Reddy K.T., Miles R.W., Yi J., Studies on ZnS. Se. buffer based thin film solar cells, Thin Solid Films, 16, (8). 3. Valeev R.G., Romanov E.A., Vorobiev V.L., Mukhgalin V.V., Kriventsov V.V., Chukavin A.I., Robouch, Structure and properties of ZnS x Se 1 x thin films deposited by thermal evaporation of ZnS and ZnSe powder mixtures, Mater. Res. Express,, 6, (1). 4. Venkata Subbaiah Y.P., Prathap P., Reddy K.T.R., Mangalaraj D., Kim K. and Junsin Yi, Growth and characterization of deposited by close-spaced evaporation, Journal of Physics D: Applied Physics, 4, (7).. Ambrico M., Perna G., Smaldone D., Spezzacatena C., Stagno V. and Capozzi V., Structural and optical parameters of ZnS x Se 1 x films deposited on quartz substrates by laser ablation, Semicond. Sci. Technol. 13, (1998). 6. Ganguly A., Chaudhury S. and A.K.Pal, Synthesis of ZnS x Se 1 x ( < x < 1) nanocrystalline thin films by high-pressure sputtering, Journal of Physics D: Applied Physics, 34, 6-13 (1). 7. Shinde M.S., Ahirrao P.B., Patil I.J, Patil R.S., Studies of nanocristalline ZnSe thin films prepared by modified chemical bach deposition method, Indian Journal of Pure and Applied Physics, 49, (11).

13 13 8. Bennett H.E., Bennett J.M., Physics of Thin Films (G.Hass and R.E.Thun, eds), Academic Press, New York, vol. 4, 1987, p Nedeoglo, D.D., Simaşkevici, A.V., Elektriceskie i liuminisţentnâe svoistva selenida ţinka, Chişinău, Ed. Ştiinţa, Bhargava R. (Ed.), Properties of Wide Bandgap II VI Semiconductors, Inspection, London, Jain M. (Ed.), II VI Semiconductor Compounds, World Scientific, Singapore, Swanepoel R. Determination of the thickness and optical constants of amorphous silicon, J. Phys. E: Sci. Instrum., 16, (1983). 13. Swanepoel R., Determination of the surface roughness and optical constants of inhomogeneous amorphous silicon films, J. Phys. E: Sci. Instrum., 17, (1984). 14. Meng L., Andritschky M., dos Santos, M., P., The effect of substrate temperature on the properties of d.c. reactive magnetron sputtered titanium oxide films, Thin Solid Films, 3, 4-47 (1993).