VARIATION OF OPTICAL CONSTANTS IN GE 10 SE 60 TE 30 THIN FILM

Transcription

1 Chalcoenide Letters Vol. 3, No. 10, October 006, p VARIATION OF OPTICAL CONSTANTS IN GE 10 SE 60 TE 30 THIN FILM P. Sharma *, V. Sharma, S. C. Katyal Department of Physics, Jaypee University of Information Technoloy Waknahat, Solon, H. P , India Thin films of a- -x Te x (x = 0, 30) lassy alloys are prepared by vacuum evaporation technique at ~10-5 Torr on lass substrate at room temperature. Optical study is performed to calculate the refractive index (n), extinction coefficient (k), optical conductivity (σ), dielectric constant (real and imainary), absorption coefficient (α) and optical band ap ( E opt ) usin transmission spectra in the wavelenth rane nm. Keywords: Chalcoenide lasses, Optical properties, Optical band ap, Defect states, Averae bond enery 1. Introduction Chalcoenide lasses are ood candidates for applications in the infrared (IR) reion [1,]. Chalcoenide lasses are vitreous materials havin one or more of the chalcoen elements (Group VI): sulfur (S), selenium (Se), and tellurium (Te). The addition of the network formers (Group IV and V) such as silicon (Si), ermanium (Ge), tin (Sn), phosphorus (P), arsenic (As), and antimony (Sb) establishes cross-linkin between the tetrahedral and pyramidal units which facilitates stable lass formation [3,4]. Dependin on the composition, the chalcoenide lasses are stable aainst crystallization and are chemically inert. They have excellent thermal stability, and are relatively easy to fabricate. In our present work we have taken the a- system and then replace the Se by 30 at. % of Te. The optical properties of the thin films of the prepared samples are calculated usin the transmission spectrum in the rane nm.. Experimental procedure Glassy alloys of and are prepared by well known quenchin technique. Materials (5N purity) are weihed accordin to their atomic percentaes and sealed in quartz ampoules in a vacuum ~ x10-5 Torr. The sealed ampoules are kept inside a furnace where the temperature is increased up to C at a heatin rate of -3 0 C/min. The ampoules are frequently rocked for 4 hours at the hihest temperature to make the melt homoeneous to avoid phase separation. The quenchin is done in ice cold water. Thin films are deposited on lass substrates which are first cleaned with soap solution, vapour cleanin and then ultrasonically cleaned by trichloroethylene, acetone followed by methyl alcohol. Finally the substrate is washed by DI water and dried in oven. On the cleaned substrate thin films of the lassy alloys are deposited by vacuum evaporation technique at room temperature and base pressure of ~ 10-5 Torr usin a molybdenum boat. The normal incidence transmission spectra of thin films of the samples have been measured by a double beam UV/VIS/NIR spectrometer [Hitachi-330], in the transmission rane nm. The spectrometer was set with a suitable slit width of 1 nm in the measured spectral rane. * Correspondin author: pks_phy@yahoo.co.in

2 T % λ (nm) Fi. 1. Variation of Transmittance (T) with wavelenth (λ) for -x Te x (x = 0, 30) thin films. 3. Results and discussion Optical transmission spectrum of the thin films is observed to be shifted towards hiher wavelenth with the addition of 30 at. % of Te content. The variation of transmission with wavelenth for -x Te x (x =0, 30) thin films is shown in Fi. 1. Transmission spectrum is used to calculate the refractive index usin the envelope method proposed by Swanepoel [5,6]. The refractive index (n) has been obtained usin the followin expressions, n 1/ 1/ = [ M + ( M s ) ] (1) where M and M s ( s = T m TM T = s T T M m + m 1) ( s + for transparent reion () + 1) for weak and medium absorption reion. (3) T M and T m are the values of maximum and minimum transmission values at a particular wavelenth, s is the refractive index of the substrate. Refractive index can be estimated by extrapolatin envelops correspondin to T M and T m. As the thickness of film is uniform, interference ive rise to the spectrum as shown in Fi. 1. These frines can be used to calculate the refractive index (n) of the thin films usin equations (1), () & (3). The refractive index correspondin to T M and T m for same wavelenths are calculated. The variation of refractive index with enery of incident radiation ( h ν ) for -x Te x (x = 0, 30) thin films is shown in Fi.. Refractive index found to decrease with the addition of Te. This may be due to the chane in crystallite size, stoichiometry and internal strain [7-9] with the addition of Te to the Ge-Se network. The values of refractive indices correspondin to wavelenth 1000 nm are iven in Table 1.

3 Refractive Index Fi.. Variation of Refractive index with hν for -x Te x (x = 0, 30) thin films. Table 1. Values of refractive index (n), extinction coefficient (k), optical conductivity (σ), real (ε r ) and imainary (ε i ) dielectric constant for -x Te x (x = 0, 30) at 1000 nm. Sample n k σ ε r ε i x x The extinction coefficient can be calculated by usin the relation [10] αλ k = (4) 4π where 'α' is the absorption coefficient and is calculated from the relation ( 1 d ) ln( 1 T ) α = (5) where d is the thickness of the film and T is the transmittance [11]. The thickness of the thin film can be calculated by knowin the values of the refractive indices n 1 and n at two adjacent maxima or minima correspondin to their wavelenths λ 1 and λ. Then the thickness is iven by d ( λ n λ ) = λ (6) 1λ 1 n1 Fi. 3 shows the variation of extinction coefficient with ν h for -x Te x (x = 0, 30) thin films. The value of extinction coefficient increases with the increase in enery of the incident beam. This may be due to the lare absorption coefficient for hiher enery values.

4 Extinction coefficient Fi. 3. Variation of Extinction coefficient with hν for -x Te x (x = 0, 30) thin films. Fi. 4 shows the variation of optical conductivity with the incident photon enery. The optical conductivity is determined usin the relation [1] σ = αnc 4π (7) where c is the velocity of liht. The optical conductivity directly depends on the absorption coefficient and found to increase sharply for hiher enery values due to lare absorption coefficient for these values..50e e E+011 σ 1.00E E E Fi. 4. Variation of Optical conductivity (σ) with hν for -x Te x (x = 0, 30) thin films.

5 77 Fi. 5 and 6 shows the variation of the real and imainary dielectric constants for - xte x (x = 0, 30) thin films. The complex dielectric constant is fundamental intrinsic material property. The real part of it is associated with the term that how much it will slow down the speed of liht in the material and imainary part ives that how a dielectric absorb enery from electric field due to dipole motion. The real and imainary parts of the dielectric constant were determined usin the relation [13], ε = ε r + ε i = (n + ik) (8) where ε r and ε i are the real and imainary parts of the dielectric constant respectively and are iven by ε r = n k (9) and ε i = nk (10) The calculated values of ε r and ε i correspondin to wavelenth 1000 nm are iven in Table ε r Fi. 5. Variation of real part of dielectric constant (ε r ) with hν for -x Te x (x = 0, 30) thin films ε i Fi. 6. Variation of imainary part of dielectric constant (ε i ) with hν for -x Te x (x = 0, 30) thin films.

6 78 Fi. 7 shows the variation of the absorption coefficient (α) with h ν for the -x Te x (x = 0, 30) thin films. The absorption coefficient is calculated by usin the equation (5) and the values of α correspondin to wavelenth 1000 nm are iven in Table. opt Table. Values of absorption coefficient (α) and optical band ap ( E ) for -x Te x (x = 0, 30) thin films. Sample α (cm -1 ) opt E (ev) 0.34 x x x x x x x x10 4 α 3.0x10 4.5x10 4.0x x x x Fi. 7. Variation of absorption coefficient (α) with hν for -x Te x (x = 0, 30) thin films. Fi. 8 shows the variation of ( αh ν ) 1/ with h ν which is used to calculate optical band opt ap ( E ). The optical ap is determined by the intercept of the extrapolations to zero with the 1/ photon enery axis ( αh ν ) 0, Tauc extrapolation [14]. The optical band ap decreases with the addition of 30 at. % of Te to. The values are iven in Table. This may be due to the presence of localised states in the forbidden ap. Accordin to Mott and Devis [15] the width of mobility ede depends on the deree of disorder and defects present in the amorphous structure. In amorphous solids unsaturated bonds are responsible for the formation of these defects. Such defects produce localised states in the forbidden ap. The presence of such states is responsible for the decrease of optical band ap. The addition of Te increases the concentration of localised states leadin to lowerin the optical bad ap. The decrease in optical band ap can also be explained on the basis of averae bond enery of the system. The addition of Te to system by replacin Se may lead to the formation of Ge-Te bonds (37.4 kcal/bond), Te-Te bonds (33.0 kcal/mol) and Se-Te bonds (40.6 kcal/bond) at the cost of Ge-Se bonds (49.1 kcal/bond) and Se-Se bonds (44.0 kcal/bond) [16]so the averae bond enery of the system decreases. As the optical band ap is a bond sensitive property so the decrease in averae bond enery of the system leads to decrease in the optical band ap [17].

7 (αhν) 1/ Fi. 8. Variation of (αhν) 1/ with hν for -x Te x (x = 0, 30) thin films. 4. Conclusion Optical transmission spectrum is used to calculate the optical properties for -x Te x (x = 0, 30) thin films. Swanepoel method is used to calculate refractive index and it is found to decrease with the addition of Te. The optical constants i.e. extinction coefficient, optical conductivity, real and imainary parts of dielectric constant, absorption coefficient and optical band ap are calculated. The decrease in optical band is explained on the basis of defect states and the averae bond enery of the system. References [1] J. R. Gannon, Proc. SPIE, 66, 6 (1981). [] G. Lucovsky, R. M. Martin, J. Non-Cryst. Solids, 8 10, 185 (197). [3] G. J. Ball, J. M. Chamberlain, J. Non-Cryst. Solids, 9, 39 (1978). [4] V. Q. Nuyen, J. S. Sanhera, J. A. Freitas, I. D. Aarwal, I. K. Lloyd, J. Non-Cryst. Solids, 48( 3), 103 (1999). [5] R. Swanepoel, J. Phys. E 16, (1983), 114. [6] R. Swanepoel, J. Phys. E 17, (1984), 896. [7] F. Tepehan, N. Ozer, Solar Enery Mater. Solar Cells, 30, 353 (1993). [8] A. Ashour, N. El-Kdry, S.A. Mahmoud, Thin Solid Films 69, 117 (1995). [9] K. Yamauchi, N. Nakayama, H. Matsumoto, S. I. Keami, Jpn. J. Appl. Phys. 16, 103 (1977). [10] E. Marquez, J. Ramirez, P. Villares, R. Jimenez, P.J.S. Ewen, A. E. Owen, J. Phys. D: Appl. Phys. 5, 535 (199). [11] Priyamvada Bhardwaj, P. K. Shishodia, R. M. Mehra, J. Optoelectron. Adv. Mater. 3, 319 (001). [1] J. I. Pankove, Optical Processes in Semiconductors Dover Publications, Inc. New York, p- 91. [13] E. Marquez, A. M. Bernal-Oliva, J. M. Gonzalez-Leal, R. Prieto-Alcon, A. Ledesma, R. Jimenez-Garay and I. Martil, Mater. Chem. and Phys. 60, 31 (1999). [14] J. Tauc, A. Menth, J. Non-Cryst. Solids, 8, 569 (197). [15] N. F. Mott, E. A. Davis, Electronic Processes in Non-Crystalline Materials, Clarendon Press, Oxford (1979). [16] D. J. Sarrach, J. P. DeNeufville, W. L. Haworth, J. Non-Cryst. Solids,, 45 (1976). [17] A. K. Pattanaik, A. Srinivasan, J. Optoelectron. Adv. Mater. 5, 1161 (003).