Optical Properties of GeSe 4, GeSe 4 Te 0.01 and GeSe 4 Te 0.03 thin films

Transcription

1 Volume 2, Issue 5, May 213 ISSN Optical Properties of 4, 4 Te.1 and 4 Te.3 thin films aad Saied Abed 1, Hanaa Saleh Sabaa 2, Farah Joawd Khathim 3 1,2,3 Department of Physics, College of Science, Al-Mustansiriyah University, Baghdad, Iraq ABSTACT This includes the preparation of glass substrates, followed by the preparation of 4 alloy then 4 doping with Te in different compositions using vacuum evaporation method after that measure the optical properties before and after doping with Te with annealing isothermal. To measure the optical properties used visible spectro-photometer device type (uv-88 spectrometry) to measure (absorbance and transmittance spectrum) in the range (3-9)nm for 4, 4 Te.1 and 4 Te.3 thin films. In semiconductors fabrication many electronic devices depended on the energy gap, such as solar cells and detectors and optical diodes. 1. Introduction: The term of "thin film" is used to describe a layer or several layers of atoms for certain substance whose thickness ranges between (1nm) and less than or equal (1μm).The physical characteristics of the thin films are different from those of their characteristic materials in their bulk [1]. Among the amorphous chalcogenide alloys, mostly, selenium (se) based materials are preferred because it is unique property of reversible phase transformation between the amorphous and crystalline phases. The choise of (se) based alloys is due to the device application like rectifier, photocells and other application [2]. But pure selenium has disadvantages such as it is low photo-sensitivity and also selenium is not stable in standard operational conditions because its glass transition is close to room temperature. These problems can be overcome by alloying selenium with tellurium, which gives higher photo-sensitivity and smaller aging effects as compared to pure selenium [3]. However, these alloys are found to have some significant problems. The two serious problems are the limited reversibility and low glass transition. These problems can be removed by adding a third element as a chemical modifier in se-te alloys [4]. 2. Physical and chemical properties of (Ge,Se,and Te) elements: 1- Germanium (Ge): is a chemical element of forth group in periodic table, with atomic number 32,atomic weight 72.59, density 5.32gm/cm 3, melting point 122k o, Boiling point 313 k o, and energy gap.744ev discovered by C.Winkler in1886[5]. 2- Selenium (Se): is a chemical element of sixth group in periodic table, the chalcogenide glasses (sulfur,selenium, tellurium)with atomic number 34,atomic weight 78.96, density 4.81gm/cm 3, melting point 49k o, Boiling point 958 k o, and energy gap is (1.89 and 2.2)ev depended on alloy composition and amount of impurities additive to selenium discovered by J.Berzelins in 1817[6]. 3- Tellurium (Te): is a chemical element of sixth group in periodic table, the chalcogenide glasses (sulfur, selenium, tellurium)with atomic number 52,atomic weight 127.6, density 6.25gm/cm 3, melting point 723k o, Boiling point 1663 k o. crystalline and amorphous tellurium has aband gap between (.5-.8)ev while in other studies it is found that crystalline tellurium (C-Te) has aband gap between ( )ev, discovered by MVon eichenstein in 1782[7]. Tellurium thin films are also used as selective surfaces and selective surfaces are surfaces which have a high absorption in the visible to near infrared region and a low emissivity in the infrared region[8]. Previous studies : many researches studied these materials over years: In 23 Salman F.studied the A.C conductivity and dielectric properties of glassy alloys Se x Te 79-x Ge 21 with (x=11,14,and 17) at temperature range (3-45)k o and over a wide range of frequencies(5-5)hz. Experiment indicated that AC conductivity and the dielectric constant depend on temperature frequency and Se content[9]. a. In 24 kumar D. et al studied the temperature dependence of photoconductivity in amorphous thin films of Se 8- xte 2 Ge x (x=5,1,15,and 2) prepared by thermal vacuum evaporation technique and indicated that photosensitivity increases and the density of defect state decreases with increasing of germanium concentration[1]. Volume 2, Issue 5, May 213 Page 193

2 Volume 2, Issue 5, May 213 ISSN b. In 27 Maan A.S. et al studied the optical absorption of Te 5 (In x Se 1-x ) 95 glassy alloy (x=1,15,2 and 25) prepared by thermal vacuum evaporation technique. These measurements are made on the as-prepared and annealed samples; it is observed that annealing affects the optical properties and causes a reduction in the energy range of tail states and an increase in the optical energy gap in all the alloys. The optical band gap is found to decrease up to x=2 and there after it increases. This behaviour has been explained to be increasing in content and average bond energy of the system[11]. c. In 29 Mainika.et al studied the effect of impurity addition of Ag, Bi and Ge on the optical properties of vacuum evaporation thin films of Se 8 Te 2,(Se 8 Te 2 ) 98 M 2 where (M= Ag, Bi and Ge)have been studied using transmission spectra at normal incidence and they found that optical energy gap decreases while refractive index increases on the incorporation of (Bi). On the other hand optical energy gap increase while refractive index and extinction coefficient decreases on incorporation of Ag and Ge in Se-Te system[2]. d. In 28 Mean A.S. et al studied the dielectric properties of In 4 Se x Te 6-x glassy alloys (x=1,2, and 3)these measurements are carried out as a function of temperature and frequency in the range (12-35) k o and (.1-1)KHz respectively. The experiments result indicates that dielectric constant ε 1 and dielectric loss ε 2 are almost similar in nature in all the alloys. At a particular temperature and frequency, values of ε 1 and ε 2 are maximum at x=2 and decrease as x is changed to 1 and 3[12]. 3. The optical properties of amorphous semiconductors: The most interesting optical processes in thin films are the absorption and emission of light. Absorption permits a determination of the optical band gap of the semiconductor and it generates charge carriers [13]. The optical band gap, refractive index and extinction coefficient are the most significant parameters in amorphous semi conducting thin films. The optical behavior of material is utilized to determine its optical constants. Films are ideal specimen for reflectance and transmittance type measurements. Therefore, an accurate measurement of the optical constant is extremely important [14]. The optical properties: a. eflectance(): the reflectance which can be defined as the ratio of the reflected intensity to the incident intensity[15]: refl refl I =.(1) inc inc I O where I : is the intensity of reflected light at the interface where the refractive indices are different. and I o : is the intensity of incident light at the interface where the refractive indices are different[16]. b. Transmission (T): the transmission coefficient is defined as the ratio of transmitted power to incident power, IT T=.(2) I and given by [17]: 2 (1 ) exp( t) T=.(3) 2 1 exp( 2 t ) where α : absorption coefficient, t : sample thickness. O c. Absorbance: the absorption is defined as (Beer- Lambert low ): IT A= -log( )..(4) I o where I T : is the intensity of the transmitted light through the material and I o : is the incident intensity of light[16]. Optical constant: a. efractive index (n) and Extinction coefficient (k): the refractive index can be calculated by using equation depending on the reflectance () and extinction coefficient (k) as in the following equation [16] : 2 2 n 1 ( n 1) k =..(5) 2 2 n 1 ( n 1) k and the extinction coefficient (k) can be calculated from the relation : 2 Volume 2, Issue 5, May 213 Page 194

3 Volume 2, Issue 5, May 213 ISSN k= 4 (6) b. Absorption coefficient (α): Absorption coefficient [α(hν)]can be defined as the relative rate of decrease in light intensity [L(hν)] along it is propagation path[18]: 1 d[ L( h )] and given by [19]: But.(7) L( h ) dx 2.33* A.(8) t A=log ( T 1 ) (9) and t : thickness of thin film. c. eal and Imaginary part of dielectric constant ε 1 and ε 2 : The dielectric constant represents the ability of material to polarization, to obtain the dielectric constant[2]: ε = ε 1 + iε 2 (1) where ε 1 and ε 2 : are the real and imaginary parts of the dielectric function respectively. ε 1 = n2-k2..(11), ε 2 =2nk (12) n : refractive index, k : extinction coefficient. 4. The experimental work: This includes the preparation of glass substrates, followed by the preparation of 4 alloy then 4 doping with Te in different compositions using vacuum evaporation method after that measure the optical properties before and after doping with Te with annealing isothermal. To measure the optical properties used visible spectro-photometer device type (uv-88 spectrometry) to measure (absorbance and transmittance spectrum) in the range (3-9)nm for 4, 4 Te.1 and 4 Te.3 thin films. In semiconductors fabrication many electronic devices depended on the energy gap, such as solar cells and detectors and optical diodes. From equation (8) the absorption coefficient for each length is obtained and the incident photo energy E(hν) as a function of the length (λ) from the relation: 124 E(ev)= (13) ( nm) and the optical constants calculated from transmittance and absorbance spectrum at normal incidence over the range (3-9)nm for 4, 4 Te.1 and 4 Te.3 thin films. efractive index (n) is calculated using equation 1 : n = (14) 1 and extinction coefficient (k) from equation (6) and the dielectric constant can be calculated in its real and imaginary parts (ε 1 and ε 2 ) from equation (11,12)respectively. 5. The results and discussion In studying the optical properties of 4, 4 Te.1 and 4 Te.3 thin films. This part includes the results of transmission and absorption measurements and their relation to length and from these measurements it can be calculated many optical constants, like absorption coefficient, refractive index, extinction coefficient, dielectric constant. 1- Transmission measurements: the influence of tellurium concentration on 4 thin films on the optical properties of the prepared films with annealing and without annealing the transmission spectra as a function of length in the range (4-9)nm, is shown in figure (1) and from this figure the transmission increase with increasing length and have maximum transmission at(8nm)because cholcogenide glasses and especially Se-Te alloys are generally transparent from the visible up to infrared[21]. Fig(1) also shows a decrease in transmission with increasing (Te) concentration because (Te) thin films are used as selective surfaces and selective surfaces are surfaces which have a high absorption in the visible to near infrared region and a low emissivity in the infrared region and this means that the transmission decrease and absorption increase with increasing (Te) concentration[7]. And show the transmission Volume 2, Issue 5, May 213 Page 195

4 A A International Journal of Application or Innovation in Engineering & Management (IJAIEM) Volume 2, Issue 5, May 213 ISSN without (Te) increasing with increased in time annealing and stable after (55nm) and decrease with increased time annealing but with (Te) the transmission decrease with increased in (Te) concentration and time annealing. Te.3 Te.1 T% T% T % Fig.(1) 2- Absorbance measurements: it is possible to separate three distinct regions in the absorption edge spectrum of amorphous semiconductors. The first is the weak absorption tail, which originates from defects and impurities. (Existence of the weak absorption tails in the band gaps of the films under study may be attributed to the amorphous nature, and randomly distributed impurities in the films) second is the exponential edge region, which is strongly retated to the structural randomness of the amorphous compound. Third is the high absorption region from which optical energy gap width can be determined from figure (2) illustrates absorbance spectrum for 4, 4 Te.1 and 4 Te.3 thin films as a function of length, the prepared films show high absorption at visible spectrum and low absorption at the near infrared region and absorption increase with increasing (Te) concentration and time annealing for all thin films[22,23]. ג 4.3 Te.3 Te.1 A Fig.(2) 3- eflectance measurements: from figure(3) illustrates reflectance spectrum for 4, 4 Te.1 and 4 Te.3 thin films as a function of length. The prepared films show high reflection at visible spectrum and low reflection at the near infrared region and reflection increase with increasing (Te) concentration and time annealing for all thin films[23]. Te.3 Te Fig.(3) 4- Absorption coefficient measurements: Analysis of optical absorption spectra is one of the most productive tools understanding and developing the band structure and energy gap of both crystalline and amorphous non-metallic material, the threshold at the low energy side of optical absorption spectra is called optical absorption edge and corresponds to separation in energy between the bottom of the conduction band and the top of the valance band. Absorption coefficient is calculated by using equation (8), figure(4) show the variation of(α) with length, from the figure the absorption coefficient(α) increases at visible spectrum and low absorption coefficient at the near infrared region and absorption coefficient increase with increasing (Te) concentration and time annealing for all thin films. Volume 2, Issue 5, May 213 Page 196

5 E E E n n k k k a a International Journal of Application or Innovation in Engineering & Management (IJAIEM) Volume 2, Issue 5, May 213 ISSN Te.3 Ge SeTe.1 7E+11 6E+11 5E+11 4E+11 3E+11 2E+11 1E+11 7E+11 6E+11 5E+11 4E+11 3E+11 2E+11 1E+11 WAVE a 7E+11 6E+11 5E+11 4E+11 3E+11 2E+11 1E+11 Fig.(4) 5- Optical constants : Extinction coefficient (k): Extinction coefficient is measured by using equation (6), figure(5) illustrates variation of(k) as a function of length, extinction coefficient increase with increasing length in visible and near infrared region which is increased with increasing the tail states width that resulted the increasing tellurium concentration and time annealing[23]. Te.3 Ge SeTe.1 3.5E E E+14 3E E+14 3E E+14 2E+14 2E E+14 2E E E+14 1E+14 1E+14 5E+13 1E+14 5E+13 5E+13 Fig.(5) efractive index (n): efractive index can be determined using equation (14), from figure(6) it is clear that refractive index increase with increasing length (6nm) and decrease in higher lengths, and (n)increases with increasing tellurium concentration and time annealing, this may be due to the change in crystallite size, stoichiometry and internal strain and because the decreasing of optical band gap with increasing Te concentration[22,23]. Te.3 4Te n Fig.(6) Dielectric constant: 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 light in the material and imaginary part gives that how a dielectric absorb energy from electric field due to dipole motion. The real and imaginary parts of the dielectric constant were determined using the relation (11,12) respectively, so that values of (ε 1,ε 2 ) depend on refractive index (n) and extinction coefficient(k)[22], from figure (7,8) show the variation of real part of dielectric constant ε 1 and imaginary part of dielectric constant ε 2 with length respectively, from these figures it is clear that (ε 1,ε 2 ) increase with increasing length (4-7)nm with increasing in Te concentration and increasing time annealing for (ε 1 ) but in (ε 2 ) increasing reach to (4-9)nm length, that is similar behavior of the previous experiments[21,22]. Te.3 Te Fig.(7) Volume 2, Issue 5, May 213 Page 197

6 Volume 2, Issue 5, May 213 ISSN Te.3 Te.1 E2 5E+16 5E+16 1E+16 E 2 1E+16 5E+15 E 2 1E+16 5E+15 5E+15 Fig.(8) 6. Conclusions In studying the optical properties of 4, 4 Te.1 and 4 Te.3 thin films. This part includes the results of transmission and absorption measurements and their relation to length and from these measurements it can be calculated many optical constants, like absorption coefficient, refractive index, extinction coefficient, dielectric constant. eferences: [1.] Chopra k. L., "thin films phenomena",mc.gaw-hill,newyork,(1969). [2.] Mainika, Sharma p.,katyal S.C. and Thakur N.,"A study of impurities (Ag,Bi and Ge) on the optical properties of Se-Te thin films", journal of Non- oxide Glasses, vol.1,no.2(29),pp.(9-95). [3.] Sharma.S., Shukla.K and Kumar A., "Effect of Ag impurity on photoconductive properties of (Se-Te) glasses",journal ovonic esearch,vol.3,no.6,(27),pp.( ) [4.] Mehta N., Shukla.K and Kumar A., "Effect of some metallic additives on the kinetics of glass transition in Se 8 Te 2 glassy alloy", journal of optoelectronics and advanced materials,vol.6,no.4(24),pp.( ). [5.] C.F. Klingshrin, "semiconductor optics", Springer verlag Berlin(1997). [6.] Elmandouh Z.S. and Chizhikov, D.M., "Tellurium and the Tullurides" journal of applied physics, vol.78(1995),pp.( )london and welling Bough. [7.] Peter Son M.J. and Cocks F.H.," Tellurium selective absorber surfaces", journal of material science and engineering, vol.41(1979),pp.( ). [8.] Sharma N.and Kumar S. "ole of Sn in the density of defect states in a-se.75 Te o.25 and a-se.85 Te o.15 thin films" Turk journal physics,vol.31(27),pp.( ). [9.] Salman F., "Ac conductivity and dielectric study of chalcogenide glasses of Se-Te-Ge system", Turk journal physics vol.28(28)pp.(41-48) [1.] Kumar D. and Kumar S." Composition dependence of photoconductivity in Amorphous thin films of Se 8- xte 2 Ge x ", Turk journal physics, vol.29(25),pp.(91-96). [11.] Maan A.S.Goyal D..and Kumar A." Investigation of optical absorption in Te 5 (In x Se 1-x ) 95 glassy alloy ",cholcogenide letters, vol.4,no.4(27)pp.(48-53). [12.] Maan A.S.Goyal D.., "Dielectric properties of In-Se-Te glassy alloys", Indian journal of engineering and material sciences,vol.15,(28),pp.(27-21). [13.] Pankove J.I., "optical process in semiconducting thin films", thin solid films,vol.9,(1982)pp.172. [14.] Pandey V.,Mehta N., Tripathi S.K. and Kumar A," Optical band gap and optical constants in Se 85 Te 15-x Pb x thin films", journal of optoelectronics and advanced materials,vol.7,no.5,(25) pp.( ). [15.] Kitt C.," Introduction to solid state physics", 8 th edition, John Wiley and Sons,Inc.(25). [16.] Sze. S. M.," semiconductor device physics and technology ", John Wiley and Sons(27). [17.] Pankove J.," optical processes in semiconductors ", prentice Hall, Inc.(1969). [18.] Cohen M.H., Fritzsche H. and Ovshinski S..," simple band model for amorphous semiconductors alloys", physical review letters, vol.22, (1969), pp.( )and Marshall J.M. and Owen A.E., "Drift mobility studies in vitreous arsenic triselenide ", journal of philosophical magazine vol.24(1971),pp.( ) by Tan W.C.," Optical properties of amorphous selenium films", thesis for the degree of master of science, university of Saskatchewan, Canada(26). [19.] Donald A.N.," semiconductor physics and devices", Irwin USA,(1992). [2.] Dutta J and Hoffman H., "Nono materials",(25). [21.] Pandey V., Tripathi S.K. and Kumar A.," Optical band gap and optical constants in amorphous Se 7 Te 3-x Ag x thin films", journal of optoelectronics and advanced materials, vol.8, No.2,(26) pp.( ). [22.] Sharma P., Sharma V. and Katyal S.C.," Varition of optical constants in Ge 1 Se 6 Te 3 thin film",chalcogenide letters, vol.3, No.1(26),pp.(73-79). [23.] Aly K.A.,"Optical properties of Ge-Se-Te wedg shaped films by using only transmission spectra ", journal of Non-crystalline solids,vol.355(29),pp.( ). Volume 2, Issue 5, May 213 Page 198