Sol-Gel Ta2 Os filmlerinin optik ve yapısal özellikleri

Abstract

ÖZET Bu çalışmadan amaç sol-gel yöntemi ile tantalum pentoksit (Tsl^Ds) ince filmleri oluşturmak, ve film hazırlama yöntemi (daldırma kaplama ve spin kaplama yöntemi) ve ısıl işlem sıcaklığına bağlı olarak elde edilen filmlerin optik ve yapısal özelliklerini kıyaslamaktadır. Spin kaplama yönteminde önce döndürme hızı 1500-3000 devir/ dak. arasında 5 farklı hızda bir seri film kaplandı, daha sonrada en uygun hız olarak belirlenen 2500 devir / dak. hızında 1, 3, ve 5 katlı film oluşturuldu. Daldırma yönteminde ise filmler, 107 mm / dak. hızında 1, 3, 5, ve 7 kat olarak hazırlandı. Bir sonraki aşamada her iki yöntem ile hazırlanan yaklaşık aynı kalınlıktaki filmlere 100-500 °C arasında 5 ayrı sıcaklıkta üçer saat ısıl işlem uygulandı. Filmlerin optik (kırma indisi, absorpsiyon katsayısı,...) ve yapısal özelliklerini belirlemek için UV- Visible ve FITR. spektrofotometre, SEM, ve x-ray difiaktometre kullanıldı. Filmlerin optik geçirgenlik- dalga boyu değişimlerinden yararlanarak optik sabitleri kaplama yöntemi, katman sayısı, ısıl işlem sıcaklığına bağlı olarak verildi. Elde edilen sonuçlan şöyle özetleyebiliriz : Spin hızı arttıkça filmlerin kalınlığı azalmakta ancak optik sabitleri artmaktadır. Her iki kaplama yönteminde de katman sayısının artması ile kalınlık ve optik sabitleri artmaktadır. Ayrıca ısıl işlem sıcaklığının artması ile kalınlığın azalmasına karşın, filmin optik sabitleri artmaktadır. Isıl işlem uygulamadan önce daldırma yönteminde hazırlanan filmlerin optik sabitleri spin yönteminkinden daha küçük iken ısıl işlem uyguladıktan sonra bu durum tersine dönmektedir. Bütün bu özelliklerde farklılık, filmin yapısındaki gözeneklerin sayısının değişmesinden kaynaklanmaktadır. Bu özellik SEM fotoğrafları ile kanıtlanmaktadır. Filmlerin enerji band aralığı kaplama yöntemi, kalınlık ve hızdan bağımsız olup, » 3.749 ± 0. 122 eV iken ısıl işlem sıcaklığa bağlı olarak 100 °C'de * 3.711 ± 0.103 eV'den 500 °C'de « 3.506 ± 0.073 e Va değişmektedir. VI

SUMMARY OPTICAL AND STRUCTURAL PROPERTIES OF SOL-GEL DERIVED Ta2Os FDLMS Films and coatings represent the earliest commercial application of sol-gel technology. Thin films (normally < lum in thickness) formed by dipping and spinning use little in the way of raw materials and may be processed quickly without cracking, overcoming most of the disadvantages of sol-gel processing. In addition, large substrate may be accommodated and it is possible to uniformly coat both side of planar and axially symmetric substrates such as pipes, tubes, rods, and fibers not easily handled by more conventional coating processes. Table 1 shows a various application of the sol-gel thin films and coatings. Table 1. Application of films and coatings. optical Coatings Electronic Coatings Protective Coatings Other Coatings 1. Colored 2. Antireflectrve 3. Optoelectronic 4. Optical memory 1. Photo anodes 2. High-temperature superconductor 3. Conductive 4. Ferroelectric- Electro-optic 3. Dielectric films 1. Corrosion-resistant 2. Mechanical Planarization Scratch- & wear- resistant Strengthening Adhesion promoting 3. Passivation 1. Porous coatings 2. Miscellaneous ` Certainly one of the most technologically important aspects of sol-gel processing is that, prior to gellation, the fluid sol or solution is ideal for preparing thin films by such common processes as dipping, spinning, or spraying. Compared to conventional thin film forming processes such as CVD, evaporation, or sputtering, sol-gel film formation requires considerably less equipment and is potentially less expensive; however the most important advantage of sol-gel processing over conventional coating methods is the ability to control precisely the rnicrostructure of the deposited film, i.e., the pore volume, pore size, and surface area. The disadvantages of sol-gel processing include the cost of the raw materials, shrinkage that accompanies drying and sintering, and processing times. The advantages and disadvantages of the sol-gel process are summarized in Table 2. VUTable 2. Some advantages and disadvantages of the sol-gel methods. Optical coatings alter the reflectance, transmission, or absorption of the substrate. In this manner buildings appear outwardly uniformly reflective, while light transmission is controlled in accordance with sun exposure to minimize summer cooling costs. Schott Glaswerke produce million of square meters of optical coatings on glass each year. In addition to reflective or colored coatings, oxide coatings on glass and silicon substrates (single layers, multilayers, and porous layers ) have been used extensively as antireflective (AR) surfaces in solar-related applications to improve device efficiency and as laser-damage-resistant AR coatings for laser optics, especially in inertial confinement fusion application. Tantalum pentoxide films have been extensively used as the dielectrics of thin film capacitors, insulators, antireftection coating materials and optical dielectric waveguides. The tantalum pentaoxide films are quite durable chemically and mechanically with a very low optical attenuation loss and a relatively high refractive index, so they have been considered to be one of the best materials for the fabrication of optical waveguides and lenses. Tantalum pentaoxide films can be deposited by several techniques, such as, anodization and thermal oxidation from a tantalum (or tantalum oxide) target, chemical vapour deposition, reactive sputtering and sol-gel- drived. In this work, the optical and structural properties of sol-gel-derived Ta205 thin films formed on Coming 7059 glass substrates have been investigated. The samples have been prepared by spinnig and dipping processes for assigning different properties of Ta2Os thin films in three stapes, 1. The samples have been deposited by spin coating method with 1500, 2000, 2500, 3000 rpm spin speed, in three layers, and with 2500 rpm spin speed in 1, 3, 5 layers. vtu2. The samples have been deposited by dip coating method with 107 mm / min. pulling rate, in 1, 3, 5, and 7 layers. 3. The samples have been deposited by spin coating method with 2500 ipm spin speed and dip coating method with 107 mm / min. pulling rate, in 5 layers which had approximately similar thickness (d^a, = 494 nm, d^ = 516 nm), and applied heat treatment in 100, 300, 500 °C temperature. For studying the optical and structural properties of TazOs thin films the samples have been measured in different way such as following, 1. For determining composition in the film, FTIR (Fourier Transform Infra Red) Spectroscopy analysis of a film prepared on a crystalline NaCl wafer has been made. 2. For determining the optical constants (refractive index, absorption coefficient, etc.) of Ta205 thin films, UV-Visible Photospectroscopy analysis of Ta2Os thin films have been made. 3. For studying the surface's topography of Ta2Os thin films Scanning Electron Microscopy (SEM) has been used. In addition, for determining the composition of the samples, x-ray quantitative analysis has been made. 4. For determining the structural properties of TaaOs thin films x-ray difractometer has been used. In this method the qualitative analysis has been made. In thİB work as previously mentioned the optical and structural properties of Ta2Os thin films have been examined in three stapes. For calculating these properties and the experimental data a method suggested by SWANEPOEL, has been used The results of these calculations and experiments in the first and second stapes can be explained as following, Refractive index of T&^Ds thin film increases with increasing spin speed in spin coating method. For about 550 nm wavelength, with 2000, 2500, and 3000 rpm spin speed refractive index was found as 1.742, 1.758, and 1.782 respectively. Refractive index of films increases with increasing the number of deposited layers in both methods. In spinning process, for about 550 nm wavelength, with 3 and 5 layers refractive indices were found as 1.758 and 1.783 respectively. In dipping process, for the same wavelength, and with 5 and 7 layers, refractive indices were found as 1.706 and 1.717 respectively. Refractive index of thin films in the spin coating method is higher than the dip coating method for approximately the same film thickness (dgPjn=494nm,d<j1p=516 nm). For about 550 nm wavelength, refractive index in spin and dip coating methods were found as 1.783 and 1.706 respectively. IXAbsorption coefficient of Ta^s thin film increases with increasing spin speed in spin coating method For about 550 nm wavelength, with 2000, 2500, and 3000 rpm spin speed absorption coefficient was found as 589, 661, and 799 cm`1 respectively. Absorption coefficient of films increases with increasing the number of deposited layers in both methods. Li spinning process, for about 550 ran wavelength, with 3 and 5 layers absorption coefficient was found 661, and 893 cm`1 respectively. In dipping process, for the same wavelength, and with 5 and 7 layers, absorption coefficient was found as 557 and 837 cm`1 respectively. Absorption coefficient of thin films in the spin coating method is higher than the dip coating method for approximately the same film thickness (dipjn=494nm,d<jjp=5 16 nm). For about 550 nm wavelength, absorption coefficient in spin and dip coating methods were found as 893 and 557 cm`1 respectively. Extinction coefficients of Ta205 thin films increase with increasing spin speed in spin coating method For about 550 nm wavelength, with 2000, 2500, arid 3000 rpm spin speed Extinction coefficient was found as 26 x 10`*, 29 x 10`4, and 35 x 10`* respectively. Extinction coefficients of films increase with increasing the number of deposited layers in both methods. In spinning process, for about 550 nm wavelength, with 3 and 5 layers refractive index was found as 29 x 10`4, and 39 x 10`4 respectively. In dipping process, for the same wavelength, and with 5 and 7 layers, extinction coefficient was found as 24 x 10*4, and 37 x 10`4 respectively. Extinction coefficient of thin films in the spin coating method is higher than the dip coating method for approximately the same film thickness (dBan=s494nm,ddç=516 nm). For about 550 nm wavelength, extinction coefficient in spin and dip coating methods were found as 39 x 10`4, and 24 x 10`4 respectively. For determining energy band gap, Tauc relationship has been used (see Eq. 5. 1). After calculating the energy ban gap for all samples and calculating the mean value and mean error of these band gap, the result showed that the energy band gap is independent of the spin speed, the number of deposited layers, and deposition method of the films and was found about 3.749 ± 0.122 eV. By using refractive index and extinction coefficient the real, s^ and imaginary, s,, parts of dielectric constants of the samples have been calculated Both real and imaginary parts of dielectric constant of thin films in the spin coating method is higher than the dip coating method for approximately the same film thickness (dtp» - 494 nm, d*? =516 nm). For about 2.5 eV energy of photon, real and imaginary parts of dielectric constant in spin and dip coating methods were found as b,, = 3.247, Srd = 3.003, B* = 0.240, and Bid = 0.009 respectively. The thickness of the films decreases with increasing spin speed in spin coating method The film thickness which have been prepared with 2000, 2500, 3000 rpmspin speeds in spin coating method were found about 278, 251, and 223 nm respectively. The thickness of the films increases with increasing the number of deposited layers in both methods. In spin coating method the film thickness with 1, 3, and 5 layers were found about 166, 251, and 494 nm respectively. In dip coating method the film thickness with 1, 3, 5, and 7 layers were found about 178, 305, 516, and 793 nm respectively. The relation between the film thickness and the number of deposited layers is fitted to a quadratic function in both methods. The result of calculation and experiments of third stapes can be explained as following, Refractive indices of Ta2Os thin films increase with increasing temperature in both spin and dip coating methods. For about 550 ran wavelength, with 100, 300, and 500 °C heat treatment temperature, refractive index was found as 1.804, 1.872, and 2.001 in spin coating method and 1.841, 1.954, and 2.035 in dip coating method respectively. Refractive index of thin films in the dip coating method is higher than the spin coating method for approximately the same film thickness (dtpi&=:201nm,d<üp-190 nm) when heat treatment is applied. For about 550 nm wavelength, refractive indices in dip and spin coating methods were found as 2.035, and 2.001 respectively. Absorption coefficients of Ta205 thin films increase with increasing temperature in both spin and dip coating methods. For about 550 nm wavelength, with 100, 300, and 500 °C heat treatment temperature, absorption coefficient was found as 503,1031, and 4431 cm`1 in spin coating method and 241, 1048, and 5668 cm`1 in dip coating method respectively. Absorption coefficient of thin films in the dip coating method is higher than the spin coating method for approximately the same film thickness (d^ =201 nm, daip = 190 nm) when heat treatment is applied. For about 550 nm wavelength, absorption coefficient in dip and spin coating methods were found 5668, and 4431 cm`1 respectively. Extinction coefficients of Ta205 thin films increase with increasing temperature in both spin and dip coating methods. For about 550 nm wavelength, with 100, 300, and 500 °C heat treatment temperature, extinction coefficients were found 21 x 10`4, 46 x 10`*, and 194 x 10^ in spin coating method and, 11 x 10`*, 46 x 10`4, and 247 x 10^ in dip coating method respectively. Extinction coefficient of thin films in the dip coating method is higher than the spin coating method for approximately the same film thickness (0^,= 201 nm, ddip= 1 90 nm) when applied heat treatment For about 550 nm wavelength, absorption coefficient in dip and spin coating methods were found 247 xlO`4, and 194 x 10^ respectively.As previously mentioned for determining energy band gap, Tauc relationship has been used After calculating the energy ban gap for all samples and calculating the mean value and mean error of these band gap, the result showed that the energy band gap is dependent to the heat treatment temperature in both spinning and dipping processes. With 100, 300, and 500 °C heat treatment temperature, the energy band gap were found as 3.71 1 ± 0.103, 3.648 ± 0.148, and 3.508 ± 0.082 eV in spin coating method and 3.680 ± 0.087, 3.654 ±0.125, and 3.506 ±0.073 eV in dip coating process. The thickness of the filmB decreases with increasing heat treatment temperature in bom spin and dip coating methods. The film thickness which have been prepared with 60, 100, 300, and 500 °C heat treatment temperatures were found about 494, 280, 245, and 201 nm in spin coating method and 516, 272, 206, and 190 nm in dip coating method respectively. Scanning Electron Microscopy (SEM) analysis have been done, and the result showed that the number of pores in dip coating method is more than the number of pores in spin coating method before heat treatment but is less after heat treatment, and the number of pores decrease with increasing heat treatment temperature. For studying the composition of films x-ray quantitative analyses have been used. In this analysis have been found that by increasing heat treatment temperature decrease the percentage of oxygen atoms in film slowly. Another analysis has been done by qualitative x-ray difractometer. The samples which have been applied with heat treatment at 100-500 °C intervals have still amorphous structure, and the variation in the optical and structural properties is predicted because of decreasing of the numbers of pores in Ta205 thin films structure. The results of this work can be summarized as following, I. Although, with increasing spin speed in spinning processes the film thickness decrease but the optical constants increase. II. The optical constants and the thickness of the films increase with increasing the number of deposited layers and heat treatment temperature in both spinning and dipping processes. III. The optical constants in spin coating process are higher than in dip coating process before heat treatment, but are lower after heat treatment for approximately the same film thickness. XU