Reaktif buharlaştırma ve Sol Gel teknikleri ile hazırlanan filmlerinin optik ve elektriksel özellikleri

Abstract

Bu çalışmada I203 ve İTO filmler reaktif buharlaştırma, Sn02 ince filmler ise dip coating (daldırarak kaplama) ve spin coating (döndürerek kaplama) yöntemleri ile kaplanmışlardır. Farklı yöntemler kullanılarak hazırlanan filmlerin, optik ve elektriksel özellikleri tespit edilmiş, bu özelliklerin, kullanılan yönteme, film kalınlığına, ısıl işlem sıcaklığına göre değişimleri incelenmiştir. Sonuçlar daha önceki çalışmalarla karşılaştırılmıştır. Hazırlanan filmlerin optik sabitleri, Swanepoel yöntemi kullanılarak hesaplanmıştır. Maxwell denklemleri kullanılarak, Fresnell kanunları elde edilmiş, ince film optiği incelenerek, bu makalede kullanılan formüller doğrulanmıştır. Hazırlanan numunelerin yasak band aralıkları, Tauc eşitliği kullanılarak hesaplanmıştır. Hazırlanan filmlerin elektriksel iletkenliği, dört nokta probu ile tayin edilmiştir. Birbirine eşit uzaklıkta yerleştirilmiş olan ince problarm dıştaki iki ucundan akım uygulanmış ve içteki iki uçtan gerilim değeri okunmuştur. Kullanılan akım değeri ve okunan gerilim değerleri kullanılarak, filmleri yüzey dirençleri hesaplanmıştır. Elde edilen sonuçlar, deneysel sonuçlar bölümünde verilmiştir. Ayrıca bu çalışmada, saydam iletken filmlerin iletkenlik mekanizmalarına değinilmiş, enerji bant aralığı geniş olan bu filmlerin, nasıl olup da iletken olabildikleri açıklanmıştır. Bu çalışmada elde edilen sonuçlar, şu şekilde özetlenebilir; Döndürerek kaplama yönteminde spin hızı arttıkça, film kalınlığı azalmakta, buna rağmen optik sabitler büyümektedir. Ayrıca katman sayısı arttıkça filmin kırılma indisinin arttığı tespit edilmiştir. Filmlere ısıl işlem uygulanması, indislerin büyümesine sebep olmuştur. Döndürerek kaplama ve daldırarak kaplama yöntemleri kullanılarak yaklaşık aynı kalınlıkta kaplanan filmlerde optik sabitlerin, döndürerek kaplanan filmlerde daha büyük olduğu görülmüştür. Özelliklerdeki bu farklılıklar, yapısal değişiklikler sonucunda ortaya çıkmaktadır. Yasak band enerjisinin spin kaplama yönteminde devir sayışma bağlı olmadığı, ancak ısıl işlem sıcaklığına bağlı olduğu tespit edilmiştir. Isıl işlem sıcaklığı arttıkça, yasak band aralığının daraldığı gözlenmiştir.

The first reported observation that a thin solid film was immediately semitransparent to visible light and electrically conducting appears to have been made by Bâdeker in 1907. Since this time, non-stoichiometric and doped films of oxides of tin, indium and some of their alloys have exhibited high transmittance in the visible spectral region, high reflectance in the IR region and nearly metallic conductivity. The electrical as well as the optical properties of these unusual materials can be tailored by controlling the deposition parameters. These transparent conductors have found a vast number of applications in active and passive electronic and opto-electronic devices. These of range from aircraft window heaters to charge-coupled imaging devices. In this work, an overwiev of the deposition techniques that used, electrical and optical properties, solid state physics of the electron transport and optical effects of these transparent conductors have been given. With increasing sophistication of active and passive devices based on transparent conductors, there is a need for improvement of their electrical and optical properties. As a result, it is now possible to produce various transparent conductors with a range of properties. A maximum solar transmittance of about 85%-90% with a minimum resistivity as low as about 2xl0"5 Q.cm is achievable. A variety of thin film deposition techniques have been employed to deposit transparent conducting oxides, such as tin oxide (TO), indium oxide (IO) and doped with tin (ITO). Since the electrical and optical transport in these films depend strongly on their microstructure and stoichiometry and the nature of the impurities present, each deposition technique with its associated parameters yields films of different properties. In this work, the samples have been prepared by reactive evaporation, dip coating and spin coating methods, glass substrates were used. All the glass substrates were properly cleaned before depositing the substrates. Reactive evaporation using metallic sources has been employed to deposit IO and ITO films. Reactive evaporation of IO and ITO films have been achieved by the vacuum evaporation of the corresponding metal in an oxygen atmosphere onto unheated substrates. An important control parameter was annealing temperature after deposition. An increase in annealing temperature results in an increase in transparency. Vll The dip technique consists essentially of inserting the substrate into a solution containing hydrolyzable metal compounds and pulling it out at a constant speed into an atmosphere containing water vapor. In this atmosphere, hydrolysis and condensation processes take place. Finally, the films are hardened by a high temperature cycle to form transparent metal oxides. The important control parameters are the pulling speed and the firing temperature. The rate of heating also needs to be controlled to avoid cracking of the films. In the spin coating process, there are four stages, deposition, spin-up, spin-off, and evaporation. An excess of liquid is dispensed on the surface during the deposition stage. In the spin-up stage, the liquid flows radially outward, driven by a centrifugal force. In the spin-off stage, excess liquid flows to the perimeter and leaves as droplets. As the film becomes thinner, the rate of removal of excess liquid by spin-off slows down, because thinner the films have greater resistance to flow, and because the concentration of the nonvolatile components increases, raising the viscosity. In the fourth stage, evaporation takes over as the primary mechanism of thinning. In this work, different spin speeds have been employed to the substrates that have different number of layers. In principal, the determination of the amplitudes and intensities of beams of light reflected or transmitted by a thin film is straightforward. The Maxwell equations has been applied to find out the correct equations. It is important to predict the photoelectric behavior of a device in order to determine the refractive index and absorption coefficient as function of wavelength. Knowledge of these optical constants is also necessary to determine the optical gap of the films. The thickness of films have been determined from the interference fringes of the transmission spectrum. The physical mechanism involved in conduction of semiconducting transparent materials is not completely clear. Stoichiometric transparent bulk oxides are known to be wide band gap insulators. Energy band investigations for SnOz and ln203 indicate a multiplicity of conduction and valence bands which are in general non-parabolic and allow both direct and indirect optical transitions. In our case, the optical transitions are indirect. In this work, the samples have been prepared by spinning, dipping and reactive evaporation processes in order to assigning different optical and electrical properties of IO, TO and ITO thin films. The film preparation procedures are as flowing; 1. The SnOz samples were deposited by the spin coating method with 1500, 2000, 2500, 3000 and 3500 rpm spin speeds and in addition 2000 rpm spin speed in 3, 5, and 7 layers. 2. The Sn02 samples were deposited by the dip coating method with 10, 26, 53, 79 and 107 mm / min pulling rates and for 107 mm / min in 1, 3, 5 and 7 layers. vm 3. The ln203 and ITO samples have been deposited by the reactive evaporation method. 4. All the samples were annealed at different temperatures to achieve oxide films. To calculate the optical constants, the Swanepoel method have been used. In this method, the rigorous expression for the transmission of a thin absorbing film on a transparent substrate is manipulated to yield formulae in closed form for the refractive index and absorption coefficient. The thicknesses of the films, refractive indexes, absorption and extinction coefficients have been calculated using only data from the transmission spectrum. The band gap energies have been calculated using the Tauc equation given below; a = B !_ hv where a is the absorption coefficient, B is a constant, hv is the photon energy and Eg is the band gap, r is 1/2 for direct transitional elements and 2 for indirect transitional elements. To calculate the resistivity of the samples a four point probe method was used. A known current was applied to the samples and the generated voltage is measured using a differential method. For studying optical, structural and electrical properties of IO, TO and ITO films, the samples have been measured using the methods below; 1. To determine the optical constants (refractive index, absorption, extinction coefficients) of the samples, UV - Visible Spectrophotometer was used. The method suggested by Swanepoel was applied. 2. To determine the composition of the samples, micro probe analyses were carried out and Joel 733 Electron probe X - Ray Microanalyzer was used. 3. To determine the structural properties of the samples, Philips Glancing angle X -Ray Defractometer was used. 4. To determine the electrical properties, the four point probe method has been used. The optical and electrical properties of the samples can be explained as follows; The optical transmittance of Sn02 thin films decreases with increasing number of deposited layers. Between 400 and 900 nm, the average transmittance for 1, IX 3 and 5 layer samples were found to be 89.45%, 88.95% and 84.403% respectively. The optical transmittance Sn02 thin films increases with the spin speed in the spin coating method. Between 400 - 900 nm, with 1500, 2000, 2500 and 3000 rpm spin speeds the average tranmittance were found to be 86.76%, 87.59%, 88.35% and 89.251% respectively. The refractive index of Sn02 thin films increases with the spin speed in the spin coating method. At 550 nm wavelength, with 1500, 2000, 2500, 3000 and 3500 rpm spin speeds the refractive indices were found to be 1.610, 1.628, 1.644, 1.662 and 1.676 respectively. The refractive index of SnOz films increases with an increasing number of deposited layers during the spinning process. At 550 nm wavelength and 2000 rpm spin speed with 3 and 5 layers, the refractive indices were found to be 1.628 and 1.6305 respectively. The refractive indices of the Sn02 thin films in the spin coating process is higher than the dip coating method for approximately the same thicknesses of d^ = 466 nm and dspin = 428 nm. At 550 nm, refractive indices in spin coating and dip coating were found to be 1.644 and 1.642 respectively. The refractive index of the sol-gel deposited SnO, thin film increases with the temperature treatment but if the temperature was high at about 600°C, the lower refractive index is achieved. For the 3 layer sample using 3000 rpm spin speed and 100 °C annealing temperature, the Sn02 thin films had a refractive index of 1.662 which became 1.625 after annealing for a further hour at 600°C. For low temperature annealing, the refractive index is always higher. The 3 layer sample, at 2500 rpm spin speed deposited Sn02 films were treated at three different temperatures, 100 °C, 300 °C, 500 °C and the refractive indices were found to be 1.644, 1.709, and 1.693 respectively. The refractive index of the reactively evaporated ln203 thin films decreases with increasing annealing temperature. The indices for 300 °C and 450 °C heat treated ln203 films are 2.007 and 1.971 respectively. The absorption coefficient of Sn02 films increases with increasing spin speed. For approximately 550 nm wavelength, with 3000 and 3500 rpm spin speeds, the absorption coefficients were found to be 370 cm"1 and 493 cm"1 respectively. The absorption coefficient of Sn02 thin films using the spin coating method is lower than the dip coating method for approximately the same thicknesses of ddiP = 466 nm and dspin = 428 nm. For approximately 550 nm wavelength, the absorption coefficients using the spin and dip coating methods were found to be 395 cm"1 and 380 cm"1 respectively. The absorption coefficients of Sn02 thin films increase with increasing annealing temperature. At 2500 rpm spin speed deposited films were annealed at 100, 500 °C and absorption coefficients were found to be 370 cm"1 and 430 cm"1 respectively. The extinction coefficients of Sn02 films decrease with increasing spin speed. At approximately about 550 nm wavelength with 3000 and 3500 rpm spin speed extinction coefficients were found to be 1.2xl0"3 and 1.45xl0"3 respectively. The extinction coefficients of Sn02 thin films using the spin coating method is higher than that obtained with the dip coating method for approximately the same thicknesses of ddip = 466 nm and dspin = 428 nm. At approximately 550 nm wavelength, extinction coefficients in spin and dip coating methods were found to be 1.62xl0"3 and 1.56xl0"3 respectively. Extinction coefficient of Sn02 thin films increases with an increasing annealing temperature. At 2500 rpm spin speed deposited films were annealed at 100, 500°C and extinction coefficients were found to be 1.47xl0"3 and 1.89xl0"3 respectively. To calculate the Energy band gap, the Tauc equation was used. It was found that the energy band gap is independent of spin speed. The energy band gaps calculated for the films with 1500, 2000 and 2500 spin speed were found to be 4.42 ± 0.44 ev, 4.425 ± 0.46 ev and 4.439 ± 0.520 ev respectively. It is found that the energy band gap is dependent to the annealing temperature. The energy band gaps were calculated for heat treated samples with 100°C, 300°C and 500 °C and it is found to be 4.439 ± 0.5204 ev, 4.41 ± 0.406 ev and 4.395 ± 0.365 ev respectively. The thicknesses of the films decreases with increasing spin speed. The thicknesses of the 1500, 2500, 3000, 3500 rpm spin speed deposited films were found to be 512, 430, 267, and 265 nm respectively. The thicknesses of the films increases with increasing the number of deposited layers. In the spin coating method, the film thickness with 3, 5, 7 layers were found to be about 329 nm, 559 nm, and 577 nm respectively. The thicknesses of the films decreases with increasing the annealing temperature. The thicknesses of the films annealed at 100°C, 300°C and 500°C were found to be 430, 309 and 235 nm respectively. The sheet resistances of the films decreases with increased annealing temperature. For reactively evaporated ln203 thin films, as low as about 80 Q.sq xi is achieved. While the sheet resistance of the as deposited ln203 film was 3.26 kfi/sq, the sheet resistances of the heat treated with 100 °C, 300 °C and 450 °C films were found to be 2.5 kO/sq, 85.88 Q/sq and 73.139 Q/sq respectively. It was found that doping the IO films with tin decreases the sheet resistivity. While the sheet resistance was 3.26 kQ/sq for undopped ln203, it was 1 kO/sq for Sn dopped In (ITO).