6 th International Advanced Technologies Symposium (IATS 11), 16-18 May 011, Elazığ, Turkey Production of Al O 3 Thin films for FET and MOSFET Transistor Gate Applications B. Gündüz 1, M. Cavaş, F. Yakuphanoğlu 3 1 Physics Department, Faculty of Arts and Sciences, Muş Alparslan University, 49100 Muş, Turkey, bgunduz83@hotmail.com Elbistan Higher Vocational School, KahramanMaraş Sütçü İmam University, Elbistan, Turkey, mcavas3@hotmail.com 3 Department of Metallurgical and Materials Engineering, Firat University, Elazığ, Turkey, fyhan@hotmail.com Production of Al O 3 Thin Films for FET and MOSFET Transistor Gate Applications Abstract The paper reports the obtaining and characterization of Al O 3 thin films deposited by sol-gel technique. The optical properties were investigated as function of the deposition parameters and annealing. Transparent Al O 3 sol was prepared with Aluminium Nitrate (Al(NO 3 ) 3. Al O 3 sol spin-coated on the glass at different coating speed. The optical properties of the Al O 3 thin films spin-coated on glasses were investigated by taking measurements of UV-visible spectra and some important optical parameters of the Al O 3 thin films were calculated. Also, effect of the annealing on the optical properties of the Al O 3 thin film was investigated. The magnitude of the reflectance decreases with annealing, but reflectance values of the Al O 3 thin films increase with increasing coating-speed. Refractive index values of Al O 3 thin films were calculated for thin films. Refractive index values vary with coating speed and annealing. The magnitude of the refractive index decreases with annealing, but refractive index values of the Al O 3 thin films increase with increasing coatingspeed. The imaginary and real parts of dielectric constant of Al O 3 thin films dependence on photon energy were investigated. The real parts of the dielectric constant are higher than that of imaginary parts of the dielectric constant and both real parts and imaginary parts of the dielectric constant decrease with annealing, but these values of the Al O 3 thin films increase with increasing coating-speed. The real part and imaginary parts of the optical conductivity of Al O 3 thin films were calculated. The real part conductivity increases with increasing frequency until certain point, but the imaginary part conductivity increases with increasing frequency. Also, the real part and imaginary parts of the optical conductivity of Al O 3 thin films increase with coating-speed but decrease with annealing. The imaginary parts of the optical conductivity are higher than that of real parts of the optical conductivity. The electric susceptibility dependence of photon energy of the alumina films was investigated. The long wavelength refractive index n, average oscillator wave length λ o and S o values of the alumina films were calculated. These values vary with annealing and coating speed. Keywords Al O 3 film, long wavelength refractive index, electrical susceptibility, average oscillator wavelength, sol-gel technique, refractive index, excitation energy, dispersion energy. I. INTRODUCTION Alumina is the most cost effective and widely used material in the family of engineering ceramics. The raw materials from which this high performance technical grade ceramic is made are readily available and reasonably priced, resulting in good value for the cost in fabricated alumina shapes. With an excellent combination of properties and an attractive price, it is no surprise that fine grain technical grade alumina has a very wide range of applications. Aluminum oxide, commonly referred to as alumina, possesses strong ionic interatomic bonding giving rise to it s desirable material characteristics. It can exist in several crystalline phases which all revert to the most stable hexagonal alpha phase at elevated temperatures. This is the phase of particular interest for structural applications and the material available from Accuratus [1]. The ceramic matrix must have a controlled morphology, able to be further infiltrated with metal nano-particles and good thermal properties. The aluminium oxide is used as ceramic matrix because of its high chemical and thermal stability. Alumina thin films can be prepared by various techniques []. Sol gel processing is widely used to prepare various functional thin films in which high-temperature oxidation resistant coatings are included [3-6]. The sol gel method applied to Al O 3 sol is well known to enable the preparation of compounds of very high optical quality and homogeneity. This is essential for optical studies and applications. Al O 3 has many interesting physical properties that make it suitable for thin film applications. Because of their good transmittance in the visible region and chemical stability, Al O 3 films have found wide application various antireflective and protective coatings [7]. II. EXPERIMENTAL Aluminum nitrate (Al(NO 3 ) 3 was dissolved in -methoxy ethanol. Al O 3 sol gel alumina sol-gel with excellent uniformity were prepared by sol-gel method. Sol gel solution was prepared by mixing Aluminum nitrate, - methoxy ethanol and monoethanolamine. Al O 3 sol-gel 574
Production of AlO3 Thin films for FET and MOSFET Transistor Gate Applications solution was prepared by starting from 1M Aluminum nitrate (Al(NO 3 ) 3 dissolved in -methoxy ethanol (10 ml). Then, this solution was stirred using a magnetic stirrer at room temperature for 30 min, then monoethanolamine (0.60 ml) was added to solution and solution was agitated using a magnetic stirrer at room temperature for h to get a transparent sol. Then, prepared alumina sol-gel solution was filtered through PTFE membrane filter before spin coating. To prepare the thin films, alumina sol-gels were spin-coated on glasses at a coating speed of 1000, 000 and 3000 rpm for 60 s and sol-gel films were dried at 150 o C for 15 minute on hot-plate. This process is repeated three times. Before annealing, the UV-Visible spectra of the thin films were recorded by a UV-Visible spectrophotometer at room temperature. After all films annealed at 400 o C for 1 h, the UV-Visible spectra of the thin films were again recorded by a UV-Visible spectrophotometer at room temperature. III. RESULTS AND DISCUSSION 3.1. Determination of the optical constants The transmittance spectra of the alumina films were measured to investigate their optical properties and they are shown in Fig. 1a. As seen in Fig. 1a, the transmittance spectra of the Al O 3 films show a sharp absorption edge in the wavelength range 95-335 nm. In the visible region, the average transmittance of the Al O 3 films varies from about 80.747% to 91.015% (Table 1). The average transmittance of alumina films decreases with annealing. The decrease of average transmittance may be due to the thickness of the anneling of the alumina films. It is well known that the low transmittance of the deposited films mainly results from the high film thickness. To estimate the absorption band edge of the films, the first derivative of the optical transmittance can be computed. The curves of dt/dλ versus wavelength were plotted, as shown in Fig. 1b. As seen in Fig. 1b, the maximum peak position corresponds to the absorption band edge and it shifts to longer wavelengths. The maximum peak values of the films varies from about 350 to 400 nm (Table 1). This suggests that the absorption band edge shifts from 3.105 to 3.549 ev with effect of the different coating speed and annealing. The reflectance spectra of the Al O 3 films is shown in Fig.. T and R spectra of the Al O 3 films change with annealing and different coating speed. This suggests that a change in the structure of thin films is taking place and both prepared alumina films at different coating speed and annealing affect optical constants of the films. The magnitude of the reflectance decreases with annealing. The refractive index is an important parameter for optical applications. Thus, it is important to determine optical constants of the films studied and the complex optical refractive index of the films is expressed as, nˆ n( ik( (1) where n is the real part and k is the imaginary part of complex refractive index. The optical properties of the films are characterized by refractive index. The refractive index of the films can be obtained from the following equation [8-10], 1/ 4R R 1 n k () ( R 1) R 1 The refractive index values of the films were calculated from the T and R spectra of alumina films. Fig. 3 show spectral dependence of refractive index. The refractive index changes with annealing and with different coating speed. The magnitude of the refractive index decreases with annealing. The complex dielectric constant is described as, i n ( n ik) ( n k ) ink (3) 1 where ε 1 is the real part and ε is the imaginary of the dielectric constant. The imaginary and real parts of dielectric constant are given as [11] 1 n k (4) and nk (5) where k=/4fig. 4(a-b) show the real and imaginary parts of dielectric constant dependence on photon energy, respectively. The real parts of the dielectric constant are higher than that of imaginary parts of the dielectric constant and both real parts and imaginary parts of the dielectric constant decrease with annealing but these values increase with increasing coating-speed. Table 1: Some optical parameters of the alumina films. Sol Type T ave (%) Max. peak (nm) Absorption band edge (ev) 1000 rpm 89.619 350 3.549 000 rpm 91.015 35 3.58 3000 rpm 89.917 350 3.549 1000 rpm 000 rpm 3000 rpm 80.747 400 3.105 89.1 360 3.450 87.935 360 3.450 575
B. Gündüz et al. Fig. 3 Refractive index dispersion of the alumina thin films. The susceptibility associated with electron transitions from band i to band j is expressed as [1] e f jik (6) m k jik 4 Fig. 1 Transmittance spectra (insert: Absorption edge shift) and the plot of the first derivative of the transmittance spectra of the alumina thin films. where k is the wave number vector, h jik is the energy difference between the state k in band i and the state k in band j and f jik is the oscillator strength for this transition. The electrical susceptibility due to the intraband transitions of free carriers can be obtained from the optical constants and it is determined as [13]; 1 c ( n k o ) (7) 4 where o is the dielectric constant in the absence of any contribution from free carriers. The electric susceptibility dependence of photon energy is shown in Fig. 5. The refractive index can be also analyzed using the following relation [14], ( n 1) o 1 ( n 1) (8) where n is the long wavelength refractive index and o is the average oscillator wave length. The n and o values were obtained from the linear Fig. Reflectance spectra of the alumina thin films. part of plotted 1/(n -1) vs. - curve and were calculated and given in Table. When rearranging of Eq. (8), gives, S o n 1 o 1 o / (9) 576
Production of AlO3 Thin films for FET and MOSFET Transistor Gate Applications where So ( n 1) / were calculated and given in Table.. The S o values for the alumina films o Table : Some optical parameters of the Al O 3 films. Sol Type n λ 0 (nm) S 0 (m - ) x10 13 1000 rpm 1.111 1.068 5.0 000 rpm 1.180 07.938 0.908 3000 rpm 1.4 01.518 1.336 1000 rpm 000 rpm 3000 rpm 1.085 183.419 0.54 1.079 186.905 0.470 1.064 190.134 0.464 Fig. 5 The electric susceptibility dependence of photon energy of the alumina films. 3.. The conductivity properties of the Alumina thin films The optical properties of the alumina films can be analyzed by a complex optical conductivity [15,16], ( 1( i ( (10) where σ 1 is the real part of conductivity and σ is the imaginary part of conductivity. The real part and imaginary parts of the optical conductivity are shown in Fig. 6a and b, respectively. The real part and imaginary parts of the optical conductivity of alumina thin films were calculated. The real part conductivity increases with increasing frequency until certain point, but the imaginary part of the optical conductivity increases with increasing frequency. Also, the real part and imaginary parts of the optical conductivity of alumina thin films decreases with annealing. The imaginary parts of the optical conductivity are higher than that of real parts of the optical conductivity. Fig. 4 Plots of real and imaginary parts of the dielectric constant of the alumina thin films. 577
B. Gündüz et al. [7] R.H. French, H. Müllejans, D.J. Jones, J. Am. Ceram. Soc. 81, (10) 549-57, 1998. [8] M. D. Migahed, H. M. Zidan, Current Applied Physics, 6 (006) 91-96. [9] F. Yakuphanoglu, M. Arslan, Optical Materials 7 (004) 9. [10] F. Yakuphanoglu, A. Cukurovali, I. Yilmaz, Physica B 351 (004) 53. [11] M.M. Wakkad, Ekh. Shokr, D.H. Mohamed, J. Non-Cryst. Solids 65 (000) 157. [1] W.G. Spitzer, H.Y. Fan, Phys. Rev. 106 (1957) 88. [13] J. Taue, in: F. Abeles (Ed.), Optical Properties of Solids, North- Holland, Amsterdam, 197. [14] N.A. Subrahamanyam, A Textbook of Optics, ninth ed.,brj Laboratory, Delhi, India, 1977. [15] F. Abeles (Ed.), Optical Properties of Solids, North-Holland Publishing Company, Amsterdam, 197. [16] F. Yakuphanoglu, M. Arslan and S.Z. Yildiz, Optical Materials, 7 (005) 1153. Fig. 6 The real part and imaginary parts of the optical conductivity. IV. CONCLUSION The magnitude of the reflectance decreases with annealing but reflectance values of the Al O 3 thin films increase with increasing coating-speed. Refractive index values of Al O 3 thin films were found for thin films. Refractive index values vary with coating speed and annealing. The magnitude of the refractive index decreases with annealing, but refractive index values of the Al O 3 thin films increase with increasing coating-speed. The real parts of the dielectric constant are higher than that of imaginary parts of the dielectric constant and both real parts and imaginary parts of the dielectric constant decrease with annealing, but these values of the Al O 3 thin films increase with increasing coating-speed. The real part and imaginary parts of the optical conductivity of Al O 3 thin films were calculated. The real part conductivity increases with increasing frequency until certain point, but the imaginary part of the optical conductivity increases with increasing frequency. Also, the real part and imaginary parts of the optical conductivity of Al O 3 thin films increases with coating-speed but decrease with annealing. The imaginary parts of the optical conductivity are higher than that of real parts of the optical conductivity. The electric susceptibility dependence of photon energy of the alumina films was investigated. The long wavelength refractive index n, average oscillator wave length λ o and S o values of the alumina films were calculated. ACKNOWLEDGMENT This work was supported by The Management Unit of Scientific Research Projects of Firat University (FUBAP) under project 1983. REFERENCES [1] I.S. Ahmed Farag, M.F. Kotkata, M.M. Selim, I.K. Battisha, and M.M. El- Rafaay, Egypt. J. Solids, Vol. (7), No. (), (004). [] Elena IENEI, Luminiţa ISAC and Anca DUŢA, Rev. Roum. Chim., 010, 55(3), 161-165. [3] E.A. Loria, Intermetallics 8 (000) 1339. [4] A. Rahmel, P.J. Spencer, Oxide Met. 35 (1991) 53. [5] H. Li, K. Liang, L. Mei, S. Gu, S. Wang, Mater. Lett. 51 (001) 30. [6] S. Zhang, W.E. Lee, J. Eur. Ceram. Soc. 3 (003) 115. 578