Synthesis of High-K Titanium Oxide Thin Films Formed by Metalorganic Decomposition
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EXPERIMENTAL The starting materials were p-type (1-2 g cm) silicon (100)-oriented wafers, which were cut into 1.0 cmX 1.0 cm squares. These wafers were first rinsed in deionized water and methyl alcohol and then cleaned by a standard RCA method. TiO 2 thin films were fabricated by MOD technique using titanium tetrakis isopropoxide (TTIP: Ti(i-OC 3H7 )4) as a precursor. Isopropyl acetate (CHC)CO)O-iPr) was used as a solvent. The precursor films was directly deposited onto the Si wafers by the spin-coating technique. The thickness of the film was controlled by the spin-coating conditions. After deposition, the films were dried in air on a hot plate at 1101C for 10 min, then preannealed in nitrogen at 400'C for 30 min in a furnace tube. Crystallization of the films was carried out in nitrogen and/or oxygen atmosphere at temperatures ranging from 600 to 10000C for 30 min. The crystallinity of the films was examined using a Raman spectrometer excited with the 514.5 nm line of an Art laser (100 mW) as the excitation source, and a X-ray diffractometer (XRD) using Cu Ka radiation at 40 kV. The TiC) 2 and TiO 2/Si interface structure was analyzed using a variable-angle spectroscopic ellipsometer at wavelengths ranging from 350 to 1250 nm at 10 nm intervals. The directly measured values are ellipsometric angles I and A as a function of wavelength. Unknown model parameters, such as layer thickness and volume fraction of each constituent in the composite layer, were designated as fitting parameters. Electrical measurements were conducted on films in a metal-insulator-semiconductor (MIS) configuration. Several aluminum dot electrodes of 0.5 mm diameter were evaporated over the area of the TiO 2 films. Capacitance-voltage (C-V) and current-voltage (I-V) measurements were carried out using an electrical analyzer. RESULTS AND DISCUSSION 0 Figure 1 shows typical T and A curves obtained for TiO 2 films after annealing at 700 C. The broken lines represent measured values and the solid lines are curves simulated using the mathematical model called the Cauchy dispersion model [19]. The ellipsometric parameters, T and A, are defined by the relation
(1)
RP/R, = tan ['(X)] exp [i A(%)],
where RP and R. are the parallel and perpendicular reflection coefficients of incident light on the sample as a function of wavelength X. Ellipsometry is a technique based on measurements of the state of polarization of the incident and reflected light, leading to the determination of the ratio R1/R.. In Fig. 1, 'P and A are modelled by calculating reflectance spectra based on trial values of thickness di using the complex index of reflection N (=n- ik) where n is the real part and k is the imaginary part of the complex refractive index for a particular layer. The quantity used to describe the agreement between measured and calculated values is given by the mean square error (MSE) defined as 2 +(Ai. MSE = (1/N) I [L'i~ -_t"icC)
2
_Ai•)
],
(2)
i=1
where N is the number of data points, ' t ir and A&iare the measured values at ki, and Tic and Ai' are
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