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Capacitance-voltage spectroscopy in metal-tantalum pentoxide (Ta-O)-silicon mos capacitors
The electronic properties of Al-Ta2O5-Si MOS capacitors with oxide layers prepared by RF magnetron sputtering with or without a prior nitridation process in N2O or NH3 gas environments at temperature ranges between 700 °C to 850 °C were investigated using Capacitance-Voltage (C-V) Spectroscopy to determine the quality of oxide layer and oxide-silicon interface. The theoretical ideal capacitance-voltage calculations were compared with the experimental capacitance-voltage results in order to evaluate effective oxide charges, Qeff , present inside Ta2O5 insulating layer and density of interface trap states, Dit, present at the Ta2O5-Si interface. In addition,dielectric constant, doping concentration, flat band voltage values were determined by using the experimental data. Finally, the effects of deposition conditions on Ta2O5 MOS capacitors were compared by using a reference sample of a MOS capacitor with native oxide SiO2. It has been found that dielectric constant value up to 12 have been reached for Ta2O5 insulating layers which increases the capacitance value several times than that of MOS capacitor with native oxide SiO2. The density of interface trap states, Dit, for unnitrided Ta2O5 MOS capacitors, values around 1.6x1012 cm-2 eV-1 have been detected which is much higher than that of MOS capacitor with native oxide SiO2. However, prior nitridation process enhances the interface properties and Dit values down to 2-5x1011 cm-2 eV-1 have been reached for the nitrided samples which is in the limits for MOS capacitors with high quality insulating layers. In addition, the effective oxide charges, Qeff, for unnitrided samples, values as high as 3x1012 cm-2 were detected. Even though nitridation process enhances interface properties, the effective oxide charges are found to be higher for nitrided samples. Best electrical and interface properties are obtained by nitridation process at 800 °C in N2O and NH3. It can be inferred that samples nitrided in N2O gas at 800 °C improves the dielectric constant above the level of SiO2 and decreases both Qeff and Dit levels to that of native oxide SiO2. These results show that a prior nitridation of p-silicon surface is a promising approach to improve both oxide and interface properties of Al-Ta2O5-Si MOS devices. However, further investigation is necessary to understand the nature of these oxide charges and interface properties of MOS devices with high dielectric constant oxide layers before integration into large scale fabrication.