Papers by Author: Sadafumi Yoshida

Abstract: Thermal oxidation process of silicon carbide in ultra-thin oxide regime has been studied by performing in-situ and real time spectroscopic ellipsometry. We found the thermal oxidation at 700°C forms no or extremely thin interface layers between SiC and oxide layers. In contrast, the oxidation at 850°C forms an interface layer of around 1 nm in thickness, having similar thickness and optical constants of the interface layers formed by the oxidation at higher temperature than 1000°C. To make clear the conditions no interface layer is formed, i.e., whether low temperature growth or thin oxide thickness is crucial, we have performed the oxidation at 850°C in the reduced oxygen pressure. Based on the results of these experiments, we discussed the origin of the formation of interface layers as well as the oxidation mechanism of SiC.

Abstract: We have characterized 4H-SiC–oxide interfaces fabricated by thermal oxidation of SiC using spectroscopic ellipsometry in the wide spectral range from visible to deep UV region. It was found that there exists an interface layer, around 1 nm in thickness, regardless of the oxide thickness from 15 nm to 40 nm. The optical constants of the interface layer have similar spectral dependence to those of SiC, though the absolute value of the refractive indices is 0.5–1 larger than that of SiC. We have discussed the structure of the interface layer based on the oxidation mechanism of SiC, like the Si-emission model.

Abstract: We proposed a kinetic model for SiC oxidation, named ‘silicon and carbon (Si-C) emission model’, taking into account the emission of Si and C atoms from the SiC–oxide interface, which suppresses the oxidation rate at the interface. Based on the model, we calculated oxide growth rates for SiC (0001) Si- and (000–1) C-face and found that the calculated values exhibit good fits to the measured ones in the entire oxide thickness range for both faces. We also calculated depth profiles of Si and C interstitials and oxidants, and discussed the oxidation mechanism of SiC as well as the difference in the oxidation process of Si-face and C-face.

Abstract: Real time observations of SiC (000–1) C-face and (0001) Si-face oxidation were performed using an in-situ ellipsometer over the oxygen-partial-pressure range from 0.1 to 1.0 atm. We analyzed the relations between oxide growth rate and oxide thickness by applying an empirical relation proposed by Massoud et al. We found the occurrence of oxidation enhancement in the thin oxide regime also for Si-face as well as for C-face. We have discussed the oxygen-partial-pressure dependence of the oxidation rate constants between SiC C- and Si face, comparing with that of Si.

Abstract: To explain the growth rate enhancement of SiC oxidation in the thin oxide regime, which was recently found from the real time monitoring experiments of the initial oxidation stage of SiC (000–1) C-face using an in-situ spectroscopic ellipsometer, we tried to apply the interfacial Si emission model, which has been originally proposed for Si oxidation, and found that the Si emission model successfully reproduced the SiC oxidation rates at the whole range of oxide thickness and at oxidation temperatures measured. By comparing with the simulations for Si oxidation, we have discussed the oxidation mechanism of SiC.

Abstract: 4H-SiC p-type MOS capacitors fabricated by wet oxidation of SiC preamorphized by nitrogen ion (N+) implantation have been investigated. The oxidation rate of the SiC layer preamorphized by high-dose N+ was much larger than that of crystalline SiC, allowing us to reduce the fabrication time of SiC MOS devices. We found that the presence of the surface amorphous SiC layer before the oxidation process did not influence the interface state density in MOS capacitors. Moreover, the shift of the flat-band voltage is not correlated to the amount of nitrogen in the oxide. On the contrary the density of interface states near the valence band edge increased according with the high concentration of the implanted N at the oxide–SiC interface, as in the case of dry oxidation reported by Ciobanu et al. The generation of positive charges due to the nitrogen embedded inside the oxide layer was smaller compared with dry oxidation. We discuss the difference between wet and dry oxidation for MOS capacitors fabricated with N+ implantation.

Abstract: We have simultaneously determined the carrier concentration, mobility, and thickness of 4H-SiC homo-epilayers with carrier concentration of 1016–1018 cm-3 from reflectance spectroscopy in the wavenumber range of 20–2000 cm-1. The spectra at 20–100 cm-1 and at 80–2000 cm-1 were measured by using the terahertz time domain spectrometer (THz-TDS) and the Fourier-transform infrared (FTIR) spectrometer, respectively. A modified classical dielectric function (MDF) model was employed for the curve fitting. We have compared the values of free carrier concentrations estimated from the reflectance spectroscopy with the net doping concentrations obtained from C–V measurements, and have discussed the validity of the electrical properties estimated from the reflectance spectroscopy.

Abstract: Real time observation of SiC oxidation was performed using an in-situ ellipsometer over the temperature range from 900°C to 1150°C. The relations between oxide thickness and oxidation time were obtained precisely by virtue of the real time measurements. We analyzed the relations between oxide thickness and oxidation time by applying the Deal and Grove model to obtain the linear and parabolic rate constants. Taking advantage of in-situ measurements, we successfully obtained the oxidation rate constants with high accuracy.