Materials Science Forum Vols. 778-780

Abstract: To verify the Si emission phenomenon during oxidation of SiC, the behavior of Si atoms was investigated using HfO₂/SiC structures. At low oxygen pressure, i.e. the oxidation condition predominant to active oxidation, Si emission into oxide layer and the growth of SiO₂ on the oxide surface were clearly observed by TOF-SIMS. On the other hand, the growth of SiO₂ on the surface was suppressed under an ordinary pressure. These results evidence the Si emission during oxidation that is proposed in the Si and C emission model.

Abstract: In this work, we have developed a novel gate stack to enhance the mobility of Si face (0001) 4H-SiC lateral MOSFETs while maintaining a high threshold voltage. The gate dielectric consists a thin lanthanum silicate layer at SiC/dielectric interface and SiO₂ deposited by atomic layer deposition. MOSFETs using this interface engineering technique show a peak field effect mobility of 133.5 cm²/Vs while maintaining a positive threshold voltage of above 3V. The interface state density measured on MOS capacitor with lanthanum silicate interfacial layers is reduced compared to the capacitors without the silicate. It is shown that the presence of the lanthanum at the interface reduces the formation of a lower quality SiO_x interfacial layer typically formed at the SiC surface during typical high temperature anneals. This better quality interfacial layer produces a sharp SiC/dielectric interface, which is confirmed by cross section Z-contrast STEM images.

Abstract: The impact of a sacrificial oxidation treatment on subsequent gate oxide formation on 4H-SiC(0001) substrates was investigated. Although x-ray photoelectron spectroscopy (XPS) analysis revealed that the SiC surface after removing a 40-nm-thick sacrificial oxide by diluted HF solution was almost identical to that of an as-grown epilayer, the subsequent dry O₂ oxidation resulted in a thinner SiO₂ layer for the sample with the sacrificial oxidation in the ultrathin film regime (~3 nm). The metal-oxide-semiconductor (MOS) capacitor with sacrificial oxidation also exhibited a larger frequency dispersion in capacitance-voltage (C-V) characteristics, indicating that interface property had been degraded. However, when the oxide thickness reached about 10 nm, there was no difference in frequency dispersion with and without sacrificial oxidation. This means that the SiO₂ growth in the initial stage of oxidation was significantly affected by the sacrificial oxidation treatment.

Abstract: Native oxide layer with thickness of about 1 nm was found easy to form on 6H-SiC surface during transporting from cleaning process to vacuum chambers, which was examined by x-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM). The interface band bending was studied by synchrotron radiation photoelectron spectroscopy (SRPES). For the native-oxide/SiC surface, after Ni deposition, the binding energy of Si 2p red-shifted about 0.34 eV, which suggested the upward bending of the interface energy band. Therefore, the native oxide layer should be considered on the study of SiC devices because it may affect the electron transport properties significantly.

Abstract: The electron mobility and the surface carrier density in the channel layer of a C-face 4H-SiC MOSFET was studied using a Hall measurement and a split C-V measurement. Total surface carrier density and the effective mobility were extracted experimentally using the split C-V measurement. It was found that the difference of the peak Hall mobility and the peak effective mobility was less than 20%. And the total trap density was the order of the 10¹² cm^-2. The cause of the relatively small difference between the Hall mobility and the effective mobility was discussed.

Abstract: The effect of phosphorus (P) on the electrical properties of the 4H-SiC / SiO₂ interface was investigated. Phosphorus was introduced by surface-near ion implantation with varying ion energy and dose prior to thermal oxidation. Secondary ion mass spectrometry revealed that only part of the implanted P followed the oxidation front to the interface. A negative flatband shift due to residual P in the oxide was found from C-V measurements. Conductance method measurements revealed a significant reduction of density of interface traps D_it with energy E_C - E_it > 0.3 V for P⁺-implanted samples with [P]_interface = 1.5 10¹⁸ cm^-3 in the SiC layer at the interface.

Abstract: The conduction mechanism of the leakage current in thermal oxide on 4H-SiC was identified. The carrier separation current-voltage method clarified that electrons are the dominant carriers of the leakage current. The temperature dependence of the currentvoltage characteristics indicated that the conduction mechanism of the leakage current involved not only Fowler-Nordheim tunneling (FN) but also Poole-Frenkel (PF) emission. The PF emission current due to the existence of defects in the oxide increased with temperature.

Abstract: In this work, we investigate the impact of Al-implantation into n-MOSFET channel regions together with its p-doping concentration upon the mobility limiting scattering mechanisms in the channel. For this purpose, a study of the interface trap density, interface trapped charge density, field-effect mobility, and Hall mobility is carried out for normally-off n-MOSFETs with different doping profiles and concentrations in the channel region. The trend of the field-effect and the Hall mobility as well as the differences thereof will be discussed. Based on the determined mobilities in the range from 11.9 cm²/Vs to 92.4 cm²/Vs, it will be shown that for p-doping concentrations above 5·10¹⁶ cm^-3 Coulomb scattering is the dominant scattering mechanism for both, low- and high-field mobility. In contrast, for p-doping concentrations below 5·10¹⁶, cm^-3 further scattering mechanisms will be considered that may account for the observed mobility trend at high electric fields.

Abstract: 4H-SiC is difficult to be polishing due to its high hardness and chemical inertness. We proposed a novel polishing technique named plasma assisted polishing (PAP), in which oxidation by water plasma and polishing soft abrasive were combined. In order to increase the material removal rate of PAP and clarify the atomic-scale flattening mechanism, experimental studies on water vapor plasma oxidation and thermal oxidation of 4H-SiC (0001) were conducted. Experimental results indicated that the initial oxidation rate of water vapor plasma oxidation (185 nm/h) was much higher than that of thermal oxidation (29nm/h). In the case of water vapor plasma oxidation, the oxide/SiC interface was rough when the oxide layer was thin and it became flatter along with the increase of the thickness of the oxide layer. In contrast, the oxide/SiC interface was atomically flat regardless of the thickness of the oxide layer in the case of thermal oxidation. CeO₂ abrasive polishing was conducted on the oxidized SiC surfaces, well-ordered step/terrace structures were obtained in both cases. The step height was about 0.25 nm, which corresponds to a one-bilayer structure of 4H-SiC.

Abstract: This paper deals with the comparison of several MOS structures with different rapid thermal oxidation processes (RTO) carried out on Off and On-axis SiC material. A first set contains MOS capacitance structures on n-epitaxial layers, while a second set of MOS capacitance are built on p-implanted layers. Both sets include On and Off-Axis angle cuts. Furthermore, n-MOSFETs have been fabricated on On-axis p-implanted layers with the best oxidation process selected from the MOS capacitance study. The final objective is to show the performances of these On-axis p-implanted n-MOSFETs and to evidence the associated lower surface roughness at the SiO₂/SiC interface.