Materials Science Forum Vols. 600-603

Abstract: We report investigations on the fabrication and electrical characterization in the range 27°C -290 °C of normally off 4H-SiC circular MOSFET devices manufactured on p-type semiconductor. An high quality SiO2/SiC interface is obtained by nitrogen ion implantation conducted before the thermal oxidation of SiC. Two samples with different nitrogen concentration at the SiO2/SiC interface and one un-implanted have been manufactured. The sample with the highest N concentration at the interface presents the highest channel mobility and the lowest threshold voltage. With increasing temperature, in all the samples the threshold voltage decreases and the electron channel mobility increases, reaching the maximum value of about 40 cm2/Vs at 290 °C for the sample with the highest N concentration. The observed improvement of the mobility is related to the beneficial effect of the N presence at the SiO2/SiC interface, which leads to the reduction of the interface trap density with energy close to the conduction band. Our results demonstrate that N implantation can effectively be used to improve the electrical performance of surface n-channel 4H-SiC MOSFETs.

Abstract: N-channel MOSFETs are irradiated with gamma-rays (g-rays) up to 3.16 MGy(SiO2) at room temperature. Above 1 MGy, the effective channel mobility increases with increasing absorbed dose. A similar increase is observed for the Hall mobility in the inversion layer. In addition, the Hall-effect measurements indicate a reduction of the interface trap density.

Abstract: The effect of the fabrication process of n-channel 6H-SiC MOSFETs on their radiation resistance is investigated. MOSFETs that post implantation annealing for source and drain was carried out with carbon coating on the sample surface are compared to MOSFETs that post implantation annealing was carried out without carbon coating. The radiation resistance (gamma-rays) of the carbon-coated MOSFETs is higher than that of non-coated ones. The channel mobility for MOSFETs whose gate oxide was formed using pyrogenic or dry oxidation process dose not change by gamma-ray irradiation below 1x105 Gy. The value of channel mobility slightly increases with increasing dose above 1x105 Gy. No significant increase in irradiation induced interface traps is observed.

Abstract: From a viewpoint of device application using p-channel SiC MOSFETs, control of their channel properties is of great importance. We aimed to control the electrical properties of 4H-SiC p-channel MOSFETs through locating the p-type epitaxial layer at the channel area, so called “epi-channel MOSFET” structure. We varied the dopant concentrations and the thickness of the epi-channel layer, and investigated their electrical properties. In case of heavily doped epi-channel samples, the devices indicated “normally-on” characteristics, and their channel mobility decreased slightly in comparison with the inversion-type devices. As for lightly doped epi-channel samples, the subthreshold current increased with thickness of the epi-channel layer keeping their “normally-off” characteristics. Their channel mobility also increased with thickness of the epi-channel layer. The peak value of field effect channel mobility of the sample with 2.5 μm thickness and 5×1015 /cm3 dopant concentration epi-channel was 18.1 cm2/Vs.

Abstract: We have identified the crystal planes of 4H-SiC, interfacing with gate oxide, which will lead to minimum defect density and lowest interface corrugations. The atomic surface density, surface energy, and surface roughness of various crystal planes of 4H-SiC have been computationally characterized using Molecular Dynamics simulations. We have also investigated the screening distances of defects in SiO2 and SiC using a multiscale approach.

Abstract: We apply electrically detected magnetic resonance (EDMR) to variously processed 4H SiC MOSFETs from two vendors. Although, the EDMR line shapes observed are nearly the same for vendor 1 devices subjected to a nitric oxide (NO) anneal and devices without it, the relationship between EDMR and gate voltage differs greatly between these samples. Furthermore, the EDMR response versus gate bias varies dramatically. EDMR results from a third device produced by a second vendor are very different from those provided by the first vendor. This result implies that significantly different defect populations are present in devices fabricated by different vendors.

Abstract: Recently, Wang et. al. formulated a criterion for identifying the source of deep interface sates at the SiC/SiO2 interface, based on the known inefficiency of H2 passivation. We apply this criterion to a variety of excess carbon defects at the interface, which we have predicted to be energetically feasible. We find that, also with respect to this criterion, the simple C split interstitials are the most likely cause of the deep interface states among the examined defects.

Abstract: 3C-SiC/SiO2-capacitors are fabricated by over-oxidation of an implanted Gaussian nitrogen (N) profile and investigated by conductance spectroscopy. A double peak structure is observed in the conductance spectra indicating two types of traps, which change their charge state at identical time constant, however, which are located at different energy positions in the bandgap of 3C-SiC. The experimental G/w-V and C-V characteristics are simulated and the existence of two types of traps is verified in the framework of a theoretical model.

Abstract: We have investigated the electrical and physical properties of high-temperature (1300, 1400 oC) grown dry oxide with or without post oxidation annealing (POA) in nitric oxide (NO) gas. A significant reduction in interface-trap density (Dit) has been observed in 1300 oC-grown dry oxide with or without NO POA if compared with the Dit of 1400 oC-grown dry oxide. The reason for this has been explained in this paper.

Abstract: A metal-oxide-semiconductor (MOS) capacitor was fabricated using 4H-SiC epitaxial layer, and the interface state was evaluated in oxygen and hydrogen ambient under high-temperature conditions by the AC conductance technique. The relationships between interface state density (Dit), and corresponding time constant (tit) were obtained. Influences of oxide thickness and of gate metal (Pt or Al) were studied. Dit of Pt gate capacitor is influenced by ambient gas at higher temperature but Dit of Al gate capacitor is little affected by ambient gas. Dit of capacitor with thicker oxide layer tends to be lower than that of capacitor with thinner oxide layer. Interface states with larger time constant are decreased for hydrogen ambient comparing with oxygen ambient in the Pt gate capacitor.