Papers by Author: S. Dhar

Abstract: Large group-I elements such as sodium, rubidium and cesium have recently been incorporated in the gate oxide of SiC power MOSFETs. In the case of sodium incorporation, enhanced field effect mobilities have been definitively observed. Based on density functional calculations, we find large group-I elements serve as a shallow impurities near the interface. The enhanced mobility, observed in the case of sodium, can be explained in terms of an impurity band model.

Abstract: At sufficiently high temperatures PLD deposited TaC films can be grown epitaxially on 4H-SiC (0001) substrates; at lower temperatures the films recrystallize and ball up forming a large number of pinholes. The growth temperature for epitaxy was found to be 1000°C, and it was facilitated by the epitaxial growth of a thin (2 nm) transition layer of hexagonal Ta2C. High temperature annealing produced changes in the surface morphology, caused grain growth, and created pin holes through a recrystallization process in the films deposited at the lower temperatures, while the films deposited at the higher temperatures remained virtually unchanged. Using TEM it is shown that the (0001) basal planes of the hexagonal 4H-SiC and Ta2C phases are aligned, and they were also parallel to the (111) plane in the cubic TaC with the [101] cubic direction being parallel to the hexagonal [2110] hexagonal direction. The Ta2C interlayer most likely is formed because its lattice parameter in the basel plane (3.103 Ǻ) is intermediate between that of the 4H-SiC (3.08 Ǻ) and the TaC (3.150 Ǻ). Given that Al.5Ga.5N is lattice matched to TaC, it could be an excellent substrate for the growth of GaN/AlGaN heterostructures.

Abstract: 4H-SiC samples implanted with 1020 Al were annealed at various temperatures with a BN/AlN or graphite cap, and there morphological, structural, and electrical properties are compared. No blow holes were observed in either cap. Some Si out-diffuses through the graphite cap which results in a rougher surface and a structurally modified region near the surface. The BN/AlN cap annealed at 1800°C cannot be readily removed, whereas the graphite cap can be removed easily after any annealing temperature. The sheet resistances for both types of samples were about the same.

Abstract: Post-oxidation anneals that introduce nitrogen at the SiO2/4H-SiC interface have been most effective in reducing the large interface trap density near the 4H-SiC conduction band-edge for (0001) Si face 4H-SiC. Herein, we report the effect of nitridation on interfaces created on the (11 20) a-face and the (0001) C-face of 4H-SiC. Significant reductions in trap density (from >1013 cm-2 eV-1 to ~ 1012 cm-2 eV-1 at EC-E ~0.1 eV) were observed for these different interfaces, indicating the presence of substantial nitrogen susceptible defects for all crystal faces. Annealing nitridated interfaces in hydrogen results in a further reduction of trap density (from ~1012 cm-2 eV-1 to ~5 x 1011 cm-2 eV-1 at EC-E ~0.1 eV). Using sequential anneals in NO and H2, maximum field effect mobilities of ~55 cm-2 V-1s-1 and ~100 cm-2 V-1s-1 have been obtained for lateral MOSFETs fabricated on the (0001) and (11 20) faces, respectively. These electronic measurements have been correlated to the interface chemical composition.

Abstract: Nickel ohmic contacts to p-type epitaxial and heavily implanted 4H-SiC are described. Room and elevated temperature results are presented. Elevated temperature measurements of specific contact resistance are compared to theoretical calculations. The calculations require the acceptor doping concentration and the contact’s barrier height. Epitaxial material has a known acceptor value thereby allowing the barrier height to be deduced by requiring agreement between the calculated and measured values of the contact resistance. Calculations of the contact resistance for implanted material use the barrier height from the epitaxial results along with a variable activated acceptor doping concentration which is adjusted to give agreement with measured room temperature specific contact resistances. Specific contact resistances as low as 7x10-6 ohm-cm2 fabricated on the Si face have been obtained to epitaxial 4H p-type material whereas contacts to implanted material result in much larger contact resistance values of 4x10-5 ohm-cm2. These results, when compared to theoretical calculations, indicate that activated acceptor doping concentrations in heavily implanted material are on the order of 2% of the implant concentration.

Abstract: Silicon has been the semiconductor of choice for microelectronics largely because of the unique properties of its native oxide (SiO2) and the Si/SiO2 interface. For high-temperature and/or high-power applications, however, one needs a semiconductor with a wider energy gap and higher thermal conductivity. Silicon carbide has the right properties and the same native oxide as Si. However, in the late 1990’s it was found that the SiC/SiO2 interface had high interface trap densities, resulting in poor electron mobilities. Annealing in hydrogen, which is key to the quality of Si/SiO2 interfaces, proved ineffective. This paper presents a synthesis of theoretical and experimental work by the authors in the last six years and parallel work in the literature. High-quality SiC/SiO2 interfaces were achieved by annealing in NO gas and monatomic H. The key elements that lead to highquality Si/SiO2 interfaces and low-quality SiC/SiO2 interfaces are identified and the role of N and H treatments is described. More specifically, optimal Si and SiC surfaces for oxidation are identified and the atomic-scale processes of oxidation and resulting interface defects are described. In the case of SiC, we conclude that excess carbon at the SiC/SiO2 interface leads to a bonded Si-C-O interlayer with a mix of fourfold- and threefold-coordinated C and Si atoms. The threefold coordinated atoms are responsible for the high interface trap density and can be eliminated either by H-passivation or replacement by N. Residual Si-Si bonds, which are partially passivated by H and N remain the main limitation. Perspectives for the future for both Si- and SiC-based MOSFETs are discussed.

Abstract: The effect of thermal treatments in nitric oxide (NO) on the paramagnetic defects at the 4H-SiC/SiO2 interface are analyzed by EPR in oxidized porous samples. The results on ultrathin thermal oxides show that the NO treatment at 1000°C is insufficient for an efficient reduction of the two dominant paramagnetic interface defects: PbC centers and carbon clusters. From the NRA and XPS analysis of bulk samples treated under the same conditions we attribute the weak effect to the low nitrogen concentration of only 1% at the interface.