Silicon Carbide and Related Materials 2006

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Authors: Wook Bahng, Hui Jong Cheong, In Ho Kang, Seong Jin Kim, Sang Cheol Kim, Sung Jae Joo, Nam Kyun Kim
Abstract: We have investigated the influence of surface modification on the electrical properties of SiC diodes. Schottky diodes (SBDs) as well as PiN diodes were fabricated on n-type SiC substrate with an epilayer, and electrically characterized before and after high temperature annealing, and after removing the surface modified layer, respectively. The devices annealed without graphite cap layer showed ohmic behavior. The surface layer was modified to a conductive layer possibly due to the preferred sublimation of Si species. In order to confirm the existence of modified surface conductive layer, diode was fabricated on the same substrate and electrically characterized after removing 30nm-thick damaged layer by ICP-RIE. The leakage current reduced dramatically, as much as 7 orders of magnitude. The PiN diodes fabricated on the damaged surface layer showed the reverse leakage current and the breakdown voltage of 50mA and 1250V, respectively. While those of the diode fabricated after removing the damaged surface layer were 200nA at the breakdown voltage of 2100V, respectively.
Authors: Ivan V. Ilyin, Marina V. Muzafarova, P.G. Baranov, B.Ya. Ber, A.N. Ionov, E.N. Mokhov, Pavel A. Ivanov, M.A. Kaliteevskii, P.S. Kop'ev
Abstract: High concentration of two types of P donors up to 1017 cm-3 in SiC enriched with 30Si after neutron transmutation doping (NTD) has been achieved. It was established that annealing at sufficiently low temperature of 1300oC, that is 500-600°C lower compared with annealing of NTD SiC with natural isotope composition, gives rise to the EPR signal of shallow P donors, labeled sPc1, sPc2 and sPh. The correlated changes of the EPR spectra of the three sP centres in all the experiments and the qualitative similarities with spectra of shallow N donors prove that these centres have shallow donor levels and a similar electronic structure and belong to different lattice sites. The annealing at 1700°C results in a transformation of one type of P donors (sPc1, sPc2 and sPh) into another type having low temperature EPR spectra labeled dP.
Authors: L. Storasta, Hidekazu Tsuchida
Abstract: Reduction in deep level defects and increase of carrier lifetime in 4H-SiC epilayer was observed after carbon ion implantation into the shallow surface layer of 250 nm and subsequent annealing above 1400 °C. The concentration of Z1/2 and EH6/7 traps was determined by deep level transient spectroscopy 4 μm below the implanted layer. After annealing, concentration of both traps decreased from 1013 cm-3 range to below the detection limit. Minority carrier lifetime almost doubled in the implanted samples compared to the unimplanted samples. We suggest that carbon interstitials from the implanted layer in-diffuse into the layer underneath during annealing and annihilate with carbon vacancies. Our results indicate that Z1/2 and EH6/7 traps are most likely carbon vacancy related.
Authors: Sergey A. Reshanov, Heiko B. Weber, Gerhard Pensl, Adolf Schöner, Hiroyuki Nagasawa
Abstract: Selenium (Se) and tellurium (Te) ions are implanted into n-type 6H-, 4H- and 3C-SiC epilayers. Double-correlated deep level transient spectroscopy investigations reveal that both Se and Te atoms form double donors in SiC. The number of double donors observed corresponds to the number of inequivalent lattice sites of the particular SiC polytype. This observation is a strong hint that Se and Te atoms reside on lattice sites. The activation energies FEa of Te double donors are larger than the corresponding ones of Se double donors.
Authors: Erwan Oliviero, Mihai Lazar, Heu Vang, Christiane Dubois, Pierre Cremillieu, Jean Louis Leclercq, Jacques Dazord, Dominique Planson
Abstract: 6H and 4H–SiC epilayers were Al-implanted at room temperature with multiple energies (ranging from 25 to 300 keV) in order to form p-type layers with an Al plateau concentration of 4.5×1019 cm-3 and 9×1019 cm-3. Post-implantation annealing were performed at 1700 or 1800 °C up to 30 min in Ar ambient. During this process, some samples were encapsulated with a graphite (C) cap obtained by thermal conversion of a spin-coated AZ5214E photoresist. From Atomic Force Microscope measurements, the roughness is found to increase drastically with annealing temperature for unprotected samples while the C capped samples show a preservation of their surface states even for the highest annealing temperature. After 1800°C/30 min annealing, the RMS roughness is 0.46 nm for the lower fluence implanted samples, slightly higher than for unimplanted samples (0.31 nm). Secondary Ion Mass Spectroscopy measurements confirm that the C cap was totally removed from the SiC surface. The total Al-implanted fluence is preserved during postimplantation annealing. A redistribution of the Al dopants is observed at the surface which might be attributed to Si vacancy-enhanced diffusion. An accumulation peak is also observed after annealing at 0.29 9m, depth corresponding to the amorphous/crystalline interface that was determined on the as-implanted samples by Rutherford Backscattering Spectroscopy in channeling mode. The redistribution of the dopants has an impact on their electrical activation. A lower sheet resistance (Rsh= 8 k) is obtained for samples annealed without capping than for samples annealed with C capping (Rsh= 15 k ).
Authors: Toshiharu Ohnuma, Atsumi Miyashita, Misako Iwasawa, Masahito Yoshikawa, Hidekazu Tsuchida
Abstract: We performed the dynamical simulation of the SiO2/4H-SiC(0001) interface oxidation process using first-principles molecular dynamics based on plane waves, supercells, and the projector augmented wave method. The slab model has been used for the simulation. The heat-and-cool method is used to prepare the initial interface structure. In this initial interface structure, there is no transition oxide layer or dangling bond at the SiO2/SiC interface. As the trigger of the oxidation process, the carbon vacancy is introduced in the SiC layer near the interface. The oxygen molecules are added one by one to the empty sphere in the SiO2 layer near the interface in the simulation of the oxidation process. The molecular dynamics simulation is carried out at 2500 K. The oxygen molecule is dissociated and forms bonds with the Si atom in the SiO2 layer. The atoms of Si in the SiC layer at the SiO2/4H-SiC(0001) interface are oxidized to form the SiO2 layer. Carbon clusters, which are considered one of the candidate structures of the interface traps, are formed in the interface layer. Oxygen molecules react with the carbon clusters and formed CO molecules.
Authors: Alton B. Horsfall, Ming Hung Weng, Rajat Mahapatra, Nicolas G. Wright
Abstract: We present the variation of trap assisted conduction current through a dielectric stack comprising TiO2 and SiO2 on SiC as a function of both temperature and hydrogen gas concentration. We show that the current can be modeled by the use of a single barrier height across the temperature range of interest (>300oC ambient). Upon exposure to hydrogen gas, this barrier height is reduced from 0.405 to 0.325eV, whilst the density of traps in the bulk of the TiO2 remains unaffected. We conclude that the formation of a charge dipole layer under the palladium contact is responsible for this change in barrier height, as has been observed in Schottky diode sensor structures. Further, sensing the gas concentration by monitoring of the trap assisted conduction current appears not to be influenced by the existence of interfacial traps, offering the chance to fabricate low drift sensors for deployment in extreme environments.
Authors: Svetlana Beljakowa, Thomas Frank, Gerhard Pensl, Kun Yuan Gao, Florian Speck, Thomas Seyller
Abstract: An alternative oxidation technique is developed and built up, which provides monatomic oxygen during the whole oxidation process. The set-up consists of a tungsten lamp furnace and a microwave-plasma. A number of different gases can be introduced into the oxidation quartz tube. In addition, an Al2O3-layer is deposited on a part of the oxide layers by atomic layer chemical vapor deposition (ALCVD). First oxidation runs result in encouraging low values of the density of interface states Dit and in the flatband voltage UFB. It turns out that with the present experimental conditions, the comparison of MOS capacitors fabricated with different dielectric layers favors gate dielectrics grown in O2/N2-ambient.
Authors: Hyung Seok Lee, Martin Domeij, Carl Mikael Zetterling, Mikael Östling, Einar Ö. Sveinbjörnsson
Abstract: The effect of the different types of passivation layers on the current gain of SiC BJTs has been investigated. Measurements have been compared for BJTs passivated with thermal SiO2, plasma deposited (PECVD) SiO2 and BJTs without passivation. The maximum DC current gain of BJTs with thermal SiO2 was about 62 at IC=20 mA and Vce=40 V. On the other hand, the BJTs with a passivation by PECVD SiO2 had a DC current gain of only 25. The surface recombination current was extracted from measurements with BJTs of different emitter widths. The surface recombination current of BJTs with a thermally grown oxide was about 25 % lower than unpassivated BJTs and 65 % lower than that of PECVD passivated BJTs.
Authors: Junji Senzaki, Atsushi Shimozato, Kenji Fukuda
Abstract: Acceleration factors in acceleration life test of thermal oxides grown on 4H-SiC(0001) wafers and influences of dislocations on oxide reliability have been investigated using time-dependent dielectric breakdown measurements. The thermal oxides are formed by dry oxidation at 1200°C followed by annealing in nitrogen atmosphere. Then, post oxidation annealing in wet ambient at 950°C or hydrogen atmosphere at 800°C were carried out for some of the oxides. Aluminum or poly-Si films with thickness of 300 nm were formed as gate electrodes. The temperature dependence of time-to-breakdown (tBD) indicates that activation energy (Ea) values for the Al-gate and Poly-Si-gate thermal oxides are 0.59 eV - 0.79 eV and 0.34 eV - 0.72 eV, respectively. Analyzing the electric field dependence of tBD, it was indicated that the values of electric acceleration parameters (β) are 2.7 cm/MV - 7.0 cm/MV and 5.8 cm/MV - 7.1 cm/MV for the Al-gate and poly-Si-gate thermal oxides, respectively. In addition, the charge-to-breakdown decreases with increase in the density of basal plane dislocation.

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