Takeshi Tawara

Growth of Shockley type stacking faults upon forward degradation in 4H-SiC p-i-n diodes

The growth of Shockley type stacking faults in p-i-n diodes fabricated on the C-face of 4H-SiC during forward current operation was investigated using Berg-Barrett X-ray topography and photoluminescence imaging. After forward current experiment, Shockley type stacking faults were generated from very short portions of basal plane dislocations lower than the conversion points to threading edge dislocations in the epitaxial layer. The growth behavior of Shockley type stacking faults was discussed. Growth of stacking faults in the substrates was not observed.

Analyses of High Leakage Currents in Al+ Implanted 4H SiC pn Diodes Caused by Threading Screw Dislocations

The authors fabricated pn diodes with Al+ implantation in p-type epitaxial layers, and investigated the influence of the implantation dose on reverse leakage currents. Only in the highest dose with the Al concentration of 2x1020cm-3, more than 90% of the devices showed high leakage currents above 10-4A at the maximum electric field of 3MV/cm. In such devices, almost all of the emissive spots corresponded to threading screw dislocations (TSDs) by the analysis of emission microscopy and X-ray topography. These TSDs were defined as killer defects with the estimated density of 500cm-2 in the case of the highest dose. The emissions were supposed to be due to microplasmas, since the spectra of the emissions were different from those of heat radiation. Condensation of Al atoms, nitrogen atoms and DI defects were excluded as the origin of the emissions by secondary ion mass spectrometry and low temperature photoluminescence analyses.

Short minority carrier lifetimes in highly nitrogen-doped 4H-SiC epilayers for suppression of the stacking fault formation in PiN diodes

We investigated the dependency of minority carrier lifetimes on the nitrogen concentration, temperature, and the injected carrier concentration for highly nitrogen-doped 4H-SiC epilayers. The minority carrier lifetimes greatly shortened when the nitrogen concentration exceeded 1018 cm−3 through enhancing direct band-to-band and Auger recombination and showed a slight variation in the temperature range from room temperature (RT) to 250 °C. The epilayer with a nitrogen concentration of 9.3 × 1018 cm−3 exhibited a very short minority carrier lifetime of 38 ns at RT and 43 ns at 250 °C. The short minority carrier lifetimes of the highly nitrogen-doped epilayer were confirmed to maintain the values even after the subsequent annealing of 1700 °C. 4H-SiC PiN diodes were fabricated by depositing a highly nitrogen-doped epilayer as a “recombination enhancing layer” between an n− drift layer free from basal plane dislocations and the substrate. The PiN diodes showed no formation of stacking faults and no increase i...

Shape Control and Roughness Reduction of SiC Trenches by High-Temperature Annealing

The high-temperature annealing of SiC trenches has been investigated for improving the shape of trenches and the smoothness of trench sidewalls. In a SiH4-added Ar (SiH4/Ar) atmosphere, the transformation of SiC trenches required a pressure of 80 Torr and a temperature of 1700 °C; The inner surface of the trenches became smoother without significant etching, while the sample surface became rougher. From the time dependence of the curvature at the trench upper corner, the authors consider that both surface diffusion and evaporation–condensation contribute to the transformation, as opposed to the annealing in H2 reported by another group where transformation is driven mainly by evaporation–condensation. The present authors did not observe a significant change in trench shape in a H2 atmosphere at annealing temperatures up to 1400 °C, except for a smoothening of sample surfaces. The proposed two-step annealing process, consisting of annealing at 1700 °C in SiH4/Ar followed by annealing at 1400 °C in H2, realized rounded trench corners and smooth surfaces simultaneously without significant etching.

High Performance SiC IEMOSFET/SBD Module

SiC power module with low loss and high reliability was developed by utilizing IEMOSFET and SBD. The IEMOSFET is the SiC MOSFET with high channel mobility in which the channel region is the p-type carbon-face epitaxial layer with low acceptor concentration. Elemental technologies for the high channel mobility and the high reliability of the gate oxide have been developed to realize the excellent characteristics by the IEMOSFET. The SBD was designed so as to minimize the forward voltage drops and the reverse leakage current. For the fabrication of these SiC power devices, the mass production technology such as gate oxidation, ion implantation and following activation annealing have been also developed.

Vanadium doping in 4H-SiC epitaxial growth for carrier lifetime control

We demonstrate controlled vanadium doping in 4H-SiC epitaxial growth, aimed at reducing the carrier lifetime in the epitaxial layers (epilayers), toward quenching the injection of minority carriers from the drift layer into the substrate in the forward operation of bipolar devices. The doping efficiency of vanadium and the quality of the epilayers were investigated for different gas systems and growth conditions. The photoluminescence spectra and decay curves of band-edge luminescence were evaluated for nitrogen- and vanadium-doped epilayers. The epilayers doped with nitrogen and vanadium demonstrated much shorter minority carrier lifetimes (<20 ns) compared with the epilayer doped with nitrogen only.

Injected carrier concentration dependence of the expansion of single Shockley-type stacking faults in 4H-SiC PiN diodes

We investigated the relationship between the dislocation velocity and the injected carrier concentration on the expansion of single Shockley-type stacking faults by monitoring the electroluminescence from 4H-SiC PiN diodes with various anode Al concentrations. The injected carrier concentration was calculated using a device simulation that took into account the measured accumulated charge in the drift layer during diode turn-off. The dislocation velocity was strongly dependent on the injected hole concentration, which represents the excess carrier concentration. The activation energy of the dislocation velocity was quite small (below 0.001 eV between 310 and 386 K) over a fixed range of hole concentrations. The average threshold hole concentration required for the expansion of bar-shaped single Shockley-type stacking faults at the interface between the buffer layer and the substrate was determined to be 1.6–2.5 × 1016 cm−3 for diodes with a p-type epitaxial anode with various Al concentrations.

Anisotropic Transformation of 4H-SiC Etching Shapes by High-Temperature Annealing and Its Enhancement by Ion Implantation

The transformation of 4H-SiC etching shapes by high-temperature annealing was investigated. Although the opening of the etching mask was circular, the resulting etched shape was a hexagon, dodecagon, or rounded polygon with more edges, depending on the diameter. A hexagon was transformed into a dodecagon following high-temperature annealing, and a dodecagon was transformed into a rounded polygon.

Suppression of the forward degradation in 4H-SiC PiN diodes by employing a recombination-enhanced buffer layer

Application of highly N-doped buffer layers or a (N+B)-doped buffer layer to PiN diodes to suppress the expansion of Shockley stacking faults (SSFs) from the epilayer/substrate interface was studied. These buffer layers showed very short minority carrier lifetimes of 30–200 ns at 250°C. The PiN diodes were fabricated with buffer layers of various thicknesses and were then tested under high current injection conditions of 600A/cm2. The thicker buffer layers with shorter minority carrier lifetimes demonstrated the suppression of SSFs expansion and thus that of diode degradation.

Carrier lifetime control of 4H-SiC epitaxial layers by boron doping

An epitaxial growth technique for 4H-SiC with B doping was developed to control the carrier lifetimes of the epilayers. A linear relationship was observed between the B doping concentration and the flow rate of tri-ethyl-boron, which was used as the B doping source. A room temperature photoluminescence spectrum of a N-and B-doped epilayer showed a broad B-related peak at 2.37 eV instead of a band-edge luminescence, which indicates that the carrier recombination path was changed by the B doping. The minority carrier lifetime decreased (< 30 ns at 250°C) with increasing B doping concentration. The thermal stability of the short carrier lifetime was compared with a conventional carrier lifetime reduction method, namely an electron irradiation technique. After thermal annealing at 1700°C, the carrier lifetime of the electron irradiated epilayer recovered while that of the B-doped epilayer remained, indicating that the carrier lifetime controlled by the B doping technique was more stable against the thermal processes.