Takasumi Ohyanagi

Electron-beam-induced current study of stacking faults and partial dislocations in 4H-SiC Schottky diode

Electrical properties of stacking faults and bounding partial dislocations in 4H-SiC Schottky diode were investigated by using electron-beam-induced current (EBIC) and cathodoluminescence (CL) techniques. EBIC images show that basal plane dislocation is easily dissociated into two partial dislocations [Si(g) 30° and C(g) 30° partials], with a stacking fault between them. The EBIC contrast of C(g) 30° partial is always several percent higher than that of Si(g) 30° partial. The stacking fault is brighter than the background, having the negative EBIC contrast. CL spectrum shows that a new peak (417nm) appears at stacking fault position. The origin of bright stacking fault in EBIC image is discussed according to its quantum-well state.

Time-and-space resolved X-ray absorption spectroscopy of laser-ablated Si particles

We have performed time-and-space resolved X-ray absorption spectroscopy with a time resolution of 10 ns to study laser-ablated Si particles in a time scale ranging from 0 ns to 120 ns. Neutral and charged particles produced by laser ablation are observed through X-ray absorption spectra. Assignments of transitions from 2s and 2p initial states to higher Rydberg states of Si atom and ions are achieved, and we experimentally determine the LI \kern-1ptI,I \kern-1ptI \kern-1ptI absorption edges of neutral Si atom ( Si0) and Si ions such as Si+, Si2+, Si3+ and Si4+. The main ablated particles are found to be Si atom and Si ions in the initial stage of 0 ns to 120 ns. The relative amounts depend strongly on times and laser energy densities. We find that the spatial distributions of particles produced by laser ablation are changed with supersonic helium gas bombardment, but no cluster formation takes place. This suggests that a higher-density region of helium gas is formed at the top of the plume of ablated particles, and free expansion of particles is restrained by this helium cloud, and that it takes more than 120 ns to form Si clusters.

Electron-beam-induced current and cathodoluminescence study of dislocation arrays in 4H-SiC homoepitaxial layers

The electrical and optical properties of dislocation arrays (DAs) in 4H-SiC homoepitaxial layers were studied by using electron-beam-induced current (EBIC) and cathodoluminescence (CL) techniques. EBIC observations show that under electron-beam irradiation, the DAs are easily dissociated to form rhombic stacking faults (SFs), whereas the single threading dislocations are not. CL results demonstrate that a new peak (417 nm) appears at the formed SFs, which is the same as the phenomenon observed from dissociating basal plane dislocations. The dissociation mechanisms of DAs are proposed based on the assumption that small basal segments exist. The dissociation velocity of each dislocation in the DAs is discussed according to its recombination activity.

Ti∕Ni bilayer Ohmic contact on 4H-SiC

Electric contact properties and interface structures of titanium/nickel (Ti∕Ni) bilayer Ohmic contact and the aluminum (Al) overlay on 4H silicon carbide (4H-SiC) were investigated. The Al overlay was used for wiring and bonding pads to pass high currents, and usually deposited after the high temperature annealing of the Ohmic contact. We found that the adhesion between the Ti∕Ni Ohmic contact and the Al overlay was improved compared to that of the Ni Ohmic contact and the Al overlay. Auger electron spectroscopy depth profiles showed that in the case of the Ti∕Ni Ohmic contacts, titanium carbide (TiC) exists at the interface between Ni silicide and the Al overlay. This TiC leads the Al to diffuse into the Ni-silicide layer, and the adhesion between Ni silicide and the Al is improved. Moreover, the authors compared the contact resistance before/after 500°C annealing after the Al deposition, and the authors found that the contact resistance was affected by the interface carbon structures.

Superlattice Phase Change Memory Fabrication Process for Back End of Line Devices

The superlattice film with the periodical thin film layers of Sb2Te3/GeTe used as a phase change memory was studied for deposition in the crystal phase. We successfully fabricated the superlattice structure with the sputtering temperature of 200 °C. Moreover, the pillar structure with the size of 70 nm was dry-etched using a HBr/Ar gas mixture.

Pinning of recombination-enhanced dislocation motion in 4H–SiC: Role of Cu and EH1 complex

We report on the pinning of recombination-enhanced dislocation motion in 4H–SiC by the implantation of Cu. The Cu was found to be preferentially gettered at basal plane dislocations (BPDs). Both EH1 and Z1/2 center were detected in 4H–SiC by cathodoluminescence. It was noticed that the EH1 has high luminescence intensity at the central part of the BPDs, while the Z1/2 does not. The complex of Cu and EH1 is regarded to be the cause for the pinning effect. The possible reason for the pinning is discussed.

Influence of Processing and of Material Defects on the Electrical Characteristics of SiC-SBDs and SiC-MOSFETs

The influences of processing and material defects on the electrical characteristics of large-capacity (approximately 100A) SiC-SBDs and SiC-MOSFETs have been investigated. In the case of processing defects, controlled activation annealing is the most important factor. On the other hand for material defects, the number of epitaxial defects must be decreased to zero for both SBDs and MOSFETs. The dislocation defects in SiC wafers are dangerous for the breakdown voltage of MOSFETs. However, they are not killer defects. If the epitaxial defect density is sufficiently low and the dislocation density is in the order of 10000cm-2, the long- term reliability of the gate oxide at the electric field of 3MV/cm can be guaranteed.

Shallow Donor Clusters in InP

Electronic states of a shallow donor in InP are investigated as functions of donor concentration (N D–N A) from 4.0×1015 to 5.5×1016/cm3 by electron spin resonance (ESR) at low temperatures. The Overhauser shift, nuclear spin-lattice relaxation time T 1N, and g-value are found to strongly depend on N D–N A; i.e., T 1N has a maximum at around 7×1015/cm3, while the g-value increases with increasing donor concentration, ranging from 1.2024 to 1.2143. Sweeping the magnetic field at appropriate speeds yields significantly inhomogeneous broadening of the ESR line, indicating that the ESR comes from bound donor electrons at least below N D–N A of 1.5×1016/cm3. A model of donor clusters is proposed to interpret the obtained results. The donor-concentration dependence of the g-values indicates that the donor states are significantly changed with the average donor-cluster size.

Influence of Surface Roughness on Breakdown Voltage of 4H-SiC SBD with FLR Structure

The Ti/4H-SiC Schottky barrier diodes with a field limiting ring (FLR) structure are fabricated. Two types of SBDs are prepared; one (SBD-A) is covered and another (SBD-B) isn’t covered with a carbon cap during high temperature annealing after ion implantation. The breakdown voltage at room temperature for SBD-A and SBD-B are 1400 V and 1000 V, respectively. The breakdown for both SBDs occurs due to an avalanche breakdown. The light emission images are obtained at the breakdown voltage by photo emission microscope (PEM). The light emission is observed along an FLR of the SBD-A as designed. On the other hand, the spot of light emission is observed on a FLR structure of the SBD-B. This light emission spot indicates that leakage current is concentrated because an electrical field concentration is generated at this one for the SBD-B. The root-mean-square roughness of the Al-implanted region on the FLR structure calculated from the atomic force microscopy (AFM) images for the SBD-A and the SBD-B are 0.697 nm and 5.58 nm, respectively. Therefore it is considered that large surface roughness on the FLR decreases breakdown voltage of SBD because an electrical field concentration is generated at a spot.

EBIC Analysis of Breakdown Failure Point in 4H-SiC PiN Diodes

The breakdown failure points in the 4H-SiC PiN diodes were analyzed by the electron beam induced current (EBIC). We focused on the failure, which showed the avalanche breakdown, and we determined the failure points by an emission microscopy. We observed the basal plane dislocation around the failure point and at measured temperatures below 200K we found the dark spots in the EBIC. However, in the X-ray topography image, no spots were found around the dislocations. We therefore think that these spots originated from the metal contamination. The electric field was multiplied due to a permittivity change, and this multiplication caused the avalanche breakdown.