Ryohei Tanuma

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.

Three-dimensional imaging and tilt-angle analysis of dislocations in 4H-SiC by two-photon-excited band-edge photoluminescence

We demonstrate the three-dimensional imaging of threading screw dislocations (TSDs), threading edge dislocations (TEDs), and basal plane dislocations (BPDs) in 4H-SiC using two-photon-excited band-edge photoluminescence. Three-dimensional images of TSDs, TEDs, and BPDs are obtained successfully as dark contrasts on a bright background of band-edge emission. Dislocation images extending ~200 µm from the surface are demonstrated. The tilt angles of TSDs and TEDs in 4H-SiC epilayers are also measured, and the mechanisms governing the line directions of TEDs and TSDs are discussed.

X-ray Microbeam Three-Dimensional Topography Imaging and Strain Analysis of Basal-Plane Dislocations and Threading Edge Dislocations in 4H-SiC

This paper demonstrates the X-ray microbeam three-dimensional (3D) topography of basal-plane dislocations (BPDs) and threading edge dislocations (TEDs) in 4H-SiC. Stereographic images showing the propagation of BPDs from a substrate to an epilayer and the conversion of BPDs into TEDs near the epilayer/substrate interface are successfully obtained. The narrowing of BPD images is observed just before the BPD-TED conversion points. The images of effective misorientations Δω provide a spatial resolution of 1–2 µm for a TED, and the range of Δω corresponds to strains on the order of ±10-5. We also discuss the image-formation mechanism in 3D topography.

Characterization of Screw Dislocations in a 4H-silicon Carbide Diode using X-ray Microbeam Three-dimensional Topography

This paper describes the study of non-hollow-core elementary screw dislocations (SDs) in silicon carbide (SiC) diodes using X-ray microbeam three-dimensional topography. Strain analysis shows that typical screw dislocations having a symmetric strain field tend to cause microplasma breakdown, whereas deformed SDs do not. The symmetry break in SDs will relax the focussing of strain and lessen the formation of defects, thereby leading to the desirable non-leak property.

Three-Dimensional Imaging of Extended Defects in 4H-SiC by Optical Second-Harmonic Generation

This paper demonstrates optical second-harmonic generation (SHG) and two-photon excited photoluminescence (2P-PL) imaging of 3C-SiC inclusions forming triangular and carrot-type defects in 4H-SiC epilayers. Triangular defects exhibit clear SHG images because 3C-SiC is SHG active, but not 4H-SiC host crystal in c-axis incidence. A carrot defect provides SHG and 2P-PL images in different regions in a basal-plane fault area. The spectrums of the SHG and 2P-PL are also investigated, and their emission mechanisms discussed.

X-ray microbeam three-dimensional topography for dislocation strain-field analysis of 4H-SiC

This paper describes the strain-field analysis of threading edge dislocations (TEDs) and basal-plane dislocations (BPDs) in 4H-SiC using x-ray microbeam three-dimensional (3D) topography. This 3D topography enables quantitative strain-field analysis, which measures images of effective misorientations (Δω maps) around the dislocations. A deformation-matrix-based simulation algorithm is developed to theoretically evaluate the Δω mapping. Systematic linear calculations can provide simulated Δω maps (Δωsim maps) of dislocations with different Burgers vectors, directions, and reflection vectors for the desired cross-sections. For TEDs and BPDs, Δω maps are compared with Δωsim maps, and their excellent correlation is demonstrated. Two types of asymmetric reflections, high- and low-angle incidence types, are compared. Strain analyses are also conducted to investigate BPD-TED conversion near an epilayer/substrate interface in 4H-SiC.

Axicon-Focused Noncritically Phase-Matched Second-Harmonic Generation

This paper describes second-harmonic generation where a conical axicon focuses a 1.06-µm YAG-laser-radiated beam into a MgO:LiNbO3 crystal, and a frequency-doubled beam is generated along the cone axis. This method can reduce a 90°-phase-matching temperature to room temperature. We obtained the matching temperatures of 35–36°C and 58–59°C by exchanging two axicons with different base angles, whereas 102°C in their absence.

Three-Dimensional Imaging of Extended Defects in 4H-SiC

This paper describes 3D imaging of extended defects in 4H-SiC using optical second-harmonic generation (SHG) and two-photon-exited photoluminescence (2PPL). SHG selectively yields the 3D images of 3C-inclusions in a 4H-SiC epilayer, while 2PPL provides 3D images of 3C-inclusions, 8H stacking faults and single Shockley stacking faults. 2PPL band-edge emission visualizes dislocation lines of threading screw dislocations and threading edge dislocations, the tilt angles of which are evaluated.

Three-Dimensional Imaging of Extended Defects in 4H-SiC by Two-Photon-Excited Band-Edge Photoluminescence

This paper demonstrates three-dimensional imaging of threading screw dislocations (TSDs) and threading edge dislocations (TEDs) in 4H-SiC using two-photon-excited photoluminescence (2PPL) band-edge emission. Three-dimensional (3D) images of TSDs and TEDs are successfully obtained as dark contrasts on a bright background of band-edge emission. The intensity inversion of a 2PPL 3D image yields a perspective to visually examine the propagation behavior of dislocations. The tilt angles of TEDs are also measured and shown to correlate with the directions of the extra half planes of TEDs.

Three-dimensional imaging of extended defects in 4H-SiC by optical second-harmonic generation and two-photon-excited photoluminescence

We demonstrate three-dimensional (3D) defect imaging in 4H-SiC epilayers by optical second-harmonic generation (SHG) and two-photon-excited photoluminescence (2PPL). The SHG method provides clear 3D images of 3C inclusions because 3C-SiC is SHG-active, but not 4H-SiC host crystal in c-axis incidence. The 2PPL method also yields 3D images of 3C inclusions, 8H stacking faults, and single Shockley stacking faults in the epilayers. The spectra obtained by the SHG and 2PPL imaging methods are investigated, and the emission mechanisms discussed.