We investigated limiting factors of carrier lifetimes and their enhancement by post-growth processes in lightly-doped
p
-type 4H-SiC epitaxial layers (
N
A
∼ 2 × 10
14
cm
−3
). We focused on bulk ...recombination, surface recombination, and interface recombination at the epilayer/substrate, respectively. The carrier lifetime of 2.8
μ
s in an as-grown epilayer was improved to 10
μ
s by the combination of V
C
-elimination processes and hydrogen annealing. By employing surface passivation with deposited SiO
2
followed by POCl
3
annealing, a long carrier lifetime of 16
μ
s was obtained in an oxidized epilayer. By investigating carrier lifetimes in a self-standing
p
-type epilayer, it was revealed that the interface recombination at the epilayer/substrate was smaller than the surface recombination on a bare surface. We found that the V
C
-elimination process, hydrogen annealing, and surface passivation are all important for improving carrier lifetimes in lightly-doped
p
-type epilayers.
The switching behavior of semiconductor devices responds to charge/discharge phenomenon of terminal capacitance in the device. The differential capacitance in a semiconductor device varies with the ...applied voltage in accordance with the depleted region thickness. This study develops a C - V characterization system for high-voltage power transistors (e.g., MOSFET, insulated gate bipolar transistor, and JFET), which realizes the selective measurement of a specified capacitance from among several capacitances integrated in one device. Three capacitances between terminals are evaluated to specify device characteristics-the capacitance for gate-source, gate-drain, and drain-source. The input, output, and reverse transfer capacitance are also evaluated to assess the switching behavior of the power transistor in the circuit. Thus, this paper discusses the five specifications of a C - V characterization system and its measurement results. Moreover, the developed C - V characterization system enables measurement of the transistor capacitances from its blocking condition to the conducting condition with a varying gate bias voltage. The measured C - V characteristics show intricate changes in the low-bias-voltage region, which reflect the device structure. The monotonic capacitance change in the high-voltage region is attributable to the expansion of the depletion region in the drift region. These results help to understand the dynamic behavior of high-power devices during switching operation.
Designing and fabrication of 10-kV 4H-SiC PiN diodes with an improved junction termination structure have been investigated. An improved bevel mesa structure and a single-zone junction termination ...extension (JTE) have been employed to achieve a high breakdown voltage (ges 10 kV). The improved bevel mesa structure, nearly a vertical sidewall at the edge of the p-n junction and a gradual slope at the mesa bottom, has been fabricated by reactive ion etching. The effectiveness of the improved bevel mesa structure has been experimentally demonstrated. The JTE region has been optimized by device simulation, and the JTE dose dependence of the breakdown voltage has been compared with experimental results. A 4H-SiC PiN diode with a JTE dose of 1.1 times 10 13 cm -2 has exhibited a high blocking voltage of 10.2 kV. The locations of electric field crowding and breakdown are also discussed.
The carbon vacancy (V sub(C)) has been suggested by different studies to be involved in the Z sub(1)/Z sub(2) defect-a carrier lifetime killer in SiC. However, the correlation between the Z sub(1)/Z ...sub(2) deep level with V sub(C) is not possible since only the negative carbon vacancy (V super(-) sub(C)) at the hexagonal site, V super(-) sub(C)(h), with unclear negative-U behaviors was identified by electron paramagnetic resonance (EPR). Using freestanding n-type 4H -SiC epilayers irradiated with low energy (250 keV) electrons at room temperature to introduce mainly V sub(C) and defects in the C sublattice, we observed the strong EPR signals of V super(-) sub(C) (h) and another S = 1/2 center. Electron paramagnetic resonance experiments show a negative-U behavior of the two centers and their similar symmetry lowering from C sub(3v) to C sub(1h) at low temperatures. Comparing the Si and C ligand hyperfine constants observed by EPR and first principles calculations, the new center is identified as V super(-) sub(C) (k). The negative-U behavior is further confirmed by large scale density functional theory supercell calculations using different charge correction schemes. The results support the identification of the lifetime limiting Z sub(1)/Z sub(2) defect to be related to acceptor states of the carbon vacancy.
Since we reported the first successful case of allogeneic hematopoietic SCT (allo-HSCT), we have performed allo-HSCT for 29 patients with chronic active EBV infection (CAEBV), using either ...myeloablative conditioning (MAC) allo-HSCT (MAST) or reduced-intensity conditioning (RIC) allo-HSCT (RIST). In this retrospective analysis we compared the outcomes after MAST and RIST to identify the optimal conditioning for patients with CAEBV. Of 29 patients, 11 underwent allo-HSCT with MAC, consisting of TBI (12 Gy), etoposide (900 mg/m²) and CY (120 mg/kg) or melphalan (210 mg/m²), and the remaining 18 patients received allo-HSCT after RIC, consisting of fludarabine (∼ 180 mg/m²) and melphalan (140 mg/m²) or CY (120 mg/kg), with/without antithymocyte globulin and low-dose irradiation. Donor sources were 8 related BM, 2 related peripheral blood, 5 CD34 selected cells from HLA-haploidentical donors, 8 unrelated BM and 8 unrelated cord blood. The 3-year-EFS rate was 54.5 ± 15.0% for MAST group and 85.0 ± 8.0% for RIST group, and the 3-year OS rate was 54.5 ± 15.0% for MAST group and 95.0 ± 4.9% for RIST group (P = 0.016). Allo-HSCT after RIC seems to be a promising approach for the treatment of CAEBV.
PDF
