Takafumi Okuda

21-kV SiC BJTs With Space-Modulated Junction Termination Extension

We report here 20-kV-class small-area (0.035 mm2) 4H-SiC bipolar junction transistors. We implemented edge termination techniques featuring two-zone junction termination extension and space-modulated rings. On-state characteristics showed a current gain of 63 and a specific on-resistance of 321 mΩ·cm2, which is slightly below the SiC unipolar limit. We achieved the open-base blocking voltage of 21 kV at a leakage current of 0.1 mA/cm2, which is the highest blocking voltage among any semiconductor switching devices.

Interface Properties of 4H-SiC (

Interface properties of 4H-SiC (112̅0) and (11̅00) walls, the absence of junction FET resistance, and the higher metal-oxide-semiconductor (MOS) structures annealed in nitric oxide are characterized by conductance, high-low, and C-ψ_s methods. Compared with 4H-SiC (0001) MOS structures, generation of very fast interface states by nitridation is much smaller in 4H-SiC (112̅0) and (11̅00). The effective mobility of planar MOSFETs fabricated on Al+-implanted p-body doped to 1×10¹⁷ cm^-3 is 10³ cm²/Vs on (11̅00), 92 cm²/Vs on (112̅0), and 20 cm²/Vs on (0001). The mobility-limiting factors are discussed on the basis of experimental results. The high channel mobilities for (112̅0) and (11̅00) MOSFETs can be correlated with the lower density of fast interface states generated by nitridation.

11\bar {2}0

We investigated the enhancement of carrier lifetime in lightly Al-doped p-type 4H-SiC epilayers (NA ≃ 2 × 1014 cm−3) by postgrowth processing. A carrier lifetime of 2.8 µs in an as-grown epilayer is increased to 5.1 µs by carbon vacancy elimination, i.e., thermal oxidation at 1400 °C for 48 h. It reaches 10 µs by subsequent hydrogen annealing at 1000 °C for 10 min. The carrier lifetime in the as-grown epilayer is also increased to 4.0 µs by only hydrogen annealing. These results suggest that, in addition to carbon vacancy, there is another lifetime killer in p-type SiC, which cannot be eliminated by thermal oxidation but can be passivated by hydrogen annealing.

) and (

Carrier lifetimes in a highly Al-doped p-type epilayer (NA = 1×1018 cm-3) are investigated by differential microwave photoconductance decay (µ-PCD) measurements. A carrier lifetime of 310 ns in the as-grown p-type epilayer decreases to 90 ns by thermal treatment in Ar, O2, or N2 atmospheres (>700 °C), and recovers to 300 ns by H2 annealing (>750 °C). Hydrogen is detected at a concentration of (2–3)×1015 cm-3 in the H2-annealed epilayer. These results suggest that a lifetime killer exists in the p-type epilayer, limiting the carrier lifetime to 90 ns and is passivated by hydrogen annealing, resulting in the improved carrier lifetime of 300 ns.

1\bar {1}00

Interface properties of heavily Al-doped 4H-SiC ( 0001) (Si-face), ( 112¯0) (a-face), and ( 11¯00) (m-face) metal-oxide-semiconductor (MOS) structures were characterized from the low-temperature gate characteristics of metal-oxide-semiconductor field-effect transistors (MOSFETs). From low-temperature subthreshold slopes, interface state density (Dit) at very shallow energy levels (ET) near the conduction band edge (Ec) was evaluated. We discovered that the Dit near Ec (Ec − 0.01 eV < ET < Ec) increases in MOS structures with higher Al doping density for every crystal face (Si-, a-, and m-face). Linear correlation is observed between the channel mobility and Dit near Ec, and we concluded that the mobility drop observed in heavily doped MOSFETs is mainly caused by the increase of Dit near Ec.

) MOS Structures Annealed in NO

We investigated surface passivation on 4H-SiC epitaxial layers with deposited or thermally grown SiO2 followed by POCl3 annealing. The measured carrier lifetime in a p-type epilayer with deposited SiO2 was limited to 0.5 µs and it was improved to 3.0 µs after POCl3 annealing. In an n-type epilayer, a measured carrier lifetime of 5.8 µs was improved to 12 µs after POCl3 annealing. We found a clear relationship between the measured carrier lifetime and the interface state density at SiO2/n-SiC after POCl3 annealing, suggesting that the reduction in interface state density lowered the surface recombination velocity on the 4H-SiC.

Enhancement of carrier lifetime in lightly Al-doped p-type 4H-SiC epitaxial layers by combination of thermal oxidation and hydrogen annealing

The impacts of the ambipolar diffusion constant and surface recombination in carrier lifetime measurements of p- and n-type 4H-SiC epilayers are investigated in detail by comparing a numerical simulation based on a diffusion equation and the measurement of microwave photoconductance decay (µ-PCD) curves measured from 4H-SiC epilayers. The simulation reveals that the shapes of decay curves of excess carrier concentration in epilayers, which defines the effective carrier lifetime, are different between p- and n-type 4H-SiC under a low-level injection condition, even when the bulk lifetime and the surface recombination velocity are fixed to the same values for p- and n-type epilayers. In experiments, the shapes of the microwave photoconductance decay curves measured from p- and n-type 4H-SiC epilayers show a similar tendency to the simulation results under a low-level injection condition. This is attributed to the difference in the dependence of the ambipolar diffusion constant on the excess carrier concentration for p- and n-type 4H-SiC. The comparison of µ-PCD decay curves obtained from 50-µm-thick epilayers with different surface passivation indicates that the surface recombination velocity on the epilayer passivated with deposited SiO2 followed by NO annealing is about one order of magnitude lower than that of the epilayer passivated with the dry oxide.

Improvement of Carrier Lifetimes in Highly Al-Doped p-Type 4H-SiC Epitaxial Layers by Hydrogen Passivation

The Hall scattering factor (γH) in p-type 4H-SiC with various aluminum doping concentrations of 5.8 × 1014–7.1 × 1018 cm−3 was investigated from 300 to 900 K. γH was determined by comparing the Hall coefficient with the theoretical carrier concentration derived from acceptor and donor concentrations obtained from secondary ion mass spectrometry and capacitance–voltage measurements. γH decreased with increasing temperature or doping concentration; γH = 1–0.4 for the doping concentration of 5.8 × 1014 cm−3 and γH = 0.5–0.2 for the doping concentration of 7.1 × 1018 cm−3. The dependence might be caused by the anisotropic and nonparabolic valence band structure of 4H-SiC.

Interface state density of SiO2/p-type 4H-SiC (0001), (112¯0), (11¯00) metal-oxide-semiconductor structures characterized by low-temperature subthreshold slopes

We investigated the photoconductivity decay characteristics of p-type 4H-SiC bulk crystals by differential microwave photoconductance decay (µ-PCD) measurements using a 349-nm laser as an excitation source. The decay time at room temperature was 2600 µs, which is much longer than that of n-type 4H-SiC bulk crystals (40 ns). Decay time decreased with increasing temperature, resulting in 120 µs at 250 °C, and the activation energy of decay time was determined to be 140±10 meV. Long decay characteristics were also observed by below-band-gap excitation at 523 nm.

Surface passivation on 4H-SiC epitaxial layers by SiO2 with POCl3 annealing

The temperature dependence of current gain from 140 to 460 K in 4H-SiC bipolar junction transistors (SiC BJTs) was investigated. The current gain increased from 110 to 1200 with decreasing temperature from 460 to 200 K. The high current gain at the low temperature can be ascribed to the enhanced incomplete ionization of aluminum acceptors in the base layer, resulting in the increase in injection efficiency. However, when the temperature was further reduced from 200 to 140 K, the current gain decreased from 1200 to 515, which is caused by high injection condition in the base layer, because of a very low hole concentration below 200 K.