Koutarou Kawahara

Detection and depth analyses of deep levels generated by ion implantation in n- and p-type 4H-SiC

The authors investigated deep levels in the whole energy range of bandgap of 4H-SiC, which are generated by low-dose N+, P+, and Al+ implantation, by deep level transient spectroscopy (DLTS). Ne+-implanted samples have been also prepared to investigate the pure implantation damage. In the n-type as-grown material, the Z1∕2 (EC−0.63eV) and EH6∕7 (EC−1.6eV) centers are dominant deep levels. At least, seven peaks (IN1, IN3–IN6, IN8, and IN9) have emerged by implantation and annealing at 1000°C in the DLTS spectra from all n-type samples, irrespective of the implanted species. After high-temperature annealing at 1700°C, however, most DLTS peaks disappeared, and two peaks, IN3 and IN9, which may be assigned to Z1∕2 and EH6∕7, respectively, survive with a high concentration over the implanted atom concentration. In the p-type as-grown material, the D (EV+0.40eV) and HK4 (EV+1.4eV) centers are dominant. Two peaks (IP1 and IP3) have emerged by implantation and annealing at 1000°C, and four traps IP2 (EV+0.39eV), ...

Analytical model for reduction of deep levels in SiC by thermal oxidation

Two trap-reduction processes, thermal oxidation and C+ implantation followed by Ar annealing, have been discovered, being effective ways for reducing the Z1/2 center (EC – 0.67 eV), which is a lifetime killer in n-type 4H-SiC. In this study, it is shown that new deep levels are generated by the trap-reduction processes in parallel with the reduction of the Z1/2 center. A comparison of defect behaviors (reduction, generation, and change of the depth profile) for the two trap-reduction processes shows that the reduction of deep levels by thermal oxidation can be explained by an interstitial diffusion model. Prediction of the defect distributions after oxidation was achieved by a numerical calculation based on a diffusion equation, in which interstitials generated at the SiO2/SiC interface diffuse to the SiC bulk and occupy vacancies related to the origin of the Z1/2 center. The prediction based on the proposed analytical model is mostly valid for SiC after oxidation at any temperature, for any oxidation tim...

Carrier Recombination in n-Type 4H-SiC Epilayers with Long Carrier Lifetimes

A longest carrier lifetime of 33.2 µs was achieved by eliminating the Z1/2 center via thermal oxidation at 1400 °C for 48 h and subsequent surface passivation with a nitrided oxide on a 220-µm-thick n-type 4H-SiC epilayer. By deep-level elimination, photoluminescence (PL) in the infrared region (wavelength: 700–950 nm) was remarkably enhanced at locations of threading dislocations. A threading screw dislocation exhibited much stronger infrared PL than a threading edge dislocation. The present results indicate that carrier recombination at extended defects becomes pronounced through the elimination of the Z1/2 center in the epilayers.

Deep levels induced by reactive ion etching in n- and p-type 4H–SiC

In this study, the authors investigate deep levels, which are induced by reactive ion etching (RIE) of n-type/p-type 4H–SiC, by deep level transient spectroscopy (DLTS). The capacitance of a Schottky contact fabricated on as-etched p-type SiC is abnormally small due to compensation or deactivation of acceptors extending to a depth of ∼14 μm, which is nearly equal to the epilayer thickness. The value of the capacitance can recover to that of a Schottky contact on as-grown samples after annealing at 1000 °C. However, various kinds of defects, IN2 (EC−0.30 eV), EN (EC−1.6 eV), IP1 (EV+0.30 eV), IP2 (EV+0.39 eV), IP4 (HK0: EV+0.72 eV), IP5 (EV+0.85 eV), IP7 (EV+1.3 eV), and EP (EV+1.4 eV), remain at a high concentration (average of total defect concentration in the region ranging from 0.3 μm to 1.0 μm:∼5×1014 cm−3) even after annealing at 1000 °C. The concentration of all these defects generated by RIE, except for the IP4 (HK0) center, remarkably decreases by thermal oxidation. In addition, the HK0 center can...

Investigation on origin of Z(1/2) center in SiC by deep level transient spectroscopy and electron paramagnetic resonance

The Z(1/2) center in n-type 4H-SiC epilayers-a dominant deep level limiting the carrier lifetime-has been investigated. Using capacitance versus voltage (C-V) measurements and deep level transient ...

Major deep levels with the same microstructures observed in n-type 4H-SiC and 6H-SiC

Major deep levels observed in as-grown and irradiated n-type 4H–SiC and 6H–SiC epilayers have been investigated. After low-energy electron irradiation, by which only carbon atoms are displaced, five traps, EH1 (EC−0.36 eV), Z1/Z2 (EC−0.65 eV), EH3 (EC−0.79 eV), EH5 (EC−1.0 eV), and EH6/7 (EC−1.48 eV), were detected in 4H–SiC and four traps, E1/E2 (EC−0.45 eV), RD5 (EC−0.57 eV), ES (EC−0.80 eV), and R (EC−1.25 eV), were detected in 6H–SiC. The Z1/Z2, EH6/7 centers in 4H–SiC and the E1/E2, R centers in 6H–SiC exhibit common features as follows: their generation rates by the e−-irradiation were almost the same each other, their concentrations were not changed by heat treatments up to 1500 °C, and they showed very similar annealing behaviors at elevated temperatures. Furthermore, these defect centers were almost eliminated by thermal oxidation. Taking account of the observed results and the energy positions, the authors suggest that the Z1/Z2 center in 4H–SiC corresponds to the E1/E2 center in 6H–SiC, and the...

Reduction of deep levels generated by ion implantation into n- and p-type 4H-SiC

The authors have investigated effects of thermal oxidation on deep levels in the whole energy range of the band gap of 4H–SiC by deep level transient spectroscopy. The deep levels are generated by ion implantation. The dominant defects in n-type samples after ion implantation and high-temperature annealing at 1700 °C are IN3 (Z1/2: EC−0.63 eV) and IN9 (EH6/7: EC−1.5 eV) in low-dose-implanted samples, and IN8 (EC−1.2 eV) in high-dose-implanted samples. These defects can remarkably be reduced by thermal oxidation at 1150 °C. In p-type samples, however, IP8 (HK4: EV+1.4 eV) survives and additional defects such as IP4 (HK0: EV+0.72 eV) appear after thermal oxidation in low-dose-implanted samples. In high-dose-implanted p-type samples, three dominant levels, IP5 (HK2: EV+0.85 eV), IP6 (EV+1.0 eV), and IP7 (HK3: EV+1.3 eV), are remarkably reduced by oxidation at 1150 °C. The dominant defect IP4 observed in p-type 4H–SiC after thermal oxidation can be reduced by subsequent annealing in Ar at 1400 °C. These pheno...

Quantitative comparison between Z1∕2 center and carbon vacancy in 4H-SiC

In this study, to reveal the origin of the Z1∕2 center, a lifetime killer in n-type 4H-SiC, the concentrations of the Z1∕2 center and point defects are compared in the same samples, using deep level transient spectroscopy (DLTS) and electron paramagnetic resonance (EPR). The Z1∕2 concentration in the samples is varied by irradiation with 250 keV electrons with various fluences. The concentration of a single carbon vacancy (VC) measured by EPR under light illumination can well be explained with the Z1∕2 concentration derived from C-V and DLTS irrespective of the doping concentration and the electron fluence, indicating that the Z1∕2 center originates from a single VC.

Deep levels generated by thermal oxidation in p-type 4H-SiC

Thermal oxidation is an effective method to reduce deep levels, especially the Z1∕2-center (EC−0.67 eV), which strongly suppresses carrier lifetimes in n-type 4H-SiC epilayers. The oxidation, however, simultaneously generates other deep levels, HK0 (EV+0.79 eV) and HK2 (EV+0.98 eV) centers, within the lower half of the bandgap of SiC, where the HK0 center is a dominant deep level with a concentration of about 1×1013 cm−3 after oxidation. By comparing deep levels observed in three sets of p-type 4H-SiC: oxidized, electron-irradiated, and C+- or Si+-implanted samples, we find that the HK0 and HK2 centers are complexes including carbon interstitials such as the di-carbon interstitial or di-carbon antisite. Other defects observed in p-type 4H-SiC after electron irradiation or after C+/Si+ implantation are also studied.

Defect Control in Growth and Processing of 4H-SiC for Power Device Applications

Extended defects and deep levels generated during epitaxial growth of 4H-SiC and device processing have been reviewed. Three types in-grown stacking faults, (6,2), (5,3), and (4,4) structures, have been identified in epilayers with a density of 1-10 cm-2. Almost all the major deep levels present in as-grown epilayers have been eliminated (< 1x1011 cm-3) by two-step annealing, thermal oxidation at 1150-1300oC followed by Ar annealing at 1550oC. The proposed two-step annealing is also effective in reducing various deep levels generated by ion implantation and dry etching. The interface properties and MOSFET characteristics with several gate oxides are presented. By utilizing the deposited SiO2 annealed in N2O at 1300oC, a lowest interface state density and a reasonably high channel mobility for both n- and p-channel MOSFETs with an improved oxide reliability have been attained.