High-purity Semi-insulating 4H-SiC Research Articles

Measurements of optical cross sections of the carbon vacancy in 4H-SiC by time-dependent photoelectron paramagnetic resonance

Time-dependent photoinduced electron paramagnetic resonance measurements have been made on high purity semi-insulating 4H-SiC to develop a more complete understanding of the optical transitions of the positively charged carbon vacancy VC+. The single defect model originally proposed is given validity by demonstrating that the time dependence of the photoinduced changes in VC+ may be fitted by a first order kinetic process. In addition, the photon energy dependence of the optical cross sections for capture and emission of electrons from VC+ is extracted by incorporating both processes into one expression for charge transfer. The data are interpreted by considering the role of the electronic density of states as well as participation of phonons. Analysis assuming only phonon participation yields thermal and optical energies of 1.6 and 2.15 eV, respectively, for charge transitions between VC+ and one of the band edges. Charge transfer between VC+ and the opposite band edge is associated with a thermal and an optical energy of 1.9 and 2.45 eV, respectively. An upper limit for the Franck–Condon shift of 0.55 eV is extracted from the difference between the thermal and optical energies.

Characterization of deep centers in semi-insulating SiC and HgI2: Application of discharge current transient spectroscopy

In order to characterize traps in semi-insulating 4H-SiC and HgI2 regarded as an attractive semiconductor for X-ray detectors, we apply discharge current transient spectroscopy (DCTS), which is a graphical peak analysis method based on the transient reverse current of a diode. In high-purity semi-insulating 4H-SiC whose diode may detect X-rays or γ-rays, DCTS can detect two types of trap species, and can determine those densities and emission rates. The emission rates of detected traps are close to each other, suggesting that thermally stimulated current (TSC) may not distinguish between these two types of traps. In semi-insulating HgI2, the densities and emission rates of two types of trap species are also determined by DCTS. Therefore, it is demonstrated that DCTS is a powerful method for determining the densities and emission rates of traps in semi-insulating semiconductors

Influence of Cooling Rate after High Temperature Annealing on Deep Levels in High-Purity Semi-Insulating 4H-SiC

The influence of different cooling rates on deep levels in 4H-SiC after high temperature annealing has been investigated. The samples were heated from room temperature to 2300°C, followed by a 20 minutes anneal at this temperature. Different subsequent cooling sequences down to 1100°C were used. The samples have been investigated using photoluminescence (PL) and IV characteristics. The PL intensities of the silicon vacancy (VSi) and UD-2, were found to increase with a faster cooling rate.

Evaluation of Unintentionally Doped Impurities in Silicon Carbide Substrates Using Neutron Activation Analysis

Impurity atoms in a high-purity semi-insulating 4H-SiC substrate fabricated by sublimation and an n-type 3C-SiC substrate fabricated by Chemical Vapor Deposition (CVD) were evaluated by neutron activation analysis. Cr, Fe, Zn, As, Br, Mo, Sb, Eu, Yb, Hf, Ta, W and Au atoms were detected in the 4H-SiC fabricated by sublimation. In the 3C-SiC fabricated by CVD, Cr, Zn, As, Br, Mo, Sb, La Sm and Hf atoms were found. The concentration of these atoms tends to decrease with increasing atomic number.

Prominent defects in semi-insulating SiC substrates

Progresses in defects characterization of high-purity semi-insulating (HPSI) SiC substrates are reviewed. The vacancies, divacancy, and carbon vacancy–carbon antisite pairs were found to be dominating defects in HPSI 4H-SiC substrates with the thermal activation energy E a in the range ∼0.6–1.6 eV. Several vacancy-related deep acceptor levels were estimated and suggested to be responsible for the semi-insulating (SI) behavior in substrates with different E a. Deep levels associated with different activation energies are suggested. Annealing behavior of vacancy-related defects and the stability of the SI properties were studied. Prominent defects in SI 4H-SiC substrates doped with vanadium (V) were identified. Activation energies of E a∼1–1.1 eV were observed for V-doped samples with the V concentration ranging from 5.5×10 15 to 1.1×10 17 cm −3. Our results show that only in heavily V-doped samples, the activation energy E a∼1 eV is related to V, whereas in moderate V-doped materials, the SI properties are determined by intrinsic defects.

Effects of Electron Irradiation on Deep Centers in High-Purity Semi-Insulating 6H-SiC

Many point-defect–related centers have been investigated in electron-irradiated 6H-SiC by deep-level transient spectroscopy (DLTS). Most of them are believed to be related to vacancies. Our DLTS studies on deep centers produced by electron-irradiation (EI) in conductive epi-6H-SiC are in agreement with the literature data. However, for semi-insulating SiC, DLTS cannot be used for trap studies so we have applied thermally stimulated current (TSC) spectroscopy. At least nine TSC traps have been observed in high-purity/semi-insulating (HPSI) 6H-SiC. To understand the nature of these centers, 1-MeV EI and postirradiation annealing at 600°C were applied to the sample. The TSC spectroscopy and 4.2 K photoluminescence (PL) have been used to study the effects of EI and annealing on the centers in HPSI 6H-SiC. It was found that (1) some of the major EI-induced DLTS centers in conductive 6H-SiC, such as ED1, E1/E2, Ei, Z1/Z2, and L9, have TSC counterparts even in as-grown HPSI 6H-SiC; (2) EI-induced TSC centers in HPSI 6H-SiC are due to point defects, which have been confirmed by typical PL lines (such as S, L, and V lines); and (3) the concentration of the 1.1-eV center, which controls material conductivity, can be increased by 1-MeV EI and decreased by 600°C annealing.

Compensation mechanism in high purity semi-insulating 4H-SiC

A study of deep levels in high purity semi-insulating 4H-SiC has been made using temperature dependent Hall effect (TDH), thermal and optical admittance spectroscopies, and secondary ion mass spectrometry (SIMS). Thermal activation energies from TDH varied from a low of 0.55eV to a high of 1.65eV. All samples studied showed n-type conduction with the Fermi level in the upper half of the band gap. Fits of the TDH data to different charge balance equations and comparison of the fitting results with SIMS measurements indicated that the deep levels are acceptorlike even though they are in the upper half of the band gap. Carrier concentration measurements indicated that the deep levels are present in concentrations in the low 1015cm−3 range, while SIMS results demonstrate nitrogen and boron concentrations in the low to mid-1015-cm−3 range. The results suggest that compensation in this material is a complex process involving multiple deep levels.

Possible Role of Hydrogen within the So-Called X Center in Semi-Insulating 4H-SiC

Multi-frequency electron paramagnetic resonance (EPR) measurements between 9 and 140 GHz as well as Hall measurements were performed on a series of nominally undoped high-purity semi-insulating (HPSI) 4H-SiC. The investigations in a temperature range from 4 K to 300 K were focused on the photosensitive intrinsic X-defect residing at two inequivalent lattice sites (X h and X k ). Photo-EPR and Hall effect measurements indicate donor-like energy levels at 1.26 ± 0.06 eV and 1.36 ± 0.06 eV below the conduction band for X h and X k , respectively. The EPR spectra recorded at 77 K and at 9 GHz consist of superimposed EPR lines, which were characterized by axially symmetric g-tensors and resolved hyperfine (hf) lines. The hf interactions determined are identical to those published earlier in two independent works for the carbon vacancy in a single positive charge state. However, it is a surprising fact that for one hf line the intensity ratio is different in all works. With the help of additional theoretical investigations, these variations are tentatively assigned to the contamination of a sample-dependent fraction of carbon vacancies with hydrogen.

Deep Traps in High-Purity Semi-Insulating 6H-SiC Substrates: Thermally Stimulated Current Spectroscopy

Thermally stimulated current spectroscopy (TSC) has been applied to characterize deep traps in high-purity semi-insulating 6H-SiC substrates. By using above bandgap to sub-bandgap light for illumination at 83 K and different applied biases, at least nine TSC traps in the temperature range of 80 to 400 K can be consistently observed. It is found that TSC peaks for T < 130 K are significantly affected by light and some peaks are strongly enhanced by the applied bias. Measured trap activation energies range from 0.15 eV to 0.76 eV. Theoretical fittings of selected traps give more accurate trap parameters. Based on literature results connected with deep traps in conductive 6H-SiC, the origin of these TSC traps is discussed.

Crystal growth of micropipe free 4H–SiC on 4H–SiC [formula omitted] seed and high-purity semi-insulating 6H–SiC

Micropipe free c-plane 4H–SiC wafers were achieved by sublimation growth on the 4H–SiC { 0 3 3 ¯ 8 } seed. 4H–SiC { 0 3 3 ¯ 8 } seeds were obtained by inclining the c-plane to 〈 0 1 1 ¯ 0 〉 at 54.7°. A transmission X-ray topograph of the micropipe free c-plane wafer revealed that there were no macroscopic defects with lattice displacements. Crystal growth of undoped (vanadium-free) semi-insulating 6H–SiC was carried out by our sublimation system. In order to achieve high resistivity, high-purity SiC source and controlled instruments were used for the reduction of nitrogen, boron and metal impurity backgrounds. Hence high-purity and high-resistivity 6H–SiC 2 and 3 in in diameter were developed for high-frequency power transistors.

Deep Levels and Compensation in High Purity Semi-Insulating 4H-SiC

AbstractA study of temperature dependent Hall effect (TDH), electron paramagnetic resonance (EPR), photoluminescence (PL) and secondary ion mass spectrometry (SIMS) measurements has been made on high purity semi-insulating (HPSI) 4H-SiC crystals grown by the physical vapor transport technique. Thermal activation energies from TDH varied from a low of 0.55 eV to a high of 1.5 eV. All samples studied showed n-type conduction with the Fermi level in the upper half of the band gap. Carrier concentration measurements indicated the deep levels had to be present in concentrations in the low 1015 cm-3 range. Several defects were detected by EPR including the carbon vacancy and the carbon-silicon divacancy. PL measurements in the near IR showed the presence of the UD-1, UD-2 and UD-3 emission lines that have been found in HPSI material. No correlation between the relative intensities of the PL lines and the TDH activation energies was seen. SIMS measurements on nitrogen, boron and other common impurities indicate nitrogen and boron concentrations higher than those of individual deep levels as determined by TDH or of intrinsic defects as determined by EPR such as the carbon vacancy or the divacancy. It is determined that several different defects with concentrations greater than or equal to 1x1015 cm-3 are required to compensate the residual nitrogen and boron.

Intrinsic defects in high purity semi-insulating 6H SiC

AbstractElectron paramagnetic resonance (EPR) and photo EPR measurements were performed on undoped semi-insulating (SI) 6H SiC material grown by the physical vapor transport (PVT) method. EPR lines from a carbon-related surface defect, two photosensitive high temperature stable intrinsic defects with S=1/2 and shallow nitrogen and boron were observed. The EPR spectrum of the intrinsic defect labeled XX was observed in the dark and consists of three single lines XX1, XX2, XX3 due to three inequivalent sites in the 6H SiC. The second defect labeled PP appeared in EPR spectrum during photo-excitation of the SI 6H SiC and consisted of a single EPR line. The photo EPR data placed the energy level of the defects in the region EV + 1.24 ÷ 1.29 eV. The EPR parameters and symmetry behavior of XX agree well with those of the carbon vacancy-related Ky center observed in p-type electron irradiated 6H SiC and we tentatively identify the XX defect with VC+. The electronic processes occurring in undoped SI 6H SiC under photo-excitation are also described.

Observation of a spin one native defect in as-grown high-purity semi-insulating 4H SiC

Electron paramagnetic resonance measurements of high-purity semi-insulating 4H SiC reveals a spectrum characteristic of an S=1 defect, which appears only after exposure to light with a wavelength less than 690 nm. Analysis of the hyperfine structure of the spectrum suggests that the defect is an intrinsic pair or defect∕impurity complex. The center is stable in an inert ambient up to temperatures of 1200 °C, but a 1-h, 1600 °C anneal reduces the concentration by at least an order of magnitude. Because the spectrum is not affected by removal of the excitation light, it is concluded that the center is the ground state of an S=1 defect. A study of the angular dependence of the spectrum shows that g=2.0052, ∣D∣=(329±14)×10−4cm−1, and ∣E∣<19×10−4cm−1.

Thermally stimulated current spectroscopy of high-purity semi-insulating 4H-SiC substrates

High-purity semi-insulating (HPSI) 4H-SiC has been widely used as a substrate material for AlGaN/GaN high electron-mobility transistors because of its fairly good lattice match with GaN and its high thermal conductivity. To control material quality, it is important to understand the nature of the deep traps. For this purpose, we have successfully applied thermally stimulated current (TSC) spectroscopy to investigate deep traps in two HPSI 4H-SiC samples grown by physical vapor transport (PVT) and high-temperature chemical vapor deposition (HTCVD), respectively. Fundamentals of TSC spectroscopy, typical TSC spectra obtained on the two samples, and theoretical fittings of a boron-related trap (peaked at ∼ 150 K) will be presented. Based on literature data for deep traps in conductive 4H-SiC, the impurity and point-defect nature of several commonly observed TSC traps, peaked at ∼105 K (0.22 eV), ∼150 K (0.29 eV), ∼175 K (∼0.33 eV), ∼260 K (∼0.53 eV), ∼305 K (∼0.63 eV), and ∼360 K (0.91 eV), in the HPSI 4H-SiC will be discussed.

SiC MESFETs for High-Frequency Applications

AbstractSignificant progress has been made in the development of SiC metal semiconductor field-effect transistors (MESFETs) and monolithic microwave integrated-circuit (MMIC) power amplifiers for high-frequency power applications. Three-inch-diameter high-purity semi-insulating 4H-SiC substrates have been used in this development, enabling high-volume fabrication with improved performance by minimizing surface- and substrate-related trapping issues previously observed in MESFETs. These devices exhibit excellent reliability characteristics, with mean time to failure in excess of 500 h at a junction temperature of 410°C. A sampling of these devices has also been running for over 5000 h in an rf high-temperature operating-life test, with negligible changes in performance. High-power SiC MMIC amplifiers have also been demonstrated with excellent yield and repeatability. These MMIC amplifiers show power performance characteristics not previously available with conventional GaAs technology. These developments have led to the commercial availability of SiC rf power MESFETs and to the release of a foundry process for MMIC fabrication.

Defects in SiC substrates and epitaxial layers affecting semiconductor device performance

The current status of SiC bulk growth is reviewed, while specific attention is given to the effect of defects in SiC substrates and epitaxial layers on device performance and yield. The progress in SiC wafer quality is reflected in the achievement of micropipe densities as low as 0.92 cm−2 for a 3-inch n-type 4H-SiC wafer, which provides the basis for a high yielding fabrication process of large area SiC power devices. Using a Murphy Probe Yield Analysis for the breakdown characteristics of 10 kV PiN diodes we have extracted an “effective” defect density for 4H-SiC material to be as low as 30 cm−2, providing valuable information to further isolate and address the specific material defects critical for device performance. We address the problematic degradation of the forward characteristics (Vf-drift) of bipolar SiC PiN diodes [CITE]. The underlying mechanism due to stacking fault formation in the epitaxial layers and possible effects of device processing are investigated. An improved device design is demonstrated, which effectively stabilizes this Vf-drift. We show the progression in the development of semi-insulating SiC grown by the sublimation technique from extrinsically doped material to high purity semi-insulating (HPSI) 4H-SiC bulk crystals of up to 100 mm diameter without resorting to the intentional introduction of elemental deep level dopants, such as vanadium. Uniform resistivities in 3-inch HPSI wafers greater than 3 × 1011 Ω-cm have been achieved. Secondary ion mass spectrometry, deep level transient spectroscopy and electron paramagnetic resonance data suggest that the semi-insulating behavior in HPSI material originates from deep levels associated with intrinsic point defects. MESFETs produced on HPSI wafers are free of backgating effects and have resulted in the best combination of power density and efficiency reported to date for SiC MESFETs of 5.2 W/mm and 63% power added efficiency (PAE) at 3.5 GHz.

Optically Induced Transitions among Point Defects in High Purity and Vanadium-Doped Semi-Insulating 4H SiC

Optically Induced Transitions among Point Defects in High Purity and Vanadium-Doped Semi-Insulating 4H SiC

Photo-EPR and Hall Measurements on Undoped High Purity Semi-Insulating 4H-SiC Substrates

Photo-EPR and Hall Measurements on Undoped High Purity Semi-Insulating 4H-SiC Substrates

Development of Large Diameter High-Purity Semi-Insulating 4H-SiC Wafers for Microwave Devices

Development of Large Diameter High-Purity Semi-Insulating 4H-SiC Wafers for Microwave Devices

Silicon vacancy relatedTV2acenter in 4H-SiC

Electron paramagnetic resonance (EPR) was used to study the ${\mathrm{T}}_{V2a}$ center in 4H-SiC, which was previously attributed to the isolated Si vacancy but with different charge states: neutral, single negative, and triple negative, corresponding to different spin states $S=1,$ $S=3/2,$ and $S=1/2,$ respectively. The ${\mathrm{T}}_{V2a}$ EPR spectra observed in dark and under light illumination in as-grown high-purity semi-insulating 4H-SiC in the absence of the negatively charged Si vacancy ${(V}_{\mathrm{Si}}^{\ensuremath{-}})$ provide direct evidence confirming the spin triplet $(S=1)$ ground state of the center. A model with a triplet ground state and a singlet excited state is proposed to explain previously obtained results. The ${\mathrm{T}}_{V2a}$ center can be detected in as-grown material annealed at $1600\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$