Björn Magnusson

Photoexcitation-electron-paramagnetic-resonance studies of the carbon vacancy in 4H-SiC

Photoexcitation-electron-paramagnetic-resonance (photo-EPR) studies were performed on p-type 4H-SiC irradiated with 2.5 MeV electrons. At W-band frequencies (∼95 GHz) different EPR spectra could be well separated, allowing a reliable determination of the ground state levels of the associated defects. The photo-EPR results obtained for the positively charged carbon vacancy (VC+) can be explained by a deep donor model with the (+/0) level located at (1.47±0.06) eV above the valence band.

HTCVD Grown Semi-Insulating SiC Substrates

The low residual doping of HTCVD grown semi-insulating SiC crystals enables the use of decreased concentrations of compensating deep levels, thereby providing new material solutions for microwave devices. Depending on the growth conditions, high resistivity crystals with either a dominating Si-vacancy absorption or with an EPR signature of intrinsic defects such as the C-vacancy and the Si-antisite are obtained. The electrical properties of substrates with resistivities above 10(11) Omega-cm are shown to be stable upon annealing during SiC epitaxy conditions. Micropipe closing at the initial growth stage enables the demonstration of low defect density off- and on-axis 2 2-inch semi-insulating 4H SiC substrates with micropipe densities down to 1.2 cm(-2).

Defects in High-Purity Semi-Insulating SiC

Defects and impurities in high-purity semi-insulating (HPSI) SiC substrates grown by high temperature chemical vapour deposition (HTCVD) and physical vapour transport (PVT) are studied using electron paramagnetic resonance (EPR) and photoluminescence (PL). The carbon vacancy in the positive charge state ( + C V ) is observed in all HTCVD and PVT HPSI substrates. EPR signals of (CSi–VC) pairs are often detected in HPSI samples. The TV2a, which was previously attributed to 0 Si V , is often observed with different concentrations in HTCVD material. The (+/0) donor level of VC at 1.47 eV above the valence band is suggested to be important for the SIproperties of HPSI 4H-SiC substrates with the activation energies Ea~1.4-1.5 eV. The SI-5 center may be the vacancy pair in the negative charge state (VC-VSi) – and its acceptor level (-1/-2) is in the region ~1.24-1.51 eV below the conduction band. This center is stable at annealing temperature of 1600 oC. After annealing, + C V and VSi-related signals decrease but can still be observed, whereas the (CSi–VC) pairs completely disappear.

Optical Characterization of Deep Level Defects in SiC

Deep levels in 4H- and 6H-SiC are characterized by FTIR spectroscopy. Vanadium, chromium and the silicon vacancy related center are listed together with the unidentified defects with emission and absorption in the near IR region. We suggest the UD-1, UD-3 and I-1 to be impurity related while the UD-2 and UD-4 to be intrinsic defects based on annealing behavior and the possibility to create the defect with irradiation. We have also tentatively assigned a new defect center around 1.0 eV to the carbon vacancy-antisite pair instead of the earlier assignment to the UD- 2 defect in 4H-SiC. We have shown that to get more information about the SiC samples a combination of absorption and luminescence techniques are very useful. Further, the use of below bandgap selective excitation is necessary to obtain more information about the defects present in the sample. FTIR absorption and luminescence measurements are useful tools to characterize deep levels important for both semi-insulating material as well as low doped conducting material where the free carrier lifetime is limited by deep levels.

Material characterization need for SiC-based devices

The simultaneous development of suitable characterization techniques that provide fast feedback to the growth as well as basic material understanding have enabled the fast development of epitaxial and bulk growth of SiC. The characterization techniques can roughly be divided into two different categories, routine characterization that are made on most grown material and specialized characterization that are performed in order to study and understand specific material properties. The routine measurements described in this paper are all based on optical and non-destructive techniques. The main effort in this field is currently to study and understand the role of structural defects, often replicated from the substrate into the epilayer.

Defects in Semi-Insulating SiC Substrates

Electron paramagnetic resonance (EPR) was used to study defects in semi-insulating (SI) SiC substrates grown by high-temperature chemical vapour deposition (HTCVD) and physical vapour transport (PVT). The C vacancy, Si antisite and several other EPR centers, labelled SI-I to SI-8, were observed in the HTCVD and/or PVT 4H-SiC substrates. Photo-EPR has revealed several deep levels responsible for the SI properties in different types of SI 4H-SiC. Annealing behaviour of the defects and the stability of the SI properties with high temperature annealing were also studied.

Optical and structural characteristics of virtually unstrained bulk-like GaN

Bulk-like GaN with high structural and optical quality has been attained by hydride vapor-phase epitaxy (HVPE). The as-grown 330 µm-thick GaN layer was separated from the sapphire substrate by a laser-induced lift-off process. The full width at half maximum values of the X-ray diffraction (XRD) ω-scans of the free-standing material are 96 and 129 arcsec for the (1 0 -1 4) and (0 0 0 2) reflection, respectively, which rank among the smallest values published so far for free-standing HVPE-GaN. The dislocation density determined by plan-view TEM images is 1–2×107 cm-2. Positron annihilation spectroscopy studies show that the concentration of Ga vacancy related defects is about 1.5×1016 cm-3. The high-resolution XRD, photoluminescence, µ-Raman, and infrared spectroscopic ellipsometry measurements consistently prove that the free-standing material is of high crystalline quality and virtually strain-free. Therefore it is suitable to serve as a substrate for stress-free growth of high-quality III–nitrides based device heterostructures.

HTCVD Growth of Semi-Insulating 4H-SiC Crystals With Low Defect Density

The development of a novel SiC crystal growth technique, generically described as High Temperature Chemical Vapor Deposition (HTCVD) is reviewed. The structural, optical and electrical properties of 4H-SiC semi-insulating substrates are investigated with the aim of providing optimal microwave device performances. In particular, alternative compensation mechanisms to vanadium doping in S.I substrates are investigated to eliminate substrate induced trapping effects. Carried out at temperatures above 2100°C, the HTCVD technique uses, as in CVD, gas precursors (silane and a hydrocarbon) as source materials. The growth process can be described as “Gas Fed Sublimation” and proceeds by the gas phase nucleation of Si x -C y clusters, followed by their sublimation into active species that are condensed on a seed. Crystals with diameters up to 45 mm have been obtained with growth rates of 0.6 mm/h. The use of specific process steps, such as in-situ seed surface preparation and micropipe closing are presented and high resistivity wafers with micropipe densities down to 10 cm −2 are demonstrated. 4H-SiC substrates prepared from undoped crystals (with vanadium concentration lower than 5×10 14 cm −3 ) exhibit semi-insulating behavior with a room temperature resistivity of the order of 10 10 Ω?cm. Infrared absorption measurements show that two types of semi-insulating crystals can be grown, with a spectrum either dominated by the Si-vacancy, or by a previously unreported defect labeled UD-1. These two types of semi-insulating wafers are also differentiated by the temperature dependence of their resistivity, with activation energies of 0.85 and 1.4±0.1 eV, respectively, and by the stability of their resistivity upon an annealing at 1600°C. Initial MESFET devices processed on HTCVD grown substrates show better DC characteristics than devices processed on vanadium doped substrates.

Clustering of vacancy defects in high-purity semi-insulating SiC

Positron lifetime spectroscopy was used to study native vacancy defects in semi-insulating silicon carbide. The material is shown to contain (i) vacancy clusters consisting of four to five missing ...

Photoluminescence and Zeeman effect in chromium-doped 4H and 6H SiC

Photoluminescence (PL) and Zeeman effect measurements in near-infrared luminescence bands in Cr-doped 4H and 6H SiC are presented. The PL spectrum consists of two no-phonon lines (NPLs) at 1.1583 and 1.1898 eV in 4H SiC and three NPLs at 1.1556, 1.1797, and 1.1886 eV in 6H SiC. The observed Zeeman splittings and temperature dependence studies reveal the spin triplet of the ground state and the orbital doublet structure of the excited state of the Cr-related center. All the triplets have almost isotropic g values close to 2 with trigonal symmetry and small zero-field splitting values D. In contrast, the effective g values of the excited state of the center are very anisotropic with g∥ in the range of 0.22–0.64 and g⊥=0 for different NPLs in both polytypes. Based on the Zeeman results, the PL is attributed to the internal transition 1E(D)→3A2(F) within the d shell of a substitutional, neutral chromium (Cr4+) in the 3d2 electronic configuration.