Semi-insulating 4H Research Articles

The Electronic Structure of the UD-4 Defect in 4H, 6H and 15R SiC

The photoluminescence (PL) of the UD-4 defect is observed in semi-insulating bulk 4H, 6H and 15R SiC. In 4H and 6H SiC the UD-4 defect consists of two families of no-phonon (NP) lines, Ua and Ub, and in 15R SiC it consists of three families, Ua, Ub and U15R. The Ua family in 4H, 6H and 15R all show similar temperature behavior with higher energy NP lines becomming observable at higher temperatures. In the case of the Ub and U15R families, a luminescence line with lower energy than the prominent luminescence line appears at higher temperatures. The polarization and Zeeman measurements suggest that the defect has C3v symmetry.

XRD and Photoluminescence Whole-Wafer Mapping of 4H-SiC Wafers

The commercial availability of SiC-based devices has been limited by a number of factors including wafer supply, cost, wafer size and crystal quality. Recently a number of vendors of 4H and 6H SiC wafers have emerged and 100 mm diameter wafers are commercially available. All vendors now claim to meet or exceed the existing SEMI specifications M55-0304. Both SEMI and manufacturers’ specifications focus on wafer dimensions and gross physical defects such as micropipes that are visible by optical inspection. In this paper we describe high-density XRD and PL mapping measurements on a series of nominally identical, semi-insulating 4H wafers from a range of manufacturers. We show very large variations in crystal quality, polytypism and doping/contamination within-wafer, from wafer-to-wafer and vendor-to-vendor. It is logical to assume that these variations may be responsible for observed variations in device properties and yield.

Vanadium donor and acceptor levels in semi-insulating 4H- and 6H-SiC

The electronic levels of vanadium in semi-insulating 4H- and 6H-SiC have been reinvestigated using temperature dependent Hall effect and resistivity measurements at temperatures up to 1000K in conjunction with electron paramagnetic resonance (EPR) and optical absorption measurements which were used to identify the charge state of vanadium in the material. Two distinct thermal activation energies were found for each polytype. The shallower of the two levels correlated with the presence of both V3+ and V4+ in the EPR and absorption experiments, demonstrating that this level is the vanadium acceptor level while the deeper level is the donor level for which the V4+ charge state was observed. The results for the V4+∕5+ donor level, EC−1.57±0.09eV for 4H-SiC and EC−1.54±0.06eV for 6H-SiC, are in agreement with the generally accepted values. However, the results for the V3+∕4+ acceptor level, EC−0.85±0.03eV in 6H-SiC and EC−1.11±0.08eV in 4H-SiC, are significantly higher than previously assumed. Variations in crystal quality and purity may explain the differences in the previously reported values for the donor and acceptor levels.

Luminescence and EPR Characterization of Vanadium Doped Semi-Insulating 4H SiC

In this paper, we report on a photoluminescence (PL) and EPR study of several semiinsulating (SI) 4H SiC samples showing the different compensation regimes due to the presence of V4+ and V3+of different concentration. The samples which contain only V3+ indicates the compensation regime NV≅ND-NA>0 with the Fermi level located in the upper half of the band gap. The presence of V4+ along with V3+ in the other two set of samples indicates the SI behavior of the samples with compensation regime NV>NA-ND>0. Considering that the samples revealed EPR spectrum of vanadium V3+, position of the Fermi level should be also in the upper half of the band gap and mixed conductivity could be expected. UD-3 PL spectrum was observed in vanadium doped SI 4H SiC presented in the samples in V3+/V4+ and V4+/V5+ charge states with compensation regime NV>NA-ND>0. The data obtained prove that the PL and EPR are suitable techniques in determination SI yield in SiC crystal.

Trapping Recombination Process and Persistent Photoconductivity in Semi-Insulating 4H SiC

The decay kinetics of a persistent photoconductivity (PPC) in undoped semi-insulating 4H SiC and intercenter charge transfer were studied with EPR, photo-EPR and optical admittance spectroscopy (OAS). A thermally activated charge transfer process that occurs in the dark has been observed. The PPC effect was observed directly in changes in the quality factor of the EPR cavity before and after illumination and by the decay of the OAS signal for deep levels, and indirectly by the excitation and decay of the nitrogen and boron EPR lines that were not observed in the dark before illumination. The decay kinetics of the PPC and photo-induced carrier capture by nitrogen and boron levels were found to follow a stretched exponential form. The PPC in the temperature range from 77 to 300K was found to be produced by a thermally induced charge transfer process involving deep trap levels.

Thermal Evolution of Defects in Semi-Insulating 4H SiC

High temperature anneals were used to study the evolution of native defects in semiinsulating (SI), ultrahigh purity SiC using electron paramagnetic resonance (EPR), infrared and visible photoluminescence (PL) and COREMA (Contactless Resistivity Mapping) measurements. In EPR we observe a defect that we tentatively identify as VC-CSi-VC. The EPR intensities of this defect and the UD1 IRPL increase significantly with annealing in all samples.

A comparative study of the electrical properties of heavily Al implanted, single crystalline and nanocrystalline SiC

The electrical properties of heavily Al doped single and nanocrystalline 4H–SiC layers on semi-insulating 4H–SiC substrate, prepared by multienergy, high-fluence Al implantation and subsequent furnace annealing, are investigated by sheet resistance and Hall effect measurements. Ion beam induced crystallization is used to prepare the nanocrystalline SiC layers. The doping levels are chosen around the solid solubility limit of 2×1020cm−3 in the range from 5×1019to1.5×1021cm−3. The comparison of the results shows that heavily Al doped single crystalline SiC layers have superior conduction properties. The lowest resistivities measured at room temperature are 0.08 and 0.8Ωcm for the single crystalline and nanocrystalline samples, respectively. Recent results on enhanced Al acceptor activation in nanocrystalline SiC cannot be confirmed. There is an upper limit for the hole concentration in the nanocrystalline samples independent of the Al supersaturation level in the as-implanted state due to outdiffusion of Al in excess to the solid solubility limit during annealing. In contrast to the nanocrystalline SiC layers the as-implanted Al profile in single crystalline material remains stable after annealing even for concentrations above the solid solubility limit. Therefore, in single crystalline material efficient impurity band conduction due to strongly interacting acceptors can be achieved in the range of supersaturation. For lower doping levels impurity band conduction is more effective in nanocrystalline SiC.

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.

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.

Growth of Large Diameter SiC Crystals by Advanced Physical Vapor Transport

Semi-insulating 6H SiC substrates, 2”, 3” and 100mm in diameter, and n+ 4H SiC substrates, 2” and 3” in diameter, are grown at II-VI using the Advanced Physical Vapor Transport (APVT) technique [1]. The process utilizes high-purity SiC source and employs special measures aimed at the reduction of background contamination. Semi-insulating properties are achieved by precise vanadium compensation, which yields substrates with stable and uniform electrical resistivity reaching ~ 1011 Ω-cm and higher. Conductive n+ 4H SiC crystals with the spatially uniform resistivity of 0.02 W-cm are grown using nitrogen doping. Crystal quality of the substrates, their electrical properties and low temperature photoluminescence are discussed.

Electron Paramagnetic Resonance of Electronic‐Grade SiC Substrates

AbstractFor Abstract see ChemInform Abstract in Full Text.

Defect levels and types of point defects in high-purity and vanadium-doped semi-insulating 4H–SiC

Electron paramagnetic resonance (EPR) spectroscopy and photo-induced EPR are used to examine the point defects in vanadium-doped 4H–SiC and high-purity semi-insulating (HPSI) 4H–SiC grown by physical vapor transport. Both types of samples often exhibit a 1.1-eV activation energy, Ea, as extracted from the temperature-dependent Hall/resistivity measurements; however, different defects are related to the same Ea in each case. In the vanadium-doped wafers, the EPR data reveal both V4+ and V3+ in the same sample; thus, the 1.1-eV Hall activation energy is tentatively interpreted as the V3+∕4+ acceptor level. However, this conclusion cannot be confirmed because additional defects complicate the photoresponse of vanadium. The carbon vacancy, which is detected in all the HPSI wafers, exhibits a range of photothresholds similar to the various values measured for Ea. The photo-EPR and temperature-dependent Hall/resistivity measurements made before and after a 1600°C anneal suggest that several different types of nonuniformly distributed defects participate in compensation of the HPSI material and that annealing selectively removes those with different defect levels.

High temperature Hall effect measurements of semi-insulating 4H–SiC substrates

High temperature Hall effect and resistivity measurements have been made on semi-insulating 4H–SiC samples. Both vanadium doped and undoped materials have been studied. Resistivity measurements before and after annealing up to 1800 °C are also reported. The thermal activation energy of the resistivity in vanadium doped samples has one of two values, 1.5 and 1.1 eV, due, respectively, to the vanadium donor level and an as yet unidentified defect. The activation energies for high purity semi-insulating material (HPSI) varied from 0.9 to 1.5 eV. Hall effect measurements were made on several HPSI and 1.1 eV V-doped samples. In all cases the material was found to be n-type. Mixed conduction analysis of the data suggests that the hole concentration is negligible in all samples studied. This suggests that the defects responsible for the semi-insulating properties have deep levels located in the upper half of the bandgap. The resistivity of V-doped samples were unaffected by anneals up to 1800 °C. The annealing results for HPSI samples were mixed.

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

Optical and magnetic resonance signatures of deep levels in semi-insulating 4H SiC

We have studied semi-insulating (SI) 4H SiC grown by physical vapor transport (PVT) and by high-temperature chemical vapor deposition (HTCVD) using electron paramagnetic resonance (EPR) and infrared photoluminescence (IR-PL) to better understand the defect(s) responsible for the SI behavior. Although intrinsic defects such as the isolated carbon vacancy and in some cases the isolated Si vacancies have previously been observed by EPR in undoped SI SiC, their concentrations are an order of magnitude too low to be responsible for the SI behavior. We are able to observe the EPR signature of the carbon vacancy–carbon antisite pair (VC–CSi) pair defect in an excited state of its 2+ charge state in all PVT samples and some HTCVD samples. We also establish the IR–PL signature of this EPR center as the UD2 spectrum—a set of four sharp lines between 1.1 and 1.15eV previously observed by Magnusson et al. in neutron-irradiated 4H–SiC. We also observe the UD1 line, a pair of sharp IR–PL lines at ∼1.06eV and UD3, a single sharp line at ∼1.36eV. We propose a simple model for the SI behavior in material in which the (VC–CSi) pair defect is the dominant deep defect.

Defects in SiC

Recent results from studies of shallow donors, pseudodonors, and deep level defects in SiC are presented. The selection rules for transitions between the electronic levels of shallow donors in 4H–SiC in the dipole approximation are derived and the ionization energy for the N donor at hexagonal site is determined. Optical and electrical studies of the DI center reveal the pseudodonor nature of this defect. Defects in high-purity semi-insulating (SI) SiC substrates including the carbon vacancy (VC), silicon vacancy (VSi), and (VC−CSi) pair are studied. The annealing behavior of these defects and their role in carrier compensation in SI 4H–SiC are discussed.

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).

240 GHz electron paramagnetic resonance studies of intrinsic defects in as-grown4HSiC

240 GHz electron paramagnetic resonance (EPR) measurements of as-grown nominally semi-insulating 4H SiC detected two well-separated centers ID1 and ID2. The EPR parameters of ID1 and ID2 coincide with that of EI5 and EI6 centers previously detected in 2-MeV electron-irradiated p-type 4H SiC:Al by 95 GHz EPR (Son et al.). The defects in irradiated material were assigned to a positively charged carbon vacancy (EI5) and silicon antisite (EI6), respectively. Increased separation between the two centers at 240 GHz and the absence of additional radiation-induced spectral lines in the as-grown, unirradiated SiC facilitated analysis of the defect structure. Our data support that the IDI center is a carbon vacancy-related defect, but is not consistent with the assignment of the ID2 center to a silicon antisite. The ID2 spectrum is best fit as another carbon vacancy-related defect. Illumination with light below 1400 nm quenched both ID1 and ID2 simultaneously, suggesting that the defect energy levels are nearly the same.

Influence of AlN buffer on electronic properties and dislocation microstructure of AlGaN/GaN grown by molecular beam epitaxy on SiC

Electronic and structural properties of AlGaN/GaN heterostructures grown by molecular beam epitaxy on semi-insulating 4H–SiC substrates with and without an initial AlN nucleation layer are studied. Differences in microstructure were examined using scanning capacitance microscopy, which reveals negatively charged dislocations through capacitance variations, conductive atomic force microscopy, used to quantify the density of dislocation-related reverse-bias leakage paths, and x-ray diffraction (XRD). Samples grown without the AlN buffer exhibited features consistent with the commonly observed coalesced island structure, with negatively charged features, commonly associated with dislocations, occurring at the domain boundaries. Samples grown with an AlN buffer layer showed a more random distribution of negatively charged features, much broader symmetric XRD peak widths, and about 1 order of magnitude greater density of localized reverse-bias leakage paths. This difference in microstructure is expected to have significant influence on carrier mobilities and optical efficiencies, and may suggest improved nucleation schemes for device-quality nitride material.