15R SiC Research Articles

EPR, ESE, and pulsed ENDOR study of the nitrogen donors in 15R SiC grown under carbon‐rich conditions

X‐band field‐sweep electron spin echo and pulsed electron nuclear double resonance (ENDOR) spectroscopy were used to study n‐type 15R SiC wafers grown under carbon (C)‐rich conditions with the aim to verify the recently proposed concept that nitrogen (N) donors substitute both carbon (C) and silicon (Si) sites and may occupy nonequivalent positions at Si sites. It was found that besides the 14N ENDOR spectra of the C substituting quasicubic “k1”, “k2”, and “k3” positions five doublet lines due to 14N nuclei at other lattice positions were observed in the ENDOR spectrum of highly compensated C‐rich n‐type 15R SiC. Three of them with the hyperfine interaction (HFI) constants 34.66, 32.20, and 29.77 MHz were attributed to the N donors substituting “k1”, “k2”, and “k3” positions at Si sites. Two other additional ENDOR spectra were explained by the presence of the C antisite (CSi) defects in C‐rich 15R SiC, which leads to the formation of CSiNC complexes with donor levels close to the conduction‐band minimum and isotropic 14N HFI constants of 21.58 and 25.94 MHz. The energy levels of N donors have been evaluated by comparing the number of ENDOR spectra observed in C‐rich 15R SiC with different degrees of compensation.

Optical Properties of the Niobium Centre in 4H, 6H, and 15R SiC

A set of lines in the photoluminescence spectra of 4H-, 6H-, and 15R-SiC in the near-infrared are attributed to Nb-related defects on the ground of doping experiments conducted with 4H-SiC. A model based on a an exciton bound at the Nb-centre in an asymmetric split vacancy configuration at a hexagonal site is proposed, which explains the structure of the luminescence spectrum and the observed Zeeman splitting of the lines.

Near-atomic images of interfaces between twin-related lamellae in a synthetic 6H-SiC sample

Transmission Electron Microscopy (TEM) and High Resolution TEM (HRTEM) investigations were used to study a complex micro-structure of twin-related lamellae of 15R SiC polytype, in syntactic coalescence with 6H-SiC. The analysis of the local stacking microstructure by means of HRTEM revealed that the perfect structure of 15R polytype, (23)3 was locally interrupted by numerous adjoining stacking faults parallel to (0001) with stacking of the coupled (22) and (33) bilayers superimposed on the twin boundaries. HR images taken exactly across both the twin boundaries showed a zig-zag pattern (23) that switched to (32) by a twin coherent interfaces or, alternatively, a zig-zag pattern (32) which passed to (23) through an isolated (33), 6H like sequence. The selected area electron diffraction (SAED) patterns taken exactly above both the twin interfaces indicate classifying of the twin found in this study as a “Friedelian” reticular merohedric twinning. However, two indistinguishable twin operations matched the observed features: a reflection through rational plane (0001), and 180°-rotation around [0001]. Since individual Si and C atoms and even the SiC bilayer polarity could not be established from these HR images, the real twin law was deduced by taking into account that the coherent structural match at the interface could be guaranteed only by a 180°-rotation around [0001]. In order to explain the origin of the planar defects found in this sample, the growth mechanism and the influence of the low-energy stacking faults were considered.

Heat- and radiation-resistant contacts to SiC made on the basis of quasi-amorphous ZrB2 films

The radiation and thermal stability of the Schottky-barrier structures deposited by the magnetron sputtering of zirconium diboride onto the (0001) face of the Lely-grown n-6H(15R)SiC single crystals with the uncompensated-donor concentration of ∼1018 cm−3 was investigated by the methods of the I–V and C–V characteristics combined with the layer-by-layer Auger analysis. It is shown that the use of quasi-amorphous ZrB2 films when manufacturing contacts on n-6H(15R)SiC leads to no changes in the Schottky-barrier characteristics during rapid thermal annealing to 800°C in the range of 60Co γ-ray irradiation doses of 103–107 Gy.

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.

Synthesis of One-Dimensional Nanostructured Silicon Carbide by Chemical Vapor Deposition

Nanorods of the wide-bandgap semiconductor silicon carbide belong to a promising group of one-dimensional materials with potential applications extending from reinforcement of composites to applications as building blocks that can be logically assembled into appropriate two- (and three-) dimensional architectures, permitting researchers to exploit their unusual electronic, optical, and other properties. Specific to the most common silicon carbide polytypes are a low intrinsic carrier concentration, an exceptionally high breakdown electric field, high thermal conductivity, high-temperature stability, and resistance to an aggressive environment. This should permit one to develop even submicron-level SiC-based devices operating under high-temperature, high-power, and/or high-radiation conditions, under which conventional semiconductors cannot function. Detailed control of the conditions favorable for the nucleation and growth processes of nanorods of a given SiC polytype is necessary because the electrical and optical properties of each SiC polytype are very different. Therefore, a systematic investigation of factors that primarily influence the morphology and polytype of a vapor-phase-grown SiC has been made in the present work. These factors were the temperature, the flow rates of the gaseous precursors, and the Si/C molar ratio in the gas phase. In order to investigate the role of these factors, the “cold gas-hot substrate” chemical vapor deposition (CVD) method has been applied, because it permits them to be closely controlled in a wide range. While in the overwhelming majority of previous investigations nanorods of the 3C SiC polytype have been grown, the present work delineates conditions that are favorable for the growth of single-phase 2H, 3C, 15R, and 6H SiC nanorods, respectively.

Chemical reactions of atomic hydrogen at SiC surface and heterogeneous chemiluminescence

In studies of the surface properties of SiC polytypes and chemical reactions of hydrogen atoms at SiC surfaces the surface (chemi)luminescence of SiC has been applied excited in the reaction of hydrogen atoms due to chemical energy released (heterogeneous chemiluminescence, HCL). The bulk photoluminescence (PL) have also been used for comparison with surface HCL. All the samples showed HCL, but only α-SiC (6H, 15R), technologically or specially doped (predominantly by N, B, Al), exhibited PL ( λ ex=365 nm). Cubic polycrystalline β-SiC (or 3C–SiC) did not show PL. The general luminescence band of α-SiC (6H, 15R) and B and Al doped SiC (6H) was a broad yellow band with λ max ranged from 620 to 650 nm for PL (110 K). Another less intensive luminescence band is a blue one, which has been observed only at low temperatures for α-SiC (6H,15R) and B and Al doped SiC (6H) in PL spectra and as a shoulder in HCL spectra ( λ max=477 nm at 110 K for 15R–SiC). The green band near 540 nm was also observed sometimes in PL spectra for α-SiC. The heat of adsorption of hydrogen atoms at polycrystalline β-SiC estimated from HCL data was found to be in the range from 2 to 3 eV.

Photoluminescence Up-Conversion Processes in SiC

Efficient photoluminescence up-conversion is observed in 4H SiC samples containing both the UD-3 defect with its characteristic photoluminescence (PL) no-phonon (NP) line in the near infrared at 1.356 eV and the titanium impurity with its emission in the visible spectral region. When both defects are present, the titanium emission can be excited efficiently by tuning the laser to UD-3. In 4H samples containing either only UD-3 or only titanium, a different photoluminescence up-conversion process can be observed. This second process occurs at photon energies higher than approximately 1.5 eV without exhibiting a clear threshold. In 6H and 15R SiC only this second process was found, even when both the UD-3 defect and the titanium impurity are abundant.

UD-3 defect in4H,6H,and15RSiC: Electronic structure and phonon coupling

The UD-3 photoluminescence (PL) spectrum is observed in high-resistive or semi-insulating bulk 4H, 6H, and 15R SiC. It consists of one no-phonon (NP) line in 4H and 6H SiC and two NP lines in 15R S ...

Optical quantitative determination of doping levels and their distribution in SiC

We report the development of an absorption measurement-based characterization tool for the quantitative determination of doping levels and their lateral distribution in silicon carbide wafers. Calibration plots for the technologically important silicon carbide polytypes 4H–SiC (n-/p-type), 6H–SiC (n-/p-type) and 15R–SiC (n-type) are presented. A review of the underlying physical effects of the measurement procedure as well as a description of the experimental setup is given. The applicability of the characterization tool as a production friendly non-contact wafer quality test is demonstrated by showing several mappings of the lateral doping level distribution. The accuracy of the described measurement procedure is typically 15–20% and is of the same order as its electrical Hall measurement counterpart.

Determination of charge carrier concentration in n- and p-doped SiC based on optical absorption measurements

We have investigated the effect of doping on absorption for various SiC polytypes, i.e., n-type (N) 6H–SiC, 4H–SiC, and 15R–SiC, p-type (Al) 6H–SiC, and 4H–SiC, and p-type (B) 6H–SiC. For these polytypes the band-gap narrowing with higher doping concentration is observed. In addition, for n-type doping below band-gap absorption bands at 464 nm for 4H–SiC, at 623 nm for 6H–SiC, and at 422 and 734 nm for 15R–SiC are observed. The peak intensities of these absorption bands show a linear relation to the charge carrier concentration obtained from Hall measurements. The corresponding calibration factors are given. As an application a purely optical wafer mapping of the spatial variation of the charge carrier concentration is demonstrated.

Synthesis of Mg-α SiAlON powders from talc and halloysite clay minerals

Carbothermal reduction and nitridation (CRN) of SiO2 is an attractive method to manufacture Si3N4 powders with controlled grain morphology. Some inexpensive raw materials were previously used to synthesize β-sialon powder, resulting in cheaper products to be useful for some engineering applications. In this work, Mg-α sialon (MgxSi12−m−nAlm+nOnN16−n) powders were achieved by CRN of mixtures of talc (Mg3(Si2O5)2(OH)2) and NZ halloysite clay (Al2Si2O5(OH)4) minerals. The final products consisted mainly of α-sialon and β-sialon phases. Small amounts of 15R AlN-polytypoid and SiC were also identified in the synthesized powder. By adding a small amount of α-Si3N4 powder as seeds to the starting composition, the conversion rate of α-sialon phase was significantly enhanced and powders with up to 90 wt% of α-sialon were produced.

Neutral aluminum and gallium four particle complexes in silicon carbide polytypes

Optical studies provide a great amount of information about acceptors in SiC. The optical recombination of neutral acceptor four particle (bound exciton) complexes has been observed in 4H, 6H, 15R and 3C SiC for Al; 4H, 6H and 3C SiC for Ga; and in 4H SiC for the shallow B acceptor. A model for the electronic structure of neutral acceptor four particle complexes based on symmetry provides a description of the no-phonon spectra for Al and Ga, when the possibility of lattice strain is included. This model is consistent with the numbers of no-phonon lines observed for Al and Ga in 3C, 4H and 6H SiC, but there are too many no-phonon lines for Al in 15R SiC. The model is further developed and applied to Zeeman spectra of Al and Ga four particle complexes in 6H SiC, providing further insight into their electronic structure.

A survey of conduction and valence band edges in SiC

A brief summary gives the current status of our understanding of the structure of the conduction band edges of SiC. The latest values for the effective masses as derived from theory and verified by experiment are given. Absorption data for higher conduction bands in 4H and 6H SiC and a theoretical interpretation in terms of first principles band calculations are reviewed. Ballistic-electron emission microscopy (BEEM) experiments sensing the bottom and higher conduction bands in 4H, 6H and 15R SiC are described and compared with theory. The current picture for the shape of the edge of the valence band near the Γ point in the Brillouin zone is given. New experimental data for the spin-orbit and crystal field splittings in 6H SiC are in good agreement with one set of recent calculated values but do not agree as well with two other calculations. However, the linear relationship for the crystal field splitting, ΔCF, with hexagonality still remains to be experimentally verified by measuring values of the crystal field splitting in 4H and 15R SiC.

Progress in the study of optical and related properties of SiC since 1992

Abstract A selected review is given of some of the advances made in the understanding of the optical and related properties of SiC during the years 1992–1996. Areas which are briefly covered are ultraviolet reflectivity and band structure, phonon replicas and the positions in k -space of the conduction band minima, cyclotron resonance and effective masses, pressure dependence of the electronic energy gap, electronic Raman scattering in heavily doped SiC, acceptors and donors, transition metals, thermoluminescence, porous SiC, and erbium-implanted 3C, 4H, 6H and 15R SiC.

Global Band Structure and Near-Band-Edge States

The global band structure from ab-initio calculations using DFT-LDA concepts is presented and discussed for 2H, 3C, 4H, 6H, and 15R SiC in view of the different stacking sequences and point lattices of these polytypes. Details of the conduction and valence band structure close to the band edges are described by using {\bf k·p} models and thermal density-of-states effective masses are calculated.

Hopping conduction in heavily doped bulk n-type SiC

The electronic properties of heavily doped n-type 4H, 6H, and 15R SiC have been studied with temperature dependent Hall effect, resistivity measurements, and thermal admittance spectroscopy experiments. Hopping conduction was observed in the resistivity experiments for samples with electron concentrations of 1017 cm−3 or higher. Both band and hopping conduction were observed in all three polytypes in resistivity and Hall effect experiments. The hopping conduction activation energy e3 obtained from the resistivity measurements varied from 0.003 to 0.013 eV. The e3 value obtained from thermal admittance spectroscopy measurements were slightly lower. The nitrogen ionization levels were observed by thermal admittance spectroscopy only in those samples where hopping conduction was not detected by this experiment. Free carrier activation energy Ea for nitrogen was difficult to determine from temperature dependent Hall effect measurements because of the effects of hopping conduction. A new feature in the apparent carrier concentration vs inverse temperature data in the hopping regime was observed.

Ab initio calculations of structural and electronic properties of 15R SiC

We report on ab initio calculations of structural and electronic properties for rhombohedral 15R SiC within the density-functional theory in the local density approximation. Besides the structural parameters we present the electronic band dispersion along high-symmetry lines of the irreducible part of the Brillouin zone and compare the position of the lowest conduction-band minimum with those of cubic and hexagonal polytypes. Furthermore we derive from the conduction-band dispersion the components of the effective-mass tensor and compare these with experimental data.

Stepped morphology on 4H and 15R silicon carbide: modelling by a random walk

A random walk algorithm for modelling the development of stepped morphology in a diffusion mediated step flow growth is applied to the {0001} surface of 4H (hexagonal) and 15R (rhombic) polytype modifications of silicon carbide grown by vapour phase epitaxy. The algorithm includes variable desorption of adatoms from terraces as well as their variable accommodation at steps. By using this algorithm it is shown that characteristic step heights of 4 and 5 ML often observed on the surface of 4H and 15R SiC are the result of a specific stacking sequence in the [0001] direction. A change between ABC and ACB stacking order implies two kinds of adatom accommodation kinetics (fast and slow) at steps in any of the 〈11̄00〉 directions in the {0001} plane which originate from the different number of dangling bonds at the step edge in ABC and ACB stacked layers. Therefore during the growth fast steps can take over, occupy positions at the top of the step riser and dominate in controlling the growth rate. The results of the simulation show that 4 and 5 ML step heights as well as step bunching are intrinsic properties of the crystal structure and of its kinetical parameters.

Are polytype transitions possible during boron diffusion?

The influence of boron diffusion on the polytype structure of 3C, 6H and 15R SiC polytypes was studied in the temperature range 1600–1900 °C. In the surface region partial polytype transitions 15R → 3C and 6H → 3C were detected by reflection high energy electron diffraction. In the bulk region such diffraction spots could not be detected. A maximum of the diffraction intensities of 3C in α-SiC was observed at 1800°C, whereas in the temperature range 1600–1800°C no phase transitions in the 3C SiC crystal were detected. At 1800°C and for long diffusion times a phase transition of the type 3C → 15R occur. This reverse phase transition may be the cause of the decrease in diffraction spot intensities of 3C SiC in 15R and 6H crystals in comparison with crystals treated by boron diffusion at lower temperatures. Reasons for the observed polytype transitions are the local generation of stacking faults which lead to elimination of the hexagonal sites in the stacking order of the α-SiC polytypes and their stabilization by the diffusion distribution induced stress field.