SiC Epitaxial Layers Research Articles

Acceptor ion-implantation in SiC

Single and multiple energy Al, B and Ga ion-implantations were performed into n-type 6H–SiC epitaxial layers. Empirical formulae for the range statistics of the implant depth distributions in the energy range 50 keV–4 MeV were developed by analyzing the experimental secondary ion mass spectrometry (SIMS) implant depth profiles. The Al and Ga implants were electrically activated at an annealing temperature of 1500°C, but the B implants required an annealing temperature of 1650°C. In this study annealings were performed by using AlN or graphite encapsulant. The Al and Ga implants are thermally stable but the B implants showed redistribution during annealing.

Photothermal ionization spectroscopy of shallow nitrogen donor states in 4H–SiC

Photothermal ionization spectroscopy (PTIS) measurements were carried out on a free-standing, high purity and high quality 4H–SiC epitaxial layer at various temperatures. The two step photothermal ionization process is clearly reflected in the temperature dependence of the photoconductivity. The PTI spectrum at a temperature of 25.6 K exhibits one order of magnitude higher energy resolution than the infrared absorption spectra of 4H–SiC bulk material. It reveals five strong, well resolved electronic transition lines associated with the shallow nitrogen donor. The ionization energy of the shallow nitrogen donor is deduced to be 60.2±0.5 meV based on experimental results. Furthermore, PTI magnetospectroscopy measurements were performed to investigate the symmetry properties of these transitions in Faraday configuration. No linear Zeeman splitting is observed, however, these lines show a diamagnetic shift. It indicates that the excited states of the shallow nitrogen donor are nondegenerate at zero magnetic field, which is consistent with the fact that the effective mass tensor of 4H–SiC has three different diagonal components.

Investigations of Non-Micropipe X-Ray Imaged Crystal Defects in SiC Devices

ABSTRACTThis paper updates on-going experimental and theoretical investigations of non-micropipe defects imaged by synchrotron white beam X-ray topography (SWBXT) in SiC devices and epitaxial layers. Computer-based thermal modeling of screw-dislocation related breakdown in SiC diodes has been initiated to gain insights into internal temperature profiles as a function of microplasma power. A preliminary study of epitaxial 4H- and 6H-SiC p+n mesa diodes indicates that very low angle boundaries, whose electrical properties have not previously been reported, do not significantly impact DC I-V properties (forward and reverse) measured at biases less than 70% of the SiC breakdown field. The presence of very small growth pits on the surface of commercial 4H-SiC epitaxial layers, almost undetectable by high magnification optical microscopy, was revealed by atomic force microscopy and found to correspond to the locations of closed core screw dislocations imaged by SWBXT.

Cross-sectional cleavages of SiC for evaluation of epitaxial layers

The application of cleavages on SiC epitaxial layers are presented as a feedback for an evaluation of the growth. The preferred cleavage planes are described and discussed in relation to the atomic configuration of the SiC lattice. From the cleavages it is possible to relate defect behaviour to the growth mechanism and obtain information which can not be revealed by studying as-grown epilayer surfaces. For demonstration, a variety of defects revealed by cleavages are investigated for SiC epitaxial layers.

Growth of Si/3C–SiC/Si(100) heterostructures by pulsed supersonic free jets

We have investigated the epitaxial growth of multilayer structures of Si/3C–SiC/Si(100) by pulsed supersonic free jets of methylsilane (CH3SiH3) for SiC growth and trisilane (Si3H8) for Si growth. Epitaxial Si layers were obtained only on very thin (≈3 nm) 3C–SiC epitaxial layers, while polycrystalline Si was grown on thicker 3C–SiC layers. It was also found that the transition regions with a thickness of ≈1 nm existed at the interface between epitaxial 3C–SiC and Si layers by high-resolution transmission electron microscopy observation. These results suggest that the surface roughness and thickness of the 3C–SiC layer play an important role for epitaxial growth of Si.

Electron paramagnetic resonance of the scandium acceptor in 4H and 6H silicon carbide

In Sc-doped 6H–SiC epitaxial layers we observed several different electron paramagnetic resonance (EPR) spectra with a 45Sc hyperfine interaction pattern. The spectra are explained as being due to the isolated Sc0(S=12) acceptor on carbon sites but with different microscopic configurations. The spectra show a pronounced temperature dependence. At low temperatures the hole of the Sc0 acceptor is located in a Sc0–Si bond either along the c-axis forming an axial centre or along one of the three other C–Si bonds giving rise to a monoclinic centre. At higher temperature the hole changes place rapidly between the three Sc–Si bonds resulting in an axial centre. All three Sc0 hole configurations have different ionisation levels. The ionization level of the low-temperature monoclinic–high-temperature axial centre is significantly influenced by an entropy term. In 4H–SiC spectra were only observed from a single Sc0 centre with monoclinic symmetry.

Structural investigation of SiC epitaxial layers grown under microgravity and on-ground conditions

Thick 4H-, and 6H–SiC epitaxial layers have been grown by LPE from Si–Sc–C solvent at microgravity conditions during a space experiment, as well as on-ground. The samples are characterised by cross-sectional TEM and HRXRD. Layers grown at microgravity are relatively defect free, although their surfaces are always stepped. Control samples grown on-ground have similar surface appearance, but contain scandium carbide precipitates, nanopipes, micropipes and/or cavities as verified by TEM. However, none of the aforementioned defects was traced in the layers grown at microgravity conditions. So, samples grown at space microgravity conditions are superior in their defect structure to those ones grown on the ground. The defects called nanopipes can be described as empty pipes of about 200 nm diameter traversing the layer in the [0001] (growth) direction. The steps in the microgravity and on-ground samples have facets of {104} type crystallographic planes both in 6H–, and 4H–SiC. We suggest, that those facets are formed and preferred during growth due to a possible mechanism of decreasing the high energy of the growing Si terminated (0001) surface.

Dangling bond defects in SiC: the dependence on oxidation time

Electron paramagnetic resonance is used to study a near surface defect in oxidized 4H 6H SiC substrates and 3C SiC epitaxial layers. The defect is observed after wet oxidation and a 900° C dry thermal treatment, but the defect is not located in the oxide. The heat treatment activates the EPR center by removing a hydrogen-related species from a C bond. Oxidation studies suggest that reaction of H 2O with SiC creates the defect. However, an alternative theory in which the defect is intrinsic to SiC cannot be disregarded.

Epitaxial growth of SiC in a new multi-wafer VPE reactor

SiC epitaxial layers have been grown in a commercial multi-wafer reactor. Results from the initial growth runs are presented. The reactor is vertical and has a high speed rotating susceptor that can support up to six 50 mm diameter wafers. The surface morphology of the grown layers are specular and show no indication of step-bunching. The unintentional background doping is p-type in the low 10 15 cm −3 range, consisting mainly of Al. Both N and Al have been used for doped layers showing wide doping range capability and sharp transients. The best uniformity in thickness and doping achieved so far on the same 35 mm wafer are ±7% and ±10%, respectively.

Deep level defects in H + implanted 6H–SiC epilayers and in silicon carbide on insulator structures

To investigate the origin of the high level of electrical compensation induced by hydrogen-implantation during the fabrication of SiCOI (Silicon Carbide layer transferred On Insulator) structures we have performed a comparative investigation of the electrical and optical properties of, both, 6H–SiCOI structures and hydrogen-implanted 6H–SiC epitaxial layers. We have found that the high resistivity of SiCOI structures comes most probably from an effect of compensation by deep traps created during the implantation, rather than H-passivation of dopants. The D 1-center, observed by photoluminescence after the high temperature treatment, and the Z 1/Z 2-center or C-defect, observed by deep level transient capacitance spectroscopy, could be the signature of these compensating centers.

Kinetics and morphological stability in sublimation growth of 6H and 4H SiC epitaxial layers

Very high growth rates (>2 mm h −1) in SiC epitaxy have been achieved. The rate determining mechanism changes from diffusion to kinetics when the growth pressure decreases below 5–10 mbar. At low pressures it is shown that sublimation of the SiC source is the rate determining step and that there is a free molecular transport from source to substrate. The growth rate is constant during several hours of growth and Si losses from the crucible are very small. These facts show that our growth system is stable. The obtained apparent activation energy (130 kcal mol −1) is attributed to the sublimation rate of the SiC source material. The morphology is smooth and the surfaces are specular if the growth conditions are selected within the given parameter window for morphological stability. The origin of the growth disturbances is discussed.

Hall effect investigations of 4H–SiC epitaxial layers grown on semi-insulating and conducting substrates

Nitrogen- and aluminum-doped 4H silicon carbide epitaxial layers were grown simultaneously on semi-insulating and conducting substrates. The layers were investigated by conventional van der Pauw Hall effect measurements and for comparison also with secondary ion mass spectrometry and capacitance voltage measurements. It was found, that the carrier concentration in the layers grown on conducting substrates were overestimated by the Hall effect measurement, which leads to an underestimation of the ionization energy of the main dopant, as compared to the layer grown on semi-insulating substrates. The difference can be explained by a two-layer Hall effect model.

Epitaxial growth of 4H–SiC by sublimation close space technique

SiC is suitable for power devices and high quality SiC epitaxial layers having a high breakdown voltage are needed and thick epilayer is indispensable. In this study, we used the close space technique (CST) method which enable us to obtain thick epitaxial layers faster. Source material used was 3C–SiC polycrystalline plate with high purity and 4H–SiC with 8° off (0001) toward 11 2 ̄ 0 was used for the substrate. Step-bunching was strongly affected by pressure during crystal growth. The carrier concentration of epilayer decreased when a lower pressure was employed. Schottky diode was also made.

Determination of hydrogen in 6H–SiC epitaxial layers by the 15N nuclear reaction analysis technique

By using the 15N nuclear reaction analysis (NRA) technique the hydrogen concentrations in 6H–SiC epilayers were determined and compared with the results of infrared (IR) absorption measurements. Both methods found similar hydrogen concentration. The hydrogen depth profile measured by the NRA technique is shown. The NRA spectra show hydrogen concentrations that remain constant for depths larger than 150 nm. The measured bulk concentrations for the two SiC epilayers are 1.4 × 10 20 and 2.5 × 10 20 hydrogen atoms/cm 3.

Domain misorientation in sublimation grown 4H SiC epitaxial layers

High resolution X-ray diffractometry has been applied to study domain misorientation in 4H SiC epitaxial-layers grown by the sublimation epitaxy method. The development of mosaicity of the epitaxial layer has been studied under different growth conditions, i.e. substrate polarity and layer thickness. In the growth experiments on Si-face with a lattice having a concave curvature the mosaicity was increased in the layer. With increasing thickness the structural quality of the layer was improved. Growth of an epilayer on concave C-face resulted in a domain structure comparable to the substrate but the lattice curvature increased considerably. The concave lattice plane curvature is a probable reason for degraded structural quality.

SiC epitaxial layer growth in a novel multi-wafer vapor-phase epitaxial (VPE) reactor

Experimental results are presented for SiC epitaxial layer growths employing a unique planetary SiC-VPE reactor. The high-throughput, multi-wafer (7×2″) reactor, was designed for atmospheric and reduced pressure operation at temperatures up to and exceeding 1600°C. Specular epitaxial layers have been grown in the reactor at growth rates ranging from 3–5 μm/h. The thickest layer grown to date is 42 μm thick. The layers exhibit minimum unintentional n-type doping of ∼1×10 15 cm −3, and room temperature mobilities of ∼1000 cm 2/V s. Intentional n-type doping from ∼5×10 15 cm −3 to >1×10 19 cm −3 has been achieved. Intrawafer layer thickness and doping uniformities (standard deviation/mean at 1×10 16 cm −3) are typically 4 and 7%, respectively, on 35 mm diameter substrates. Moderately doped, ∼4×10 17 cm −3, layers, exhibit ∼3% doping uniformity. Recently, 3% thickness and 10% doping uniformity (at 1×10 16 cm −3) has been demonstrated on 50 mm substrates. Within a run, wafer-to-wafer thickness deviation averages ∼9%. Doping variation, initially ranging as much as a factor of two from the highest to the lowest doped wafer, has been reduced to ∼13% at 1×10 16 cm −3, by reducing susceptor temperature nonuniformity and eliminating exposed susceptor graphite. Ongoing developments intended to further improve layer uniformity and run-to-run reproducibility, are also presented.

Morphology and polytype disturbances in sublimation growth of SiC epitaxial layers

Epitaxial layers of 6H and 4H-SiC have been grown with high growth rate by sublimation epitaxy on SiC substrates with different surface orientation. Polytype disturbances, i.e. regions within the layer where the polytype differs from that of the underlying substrate, may be observed at domain boundaries, defects and along the edges of the samples. The formation of polytype disturbances is more pronounced as the off-axis angle of the substrate decreases. For on-axis substrates, due to a dislocation controlled growth mechanism, many nucleation centers are formed thus creating several domains with polycrystalline growth along the boundaries. This results in a severe roughening of the layer surfaces. With increasing substrate off-orientation, the morphology improves and the appearance of polytype disturbances is diminished or prevented. For thick layers obtained with high growth rates, the surfaces of the layers grown on vicinal substrates are specular with some defects which are formed from obstacles. The growth proceeds via step-flow growth mechanism. A polytype change at the edge, where the growth steps start, may occur indicating a change in the growth mechanism from step-flow growth to 2D nucleation. 3C-SiC can become the dominant polytype in the epitaxial layer grown on 6H- or 4H-SiC substrates. The polytype disturbance at the edge is considerably reduced or has completely disappeared if the layer thickness does not exceed 300–400 μm as obtained with moderate growth rate of 100–200 μm/h.

Ion-implantation in SiC and GaN

The electrical activation behavior of N- and Al-implantations in bulk V-doped semi-insulating 4H–SiC is similar to that in 4H–SiC epitaxial layers. The As- and Sb-implantations in p-type 6H–SiC epitaxial layers showed out-diffusion behavior with room-temperature sheet carrier concentrations of <20% of the implanted dose. In n-type GaN, compensating levels introduced by the high-dose implantation damage are thermally stable even at 1150°C annealing.

Properties of 4H-SiC by Sublimation Close Space Technique

ABSTRACTSiC is suitable for power devices but high quality SiC epitaxial layers having a high breakdown voltage are needed and thick epilayer is indispensable. In this study, CST method (Close Space Technique) was used to rapidly grow thick epitaxial layers. Source material used was 3C-SiC polycrystalline plate of high purity while 4H-SiC(0001) crystals inclined 8° off toward &lt;1120&gt; was used for the substrate. Quality of the epilayer was influenced significantly by pressure during growth and polarity of the substrate. A p-type conduction was obtained by changing the size of p-type source material. The carrier concentration of epilayer decreased when a lower pressure was employed. Schottky diode was also fabricated.

Reliability of metal–oxide–semiconductor capacitors on nitrogen implanted 4H-silicon carbide

4H- SiC epitaxial layers were implanted with nitrogen up to doses of 1×1015 cm−2 and annealed at different temperatures. Atomic force microscopy revealed that the roughness of the SiC surface increased with the annealing temperature. It was shown that the oxide grows thicker on substrates with doping levels exceeding 1×1018 cm−3. The barrier height at the SiC/SiO2 interface, determined by voltage ramping on metal–oxide–semiconductor capacitors, decreased with increasing implantation dose. This decrease was attributed to residual implantation damage. Constant current injection experiments revealed an opposite charge buildup at the SiC/SiO2 interface for the highest implantation dose compared to samples with no implantation. It was shown that the breakdown behavior can be improved by annealing at 1700 °C compared to 1450 °C despite a higher surface roughness.