Porous 4H-SiC Research Articles

Novel Use of Columnar Porous Silicon Carbide Structures as Nanoimprint Lithography Stamps

Columnar porous Si-face 6H-SiC substrates were prepared by a photo-electrochemical etching method and applied as nanoimprint lithography (NIL) stamps. The diameter of the pores in the porous region was about 20 nm and the center-to-center separation between pores was about 60 nm. The columnar porous SiC substrates were subjected to a vapor phase silanization treatment whereby a monolayer of perfluorooctyltrichlorosilane (FOTS) was deposited in order to keep the stamps from sticking to the substrates during the imprint step. Subsequently, the porous SiC stamps were used to imprint polymethylmethacrylate (PMMA) at elevated temperatures and pressures. The imprinted PMMA could then be used to transfer the nanopattern on the columnar porous SiC to other substrates for various purposes; e.g. templates for GaN regrowth, catalysts for nanowire growth by vapor-liquid-solid type methods (VLS), etc. SiC is not typically used for NIL stamps since etch processing of SiC is less mature than that of Si. However, as demonstrated here, there is no reason why SiC cannot be used as a material for NIL stamps. The superior mechanical properties to Si make the use of SiC alluring as a master template for NIL processing.

A STUDY ON THE PHOTOLUMINESCENCE PROPERTIES OF POROUS 6H-SiC(p) AND SiC FILM ON p-Si SUBSTRATE

P-type porous SiC layers were fabricated by anodization of a hot-pressed p-type 6H-SiC (30 kΩcm) and a 1.6 μm SiC film was deposited onto p-type Si (100) substrate by Pulsed Laser Deposition (PLD) in HF. In order to facilitate the electrochemical etching of the substrate, ethylene glycol electrolyte has been added to the solution and a thin metallic film of aluminium ( Al ) has been deposited onto a SiC prior to anodization. The structure and properties of the porous SiC layer formed by this method were investigated by Scanning Electron Microscopy (SEM) and Photoluminescence (PL). It shows that photoluminescence spectra exhibit two emission bands centered at 2.20 eV (blue band) and an extended broad band from 2.20 to 3.22 eV (green band) from porous 6H-SiC (PSC). On the contrary, porous thin 1.6 μm SiC layer exhibits only a blue band centered at 2.95 eV. Finally, the results indicate clearly that porous SiC gives off green and blue light. Also, there is a shift to blue luminescence with the same order of magnitude when using a thin SiC layer deposited onto silicon (p- Si (100)).

Raman Spectra of a 4H-SiC Epitaxial Layer on Porous and Non-Porous 4H-SiC Substrates

A lightly doped n-type homo-epitaxial layer was grown by CVD onto a heavily doped n-type 4H-SiC substrate for which half of the surface had been made porous by photoelectrochemical etching. Raman spectra are obtained in the optic phonon region using three scattering geometries. An effective medium model for the porous layer is used to assist in the interpretation of the spectra. This work demonstrates that the contributions to the Raman spectra of the various layers in a sample with multiple 4H-SiC layers can be extracted.

Heavily nitrogen-doped 4H-SiC homoepitaxial films grown on porous SiC substrates

Heavily nitrogen-doped 4H-SiC homoepitaxial films were grown on normal p-type and porous 4H-SiC (PSC) substrates. The nitrogen doping concentration and resistivity of the epitaxial films, measured by Hall measurement, were about 10 20–10 21 cm −3 and 0.002–0.006 Ω cm, respectively. From X-ray analysis, it was observed that the crystallinity of the epilayer grown on porous 4H-SiC substrates is better than that grown on p-type 4H-SiC substrates. In spite of the heavy doping, the full-width at half-maximum (FWHM) value of the epitaxial film, which was grown on the porous substrate, was about 13 arcsec, proving its excellent crystal quality. In contrast, the heavily nitrogen-doped epilayer, which was grown on the normal substrate, had a FWHM value of 16.9 arcsec. High-voltage transmission electron microscope (HVTEM) and scanning electron microscope (SEM) observations made clear that the enhancement of the crystallinity was caused by stress relaxation near pore boundary and defect terminations in the porous layer.

SiC Pore Surfaces: Surface Studies of 4H-SiC(1-102) and 4H-SiC(-110-2)

The morphology and atomic structure of 4H-SiC(1102) and 4H-SiC(1102) surfaces, i.e. the surfaces found in the triangular channels of porous 4H-SiC, have been investigated using AFM, LEED and AES. After hydrogen etching the surfaces show steps parallel and perpendicular to the caxis, yet drastically different morphologies for the two isomorphic orientations. Both surfaces immediately display a sharp LEED pattern. Together with the presence of oxygen in the AES spectra this indicates the development of an ordered oxide. Both surfaces show an oxygen free, well ordered surface after Si deposition and annealing.

SiC pore surfaces: Surface studies of 4H–SiC(11¯02) and 4H–SiC(1¯102¯)

The morphology and atomic structure of 4H–SiC(11¯02) and 4H–SiC(1¯102¯) surfaces, i.e., the surfaces found in the triangular channels of porous 4H–SiC, have been investigated using atomic force microscopy, low-energy electron diffraction (LEED), and Auger electron spectroscopy (AES). After hydrogen etching, the surfaces show steps parallel and perpendicular to the c axis, yet drastically different morphologies for the two isomorphic orientations. Both surfaces immediately display a sharp LEED pattern. Together with the presence of oxygen in the AES spectra, this indicates the development of an ordered oxide. Both surfaces show an oxygen-free well-ordered surface after Si deposition and annealing.

Modification of the Oxide/Semiconductor Interface by High Temperature NO Treatments: A Combined EPR, NRA and XPS Study on Oxidized Porous and Bulk n-Type 4H-SiC

The effect of thermal treatments in nitric oxide (NO) on the paramagnetic defects at the 4H-SiC/SiO2 interface are analyzed by EPR in oxidized porous samples. The results on ultrathin thermal oxides show that the NO treatment at 1000°C is insufficient for an efficient reduction of the two dominant paramagnetic interface defects: PbC centers and carbon clusters. From the NRA and XPS analysis of bulk samples treated under the same conditions we attribute the weak effect to the low nitrogen concentration of only 1% at the interface.

Preparation of Porous 4H-SiC by Surface Anodization

The preparation of porous 4H-SiC by electrochemical etching of SiC crystals was investigated. The porous layer was created at the porous SiC (PSC)/SiC interface but not from the SiC/electrolyte interface. The nanopores at the adjacent region of PSC/SiC interface were bigger than those at the top region. In the visible light region, the optical reflectance from PSC exhibits interference fringes. In the Reststrahlen region, the fourier transform infrared (FTIR) reflectance of porous 4H-SiC shows a splitting into more bands: a broad band with high reflectivity at low frequency and several sharp peaks near the LO frequency. The width and shape of FTIR spectra depended on the anodization current density. The anodization current density is a crucial parameter which determined the porosity, porosity depth profile, and the thickness of PSC layers. A pore transformation of porous structure was observed after chemical vapor deposition process.

X-Ray Diffraction Analysis of Epigrowth on Porous 4H-SiC Substrates

The methods of X-ray topography and diffractometry have been applied to characterize the structure of epilayers grown on porous layers. Two geometrical configurations of defects determined to be stacking faults (SF) were revealed: i) with the images of triangular shape with the edge size 560 µm along the <10-10> directions; ii) linear shape along the [11-20] direction. The sources of SFs are located within the epilayer and start from the epilayer / porous layer interface. We propose that the source of SFs is connected with graphitization of porous layer at the temperature of epitaxy.

Synthesis and structural characterization of a heterocomposition based on porous layers of SiC polytypes

We have studied the structure of transverse cross sections of a heterocomposition consisting of porous layers of silicon carbide polytypes (substrate 6H-SiC/por-6H-SiC/epi-3C-SiC/por-3C-SiC). The epitaxial layer of 3C-SiC polytype (epi-3C-SiC) was grown by sublimation in vacuum on the surface of a porous 6H-SiC sublayer (por-6H-SiC) obtained by electrochemical etching of a 6H-SiC substrate. The boundary between the 3C-SiC epilayer and the porous 6H-SiC sublayer contains no defective transition layer, and the 3C-SiC epilayer is free of dislocations.

Graphitization of porous 6H-SiC layers during thermal treatment in vacuum

We have studied the effect of thermal treatment in vacuum on the optical transmission of 6H-SiC samples with a porous layer on the Si face in the visible and near infrared spectral range. An analysis of changes in the absorption coefficient shows that the process of graphitization begins at a temperature below 70°C.

Vibrational and Emission Properties of Porous 6H-SiC

Vibrational and Emission Properties of Porous 6H-SiC

Growth of GaN Films on Porous 4H-SiC Substrate by Metal-Organic Chemical Vapor Deposition

Growth of GaN Films on Porous 4H-SiC Substrate by Metal-Organic Chemical Vapor Deposition

Improvement in the Crystalline Quality of Epitaxial GaN Films Grown by MOCVD by Adopting Porous 4H-SiC Substrate

Epitaxial GaN films were grown on a porous 4H-SiC (PSC) substrate at 1080°C using metallorganic chemical vapor deposition (MOCVD). The GaN films showed an improved epitaxial quality with the smallest threading dislocation density being demonstrated for growth on the PSC substrates, when compared to that on standard 4H-SiC substrates as well as on sapphire The full width at half-maximum values of the reflection in high-resolution X-ray diffraction of the GaN films grown on sapphire, the standard 4H-SiC, and the PSC substrate under a skew symmetric diffraction geometry were 388, 313, and 224 arcsec, respectively, showing that PSC is effective in reducing the threading edge dislocation density. 224 arcsecs is the smallest ever reported for GaN films. This may be due to a relaxation of the mismatch in the lattice parameter by the lower thermal strain in the GaN films due to the porous structure of the PSC substrate. © 2004 The Electrochemical Society. All rights reserved.

Semi-insulating silicon carbide layers obtained by diffusion of vanadium into porous 4H-SiC

Semi-insulating silicon carbide layers have been obtained by diffusion of vanadium into porous 4H-SiC. The diffusion was performed from a film deposited by cosputtering of silicon and vanadium, with the content of the latter equal to 20%. The diffusion profile of vanadium in porous silicon carbide has a complex structure with a fast diffusion coefficient of 7×10−15 cm2/s. The activation energy of the resistivity of vanadium-diffusion-doped porous SiC layers is 1.45 eV. The resistivity of vanadium-doped semi-insulating layers is 5×1011 Ω cm at 500 K, which exceeds the resistivity of undoped porous SiC by two orders of magnitude. The results obtained indicate that porous SiC is a promising material for semi-insulating substrates in device structures based on wide-bandgap semiconductors.

Characterization of 4H-SiC Homoepitaxial Films on Porous 4H-SiC from Bis(trimethylsilyl)methane Precursor

4H-SiC homoepitaxial films were grown on 8° off-axis porous 4H-SiC (0001) faces in the temperature range of by chemical vapor deposition from bis(trimethylsilyl)methane (BTMSM) precursor. The activation energy for growth was 5.6 kcal/mol, indicating that the film growth is dominated by the diffusion-limited mechanism. Triangular stacking faults were incorporated in the SiC thin film grown at low temperature of 1280°C due to the formation of 3C-SiC polytype. Moreover, super-screw dislocations appeared seriously in the SiC film grown below 1320°C. Clean and featureless morphology was observed in the SiC film grown below 25 standard cubic centimeters per minute (sccm) carrier gas flow rate of BTMSM at 1380°C while 3C-SiC polytype with double positioning boundaries grew at 30 sccm flow rate of BTMSM. The dislocation density of the epi layer was strongly influenced by the growth temperature and flow rate of BTMSM. Double axis crystal X-ray diffraction and optical microscopy analysis revealed that the dislocation density decreased at the higher growth temperature and lower flow rate of BTMSM. The full width at half maximum of the rocking curve of the film grown at optimized condition was 7.6 arcsec and the sharp free exciton and Al bound exciton lines appear in the epi layer, which indicates that the 4H-SiC film was of very high quality. © 2003 The Electrochemical Society. All rights reserved.

Oxidation of Porous 4H-SiC Substrates

Oxidation of Porous 4H-SiC Substrates

Surface, Pore Morphology, and Optical Properties of Porous 4H-SiC

Electron microscopy and spectroscopic ellipsometry are used to investigate the microstructural and optical properties of porous SiC. It is discussed that 1c dislocations are likely to play a role in the initial phases of the anodization process in terms of formation of nanometer sized holes. Pores are found to initially propagate nearly parallel with the sample surface and gradually change direction toward the c axis. Pore sizes are found to increase with depth toward the interface between the porous SiC and the substrate. A different pore morphology is found along the internal surfaces of micropipes, where structures are relatively large in size and appear spherical in shape. The anisotropy in pore propagation influences the etch rate, which varies in a nonlinear manner with the anodization time. The etching rate is also influenced by the larger absorptivity of the porous layers caused by formation of a disordered phase at the interface between the crystalline SiC and the pores. Ellipsometric analysis of porous SiC layers yields thicknesses and porosities in good agreement with the electron microscopy observations. Optical properties of the solid content of the porous layers are significantly different from those of bulk crystalline SiC, and depend on the etching time and sample thickness. © 2001 The Electrochemical Society. All rights reserved.

Porous Anodic 4H-SiC: Thickness Dependent Anisotropy in Pore Propagation and Ellipsometric Characterization

Pores in porous 4H-SiC propagate first nearly parallel with the sample surface and gradually change direction tc,wards the direction of the c-axis. A disordered region at the interface between crystalline SiC and the pores is encountered ed, which significantly influences the optical response of the material. Thickness and porosity of porous SIC as obtained using variable angle of incidence spectroscopic ellipsometry show good agreement with electron microscopy observations.

Self-organization in porous 6H–SiC

Pores in porous 6H–SiC were found to propagate first nearly parallel with the basal plane and gradually change direction and align with the c axis. As a consequence, well-defined columnar pores were formed. It was shown that the rate of change of propagation directions was influenced by the etching parameters, such as hydrofluoric acid concentration and current density. Larger currents resulted in formation of larger pores. Pore sizes were found to increase with depth due to a decrease of the acid concentration. In addition, due to chemical etching effects, larger pore sizes were obtained close to the sample surface.