4H-SiC Homoepitaxial Layers Research Articles

Spheroid 3C inclusions in 8° off-axis 4H-SiC epilayers grown by chemical vapor deposition

Spheroidal inclusions in 4H-SiC homoepitaxial layers deposited by silane-based chemical vapor deposition (CVD) process have been investigated by KOH etching, optical microscopy, and transmission electron microscopy. The inclusions consist of polycrystalline 3C silicon carbide bearing no orientational relationship with the substrate and cause characteristic corrugation of epilayer surface referred to as “arrow” defect. Their origin is interpreted as due to SiC particles deposited on the wafer surface at the initial stages of the CVD process.

Cathodoluminescence study of the properties of stacking faults in 4H-SiC homoepitaxial layers

In-grown stacking faults in n-type 4H-SiC epitaxial layers have been investigated by real-color cathodoluminescence imaging and spectroscopy carried out at room and liquid helium temperatures. Stacking faults with 8H stacking order were observed, as well as double layer and multilayer 3C-SiC structures and a defect with an excitonic band gap at 2.635 eV. It was found that 8H stacking faults and triangular surface defects can be generated from similar nucleation sources. Time-resolved measurements reveal that compared to defect-free regions, the carrier lifetimes are severely reduced by the presence of stacking faults corresponding to triangular surface defects and three-dimensional 3C-SiC inclusions.

The Effect of 4H-SiC Substrate Surface Scratches on Chemical Vapor Deposition Grown Homo-Epitaxial Layer Quality

The influence of substrate surface scratches on the quality of CVD grown 4H-SiC homo-epitaxial layers has been studied using a combination of post-growth Monochromatic Synchrotron X-ray Topography (MSXT) and KOH etching. MSXT observations suggest that the scratches on the substrate surface act as dislocation nucleation centers during the growth. When the scratch is along the off-cut direction, only TED-TED pairs are generated. As the inclination of the scratch to the off-cut direction increases, an increasing number of TED-BPD pairs are generated. A model is presented for a possible mechanism for the nucleation of dislocations at scratches.

Influence of Growth Rate and C/Si-Ratio on the Formation of Point and Extended Defects in 4H-SiC Homoepitaxial Layers Investigated by DLTS

4H-SiC epilayers are homoepitaxially grown on 4H-SiC substrates with different C/Si-ratios and different growth rates by the chemical vapour deposition method. DLTS investigations are applied in order to trace energetically deep states of electrically active point defects and extended defects, which may act as the source for the degradation of electronic devices. In addition, the dependence of the DLTS signal heights on the filling pulse length is studied.

Deep level defects in unintentionally doped 4H-SiC homoepitaxial layer

Unintentionally doped 4H-SiC homoepitaxial layers grown by hot-wall chemical vapor deposition (HWCVD) have been studied using photoluminescence (PL) technique in the temperature range of 10 to 240 K. A broadband green luminescence has been observed. Vacancies of carbon (VC) are revealed by electron spin resonance (ESR) technique at 110 K. The results strongly suggest that the green band luminescence, as shallow donor-deep accepter emission, is attributed to the vacancies of C and the extended defects. The broadband green luminescence spectrum can be fitted by the two Gauss-type spectra using nonlinear optimization technique. It shows that the broadband green luminescence originates from the combination of two independent radiative transitions. The centers of two energy levels are located 2.378 and 2.130 eV below the conduction band, respectively, and the ends of two energy levels are expanded and superimposed each other.

Transmission Electron Microscopy and Luminescence Studies of Quantum Well Structures Resulting from Stacking Fault Formation in 4H-SiC Layers

The formation of 3C-SiC staking fault inclusions in higher band-gap other SiC polytypes provides a unique possibility to obtain quantum well (QW) structures without changing of chemical composition of epitaxial layers during growth. [1] In this report we present low temperature microphotoluminescence and transmission electron microscopy (TEM) studies of 3C-SiC stacking faults inclusions in 4H-SiC homoepitaxial layers, grown by molecular vapor deposition (MOCVD) on 4H-SiC substrates with an 8o surface miscut from the basal plane. TEM investigations showed that basal plane inclusion with cubic 3CSiC stacking faults present in the substrate propagate into the 4H-SiC layer material, forming inclined QW structures. The distance between SF was found to vary from 2 nm to 1.5 μm depending on the position on the layer. The presence of SF in the layer was confirmed by photoluminescence studies. They showed that in addition to excitonic emission typical for high quality 4H-SiC layers, well defined group of several emission lines around energy of 2.9 eV, which we attribute to QW formation induced by 3C-SiC staking faults [2]. In contrary to the emission due to nitrogen-bound excitons (observed at higher energy), which remains practically unchanged when changing position on the sample, the energy of spectral lines related to SF emission shows quasi-periodic jumps between well defined values when scanning on the sample surface (laser spot on the sample is of about 1-2 μm). The observed intensity pattern changes in a similar way. The obtained experimental data are discussed in terms of single, double and multiple quantum wells formed form neighboring 3C-SiC SF layers. It is confirmed that a concept of “structure-only” QW is promising from the point of view applications, provided precise control of FS formation during the growth of SiC based nanostructures is attainable. Our results show that, at least the formation of SF related QW structures can be successfully initiated by appropriate substrate preparation.

Aspects of Dislocation Behavior in SiC

A review is presented of the current understanding of the dislocation configurations observed in PVT-grown 4H- and 6H-SiC boules and CVD-grown 4H-SiC homoepitaxial layers. In both PVT-grown boules and CVD-grown epilayers, dislocation configurations are classified according to whether they are growth dislocations, i.e., formed during growth via the replication of dislocations which thread the moving crystal growth front, or result from deformation processes (under either mechanical or electrical stress) immediately following growth, during post growth cooling, i.e., behind the crystal growth front or during device operation. Possible formation mechanisms of growth defects in the PVT grown boules, such as axial screw dislocations and threading edge dislocation walls are proposed. Similarly, possible origins of growth defect configurations in CVD-grown epilayers, such as Frank faults bounded by Frank partials, BPDs and TEDs, are also discussed. In a similar way, the origins of BPD configurations resulting from relaxation of thermal stresses during post-growth cooling of the PVT boules are discussed. Finally, the susceptibility of BPD configurations replicated into CVD grown epilayers from the substrate towards Recombination Enhanced Dislocation Glide (REDG) is discussed.

The relation between green-band luminescence of 4H-SiC homoepitaxial layer and defects

The Green-band luminescence (GL) from an Al-doped 4H-SiC Homoepitaxial layer prepared by using chemical vapur deposition (CVD) has been observed and studied. The deep pattern obtained by SEM and the buffer layer can be found in the interface between SiC substrate and epilayer. The deep profiles of Si atom in 4H-SiC epilayers obtained by using the secondary ion mass spectrometry (SIMS) and the X-ray photoelectron spectroscopy (XPS) show the relative distribution of Si and C. The results strongly suggest that the vacancy of C and the extended defects (point defects, threading dislocations, et al) from buffer layer are responsible for the GL emission. The full width at half maximum (FWHW) of GL indicates the distributions of VC and its complex defects. The quality of crystals and the distribution of defects in epitaxial layers can be characterized by the intensity and the wavelength of GL obtained from samples at room temperature.

Nitrogen doped 4H-SiC homoepitaxial layers grown by CVD

Homoepitaxial growth of 4H-SiC on off-oriented Si-face (0001) 4H-SiC substrates is performed at 1550℃, under the pressure of 100 mbar using the mbar step-controlled technique with rotation in the horizontal low-pressure hot-wall CVD (LP-HW-CVD) system to obtain high quality 4H-SiC epilayers. The surface morphology, structure and optical properties of the epilayers are characterized by SEM, AFM, FTIR and C-V measurement. The 4H-SiC epitaxial layer has a good crystalline structure and mirror-like surface with few surface defects. N type 4H-SiC epilayers are obtained by in-situ doping of N2.The uniformities of thickness are 1.74%, 1.99%, and 1.32%, and the uniformities of doping concentration are tested to be 3.37%, 2.39%, and 2.01%, respectively. The deviations in thickness and concentration between different samples are 1.54% and 3.63% under the same processing conditions, which shows that the process is repeatable and reliable.

Morphology of basal plane dislocations in 4H-SiC homoepitaxial layers grown by chemical vapor deposition

The morphology of basal plane dislocations (BPDs) in 4H-SiC homoepitaxial layers has been investigated by plan-view transmission x-ray topography and molten KOH etching. Three types of BPDs are distinguished based on their morphologies. These include interfacial dislocations, curved dislocations, and circular loop dislocations around micropipes. Their characteristics are studied in detail and possible sources of their formation during epitaxy are discussed.

Transmission electron microscopy analysis of mechanical polishing-related damage in silicon carbide wafers

The subsurface damage generated by mechanical polishing of silicon carbide wafers was investigated and quantified by plan view transmission electron microscopy (TEM) and atomic force microscopy (AFM). Damage generated during polishing using diamond abrasives with 0.5 µm particle size consists of dislocation loops with length up to 400 nm from the scratches. The total dislocation density was estimated at 5 × 1010 dislocations cm−2. TEM analysis of the Burgers vectors indicates that the initial perfect dislocations have a Burgers vector of b = a/3 ⟨11–20⟩-type with many dislocation dissociated into two partials with b = a/3 ⟨1–100⟩. The depth of damage was estimated to be up to 50 nm. 4H–SiC homoepitaxial layers grown on mechanically polished substrates without further surface treatment exhibit threading dislocation density along scratches in the order of 105 cm−1.

Structure of Carrot Defects in 4H-SiC Epilayers

Structure of the “carrot” defects in 4H-SiC homoepitaxial layers deposited by CVD has been investigated by plan-view and cross-sectional transmission x-ray topography, cross-sectional transmission electron microscopy, atomic force microscopy, and KOH etching. The carrot defects nucleate at the substrate/epilayer interface at the emergence points of threading screw dislocations propagating from the substrate. The typical defect consists of two stacking faults: one in the prismatic plane with second one in the basal plane. The faults are connected by a stair-rod dislocation with Burgers vector 1/n[10-10] with n>3 at the cross-over. The basal plane fault is of Frank-type. Carrot defects are electrically active as evidenced by contrast in EBIC images indicating enhanced carrier recombination rate. Presence of carrot defects in the p-i-n diodes results in higher pre-breakdown reverse leakage current and approximately 50% lower breakdown voltage compared to the nominal value.

Morphological defects and uniformity issues of 4H-SiC homoepitaxial layers grown on off-oriented (0 0 0 1) Si faces

The morphological defects and uniformity of 4H-SiC epilayers grown by hot wall CVD at 1500 °C on off-oriented (0 0 0 1) Si faces are characterized by atomic force microscope, Nomarski optical microscopy, and Micro-Raman spectroscopy. Typical morphological defects including triangular defects, wavy steps, round pits, and groove defects are observed in mirror-like SiC epilayers. The preparation of the substrate surface is necessary for the growth of high-quality 4H-SiC epitaxial layers with low-surface defect density under optimized growth conditions.

Effects of Different Defect Types on the Performance of Devices Fabricated on a 4H-SiC Homoepitaxial Layer

AbstractAn 8° off-axis 4H-SiC wafer with circular Schottky contacts fabricated on a CVD grown 4H-SiC homoepitaxial layer was studied to investigate the influence of various defects, including small (closed-core) screw dislocations (Burgers vector of 1c or 2c), hollow-core (micropipes; Burgers vector larger than 2c), threading edge dislocations (from conversion of basal plane dislocations from the substrate into the epilayer), grain boundaries and triangular defects, on the device performance in the form of breakdown voltages. The defects were examined using synchrotron white beam x-ray topography (SWBXT) based techniques and molten KOH etching. The devices commonly contained basal plane dislocations, small screw dislocations and threading edge dislocations, the latter two of which could give rise to low breakdown voltages for the devices. In addition, less commonly observed defects such as micropipes, grain boundaries and triangular defects are much more destructive to device performance than closed-core screw dislocations and threading edge dislocations.

2-Inch 4H-SiC Homoepitaxial Layer Grown on On-Axis C-Face Substrate by CVD Method

Homoepitaxial growth was carried out on 4H-SiC on-axis substrate by horizontal hot wall chemical vapor deposition. By using carbon face substrate, specular surface morphology of a wide area of up to 80% of a 2-inch epitaxial wafer was obtained at a low C/Si ratio growth condition of 0.6. The Micropipe in on-axis substrate was indicated to be filled with spiral growth and to be dissociated into screw dislocations during epitaxial growth. It was found that the appearance of basal plane dislocations on the epitaxial layer surface can be prevented by using an on-axis substrate.

Doping-induced strain and relaxation of Al-doped 4H-SiC homoepitaxial layers

Aluminum-doped 4H-SiC epilayers with Al concentrations in the 7.4×1018–3.8×1020cm−3 range were deposited on off-orientation (0001) wafers by chemical vapor deposition method and analyzed using high-resolution x-ray diffraction, transmission electron microscopy, and KOH etching. Reciprocal space maps of (0008) reflection revealed two distinct peaks originating from the substrate and doped epilayer. For Al concentration below 3.3×1020cm−3, 10μm thick layers were fully strained with the a-lattice parameter of the layer matching that of the substrate. The equilibrium c-lattice parameter change versus doping was determined to be 1.3±0.3×10−24cm3. The basal planes of the epilayers were tilted in respect to the substrate in the direction of the offcut with the tilt magnitude proportional to the doping concentration. The 10μm thick layers with Al concentration above 3.3×1020cm−3 underwent partial relaxation. The a-lattice parameter of the epilayer was higher than that of the substrate, the width of ω and 2θ scans of (0008) x-ray peaks broadened by a factor of 2 compared to strained layers, and the threading dislocation density increased by several orders of magnitude. Since no inclusions have been found in the relaxed epilayer, we interpret the above changes as due to strain relaxation by nucleation of dislocations.

Homoepitaxial growth of 4H-SiC on on-axis [formula omitted] C-face substrates by chemical vapor depositon

4H-SiC homoepitaxial layers were grown on on-axis (0 0 0 1 ̄ ) C-face substrates at an inclination of 0.5° or less by horizontal hot wall chemical vapor deposition. After H 2 etching, the surface morphology of the on-axis (0 0 0 1 ̄ ) C-face substrate showed a straight step structure that had the same height as a 4H-SiC lattice constant along the c-axis. Specular surface morphology of a wide area of up to 80% of a 2-in wafer was obtained at a low C/Si ratio of 0.6. It was found that that appearance of basal plane dislocations on the epitaxial layer surface can be prevented by using an on-axis substrate. Ni/4H-SiC Schottky barrier diodes fabricated on the epitaxial layer at 1×10 16 cm −3 showed a high breakdown voltage of up to 1 kV and low leakage current.

Homoepitaxial growth of 4H–SiC [formula omitted] and nitrogen doping by chemical vapor deposition

Homoepitaxial growth of 4H–SiC (0 3 3 ̄ 8) by hot-wall chemical vapor deposition has been investigated. The 4H–SiC (0 3 3 ̄ 8) is inclined by 54.74° toward [0 1 1 ̄ 0] from (0 0 0 1), which is semi-equivalent to (0 0 1) in a zincblende structure. 4H–SiC homoepitaxial layers with a specular surface can be obtained on 4H–SiC (0 3 3 ̄ 8) without intentional off angle. The lowest donor concentration of undoped 4H–SiC (0 3 3 ̄ 8) epilayers was 3×10 14 cm −3. The doping efficiency of nitrogen on 4H–SiC (0 3 3 ̄ 8) was similar to that on 4H–SiC (1 1 2 ̄ 0) , which is in between that on off-axis (0 0 0 1) and (0 0 0 1 ̄ ) faces. Growth results on this novel face are compared with those on off-axis {0 0 0 1} and (1 1 2 ̄ 0) from viewpoints of growth mechanism and impurity doping.

Sub-μm scale photoluminescence images of wide bandgap semiconductors by cryogenic scanning optical microscope

Photoluminescence images with a sub-μm spatial resolution were obtained at 15 K using a newly developed cryogenic scanning microscope with both high throughput and simplicity of measurement procedure. The objective was put in a sample chamber to reduce thermal conduction from the objective to the sample. The spatial resolution at 15 K was measured to be 300 nm at a wavelength of 488 nm, which is almost equal to the diffraction limit. The microscope demonstrated μm-scale defective regions emitting a PL signal at 3.40 eV in GaN grown on SiC. At an elongated surface defect in a 4H–SiC homoepitaxial layer, the PL image ascribed to excitons bound to neutral nitrogen atoms showed a dark line coincident with the defect, indicating that non-radiative recombination dominates over the excitonic emission at the surface defect.

半導体エレクトロニクス サブミクロン解像度極低温顕微ホトルミネセンス装置の開発とワイドギャップ半導体の欠陥検出への応用

A new photoluminescence (PL) microscope has been developed with a conventional optical system to obtain a monochromated PL image at a low temperature with a spatial resolution in sub-micron range. The objective and sample were put in the identical vacuum chamber to ensure thermal insulation between them. The spatial resolution at 15K was confirmed to be almost equal to the diffraction limit, i.e., 0.3μm, at a wavelength of 488nm. The microscope clearly visualized defective region, and polytype domain with a spatial resolution of 0.3μm in GaN. PL emissions ascribed to free excitons (EXA) and excitons bound to donors (D0X) were discriminated in μ-PL measurements at 15K on epitaxially laterally overgrown (ELO) GaN. The emission at 3.487eV ascribed to D0X became strong above the SiO2 mask within a distance of 8μm from the mask. In a μ-PL image related to excitons bound to N atoms in a 4H-SiC homoepitaxial layer, a dark line was observed at a line-shaped surface defect. Since the PL emission from donor (N)-acceptor (B) pairs was spatially uniform, N atoms were incorporated uniformly in the 4H-SiC epilayer. The dark line is probably caused by an enhancement of nonradiative recombination at the surface defect.