(Invited) Understanding of Growth Kinetics of Thermal Oxides on 4H-SiC (0001) for Control of MOS Characteristics

Control of thermal oxidation conditions is inevitable to achieve a high-quality MOS interface on SiC substrates. We investigated the kinetics and thermodynamics of 4H-SiC oxidation for nanometer-thick SiO2/SiC system, to find out thermodynamically preferred conditions for a smooth elimination of carbon byproduct from the interface. A linear regime of thermal oxidation of 4H-SiC (0001) was clearly observed with a high activation energy corresponding to direct CO ejection from the interface. Based on our understanding of oxidation kinetics, we found that nearly-ideal MOS characteristics with reduced interface state density ~1011 cm-2eV-1 or less, were achievable on 4H-SiC (0001) only by dry oxidation processes.

Source:IOPscience

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Epitaxial Deposition of SiC onto 4H SiC using a Hollow Cathode

Thin films of SiC were deposited using DC, RF and pulsed sputtering of a hollow cathode. The majority of the films were deposited using RF sputtering at temperatures ranging from 610 to 858 degrees C. Initial films were deposited onto Si substrates in order to determine deposition rates, film uniformity, and film composition. The introduction of a rotating substrate holder greatly improved the film thickness and composition uniformity. The samples were characterized using X-ray diffraction (XRD), Raman spectroscopy, optical absorption, and infrared ellipsometry. The initial films were polycrystalline in nature independent of the substrate used for deposition. The 4H/3C polytype ratio increases strongly for elevated substrate temperatures for the films which were grown homo-epitaxially on 4H SiC. This observation suggests a new avenue for homo-epitaxial growth of SiC onto 4H SiC and rapid hollow cathode sputtering is envisioned for the growth of single crystal films of 4H SiC for future device applications.

Source:IOPscience

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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.

Source:IOPscience

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Angle-Resolved Photoelectron Spectroscopy Studies of Initial Stage of Thermal Oxidation on 4H-SiC (0001) on-Axis and 4° Off-Axis Substrates

t is a key for improving the performance of SiC MOSFET to clarify SiO2/SiC interface structure formed by thermal oxidation. We have investigated the initial stage of thermal oxidation on 4H-SiC (0001) on-axis and 4° off-axis substrates using angle-resolved photoelectron spectroscopy. The changes of the Si 2p3/2 and C 1s photoelectron spectra show that the off-axis has an influence on the chemical bonding state of SiO2/SiC. On the other hand, there isn't difference in the oxidation rate between on-axis 4H-SiC(0001) and 4° off-axis 4H-SiC(0001).

Source:IOPscience

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Effect of carbon in Si oxide interlayers of the Al2O3/4H-SiC structure on interfacial reaction by oxygen radical treatment

The interface state density of Al2O3/4H-SiC can be decreased by oxygen radical treatment at room temperature, so we have systematically investigated the mechanism of the interfacial reaction at Al2O3/4H-SiC during oxygen radical treatment. In this study we focus on the characteristics of the Si oxide interlayer, namely whether carbon atoms are included in the film or not, because the interfacial reactions induced by oxygen radical treatment for Al2O3/4H-SiC and Al2O3/Si are different. It is revealed that decarbonization of the SiC x O y layer occurs when the SiC x O y surface is directly exposed to oxygen radicals. Then, the difference in the interfacial reaction caused by oxygen radical treatment between Al2O3/4H-SiC and Al2O3/Si is investigated by intentionally forming SiC x O y and SiO2 interlayers at the Al2O3/4H-SiC interface. Finally, the possible reaction mechanism is discussed on the basis of experimental results. All the results are qualitatively (but clearly) understood by considering that decarbonization of SiC x O y occurs as the starting point.

Source:IOPscience

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Mechanism of phosphorus passivation of near-interface oxide traps in 4H–SiC MOS devices investigated by CCDLTS and DFT calculation

Interfacial charge trapping in 4H–SiC MOS capacitors with P doped SiO2 or phospho-silicate glass (PSG) as a gate dielectric has been investigated with temperature dependent capacitance–voltage measurements and constant capacitance deep level transient spectroscopy (CCDLTS) measurements. The measurements indicate that P doping in the dielectric results in significant reduction of near-interface electron traps that have energy levels within 0.5 eV of the 4H–SiC conduction band edge. Extracted trap densities confirm that the phosphorus induced near-interface trap reduction is significantly more effective than interfacial nitridation, which is typically used for 4H–SiC MOSFET processing. The CCDLTS measurements reveal that the two broad near-interface trap peaks, named 'O1' and 'O2', with activation energies around 0.15 eV and 0.4 eV below the 4H–SiC conduction band that are typically observed in thermal oxides on 4H–SiC, are also present in PSG devices. Previous atomic scale ab initio calculations suggested these O1 and O2 traps to be carbon dimers substituted for oxygen dimers (CO=CO) and interstitial Si (Sii) in SiO2, respectively. Theoretical considerations in this work suggest that the presence of P in the near-interfacial region reduces the stability of the CO=CO defects and reduces the density of Sii defects through the network restructuring. Qualitative comparison of results in this work and reported work suggest that the O1 and O2 traps in SiO2/4H–SiC MOS system negatively impact channel mobility in 4H–SiC MOSFETs.

Source:IOPscience

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Fermi-level pinning at metal/4H-SiC contact induced by SiC x O y interlayer

We investigated the impact of defects formed at the SiO2/4H-SiC interface on the Schottky barrier height of metal/4H-SiC(0001) contacts. We found that an ultra-thin SiC x O y layer remains on the 4H-SiC surface after SiO2 sputtering at various powers and its removal by diluted hydro fluoride solution. Ni, Mo, or Al was deposited on 4H-SiC surface without and with a residual SiC x O y layer. It was found that metal/4H-SiC contacts without a residual SiC x O y layer exhibit ideal Schottky property, while Fermi-level pinning (FLP) is caused for metal/4H-SiC contacts with a residual SiC x Oy layer, and the degree of FLP increases increasing sputtering power of SiO2. The pinning position was estimated to be ~0.8 eV below the conduction band minimum of 4H-SiC, which does not correspond to the charge neutrality level of 4H-SiC. Finally, we proposed a physical model where a SiC x O y interlayer causes FLP, and the model was experimentally verified by intentionally forming a SiC x O y interlayer.

Source:IOPscience

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Magnetism in transition metal (Fe, Ni) co-doped 4H-SiC: a first-principles study

The electronic structure and magnetic properties of (Fe, Ni) co-doped 4H-SiC-based dilute magnetic semiconductors were investigated by first-principles calculations. The (Fe, Ni) co-doped 4H-SiC exist spin-polarization due to the introduction of dopant atoms, resulting in splitting. The co-doped system is more prone to a ferromagnetic state, and the magnetization energy is larger than some known room temperature diluted magnetic semiconductors, indicating that the room temperature ferromagnetism (FM) is higher. The results show that the (Fe, Ni) co-doped 4H-SiC system is ferromagnetic at room temperature with E FM −398.8 meV. The (Fe, Ni) co-doped 4H-SiC system exhibits strong magnetic properties due to strong coupling between Fe and Ni, resulting in strong spin polarization of nearby C atoms. We also studied the effect of silicon vacancies in the (Fe, Ni) co-doped 4H-SiC system. The results show that all the configurations are FM, and the FM is significantly reduced compared with the system without silicon vacancies. These results have potential applications in electronic or spintronic devices.

Source:IOPscience

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Structural defects in SiC ingots investigated by synchrotron diffraction imaging

4H silicon carbide as-grown ingots were investigated by diffraction imaging using synchrotron radiation. The white beam section topographs obtained for various sample geometries allowed us to reveal structural imperfections before slicing the bulky ingots to the thin wafers used as electronic device substrates. The systematic investigation indicated that the observed inclusions of different polytypes in 4H-SiC ingots are correlated with the 8° off-axis orientation of the seed. These inclusions, formed at the beginning of the crystal growth, provoke planar defects that propagate along the main vertical axis of the cylindrical crystal. New findings permitted us to understand the inclusion formation with the aim to increase the useful volume.

Source:IOPscience

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Evaluation of Model for Determining Nitrogen Doping Concentration from Resultant Strain in Heavily Doped 4H-SiC Crystals

Significant strain in 4H-SiC substrates can be introduced by impurity incorporation in heavily doped 4H-SiC crystals. The non-destructive X-ray double-crystal contour mapping method we reported recently is a powerful tool to estimate the lattice strain levels in 4H-SiC substrates with different doping concentrations. The lattice strain maps were derived from 11-20, 1-100 and 0008 reflections. Result shows that lattice strains within the basal plane and along the [0001] direction are in the order of 10-3 and 10-4 respectively. Hall effect measurements performed to determine the doping concentration of 4H-SiC substrates show an increase in doping concentration with the decrease of contact resistivity. X-ray rocking curve measurements were employed to confirm the existence of significant lattice distortion in heavily doped 4H-SiC substrates, which is in good agreement with the lattice strain analysis based on X-ray contour mapping method.

Source:IOPscience

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Ti/4H-SiC Schottky diode breakdown voltage with different thickness of 4H-SiC epitaxial layer

Breakdown voltage for Ti/4H-SiC type Schottky diode with six guard rings have been calculated theoretically and by mean of numerical simulations. It is shown that the breakdown voltage can be increase at the minimum on 100 V in case when thickness of the n-type 4H-SiC epitaxial layer increase from 18 up to 22 μm. It is established that the breakdown voltage value for Ti/4H-SiC type Schottky diode with guard rings calculated by mean simulation in ATLAS program and theoretically have good approximation. Thus, above approach gives the possibility for projection of diode structure with different 4H-SiC epitaxial layer thickness with higher breakdown voltage value.

Source:IOPscience

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Ultra high voltage MOS controlled 4H-SiC power switching devices

Ultra high voltage (UHV, >15 kV) 4H-silicon carbide (SiC) power devices have the potential to significantly improve the system performance, reliability, and cost of energy conversion systems by providing reduced part count, simplified circuit topology, and reduced switching losses. In this paper, we compare the two MOS based UHV 4H-SiC power switching devices; 15 kV 4H-SiC MOSFETs and 15 kV 4H-SiC n-IGBTs. The 15 kV 4H-SiC MOSFET shows a specific on-resistance of 204 mΩ cm2 at 25 °C, which increased to 570 mΩ cm2 at 150 °C. The 15 kV 4H-SiC MOSFET provides low, temperature-independent, switching losses which makes the device more attractive for applications that require higher switching frequencies. The 15 kV 4H-SiC n-IGBT shows a significantly lower forward voltage drop (VF), along with reasonable switching performance, which make it a very attractive device for high voltage applications with lower switching frequency requirements. An electrothermal analysis showed that the 15 kV 4H-SiC n-IGBT outperforms the 15 kV 4H-SiC MOSFET for applications with switching frequencies of less than 5 kHz. It was also shown that the use of a carrier storage layer (CSL) can significantly improve the conduction performance of the 15 kV 4H-SiC n-IGBTs.

Source:IOPscience

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Impact of crystal faces of 4H-SiC in SiO2/4H-SiC structures on interface trap densities and mobilities

The impact of crystal faces of 4H-SiC in SiO2/4H-SiC structures on interface trap densities and mobilities were examined by a method that utilizes Hall effect measurements and split capacitance–voltage measurements to clarify the mechanism of high field-effect mobilities in SiO2/4H-SiC and . The characterization results show that high field-effect mobilities in nitrided SiO2/4H-SiC  and  are caused by both lower interface trap densities near the conduction band edge and higher Hall mobilities compared to those in nitrided SiO2/4H-SiC (0001) and .

Source:IOPscience

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Chloride-based fast homoepitaxial growth of 4H-SiC films in a vertical hot-wall CVD*

Chloride-based fast homoepitaxial growth of 4H-SiC epilayers was performed on 4° off-axis 4H-SiC substrates in a home-made vertical hot-wall chemical vapor deposition (CVD) system using H2−SiH4−C2H4−HCl. The effect of the SiH4/H2 ratio and reactor pressure on the growth rate of 4H-SiC epilayers has been studied successively. The growth rate increase in proportion to the SiH4/H2ratio and the influence mechanism of chlorine has been investigated. With the reactor pressure increasing from 40 to 100 Torr, the growth rate increased to 52 μm/hand then decreased to 47 μm/h, which is due to the joint effect of H2 and HCl etching as well as the formation of Si clusters at higher reactor pressure. The surface root mean square (RMS) roughness keeps around 1 nm with the growth rate increasing to 49 μm/h. The scanning electron microscope (SEM), Raman spectroscopy and X-ray diffraction (XRD) demonstrate that 96.7 μm thick 4H-SiC layers of good uniformity in thickness and doping with high crystal quality can be achieved. These results prove that chloride-based fast epitaxy is an advanced growth technique for 4H-SiC homoepitaxy.

Source:IOPscience

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Investigation of spatial charge distribution and electrical dipole in atomic layer deposited Al2O3 on 4H-SiC

Charge distribution and electrical dipole in an Al2O3/4H-SiC structure are investigated by capacitance–voltage measurement and x-ray photoelectron spectroscopy (XPS). The charge densities in Al2O3 and at the Al2O3/4H-SiC interface are negligible and  −6.89  ×  1011 cm−2, respectively. Thus the small charge amount indicates the suitability of Al2O3 as a gate dielectric. The dipole at the Al2O3/4H-SiC interface is  −0.3 to  −0.91 V. The XPS manifests electron transfer from Al2O3 to 4H-SiC. The dipole formation is explained by a gap state model and the higher charge neutrality level of Al2O3 than the Fermi level of 4H-SiC, which confirms the feasibility of the gap state model on investigating band lineup at heterojunctions. The electrical dipole at the Al2O3/4H-SiC interface is critical for threshold voltage tuning. These results are helpful in engineering the SiC based gate stacks.

Source:IOPscience

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