First-principle calculation on the defect energy level of carbon vacancy in 4H-SiC

First, electronic structures of perfect wurtzite 4H-SiC were calculated by using first-principle ultra-soft pseudopotential approach of the plane wave based on the density functional theory; and the structure changes, band structures, and density of states were studied. Then the defect energy level of carbon vacancy in band gap was examined by substituting the carbon in 4H-SiC with carbon vacancy. The calculated results indicate the new defect energy level generated by the carbon vacancy, and its location in the band gap in 4H-SiC, which has the character of deep acceptor. A proper explanation for green luminescence in 4H-SiC is given according to the calculated results which are in good agreement with our measurement results.

Density functional theory calculation of the properties of carbon vacancy defects in silicon carbide

As a promisingmaterial for quantumtechnology,silicon carbide(SiC)has attracted great interest inmaterials science.Carbon vacancy is a dominant defect in 4H-SiC.Thus,understanding the properties of this defect is critical to its application,and the atomic and electronic structures of the defects needs to be identified.In this study,density functional theorywas used to characterize the carbon vacancy defects in hexagonal(h)and cubic(k)lattice sites.The zero-phonon line energies,hyperfine tensors,and formation energies of carbon vacancies with different charge states(2−,−,0,+and 2+)in different supercells(72,128,400 and 576 atoms)were calculated using standard Perdew-Burke-Ernzerhof and Heyd-Scuseria-Ernzerhof methods.Results show that the zero-phonon line energies of carbon vacancy defects are much lower than those of divacancy defects,indicating that the former is more likely to reach the excited state than the latter.The hyperfine tensors of VC+(h)and VC+(k)were calculated.Comparison of the calculated hyperfine tensor with the experimental results indicates the existence of carbon vacancies in SiC lattice.The calculation of formation energy shows that the most stable carbon vacancy defects in the material are VC 2+(k),VC+(k),VC(k),VC−(k)and VC 2−(k)as the electronic chemical potential increases.

Synthesis of carbon nitride in moist environments:A defect engineering strategy toward superior photocatalytic hydrogen evolution reaction

Intimate understanding of the synthesis-structure-activity relationships is an accessible pathway to overcome the intrinsic challenges of carbon nitride(g-C_(3)N_(4))photocatalysts.This work looks in the effects of humidity of the synthesis process to the morphology,chemical structure,band structure as well as the photocatalytic activity of g-C_(3)N_(4) materials.Four g-C_(3)N_(4) samples were prepared by heating melem in four gas environments:dry Ar,dry Air,moist Ar and moist Air.The photocatalytic activity measurements revealed that the samples synthesized in moist inert and oxidic gases environments displayed 20 and 10 times the photocata lytic H_(2) evolution activity of the samples synthesized in dry inert and oxidic gases environments,respectively.The reasons for this remarkable variety in photocata lytic activities had been through investigated.After all,the terminations of the carbon vacancies were identified as the dominant factor in enhancing H_(2) evolution performance.The work here thus demonstrating an example of defect engineering.

Characteristics of Ni-based ohmic contacts on n-type 4H-SiC using different annealing methods

Nickel is an excellent ohmic-contact metal on 4H-SiC.This paper discusses the formation mechanism of nickel ohmic contact on 4HSiC by assessing the electrical properties and microstructural change.Under high-temperature annealing,the phase of nickel-silicon compound can be observed with X-ray diffraction,and the contact resistance also changes.A comparative experiment was designed to use X-ray diffraction and energy-dispersive spectroscopy to clarify the difference of ohmic-contact material composition and elemental analysis between samples prepared using pulsed laser annealing and rapid thermal annealing.It is found that more Ni2Si and carbon vacancies formed at the interface in the sample prepared using pulsed laser annealing,resulting in a better ohmic-contact characteristic.

Electron paramagnetic resonance characterization of aluminum ion implantation-induced defects in 4H-SiC

Deep-level defects in silicon carbide(SiC)are critical to the control of the performance of SiC electron devices.In this paper,deep-level defects in aluminumion-implanted 4H-SiC after high-temperature annealingwere studied using electron paramagnetic resonance(EPR)spectroscopy at temperatures of 77 K and 123 K under different illumination conditions.Results showed that the main defect in aluminum ion-implanted 4H-SiC was the positively charged carbon vacancy(VC+),and the higher the doping concentration was,the higher was the concentration of VC+.Itwas found that the type of material defectwas independent of the doping concentration,although more VC+defects were detected during photoexcitation and at lower temperatures.These results should be helpful in the fundamental research of p-type 4H-SiC fabrication in accordance with functional device development.

Low thermal conductivity of dense(TiZrHfVNbTa)C_(x) high-entropy carbides by tailoring carbon stoichiometry

Transition metal carbides are promising candidates for thermal protection materials due to their high melting points and excellent mechanical properties.However,the relatively high thermal conductivity is still a major obstacle to its application in an ultra-high-temperature insulation system.In this work,the low thermal conductivity of dense(TiZrHfVNbTa)Cx(x=0.6-1)high-entropy carbides has been realized by adjusting the carbon stoichiometry.The thermal conductivity gradually decreases from 10.6 W·m^(−1)·K^(−1) at room temperature to 6.4 W·m^(−1)·K^(−1) with carbon vacancies increasing.Due to enhanced scattering of phonons and electrons by the carbon vacancies,nearly full-dense(97.9%)(TiZrHfVNbTa)C_(0.6) possesses low thermal conductivity of 6.4 W·m^(−1)·K^(−1),thermal diffusivity of 2.3 mm^(2)·s^(−1),as well as electrical resistivity of 165.5μΩ·cm.The thermal conductivity of(TiZrHfVNbTa)C_(0.6) is lower than that of other quaternary and quinary high-entropy carbide ceramics,even if taking the difference of porosity into account in some cases,which is mainly attributed to compositional complexity and carbon vacancies.This provides a promising route to reduce the thermal conductivity of high-entropy carbides by increasing the number of metallic elements and carbon vacancies.

Mechanism on Carbon Vacancies in Polymeric Carbon Nitride for CO2 Photoreduction

Defect engineering has being regarded as one of the effective ways to regulate chemical and electronic structure of semiconductors.Recently,our collaborative work has shown experimentally that carbon vacancy on polymeric carbon nitride(CV)can greatly improve the CO2 to CO conversion with a 45-fold improvement over the polymeric carbon nitride(Angew.Chem.Int.Ed.,2019,58,1134).In order to clarify the detailed mechanism of promotion,we have systematically studied the electronic properties of CV and hydrogenated CV(CV+H)as well as the effective CO2 reduction reaction through density functional theory calculations.We found that it is the synergistic effect for the CO2 reduction reaction in the CV systems,as the onset potentials of several CVs are much lower than that of the polymeric carbon nitride.In particular,the onset potentials of CV1,CV2,and CV2+H are around 0.9~1.5 eV with a strong chemisorbed CO2 on them.Combined with the analysis of the electronic properties,our results confirm that defect engineering increases the lifetime of photo-generated charges,improves photocatalytic activity,and promotes the CO2 reduction reaction on the defected polymeric carbon nitrides.

Effect of native carbon vacancies on evolution of defects in ZrC_(1-x)under He ion irradiation and annealing

The dynamic study of radiation-induced defects with annealing is critical for the material design for nextgeneration nuclear energy systems.The native vacancy could affect the development of defects,which lacks study.In the present work,the as-hot pressed ZrC_(1-x)(x=0,0.15,0.3)ceramics which comprised crystallites of a few microns in size with different amounts of carbon vacancies were irradiated by 540 ke V He^(2+)ions at room temperature with a fluence of 1×10^(17)/cm^(2).The radiation-induced lattice expansion was found to be a common phenomenon in a sequence of ZrC_(0.85)≥ZrC_(1.0)>ZrC_(0.7).Both X-ray and electron diffractions confirmed maintenance of structural integrity without amorphization after irradiation.Inside the irradiated region,only“black-dot”type defects,i.e.,clusters of point defects were observed while no helium-induced cavities,cracks,or extended dislocations were detected.The as-irradiated ZrC_(1-x)were then annealed at different high temperatures.Upon annealing at 800℃,very tiny helium-induced cavities were found to be generated and the crystal lattice recovered to a great extent,especially for the sub-stoichiometric samples.While annealed at 1500℃,all the samples almost fully recovered the crystal lattices close to those of as-hot pressed ones.Meanwhile,large cavities and extended dislocations were generated.With increasing amount of native carbon vacancies,the size of cavities increased while the length and density of extended dislocations decreased.Inverse changes of lattice parameters during irradiation and annealing processes have been interpreted by the kinetics of defects.Finally,the correlation between native vacancies and damage behavior is discussed.

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 broad-band 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.