Impact of oxide/4H-SiC interface state density on field-effect mobility of counter-doped n-channel 4H-SiC MOSFETs

We investigated the effect of interface state density on the field-effect mobility (μ FE) of 4H-SiC counter-doped MOSFETs. We fabricated counter-doped MOSFETs with three types of gate oxides i.e., SiO2, Al2O3 formed via atomic layer deposition, and Al2O3 formed via metal layer oxidation (MLO). A maximum μ FE of 80 cm2/Vs was obtained for the MLO-Al2O3 FET, and this value was 60% larger than that of the SiO2 FET. In addition, we evaluated the electron mobility in the neutral channel (μ neutral) and the rate of increase in the free electron density in the neutral channel with respect to the gate voltage (dN neutral/dV G), which are factors determining μ FE. μ neutral depended only on the channel depth, independent of the type of gate oxide. In addition, dN neutral/dV G was significantly low in the SiO2 FET because of carrier trapping at the high density of interface states, whereas this effect was smaller in the Al2O3 FETs.

Multiphoton Absorption Simulation of Sapphire Substrate under the Action of Femtosecond Laser for Larger Density of Pattern-Related Process Windows

It is essential to develop pattern-related process windows on substrate surface for reducing the dislocation density of wide bandgap semiconductor film growth. For extremely high instantaneous intensity and excellent photon absorption rate, femtosecond lasers are currently being increasingly adopted. However, the mechanism of the femtosecond laser developing pattern-related process windows on the substrate remains to be further revealed. In this paper, a model is established based on the Fokker–Planck equation and the two-temperature model (TTM) equation to simulate the ablation of a sapphire substrate under the action of a femtosecond laser. The transient nonlinear evolutions such as free electron density, absorption coefficient, and electron–lattice temperature are obtained. This paper focuses on simulating the multiphoton absorption of sapphire under femtosecond lasers of different wavelengths. The results show that within the range of 400 to 1030 nm, when the wavelength is large, the number of multiphoton required for ionization is larger, and wider and shallower ablation pits can be obtained. When the wavelength is smaller, the number of multiphoton is smaller, narrower and deeper ablation pits can be obtained. Under the simulation conditions presented in this paper, the minimum ablation pit depth can reach 0.11 μm and the minimum radius can reach 0.6 μm. In the range of 400 to 1030 nm, selecting a laser with a shorter wavelength can achieve pattern-related process windows with a smaller diameter, which is beneficial to increase the density of pattern-related process windows on the substrate surface. The simulation is consistent with existing theories and experimental results, and further reveals the transient nonlinear mechanism of the femtosecond laser developing the pattern-related process windows on the sapphire substrate.

Insights into the surface responses of graphene oxide irradiated by an infrared femtosecond laser

Graphene oxide (GO) has emerged as unique and multifaceted novel material with a wide range of applications in electrochemistry and optoelectronic engineering. In these applications, GO surface is characterized with different functional structures in the micro-nano scale, while the femtosecond laser is a promising and versatile tool for manufacturing these structures comparing with conventional approaches. However, the comprehensive surface responses and corresponding regimes of GO surface under femtosecond laser irradiation are not yet identified, which creates obstacles to the further application of femtosecond laser in programming GO surface with specific nanopatterns. Herein, theoretical models characterizing the electrical response, i.e., the transient spatial and temporal distribution of infrared femtosecond laser-excited free electron density at the GO surface layers are established. The numerical simulations are carried out using the discontinuous Galerkin finite element algorithm with a 5 fs time step. The relationship between the laser polarized electric field and free electron density is revealed. On this basis, the surface plasma distribution is characterized, the accuracy of which is verified through the comparison of experimental ablation morphology. Thermal, morphological and chemical responses of the GO surface using different parameters are analyzed correspondingly, from which the formation and evolution mechanisms of surface nanopatterns with different features are explained. This work offers a new insight into the fundamental regimes and feasibility of ultrafast patterning of GO for the application of multifunctional device engineering.

An Arecibo follow-up study of seven pulsars discovered by Five-hundred-meter Aperture Spherical radio Telescope (FAST)

We present Arecibo 327 MHz confirmation and follow-up studies of seven new pulsars discovered by the Five-hundred-meter Aperture Spherical radio Telescope (FAST). These pulsars are discovered in a pilot program of the Commensal Radio Astronomy FAST Survey (CRAFTS) with the ultra-wide-bandwidth commissioning receiver. Five of them are normal pulsars and two are extreme nulling slow pulsars. PSR J2111+2132’s dispersion measure(DM: 78.5 pc cm−3) is above the upper limits of the two Galactic free electron density models, NE2001 and YMW16, and PSR J2057+2133’s position is out of the Scutum-Crux Arm, making them uniquely useful for improving the Galactic free electron density model in their directions. We present a detailed single pulse analysis for the slow nulling pulsars. We show evidence that PSR J2323+1214’s main pulse component follows a non-Poisson distribution and marginal evidence for a sub-pulse-drift or recurrent period of 32.3±0.4 rotations from PSR J0539+0013. We discuss the implication of our finding to the pulsar radiation mechanism.

Electric Field Assisted Femtosecond Laser Preparation of Au@TiO2 Composites with Controlled Morphology and Crystallinity for Photocatalytic Degradation

TiO2 is popular in photocatalytic degradation dye pollutants due to its abundance and its stability under photochemical conditions. Au loaded TiO2 can achieve efficient absorption of visible light and deal with the problem of low conversion efficiency for solar energy of TiO2. This work presents a new strategy to prepare Au nanoparticles-loaded TiO2 composites through electric−field−assisted temporally−shaped femtosecond laser liquid-phase ablation of Au3+ and amorphous TiO2. By adjusting the laser pulse delay and electric field parameters, gold nanoparticles with different structures can be obtained, such as nanospheres, nanoclusters, and nanostars (AuNSs). AuNSs can promote the local crystallization of amorphous TiO2 in the preparation process and higher free electron density can also be excited to work together with the mixed crystalline phase, hindering the recombination between carriers and holes to achieve efficient photocatalytic degradation. The methylene blue can be effectively degraded by 86% within 30 min, and much higher than the 10% of Au nanoparticles loaded amorphous TiO2. Moreover, the present study reveals the crystallization process and control methods for preparing nanoparticles by laser liquid ablation, providing a green and effective new method for the preparation of high-efficiency photocatalytic materials.

Comparison of the results of optical and electrophysical measurements of free electron density in n-GaAs samples doped with tellurium

A theoretical model has been developed that allows one to determine free electron density in n-GaAs from the characteristic points on far-infrared reflection spectra. It was shown that, in this case, it is necessary to take into account the plasmon-phonon coupling (otherwise, the electron density is overestimated). The calculated dependence of electron density, Nopt, on the characteristic wave number, ν+, which is described by a second degree polynomial, has been obtained.Twenty-five tellurium-doped gallium arsenide samples were used to measure the electron density in two ways: according to traditional four-contact Hall method (Van der Pauw method) and using the optical method we developed (measurements were carried out at room temperature). Based on the experimental results, the dependence was constructed of the electron density values obtained from the Hall data, NHall, on the electron density obtained by the optical method, Nopt. It is shown that this dependence is described by linear function. It is established that the data of optical and electrophysical measurements coincide if the electron density is Neq = 1.07 ⋅ 1018 cm-3, for lower values of the Hall density NHall < Nopt, and for large values NHall > Nopt. A qualitative model is proposed to explain the results. It has been suggested that tellurium atoms bind to vacancies of arsenic into complexes, as a result of which the electron density decreases. On the surface of the crystal, the concentration of arsenic vacancies is lower and, therefore, the condition Nopt > NHall should be satisfied. As the doping level increases, more and more tellurium atoms remain electrically active, so electron density in the volume begins to prevail over the surface one. However, with a further increase in the doping level, the ratio NHall/Nopt again decreases, tending to unity. This, probably, is due to the fact that the rate of decomposition of the complexes “tellurium atom + arsenic vacancy” decreases with increasing doping level.

Real-time imaging of surface chemical reactions by electrochemical photothermal reflectance microscopy

The potential-dependent photothermal signal, which is sensitive to the free electron density, map the evolution of surface species on the electrode in real time.

Inevitable imprints of patchy reionization on the cosmic microwave background anisotropy

ABSTRACT Reionization of the cosmic neutral hydrogen by the first stars in the Universe is an inhomogeneous process, which produces spatial fluctuations in free electron density. These fluctuations lead to observable signatures in cosmological probes like the cosmic microwave background (CMB). We explore the effect of the electron density fluctuations on CMB using photon-conserving seminumerical simulations of reionization named SCRIPT. We show that the amplitude of the kinematic Sunyaev–Zeldovich (kSZ) and the B-mode polarization signal depends on the patchiness in the spatial distribution of electrons along with the dependence on mid-point and extent of the reionization history. Motivated by this finding, we provide new scaling relations for the amplitude of kSZ and the B-mode polarization signal which can capture the effects arising from the mean optical depth, width of reionization, and spatial fluctuations in the electron density arising from patchy reionization. We show that the amplitude of the kSZ and the B-mode polarization signal exhibits different dependency on the width of reionization and the patchiness of reionization, and hence a joint study of these CMB probes will be able to break the degeneracy. By combining external data sets from 21-cm measurements, the degeneracy can be further lifted by directly exploring the sizes of the ionized regions.

Structural, Compositional, and Plasmonic Characteristics of Ti–Zr Ternary Nitride Thin Films Tuned by the Nitrogen Flow Ratio in Magnetron Sputtering

Ternary nitride gives high diversity and tunability of the plasmonic materials. In this work, highly crystallized ternary (Ti, Zr)N x films were prepared by magnetron co-sputtering with different nitrogen gas flow ratio R n . The structural and plasmonic properties of the films tuned by R n were investigated. All the films are solid solutions of TiN x and ZrN x with a rocksalt structure and (111) preferred orientation. The films are nitrogen-overstoichiometric and the main defects are cation vacancies. Increased R n reduces the zirconium content, and therefore leads to the reduction of lattice constant and enhancement of the crystallinity. As R n increases, the screened plasma frequency decreases for the reduction of free electron density. The maximum of the energy loss spectra of (Ti, Zr)N x films shifts to long-wavelength with R n increasing. The calculated electronic structure shows that increased nitrogen content enhances the electronic density of states of nitrogen and reduces that of metal, and therefore elevates the energy level at which interband transition is exited. The results show that (Ti, Zr)N x films give a relatively high plasmonic quality in the visible and near-infrared region, and the film properties can be significantly tuned by the nitrogen content.