Power factor enhancement in Pr-substituted alloys prepared by high-pressure sintering

Bi₈₅Sb_15−xPr_x (x=0,1,2,3) alloys with partial substitution of Pr for Sb were synthesized by mechanical alloying followed by high-pressure sintering. The crystal structure was characterized by X-ray diffraction. The electrical conductivity and Seebeck coefficient were measured in the temperature range of 80–300 K. The results show that the electrical conductivity and Seebeck coefficient of Pr-substituted samples are both larger than those of the reference sample, Bi₈₅Sb₁₅, in the whole measurement temperature range. The power factor of Bi₈₅Sb₁₃Pr₂ reaches a maximum value of 3.83×10⁻³ W K⁻² m⁻¹ at 235 K, which is about four times larger than that of the reference sample, Bi₈₅Sb₁₅, at the same temperature.     

Highly-doped p-type few-layer graphene on UID off-axis homoepitaxial 4H–SiC

In this report we investigate structural and electrical properties of epitaxial Chemical Vapor Deposition quasi-free-standing graphene on an unintentionally-doped homoepitaxial layer grown on a conducting 4H–SiC substrate 4° off-axis from the basal [0001] direction towards [11-20]. Due to high density of SiC vicinal surfaces the deposited graphene is densely stepped and gains unique characteristics. Its morphology is studied with atomic force and scanning electron microscopy. Its few-layer character and p-type conductance are deduced from a Raman map and its layers structure determined from a high-resolution X-ray diffraction pattern. Transport properties of the graphene are estimated through Hall effect measurements between 100 and 350 K. The results reveal an unusually low sheet resistance below 100 Ω/sq and high hole concentration of the order of 10¹⁵ cm⁻². We find that the material's electrical properties resemble those of an epitaxially-grown SiC PIN diode, making it an attractive platform for the semiconductor devices technology.     

AlGaN/GaN HEMTs grown on silicon (001) substrates by molecular beam epitaxy

We report the realization of an AlGaN/GaN HEMT on silicon (001) substrate with noticeably better transport and electrical characteristics than previously reported. The heterostructure has been grown by molecular beam epitaxy. The 2D electron gas formed at the AlGaN/GaN interface exhibits a sheet carrier density of 8×10¹² cm⁻² and a Hall mobility of 1800 cm²/V s at room temperature. High electron mobility transistors with a gate length of 4 μm have been processed and DC characteristics have been achieved. A maximum drain current of more than 500 mA/mm and a transconductance g_m of 120 mS/mm have been obtained. These results are promising and open the way for making efficient AlGaN/GaN HEMT devices on Si(001).     

Remarkable influence on the dielectric and magnetic properties of lithium ferrite by Ti and Zn substitution

The effect of Zn and Ti substitution on the magnetic and electrical properties of Li_0.5Zn_xTi_xMn_0.05Fe_2.45−2xO₄ ferrites (x=0.0 to 0.30 in steps of 0.05) +0.5wt% Bi₂O₃ prepared by a standard ceramic technique has been investigated. Electrical conductivity and dielectric measurements at different temperatures from 300 K to 700 K in the frequency range from 100 Hz to 2 MHz have been analysed. The variation of the real part of dielectric constant (ε^′) and loss tangent (tanδ) with frequency and temperature has been studied; it follows the Maxwell–Wagner model based on the interfacial polarization in consonance with the Koops phenomenological theory. It is found that the permittivity of zinc and titanium substituted lithium ferrite improves and shows a maximum value ( 1.5×10⁵) at 100 Hz for the x=0.25 sample. The dielectric transition temperature (T_d) depends on the concentration of Ti and Zn in Li_0.5Zn_xTi_xMn_0.05Fe_2.45−2xO₄. The saturation magnetization and Curie temperature both decrease with increase in the concentration of Ti and Zn in the ferrite.     

Ultrafast relaxation of hot optical phonons in monolayer and multilayer graphene on different substrates

Hot carrier cooling in few-layer and multilayer epitaxial graphene on SiC, and chemical vapor deposition (CVD) grown graphene transferred onto a glass substrate was investigated by transient absorption spectroscopy and imaging. Coupling to the substrate was found to play a critical role in charge carrier cooling. For both multilayer epitaxial graphene and monolayer CVD graphene, charge carriers transfer heat predominantly to intrinsic in-plane optical phonons of graphene. At high pump intensity, a significant number of optical phonons are accumulated, and the optical phonon lifetime presents a bottleneck for charge carrier cooling. This hot phonon effect did not occur in few-layer epitaxial graphene because of strong coupling to the substrate, which provided additional cooling channels. The limiting charge carrier lifetimes at high excitation densities were 1.8 ± 0.1 ps and 1.4 ± 0.1 ps for multilayer epitaxial graphene and monolayer CVD graphene, respectively. These values represent lower limits on the optical phonon lifetime for the graphene samples.     

Transport characteristics of a single-layer graphene field-effect transistor grown on 4H-silicon carbide

We have investigated transport characteristics of epitaxial graphene grown on semi-insulating silicon-face 4H-silicon carbide (SiC) substrate by thermal decomposition method in relatively high N₂ pressure atmosphere. We have succeeded in forming 1–2 layers of graphene on SiC in controlled manner. The surface morphology of formed graphene was analyzed by atomic force microscopy (AFM), low-energy electron diffraction (LEED) and low-energy electron microscope (LEEM). We have confirmed single-layer graphene growth in average by this method. Top-gated, single-layer graphene field-effect transistors (FETs) were fabricated on epitaxial graphene grown on 4H-SiC. Increased on/off ratio of nearly 100 at low temperature and extremely small minimum conductance (0.018–0.3 in 4 e²/h) in gated Hall-bar samples suggest possible band-gap opening of single-layer epitaxial graphene grown on Si-face SiC.     

Scanning tunneling microscopy and spectroscopy study of charge inhomogeneities in bilayer graphene

We report a room-temperature scanning tunneling microscopy and spectroscopy study of bilayer graphene prepared by mechanical exfoliation on a SiO₂/Si surface and electrically contacted with gold pads using a mechanical mask. The bulk conductivity shows contributions from regions of varying electron density, indicating significant charge inhomogeneity. Large-scale topographic images show ripple-like structures with a roughness of ∼1 nm, while the small-scale atomic resolution images show graphite-like triangular lattices. The local () tunnel spectra have an asymmetric V-shape with the minima location showing significant spatial variation, indicating inhomogeneity in electron density of order 10¹¹ cm⁻². The minimum in spectrum at a fixed location also shifts linearly with the gate voltage with a slope consistent with the field-induced carrier density.     

Structural properties and Raman spectroscopy of orthorhombic (Eu1−xPrx)0.6Sr0.4MnO3 (0≤x≤1.0)

The effect of Pr doping on structural properties and room temperature Raman spectroscopy measurements is investigated in manganites (Eu_1−xPr_x)_0.6Sr_0.4MnO₃ (0≤x≤1.0) with fixed carrier concentration. The result of the Rietveld refinement of x-ray powder diffraction shows that these compounds crystallize in an orthorhombic distorted structure with a space group Pnma. It is evident that, with increasing Pr substitution, three types of orthorhombic structures can be distinguished. The phonon frequencies of the three main peaks, in room temperature Raman-scattering measurements, have been discussed together with their structural characteristics, such as bond-length, bond-angles, and the change of orthorhombic structure type. With the increase of Pr content, the mode at 491  cm⁻¹ also shows a corresponding change. A step effect becomes evident, which seems to indicate the close dependence between the frequency shift of this mode and the change in crystal symmetry. This further supports the notion that the mode at 491  cm⁻¹ is closely correlated with the Jahn–Teller distortion. Moreover, we have found that the lowest frequency peak (assigned as an A_1g phonon mode) depends linearly on the tolerance factor t.     

Comparative Study of Monolayer and Bilayer Epitaxial Graphene Field-Effect Transistors on SiC Substrates

Monolayer and bilayer graphenes have generated tremendous excitement as the potentially useful electronic materials due to their unique features.We report on monolayer and bilayer epitaxial graphene field-effect transistors(GFETs)fabricated on SiC substrates.Compared with monolayer GFETs,the bilayer GFETs exhibit a significant improvement in dc characteristics,including increasing current density Ids,improved transconductance g_m,reduced sheet resistance R_(on),and current saturation.The improved electrical properties and tunable bandgap in the bilayer graphene lead to the excellent dc performance of the bilayer GFETs.Furthermore,the improved dc characteristics enhance a better rf performance for bilayer graphene devices,demonstrating that the quasifree-standing bilayer graphene on SiC substrates has a great application potential for the future graphene-based electronics.     

Theoretical study of the structural, elastic, electronic and thermal properties of the MAX phase Nb2SiC

Structural, elastic, electronic and thermal properties of the MAX phase Nb₂SiC are studied by means of a pseudo-potential plane-wave method based on the density functional theory. The optimized zero pressure geometrical parameters are in good agreement with the available theoretical data. The effect of high pressure, up to 40 GPa, on the lattice constants shows that the contractions along the c-axis were higher than those along the a-axis. The elastic constants C_ij and elastic wave velocities are calculated for monocrystal Nb₂SiC. Numerical estimations of the bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, average sound velocity and Debye temperature for ideal polycrystalline Nb₂SiC aggregates are performed in the framework of the Voigt-Reuss-Hill approximation. The band structure shows that Nb₂SiC is an electrical conductor. The analysis of the atomic site projected densities and the charge density distribution shows that the bonding is of covalent-ionic nature with the presence of metallic character. The density of states at Fermi level is dictated by the niobium d states; Si element has a little effect. Thermal effects on some macroscopic properties of Nb₂SiC are predicted using the quasi-harmonic Debye model, in which the lattice vibrations are taken into account. The variations of the primitive cell volume, volume expansion coefficient, bulk modulus, heat capacity and Debye temperature with pressure and temperature in the ranges of 0-40 GPa and 0-2000 K are obtained successfully.     

First-principles study of the optical properties of BeO in its ambient and high-pressure phases

Optical properties such as the dynamic dielectric function, reflectance, and energy-loss function of beryllium oxide (BeO) in its ambient and high-pressure phases are reported for a wide energy range of 0-50 eV. The calculations of optical properties employ first-principles methods based on all-electron density functional theory together with sum over states and finite-field methods. Our results show subtle differences in the calculated optical properties of the wurtzite, zincblende, rocksalt and CsCl phases of BeO, which may be attributed to the higher symmetry and packing density of these phases. For the wurtzite phase, the calculated band gap of 10.4 eV corresponds well with the experimental value of 10.6 eV and the calculated (average) index of refraction of 1.70 shows excellent agreement with the experimental value of 1.72.     

Optical and electrical characterizations of 4H-SiC-oxide interfaces by spectroscopic ellipsometry and capacitance-voltage measurements

4H-SiC-oxide interfaces formed by various oxidation methods on SiC (0 0 0 1) Si- and () C-face substrates have been characterized by performing spectroscopic ellipsometry in wide spectral region including deep UV spectral range and capacitance-voltage measurements. The results exhibit that the refractive indices of the interface layers well correlate with interface state density in all the cases of oxidation processes. To investigate the difference in interface characteristics between wet and dry oxidation, we compared to the sample fabricated by wet oxidation followed by heating in Ar or O₂ atmosphere, aiming to remove hydrogen related species at the interface. We also tried to make clear the difference in the interface characteristics between Si- and C-faces by lowering the oxidation rate of C-face down to those for Si-face. Putting together with all of the results obtained, we discuss the origins that determine the interface characteristics in terms of both the optical and electrical characterizations.     

Electronic structure and magnetic properties of the compound CeCuIn

Magnetization, magnetic susceptibility, electrical resistivity, thermoelectric power and X-ray photoemission measurements were performed on a polycrystalline sample of CeCuIn. This compound crystallizes in a hexagonal structure of the ZrNiAl type. The magnetic data indicate that CeCuIn remains paramagnetic down to 1.9 K with a paramagnetic Curie temperature of −13 K and an effective magnetic moment equal to 2.5 μ_B. The electrical resistivity has metallic character, yet in the entire temperature range studied here, it is a strongly nonlinear function of temperature. The temperature dependence of the thermoelectric power is dominated by a small positive maximum near 76 K and a deep negative minimum at about 16 K. Above 150 K the thermopower exhibits a Mott's type behavior. The positive sign of the Seebeck coefficient in this temperature region indicates that the holes are dominant charge and heat carriers. The structure of Ce 3d_5/2 and Ce 3d_3/2 XPS spectra has been interpreted in terms of the Gunnarsson-Schönhammer theory. Three final-state contributions f⁰, f¹ and f² are clearly observed, which exhibit a spin-orbit splitting Δ_SO≈18.7 eV. The appearance of the 3d⁹f⁰ component is a clear evidence of the intermediate valence behavior of Ce. From the intensity ratio I(f⁰)/[I(f⁰)+I(f¹)+I(f²)] the 4f-occupation number is estimated to be 0.95. In turn, the ratio I(f²)/[I(f¹)+I(f²)]=0.08 yields a measure of the hybridization energy that is equal to 45 meV.     

Large area SiC substrates and epitaxial layers for high power semiconductor devices — An industrial perspective

We review the progress in the industrial production of SiC substrates and epitaxial layers for high power semiconductor devices. Optimization of SiC bulk growth by the sublimation method has resulted in the commercial release of 100 mm n-type 4H-SiC wafers and the demonstration of micropipe densities as low as 0.7 cm⁻² over a full 100 mm diameter. Modelling results link the formation of basal plane dislocations in SiC crystals to thermoelastic stress during growth. A warm-wall planetary SiC-VPE reactor has been optimized up to a 8×100 mm configuration for the growth of uniform 0.01–80-micron thick, specular, device-quality SiC epitaxial layers with low background doping concentrations of <1×10¹⁴ cm⁻³, and intentional p- and n-type doping from 1×10¹⁵ to >1×10¹⁹ cm⁻³. We address the observed degradation of the forward characteristics of bipolar SiC PiN diodes [H. Lendenmann, F. Dahlquist, J.P. Bergmann, H. Bleichner, C. Hallin, Mater. Sci. Forum 389–393 (2002) 1259], and discuss the underlying mechanism due to stacking fault formation in the epitaxial layers. A process for the growth of the epitaxial layers with a basal plane dislocation density <10 cm⁻² is demonstrated to eliminate the formation of these stacking faults during device operation [J.J. Sumakeris, M. Das, H.McD. Hobgood, S.G. Müller, M.J. Paisley, S. Ha, M. Skowronski, J.W. Palmour, C.H. Carter Jr., Mater. Sci. Forum 457–460 (2004) 1113].     

In situ scanning electron microscope electrical characterization of GaN nanowire nanodiodes using tungsten and tungsten/gallium nanoprobes

A lithography-free technique for measuring the electrical properties of n-type GaN nanowires has been investigated using nanoprobes mounted in a scanning electron microscope (SEM). Schottky contacts were made to the nanowires using tungsten nanoprobes, while gallium droplets placed in situ at the end of tungsten nanoprobes were found to be capable of providing Ohmic contacts to GaN nanowires. Schottky nanodiodes were fabricated based on single n-type nanowires, and measured current–voltage (I–V) results suggest that the Schottky nanodiodes deviate from ideal diodes mainly due to their nanoscopic contact area. Additionally, the effect of the SEM electron beam on the I–V characteristics was investigated and was found to impact the transport properties of the Schottky nanodiodes, possibly due to an increase in carrier density in the nanodiodes.     

Effect of the oxidation process on SiO2/4H-SiC interface electrical characteristics

In this work we have compared the SiO₂/SiC interface electrical characteristics for three different oxidations processes (dry oxygen, water-containing oxygen and water-containing nitrogen atmospheres). MOS structures were fabricated on 8° off-axis 4H-SiC(0 0 0 1) n- and p-type epi-wafers. Electrical characteristics were obtained by I-V measurements, high-frequency capacitance-voltage (C-V) and ac conductance (G-ω) methods. Comparing the results, one observes remarkable differences between samples which underwent different oxidation routes. Among the MOS structures analyzed, the sample which underwent wet oxidation with oxygen as carrier gas presented the higher dielectric strength and lower values of interface states density.     

The enigma of the quantum Hall effect in graphene

We apply Laughlin’s gauge argument to analyze the ν=0 quantum Hall effect observed in graphene when the Fermi energy lies near the Dirac point, and conclude that this necessarily leads to divergent bulk longitudinal resistivity in the zero temperature thermodynamic limit. We further predict that in a Corbino geometry measurement, where edge transport and other mesoscopic effects are unimportant, one should find the longitudinal conductivity vanishing in all graphene samples which have an underlying ν=0 quantized Hall effect. We argue that this ν=0 graphene quantum Hall state is qualitatively similar to the high field insulating phase (also known as the Hall insulator) in the lowest Landau level of ordinary semiconductor two-dimensional electron systems. We establish the necessity of having a high magnetic field and high mobility samples for the observation of the divergent resistivity as arising from the existence of disorder-induced density inhomogeneity at the graphene Dirac point.     

界面态电荷对n沟6H-SiC MOSFET场效应迁移率的影响

针对界面态密度在禁带中的不均匀分布，分析了界面态电荷对n沟6H碳化硅MOSFET场效应迁移率的影响.分析结果显示，界面态电荷使n沟碳化硅器件的场效应迁移率明显降低.并给出了实验测定的场效应迁移率和反型层载流子迁移率的比值与界面态密度之间关系. 关键词： 碳化硅 界面态 反型层迁移率 场效应迁移率     

Dielectric/piezoelectric properties and temperature dependence of domain structure evolution in lead free single crystal

(K_0.5Na_0.5)NbO₃ (KNN) single crystals were grown using a high temperature flux method. The dielectric permittivity was measured as a function of temperature for [001]-oriented KNN single crystals. The ferroelectric phase transition temperatures, including the rhombohedral–orthorhombic T_R−O, orthorhombic–tetragonal T_O−T and tetragonal–cubic T_C were found to be located at −149  C, 205 C and 393 C, respectively. The domain structure evolution with an increasing temperature in [001]-oriented KNN single crystal was observed using polarized light microscopy (PLM), where three distinguished changes of the domain structures were found to occur at −150  C, 213 C and 400 C, corresponding to the three phase transition temperatures.     

Graphene oxide with improved electrical conductivity for supercapacitor electrodes

Predominant few-layer graphene (FLG) sheets of high electrical conductivity have been synthesized by a multi-step intercalation and reduction method. The electrical conductivity of the as-synthesized FLG is measured to be ∼3.2 × 10⁴ S m⁻¹, comparable to that of pristine graphite. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman analysis reveal that the as-synthesized FLG sheets have large areas with single and double layers. The specific capacitance of 180 F g⁻¹ is obtained for the FLG in a 1 M Na₂SO₄ aqueous electrolyte by integrating the cyclic voltammogram. The good capacitive behavior of the FLG is very promising for the application for next-generation high-performance electrochemical supercapacitors.