Growth of Highly Conductive n-Type Al0.7Ga0.3N Film by Using AlN Buffer with Periodical Variation of Ⅴ/Ⅲ Ratio

High quality and highly conductive n-type Al0.7Ga0.3N films are obtained by using AlN multi-step layers （MSL） with periodical variation of Ⅴ/Ⅲ ratios by low-pressure metalorganic chemical vapour deposition （LP-MOCVD）. The full-width at half-maximum （FWHM） of （0002） and （1015） rocking curves of the Si-doped Al0.7Ga0.3N layer are 519 and 625 arcsec, respectively, Room temperature （RT） Hall measurement shows a free electron concentration of 2.9 × 10^19 cm^-3, and mobility of 17.8cm^2V^-1s^-1, corresponding to a resistivity of 0.0121 Ω cm. High conductivity of the Si-doped AlGaN film with such high Al mole fraction is mainly contributed by a remarkable reduction of threading dislocations （TDs） in AlGaN layer. The TD reducing mechanism in AlN MSL growth with periodical variation of Ⅴ/Ⅲ ratio is discussed in detail.     

Activation of Hydrogen-Passivated Mg in GaN-Based Light Emitting Diode Annealing with Minority-Carrier Injection

We discuss an issue on the activation of p-GaN material under different annealing conditions and study the mechanism for the p-GaN activation. Under annealing in nitrogen, it is found that hydrogen cannot be completely removed from p-GaN. The experiments also indicate that rudimental hydrogen can exist stably in a certain state where hydrogen does not passivate the Mg acceptor in the sample annealing under bias. However, making additional annealing in nitrogen, we find that the steady state hydrogen can be decomposed and the Mg-H complex could generate again. Hydrogen remaining in the layer seems to play a major role in this reversible phenomenon.     

Interface Evolution of TiN/Poly Si as Gate Material on Si/HfO2 Stack

TiN as gate electrode in Si/HfO₂/TiN/poly-Si stack is evaluated after the postmetal annealing treatments. Interface reactions are investigated usingelectron-energy-loss spectroscopy and x-ray photoelectron spectroscopy. The work function of the TiN/poly-Si stack shows strong dependence on the postmetal deposition annealing conditions. The interfacial product in TiN/poly-Si interface is inferred as TiSiN, which is beneficial for the whole high-k stack since TiSiN possesses higher work function compared to TiN and poly-Si.     

Effects of Crossed Electric and Magnetic Fields on Shallow Donor Impurity Binding Energy in a Parabolic Quantum Well

We have calculated variationally the ground state binding energy of a hydrogenic donor impurity in a parabolic quantum well in the presence of crossed electric and magnetic fields. These homogeneous crossed fields are such that the magnetic field is parallel to the heterostructure layers and the electric field is applied perpendicular to the magnetic field. The dependence of the donor impurity binding energy to the well width and the strength of the electric and magnetic fields are discussed. We hope that the obtained results will provide important improvements in device applications, especially for a suitable choice of both fields in the narrow well widths.     

The hydrostatic pressure and temperature effects on donor impurities in GaAs/Ga1 − xAlxAs double quantum well under the external fields

The combined effects of hydrostatic pressure and temperature on donor impurity binding energy in GaAs/Ga_0.7Al_0.3As double quantum well in the presence of the electric and magnetic fields which are applied along the growth direction have been studied by using a variational technique within the effective-mass approximation. The results show that an increment in temperature results in a decrement in donor impurity binding energy while an increment in the pressure for the same temperature enhances the binding energy and the pressure effects on donor binding energy are lower than those due to the magnetic field.     

Properties of AlyGa1-yN/AlxGa1-xN/AlN/GaN Double-Barrier High Electron Mobility Transistor Structure

Electrical properties of Al_yGa_1-yN/Al_xGa_1-xN/AlN/GaN structure are investigated by solving coupled Schrödinger and Poisson equation self-consistently. Our calculations show that the two-dimensional electron gas (2DEG) density will decrease with the thickness of the second barrier (Al_yGa_1-yN) once the AlN content of the second barrier is smaller than a critical value y_c, and will increase with the thickness of the second barrier (Al_yGa_1-yN) when the critical AlN content of the second barrier y_c is exceeded. Our calculations also show that the critical AlN content of the second barrier y_c will increase with the AlN content and the thickness of the first barrier layer (Al_xGa_1-xN).     

Growth and Characteristics of Epitaxial AlxGa1-xN by MOCVD

Al_xGa_1-xN epilayers with a wide Al composition range (0.2≤x≤ 0.68) were grown on AlN/sapphire templates by low-pressure metalorganic chemical vapour deposition (LP-MOCVD). X-ray diffraction results reveal that both the (0002) and (10^-15) full widths at half-maximum (FWHM) of the Al_xGa_1-xN epilayer decrease with increasing Al composition due to the smaller lattice mismatch to the AlN template. However, the surface morphology becomes rougher with increasing Al composition due to the weak migration ability of Al atoms. Low temperature photoluminescence (PL) spectra show pronounced near band edge (NBE) emission and the NBE FWHM becomes broader with increasing Al composition mainly caused by alloy disorder. Meanwhile, possible causes of the low energy peaks in the PL spectra are discussed.     

Structural, elastic and electronic properties and formation energies for hexagonal (W0.5Al0.5)C in comparison with binary carbides WC and Al4C3 from first-principles calculations

First principle calculations have been performed with the purpose to understand the peculiarities of the structural, elastic parameters and electronic properties and interatomic bonding for novel hexagonal carbide (W_0.5Al_0.5)C in comparison with binary phases WC and Al₄C₃. The geometries of all phases were optimized and their structural, elastic parameters and theoretical density were established. Besides, we have evaluated the formation energies (E_form) of W_0.5Al_0.5C for different possible preparation routes (namely for the reactions with the participation of simple substances (metallic W, Al and graphite, binary W or Al carbides and metallic Al and W, or binary W and Al carbides). The results show that the synthesis of the ternary carbide from simple substances is more favorable in comparison with the reactions with participation of W and Al carbides. Moreover, band structures, total and partial densities of states were obtained and analyzed systematically for (W_0.5Al_0.5)C, WC and Al₄C₃ phases in comparison with available theoretical and experimental data. The bonding picture in W_0.5Al_0.5C was described as a mixture of metallic, ionic and covalent contributions with the high anisotropy for the covalent W-C and Al-C bonds, where p-p like Al-C bonds become weaker than p-d like W-C bonds.     

Hydrogen indium vacancy complex VInH4 in n-type InP studied by positron-lifetime

Positron-lifetime experiments have been carried out on two undoped n-type liquid encapsulated Czochralski (LEC)-grown InP samples with different stoichiometric compositions in the temperature range 10-300 K. For temperatures below 120 K for P-rich InP and 100 K for In-rich InP, the positron average lifetime began to increase rapidly and then leveled off, which was associated with the charge state change of hydrogen indium vacancy complexes from (V_InH₄)⁺ to (V_InH₄)⁰. This phenomenon was more obvious in P-rich samples that have a higher concentration of V_InH₄. The transformation temperature of approximately 120 K suggests that the complex V_InH₄ is a donor defect and that the ionization energy is about 0.01 eV. The ionization of neutral V_InH₄ accounted for the decrease of the positron average lifetime when the sample was illuminated with a photon energy of 1.32 eV at 70 K. These results provide evidence for hydrogen complex defects in undoped LEC InP.     

Assessment of electronic properties of InNxSb1−x for long-wavelength infrared detector applications

This work assesses theoretically the potential of dilute nitride alloys of InN_xSb_1−x for long-wavelength IR applications. A 10-band k.p approximation modified to account for conduction/valence band coupling is implemented to extract the bandgap as a function of the nitrogen concentration in the alloy and the temperature. The calculations show the possibility to obtain a band closure at ∼2% of nitrogen for InSbN at 300 K. The absorption coefficient, and its temperature dependence, is then determined using an Elliot-like formalism, predicting stronger absorption properties associated with the enhancement of conduction band effective masses. This enhancement yields over an order of magnitude increase in the non-radiative Auger recombination lifetimes suggesting the potential of InNSb for significantly enhancing detectivity limits and operation temperatures of long-wavelength IR detectors.     

Study of the optical-electrical characteristics of InxGa1−xN alloy with low in doping

The electronic structures and optical properties of In doped GaN were calculated with different doping concentration, from first-principles using density function theory with the plane-wave ultrasoft pseudopotential method. The influence of In doping on the volume, interactions among atoms, density of states, electron density difference, and optical properties of GaN was analyzed. The results show that the interactions among atoms are reduced, band gap decreases, and absorption spectra have red shift along with the increase of In doping concentration.     

Electronic structure of cubic ErxGa1−xN using the LSDA+U approach

Electronic structure calculations were performed for substitutional erbium rare-earth impurity in cubic GaN using density-functional theory calculations within the LSDA+U approach (local spin-density approximation with Hubbard-U corrections). The LSDA+U method is applied to the rare-earth 4f states. The Er_xGa_1−xN is found to be a semiconductor, where the filled f-states are located in the valence bands and the empty ones above the conduction band edge. The filled and empty f-states are also shown to shift downwards and upwards in the valence and conduction bands, respectively, with increase in the U potentials.     

Effect of Quantizing Magnetic Field on Cyclotron Energy and Cyclotron Effective Mass in Size Quantized Films with Non-Parabolic Energy Band

The Fermi energy, cyclotron energy and cyclotron effective mass of degenerate electron gas in a sizeoquantized semiconductor thin film with non-parabolic energy bands are studied. The influences of quantizing magnetic field on these quantities in two-band approximation of the Kane model are investigated. It is shown that the Fermi energy oscillates in a magnetic field. The period and positions of these oscillations are found as a function of film thickness and concentration of electrons. Cyclotron energy and cyclotron effective mass are investigated as a function of film thickness in detail The results obtained here are compared with experimental data on GaAs quantum wells.     

Chemical Synthesis of C3N and BC2N Compounds

A chemical reaction for the preparation of B-C-N compounds by using carbon tetrachloride （CC14）, boron tribromide （BBr3）, lithium nitride （Li3N） and sodium as reactants has been carried out at the temperature of 400℃. Measurements of FTIR, XRD, TEM and EELS show that two kinds of compounds have been formed in the prepared sample. One is hollow sphere-like C-N with an amorphous, structure; the other is piece-like polycrystalline B-C-N with the hexagonal structure. Their determined compositions are close to C3N and BC2N, respectively.     

Composite-Collector InGaAs/InP Double Heterostructure Bipolar Transistors with Current-Gain Cutoff Frequency of 242 GHz

To eliminate the conduction band spike at the base-collector interface, an InP/InGaAs double heterostructure bipolar transistor （DHBT） with an InGaAsP composite collector is designed and fabricated using the conventional mesa structure. The DHBT with emitter area of 1.6×15μm^2 exhibits current-gain cutoff frequency ft = 242 OHz at the high collector current density Jc = 2.1 mA/μm^2, which is to our knowledge the highest ft reported for the mesa InP DHBT in China. The breakdown voltage in common-emitter configuration is more than 5 V. The high-speed InP/InGaAs DHBT with high current density digital circuits. is very suitable for the application in ultra high-speed     

Fabrication, morphology, and photoluminescence properties of GaN nanowires and nanorods by ammoniating Ga2O3/V films on Si(1 1 1)

GaN nanowires and nanorods have been successfully synthesized on Si(1 1 1) substrates by magnetron sputtering through ammoniating Ga₂O₃/V films at 900 °C in a quartz tube. X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) spectrum were carried out to characterize the structure, morphology, and photoluminescence properties of GaN sample. The results show that the GaN nanowires and nanorods with pure hexagonal wurtzite structure have good emission properties. The growth direction of nanostructures is perpendicular to the fringes of (1 0 1) plane. The growth mechanism is also briefly discussed.     

Photoluminescence investigation of ferromagnetic Ga1−xMnxN layers with GaN templates grown on sapphire (0 0 0 1) substrates

We have investigated the temperature and composition dependent photoluminescence (PL) spectra in Ga_1−xMn_xN layers (where x ≈ 0.1-0.8%) grown on sapphire (0 0 0 1) substrates using the plasma-enhanced molecular beam epitaxy technique. The efficient PL is peaked in the red (1.86 eV), yellow (2.34 eV), and blue (3.29 eV) spectral range. The band-gap energy of the Ga_1−xMn_xN layers decreased with increasing temperature and manganese composition. The band-gap energy of the Ga_1−xMn_xN layers was modeled by the Varshni equation and the parameters were determined to be α = 2.3 × 10⁻⁴, 2.7 × 10⁻⁴, 3.4 × 10⁻⁴ eV/K and β = 210, 210, and 230 K for the manganese composition x = 0.1%, 0.2%, and 0.8%, respectively. As the Mn concentration in the Ga_1−xMn_xN layers increased, the temperature dependence of the band-gap energy was clearly reduced.     

Magnetoresistance and magnetostriction of Co2Cr0.6Fe0.4Al Heusler alloy

We report a study of the magnetotransport properties of the Co₂Cr_0.6Fe_0.4Al Heusler alloy grown by means of the arc-melting technique. X-ray diffraction and energy dispersive X-ray spectroscopy have been used to analyze the crystallographic structure and the sample stoichiometry. Temperature-dependent magnetization measurements have been carried out in the range 300-850 K. Co₂Cr_0.6Fe_0.4Al is theoretically predicted to have full positive spin polarization at the Fermi level, and as a consequence its spin-dependent transport properties are being intensively studied. Low field magnetoresistance exceeding 30% has been observed very recently in Co₂Cr_0.6Fe_0.4Al compact pellets. We have performed magnetoresistance and magnetostriction measurements in both the as-grown alloy and compact pellets made of mechanically milled Co₂Cr_0.6Fe_0.4Al. The as-grown and the milled sample show negligible anisotropic magnetostriction (25 μst at saturation), whilst only the milled sample exhibits magnetoresistance (0.65% at 300 K). These results permit us to discard the magnetostrictive effects as the magnetoresistance source.     

Effects of interface on the dielectric properties of Ba0.6Sr0.4TiO3 thin film capacitors

Ba_0.6Sr_0.4TiO₃ thin films were deposited on Pt/SiO₂/Si substrate by radio frequency magnetron sputtering. High-resolution transmission electron microscopy (HRTEM) observation shows that there is a transition layer at BST/Pt interface, and the layer is about 7-8 nm thickness. It is found that the transition layer was diminished to about 2-3 nm thickness by reducing the initial RF sputtering power. X-ray photoelectron spectroscopy (XPS) depth profiles show that high Ti atomic concentration results in a thick interfacial transition layer. Moreover, the symmetry ν of ?_r-V curve of BST thin film is enhanced from 52.37 to 95.98%. Meanwhile, the tunability, difference of negative and positive remanent polarization (P_r), and that of coercive field (E_C) are remarkably improved.     

Study on preparation and fluorescence characteristic of coordinated Eu2O3/polymer hybrid films

The cyclohexane solution of TTA (trifluorothenoyl-acetone), phen (8-hydroxylquinoline) and PS (polystyrene), the ethyl acetate solution of TTA, phen and PMMA (polymethyl methacrylate) were used as flowing liquid, the coordinated Eu₂O₃/polymer hybrid colloids were successively produced by focused pulsed laser ablation of Eu₂O₃ target in interface of solid and flowing liquid. As solvent in the hybrid colloids has volatilized, the coordinated Eu₂O₃/polymer hybrid films were obtained. The hybrid colloids and films were characterized by TEM, UV-vis spectrum, fluorescence spectrum, TG-FTIR and X-ray photoelectron spectrum. The results show the coordinated Eu₂O₃ nanoparticles with average size of less than 20 nm are surrounded by the three-dimensional network and are properly incorporated into the PMMA and PS matrix, the hybrid films can emit intense red light under ultraviolet radiation, and their emission fluorescence spectra display same characteristic emission peaks of Eu³⁺ ions. The Eu₂O₃ hybrid films have better thermo stability than the related pure polymers because of strong interaction between surface europium ions of the nanoparticles and polymer. Because the coordinated Eu₂O₃ nanoparticles were wrapped by polymer, they have higher chemical stability than the related europium complex.