A Novel Kind of Transverse Micro-Stack High-Power Diode Bars

Novel transverse micro-stack semiconductor laser bars are put forward to improve the output optical power of semiconductor laser bars at low injection current. More importantly, the novel laser bars have a coupled large optical cavity, which can overcome the problem of catastrophic optical damage and improve light beam quality due to the coherently coupled emitting along the transverse direction. The micro-stack tunnel regeneration tri-active region laser structure was grown by metal organic chemical vapour deposition. For a weakly coupled uncoated device, the optical power exceeds 60 W under 50 A driving current and the slope efficiency reaches 1.55 W/A. Further experiments show that the perpendicular divergence of 23° is achieved from transverse strongly coupled devices.     

Reversibility of Temperature-Induced Liquid Transition in Pb26Sn42Bi32 Melt: Experimental Evidence with Electrical Property

Exploring nature of liquid structures and properties is becoming more interesting in various fundamental and applied fields. With different resorts including the resistivity method, temperature-induced liquid-liquid structure transitions （TI-LLST） have been suggested and verified to occur in some liquid alloys, while the reversibility of TI- LLST has rarely been examined as yet. Unlike some other investigated liquid alloys, here we show that electrical resistivity of Pb26Sn42Bi32 melt exhibit an anomalous change in the first heating run and a reversible change in the following cooling and heating cycles. Taking account of the structural sensitivity of electrical resistivity, the abnormal patterns suggest two sorts of TI-LLST that are irreversible and reversible in the liquid ternary alloy, respectively. This interesting phenomenon together with other growing evidence imply that liquid structures and their change characteristics are multiform and complex.     

Improved performance and stability by an Al/Ni bilayer cathode in organic light-emitting diodes

Al/Ni bilayer cathode was used to improve the electroluminescent (EL) efficiency and stability in N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′ biphenyl 4,4′-dimaine (NPB)/tris-(8-hydroxyquinoline) aluminum (Alq₃)-based organic light-emitting diodes. The device with LiF/Al/Ni cathode achieved a maximum power efficiency of 2.8 lm/W at current density of 1.2 mA/cm², which is 1.4 times the efficiency of device with the state-of-the-art LiF/Al cathode. Importantly, the device stability was significantly enhanced due to the utilization of LiF/Al/Ni cathode. The lifetime at 30% decay in luminance for LiF/Al/Ni cathode was extrapolated to 400 h at an initial luminance of 100 cd/m², which is 10 times better than the LiF/Al cathode.     

Correlation between Light Emissions from Amorphous-Si：H/SiO2 and nc-Si/SiO2 Multilayers

We investigate the properties of light emission from amorphous-Si：H/SiO2 and nc-Si/SiO2 multilayers （MLs）. The size dependence of light emission is well exhibited when the a-Si：H sublayer thickness is thinner than 4 nm and the interface states are well passlvated by hydrogen. For the nc-Si/Si02 MLs, the oxygen modified interface states and nanocrystalline silicon play a predominant role in the properties of light emission. It is found that the light emission from nc-Si/SiO2 is in agreement with the model of interface state combining with quantum confinement when the size of nc-Si is smaller than 4 nm. The role of hydrogen and oxygen is discussed in detail.     

High-Temperature Characteristics of Ti/Al/Ni/Au Ohmic Contacts to n-GaN

We present the high-temperature characteristics of Ti/Al/Ni/Au（15 nm/220 nm/40 nm/50 nm） multiplayer contacts to n-type GaN （Nd = 3.7 × 10^17 cm^-3, Nd = 3.0 × 10^18 cm^-3）. The contact resistivity increases with the measurement temperature. Furthermore, the increasing tendency is related to doping concentration. The higher the doped, the slower the contact resistivity with decreasing measurement temperature. Ti/Al/Ni/Au ohmic contact to heavy doping n-GaN takes on better high temperature reliability. According to the analyses of XRD and AES for the n-GaN/Ti/Al/Ni/Au, the Au atoms permeate through the Ni layer which is not thick enough into the AI layer even the Ti layer.     

Deposition of MgB2 Superconducting Films on Different Metal Substrates

By a method of hybrid physical-chemical vapour deposition （HPCVD） on three metal substrates of stainless steel, copper and niobium, we deposit MgB2 superconducting films over 1 μm thickness. All of them have zero resistance temperatures To（O） 〉 36 K and critical current densities Jc （IOK, OT） 〉106 A/cm^2. Meanwhile, in the bending test, all the MgB2 superconducting films adhere strongly to the metal substrates without peeling off. Therefore, the MgB2 superconducting films supplied by the HPCVD method exhibit preferable electrical, magnetic and mechanical properties, and have potential applications in future.     

Transport Behaviour of La0.8Sr0.2AlO3 Thin Film on Oxygen Deficient SrTiO3 Substrate

La_0.8Sr_0.2AlO₃ (LSAO) thin films are grown on SrTiO₃ (STO) and MgO substrates by laser molecular beam epitaxy. The LSAO thin film on oxygen deficient STO substrate exhibits metallic behaviour over the temperature range of 80--340K. The optical transmittance spectrum indicates that theLSAO thin films on MgO substrate are insulating at room temperature. The transport properties of LSAO thin films on STO substrates deposited in different oxygen pressure are compared. Our results indicate that oxygen vacancies in STO substrates should be mainly responsible for the transport behaviour of LSAO thin films.     

Cs^＋-K^＋ Exchange in Z-Cut KTiOPO4 Crystal Covered with Chromium Masking Layer

Cs^＋-K^＋ ion exchanges are performed on z-cut KTiOPO4 crystals with chromium coating covered. The temperature of ion exchange is 430℃, and the time range from 15min to 30min. The dark mode spectra of the samples are measured by the prism coupling method. The channel structures on the samples are observed by a microscope and the near field pattern of the channel waveguides are measured by the end-fire coupling method. The refractive index of the samples increases and the increments at surface are modulated due to the existence of Cr film. In the region covered by Cr film, the refractive index of the samples at the surface increases dramatically in a shallow layer. The results of energy dispersive x-ray spectra indicate that in the region covered with Cr film, Cr ions participate in the ion exchange process, and enhance the refractive index. The results may provide a possibility that achieves index enhancement and Cr doping synchronically.     

Electronic Structure and Optical Properties of La-Doped SrTiO3 and Sr2TiO4 by Density Function Theory

The effect of La doping on the electronic structure and optical properties of SrTiO₃ and Sr₂TiO₄ is investigated by the first-principles calculation of plane wave ultrasoft pseudopotential based on the density function theory (DFT). The calculated results reveal that the electron doping in the case of Sr_0.875La_0.125TiO₃ and Sr_1.875La_0.125TiO₄ can be described within the rigid band model. The La³⁺ ions fully acts as electron donors in Sr_0.875La_0.125TiO₃ and Sr_1.875La_0.125TiO₄ systems and the Fermi level shifts further into the conduction bands (CBs) for Sr_1.875La_0.125TiO₄ compared to Sr_0.875La_0.125TiO₃. The two systems exhibit n-type degenerate semiconductor features. At the same time, the density of states (DOS) of the two systems shift towards low energies and the optical band gaps are broadened. The Sr_1.875La_0.125TiO₄ is highly transparent with the transmittance about 90% in the visible range, which is larger than that of Sr_0.875La_0.125TiO₃(85%). The wide band gap, small transition probability and weak absorption due to the low partial density of states (PDOS) of impurity in the Fermi level result in the optical transparency of the films...     

First-Principles Calculations of Phase Transition and Stability of Si2CN4 under High Pressure

A pressure-induced phase transition and stability in Si2 CN4 polymorphs under high pressure are studied by firstprinciples calculations. The result shows that the phase transition pressure of α- and β-Si2 CN4 to the cubic spinal phase is 29.9 GPa and 27.5 GPa predicted by thermodynamic method respectively. Under ambient condition, all of the three Si2CN4 polymorphs are metastable with positive formation enthalpy. Unlike the stability of Si3N4 polymorphs, α-Si2 CN4 is more stable than the β phase.     

Element Substitution Effect in Transition Metal Oxypnictide Re(O1-xFx)TAs (Re=rare earth, T=transition metal)

Different element substitution effects in transition metal oxypnictide Re(O_1-xF_x)TAs, with Re=La, Ce, Nd, Eu, Gd, Tm, T=Fe, Ni, Ru, are studied. Similarto the La- or Ce-based systems, we find that the pure NdOFeAs shows a strong resistivity anomaly near 145K, which is ascribed to the spin-density-wave instability. Electron doping by F increases T_c to about 50K. While in the case of Gd, Tc is reduced below 10K. The tetragonal ZrCuSiAs-type structure could not be formed for Eu or Tm substitution in our preparing process. For the Ni-based case, although both pure and F-doped LaONiAs are superconducting, no superconductivity is found when La is replaced by Ce in both the cases, instead a ferromagnetic ordering transition is likely to form at low temperature in the undoped sample. We also synthesize LaO_1-xF_xRuAs and CeO_1-xF_xRuAs compounds. The metallic behaviour is observed down to 4K.     

Physical and Chemical Properties of TiOxNy Prepared by Low-Temperature Oxidation of Ultrathin Metal Nitride Directly Deposited on SiO2

Physical and chemical properties of titanium oxynitride （TiOxNy ） formed by low-temperature oxidation of titanium nitride （TIN） layer are investigated for advanced metal-oxide-semiconductor （MOS） gate dielectric application. TiOxNy exhibits polycrystalline properties after the standard thermal process for MOS device fabrication, showing the preferred orientation at [200]. Superior electrical properties of TiOxNy can be maintained before and after the annealing, probably due to the nitrogen incorporation in the oxide bulk and at the interface. Naturally formed transition layer between TiOxNy and SiO2 is also confirmed.     

Adsorption and Reaction of CO on （100） Surface of SrTiO3 by Density Function Theory Calculation

Adsorption and reaction of CO on two possible terminations of SrTiO3 （100） surface are investigated by the first-principles calculation of plane wave ultrasoft pseudopotentiai based on the density function theory. The adsorption energy, Mulliken population analysis, density of states （DOS） and electronic density difference of CO on SrTiO3 （100） surface, which have never been investigated before as far as we know are performed. The calculated results reveal that the Ti-CO orientation is the most stable configuration and the adsorption energy （0.449eV） is quite small. CO molecules adsorb weakly on the SrTiO3 （100） surface, there is predominantly electrostatic attraction between CO and the surface rather than a chemical bonding mechanism.     

Theoretical Study of Interesting Fine-Structure Splittings for 23P0, 1, 2 States along Helium Isoelectronic Sequence

Using the multi-configuration Dirac Fock method including the Breit interactions and QED corrections, we calculate the fine-structure energy levels of the 2^3 Po, 1,2 states along the helium isoelectronic sequence with atomic number up to Z = 36, where LS-coupling is appropriate. Our calculation results agree with the experimental results within about 1%. We elucidate the mechanism of the interesting fine-structure splittings for the 2^3Po,1,2 states along the helium isoelectronic sequence, i.e. the competitions between the spin-orbit interactions and the Breit interactions which represent the relativistic retardation effect of electromagnetic interactions.     

Effect of TiO2 Nanotubes on Polymer-Fullerene Bulk Heterojunction Solar Cells

We investigate the photovoltaic properties of hybrid organ/c solar cell based on the blend of poly[2-methoxy-5-（2- ethylhexoxy-l,4-phenylenevinylene） （MEH-PPV）, C60 and titanium dioxide （TiO2） nanotubes. In comparison of the composite devices with different TiO2：[MEH-PPV ＋C60] weight ratios of lw$.% （D1-1）, 2wt.% （D1-2）, 3wt.% （D1-3）, 5wt.% （D1-4）, 10wt.% （D1-5） and 20wt.% （D1-6）, it is found that the device Dl-a exhibits the best performance. The conversion efficiency is improved by a factor of 3 compared with the MEH-PPV：C60 device.     

A Kilowatt Radio-Frequency Excited Diffusively Cooled All-Metal Slab Waveguide CO2 Laser

A new type of rf excited diffusively cooled all-metal slab waveguide CO2 laser is presented, in which the waveguide channel is constructed by two copper side walls and two copper electrodes, and the discharge is confined in the slab waveguide channel in terms of the voltage division structure. From this type of structure, over 1 kW laser power is obtained with an efficieney of more than 13%.     

NaCl/Ca/Al as an efficient cathode in organic light-emitting devices

An efficient cathode NaCl/Ca/Al used to improve the performance of organic light-emitting devices (OLEDs) was reported. Standard N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′ biphenyl 4,4′-dimaine (NPB)/tris-(8-hydroxyquinoline) aluminum (Alq₃) devices with NaCl/Ca/Al cathode showed dramatically enhanced electroluminescent (EL) efficiency. A power efficiency of 4.6 lm/W was obtained for OLEDs with 2 nm of NaCl and 10 nm of Ca, which is much higher than 2.0 lm/W, 3.1 lm/W, 2.1 lm/W and 3.6 lm/W in devices using, respectively, the LiF (1 nm)/Al, LiF (1 nm)/Ca (10 nm)/Al, Ca (10 nm)/Al and NaCl (2 nm)/Al cathodes. The investigation of the electron injection in electron-only devices indicates that the utilization of the NaCl/Ca/Al cathode substantially enhances the electron injection current, which in case of OLEDs leads to the improvement of the brightness and efficiency.     

A New Resonant Tunnelling Structure of Integrated InGaAs/GaAs Very-Long-Wavelength Quantum-Well Infrared Photodetector

We perform a theoretical study on a low dark current InGaAs/GaAs very-long-wavelength （〉 12 μm） quantum well infrared photodetector （VLW-QWIP）, based on a double barrier resonant tunnelling structure （DBRTS）. The ground tunnelling state of the central quantum well （QW） of the DBRTS can resonate with the first excited bound state of the doped InGaAs QW by adjusting the structure parameters of the DBRTS. Investigation of the carrier transport performance of this device is carried out based on quantum wave transport theory. It has been shown that the dark current in this device can be significantly reduced by two orders compared to conventional InGaAs/GaAs VLW-QWIPs, while the photocurrent is almost the same as those in conventional VLW-QWIPs. This DBRTS integrated VLW-QWIP structure may stimulate the experimental investigation for VLW-QWIPs at high operation temperatures.     

Red organic light-emitting devices with dotted-line doped emitting layers

High efficiency red organic light-emitting devices (OLEDs) with several dotted-line doped layers (DLDLs) were fabricated by using an ultra-high vacuum organic molecular-beam deposition system. The red OLEDs consisted of indium-tin-oxide (ITO)/N, N′-diphenyl-N, N′-bis(1-naphthyl)-(1, 1′-biphenyl)-4, 4′-diamine (α-NPD): 40 nm/tris(8-hydroxyquinoline)aluminum (Alq₃)+4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetra-methyljuloldyl-9-enyl)-4H-pyran (DCJTB); 3%wt.: x nm/(Alq₃+DCJTB; 3%wt./ Alq₃)_n−1: (30−x) nm/ Alq₃: 30 nm/Mg:Ag with n of 2, 4, 6, or 8, and x=30/(2n−1). The luminance yield of the device with 8 DLDLs was 75% higher than that of the device with a common doped layer. This was attributed to more formation of the excitons formed in a wider region resulting from the existence of the DLDLs. The dominant mechanisms of the dopant emission for the devices with DLDLs were described on the basis of the sequential carrier trapping process.     

Effects of Electric Field on Electronic States in a GaAs/GaAlAs Quantum Dot with Different Confinements

Binding energies of shallow hydrogenic impurity in a GaAs/GaAlAs quantum dot with spherical confinement, parabolic confinement and rectangular confinement are calculated as a function of dot radius in the influence of electric field. The binding energy is calculated following a variational procedure within the effective mass approximation along with the spatial depended dielectric function. A finite confining potential well with depth is determined by the discontinuity of the band gap in the quantum dot and the cladding. It is found that the contribution of spatially dependent screening effects are small for a donor impurity and it is concluded that the rectangulax confinement is better than the parabolic and spherical confinements. These results are compared with the existing literature.