Spectroscopic and fluorescence decay behaviors of Yb3+ -doped SiO2-PbO-Na2O-K2O glass

The spectroscopic and fluorescent decay behaviors of Yb3+-doped SiO2-PbO-Na2O-K2O glass is reported in this work. Yb2O3 contents of 1, 1.5 and 2mol% are added into the glass. Through the measurement of absorption and fluorescence spectra, and fluorescent decay rate at room temperature and at low temperature (18 K), it is found that the nonradiative decay rate of Yb3+ ions is mainly determined by the interaction between residual hydroxyl groups and Yb3+ ions. Concentration quenching effect can be omitted in this glass up to the Yb3+ ion concentration of 8.98 x 10(20)/cm(3). Multiphonon decay rate is also very small because of the large energy gap between F-2(5/2) and F-2(7/2) levels of Yb3+ ions. (c) 2004 Elsevier B.V. All rights reserved.

射频磁控溅射法制备TiO2-xNX薄膜的结构性能研究

利用射频磁控溅射法室温下在Si（100）衬底上制备了N掺杂的TiO2薄膜，并且采用x射线衍射（XRD）、X射线光电子能谱（XPS）和透射光谱对薄膜进行了表征。XRD结果表明在纯Ar和N2（33．3％）／Ar气氛下制备的TiO2-xNx薄膜均为单一的金红石相，薄膜结晶性良好，呈高度（211）择优取向，而在N2（50．0％）／Ar下制备的薄膜结晶性明显变差；对于N掺杂的TiO2薄膜，XPS表明部分N原子进入TiO2晶格，并且以N—Ti—O、N—O键以及间隙式N原子形式存在；透射光谱表明掺N后的TiO2薄膜吸收边发生了红移。

Properties of electrons and holes during femtosecond cooperative emission

Spectral and energetic characteristics of the cooperative recombination of high density electrons and holes in bulk GaAs are experimentally studied at room temperature. It is shown that the properties and parameters of femtosecond superradiant pulses are conditioned by the collective properties of electrons and holes. Electron-hole pairing and the formation of a short-living coherent e-h BCS state distinguish strongly the regime of cooperative emission from all radiative e-h recombination regimes, which have been observed earlier. The dependences of the energy gap (the order parameter), the Fermi energy, and the band gap of the coherent e-h BCS state on the concentration of electron-hole pairs are obtained.

Electronic structure and optical properties of GaN with Mn-doping

Calculations of the electronic structure and the density of states of GaN with Mn are carried out by means of first-principles plane-wave pesudopotential method based on density functional theory. The results reveal a 100% spin polarized impurity band in band structure of Ga1-xMnxN due to hybridization of Mn 3d and N 2p orbitals. The material is half metallic and suited for spin injectors. In addition, a peak of refractive index can be observed near the energy gap. The absorption coefficient increases in the UV region with the increase of the Mn content.

Quantum efficiency and temperature coefficients of GaInP/GaAs dual-junction solar cell

GaInP/GaAs dual-junction solar cell with a conversion efficiency of 25.2% has been fabricated using metalorganic chemical vapor deposition (MOCVD) technique. Quantum efficiencies of the solar cell were measured within a temperature range from 25 to 160A degrees C. The results indicate that the quantum efficiencies of the subcells increase slightly with the increasing temperature. And red-shift phenomena of absorption limit for all subcells are observed by increasing the cell's work temperature, which are consistent with the viewpoint of energy gap narrowing effect. The short-circuit current density temperature coefficients dJ (sc)/dT of GaInP subcell and GaAs subcell are determined to be 8.9 and 7.4 mu A/cm(2)/A degrees C from the quantum efficiency data, respectively. And the open-circuit cell voltage temperature coefficients dV (oc)/dT calculated based on a theoretical equation are -2.4 mV/A degrees C and -2.1 mV/A degrees C for GaInP subcell and GaAs subcell.

Temperature dependent spectral response characteristic of III-V compound tandem cell

The GaInP/GaAs/Ge triple-junction tandem cells with a conversion efficiency of 27.1% were fabricated using metalorganic chemical vapor deposition (MOCVD) technique. Temperature dependence of the spectral response measurements of the GaInP/GaAs/Ge tandem cell was performed by a quantum efficiency system at temperatures ranging from 25A degrees C to 160A degrees C. The red-shift phenomena of the absorption limit for all subcells were observed with increasing temperature, which is dued to the energy gap narrowing with temperature. The short-circuit current densities (J (sc)) of GaInP, GaAs and Ge subcells at room temperature calculated based on the spectral response data were 12.9, 13.7 and 17 mA/cm(2), respectively. The temperature coefficient of J (sc) for the tandem cell was determined to be 8.9 mu A/(cm(2) center dot A degrees C), and the corresponding temperature coefficient of the open-circuit voltage deduced from the series-connected model was -6.27 mV/A degrees C.

Magneto - Transport of electron symmetric and antisymmrtric states in highly doped InGaAs/InAlAs single quantum well

Beating patterns in longitudinal resistance caused by the symmetric and antisymmetric states were observed in a heavily doped InGaAs/InAlAs quantum well by using variable temperature Hall measurement. The energy gap of symmetric and antisymmetric states is estimated to be 4meV from the analysis of beating node positions. In addition, the temperature dependences of the subband electron mobility and concentration were also studied from the mobility spectrum and multicarrier fitting procedure.

Photovoltaic effect in a photon storage cell

The response of photonic memory effect in I-V characteristics of a specially designed photonic memory cell was reported. When the cell is biased in a storage mode, the optical excitation with the photon's energy larger than the energy gap gives rise to a step-like jump in the current. A set-up was used to measure the transient photocurrent at the biases where the step-like jump showed up. It is proved that the falling transient edge of the photocurrent, as the photoexcitation turns off, mainly maps the decaying of electrons and holes, which were previously stored in the cell during the illumination. Its time constant is a measure of photonic memory time.

Temperature dependence of absorption edge in MOCVD grown GaN

We have studied the temperature dependence of absorption edge of GaN thin films grown on sapphire substrate by metal-organic chemical vapor deposition using optical absorption spectroscopy. A shift in absorption edge of about 55 meV has been observed in temperature range 273-343 K. We have proposed a theoretical model to find the energy gap from absorption coefficient using alpha = alpha(max) + (alpha(min) - alpha(max))/[1 + exp 2(E - E-g + KT)/KT]. Temperature dependence of band gap has also been studied by finding an appropriate theoretical fit to our data using E-g(T) = E-g(273 K) - (8.8 x 10(-4)T(2))/(483 + T) + 0.088 (Varshni empirical formula) and E-g(T) = E-g(273 K)-0.231447/[exp(362/T)-1] + 0.082 relations. It has been found that data can be fitted accurately after adding a factor similar to 0.08 in above equations. Debye temperature (483 K) and Einstein temperature (362 K) in the respective equations are found mutually in good agreement.

Epitaxial growth of SiC on complex substrates

Epitaxial growth of SiC on complex substrates was carried out at substrate temperature from 1200 degreesC to 1400 degreesC. Three kinds of new complex substrates, c-plane sapphire, AlN/sapphire, and GaN/AlN/sapphire, were used in this study. We obtained a growth rate in the range of 1-6 mum/h. Thick (6 mum) SIC epitaxial layers with no cracks were successfully obtained on AlN/sapphire and GaN/AlN/sapphire substrates. X-ray diffraction patterns have confirmed that single-crystal SiC was obtained on these complex substrates. Analysis of optical transmission spectra of the SIC grown on sapphire substrates shows the lowest-energy gap near 2.2 eV, which is the value for cubic SiC. The undoped SIC showed n-type electrical conductivity. (C) 2001 Elsevier Science B.V. All rights reserved.

Effect of In-mole-fraction in InGaAs overgrowth layer on self-assembled InAs/GaAs quantum dots

We have studied the optical and structural properties of InAs/GaAs QDs covered by InxGa1-xAs (0 less than or equal to x less than or equal to 0.3) layer using transmission electron microscopy, photoluminescence (PL) spectra and atomic force microscopy. We find that the strain reduces in the growth direction of InAs islands covered by InGaAs instead of GaAs layer. Significant redshift of PL peak energy and narrowing of PL linewidth are observed for the InAs QDs covered by 3 nm thick InGaAs layer. In addition, atomic force microscopy measurements indicate that the InGaAs islands will nucleate on top of InAs quantum dots, when 3 nm In0.3Ga0.7As overgrowth layer is deposited. This result can well explain the PL intensify degradation and linewidth increment of quantum dots with a higher In-mole-fraction InGaAs layer. The energy gap change of InAs QDs covered by InGaAs may be explained in terms of reducing strain, suppressing compositional mixing and increasing island height. (C) 2000 Elsevier Science B.V. All rights reserved.

Observation of field-driven blue shift in delta-doped Si n-i-p-i multiple quantum wells

We report the observation of the field-driven blue shift at near absorption edge in the photo-current response spectra of delta-doped Si n-i-p-i multiple quantum wells due to the widening of the effective energy gap. This phenomenon differs from the observed results in GaAs/AlGaAs and GeSi/Si superlattices, because the physical mechanisms of forming energy band in these superlattice samples are different. Our experimental results are interpreted satisfactorily by the theoretical calculation. (C) 1999 Elsevier Science Ltd. All rights reserved.

Quantum confinement effect in silicon quantum-well layers

The electronic states and optical transition properties of silicon quantum-well layers embedded by SiO2 layers are studied by the empirical pseudopotential homojunction model. The energy bands, wave functions, and the optical transition matrix elements are obtained for layers of thickness from 1 to 6 nm, and three oriented directions (001), (110), and (111). It is found that for Si layers in the (001) direction the energy gap is pseudodirect, for these in the (111) direction the energy gap is indirect, while for those in the (110) direction the energy gap is pseudodirect or indirect for a thickness smaller or larger than 3 nm, respectively. The optical transition matrix elements are smaller than that of diner transition, and increase with decreasing layer thickness. When the thickness of a layer is smaller than 2 nm, the Si QW layers have larger transition matrix elements. It is caused by mixing of bulk X states with the Gamma(1) state. The calculated results are compared with experimental results.

BAND-STRUCTURE OF MG1-XZNXSYSE1-Y

The empirical pseudopotential method within the virtual crystal approximation is used to calculate the band structure of Mg1-xZnySySe1-y, which has recently been proved to be a potential semiconductor material for optoelectronic device applications in the blue spectral region. It is shown that MgZnSSe can be a direct or an indirect semiconductor depending on the alloy composition. Electron and hole effective masses are calculated for different compositions. Polynomial approximations are obtained for both the energy gap and the effective mass as functions of alloy composition at the GAMMA valley. This information will be useful for the future design of blue wavelength optoelectronic devices as well as for assessment of their properties.

THEORY OF THE ELECTRONIC-STRUCTURE OF POROUS SI

A theoretical model for the electronic structure of porous Si is presented. Three geometries of porous Si (wire with square cross section, pore with square cross section, and pore with circular cross section) along both the [001] and [110] directions are considered. It is found that the confinement geometry affects decisively the ordering of conduction-band states. Due to the quantum confinement effect, there is a mixing between the bulk X and GAMMA states, resulting in finite optical transition matrix elements, but smaller than the usual direct transition matrix elements by a factor of 10(-3). We found that the strengths of optical transitions are sensitive to the geometry of the structure. For (001) porous Si the structure with circular pores has much stronger optical transitions compared to the other two structures and it may play an important role in the observed luminescence. For this structure the energy difference between the direct and the indirect conduction-band minima is very small. Thus it is possible to observe photoluminescence from the indirect minimum at room temperature. For (110) porous Si of similar size of cross section the energy gap is smaller than that of (001) porous Si. The optical transitions for all three structures of (110) porous Si tend to be much stronger along the axis than perpendicular to the axis.