摘要:

The optical and electrical properties of the lead bearing glasses in thePbO-TiO2,PbO-TeO2,and PbO-CdO systems have been investigated. The density, refractive index, optical band-gap, and Urbach energies of these glasses are reported. It is shown that the additive nature of oxide ion polarizabilities reported for the simple oxides can be extended to glass systems as well. A modified linear relationship between(Eopt)1/2and1−(Rm/Vm)holds good for glasses. The oxide ion polarizabilities deduced from two different experimental quantities viz., refractive index and optical band-gap energy agree well. The dc electrical conductivities have been measured in the temperature range 350–700 K. The activation energies are close to 1 eV. The low melting70PbO-30TeO2glass shows higher isothermal conductivity compared to other glasses of similar composition. InPbO-TiO2glass the PbO content contributes significantly to conductivity and composition dependence of pre-exponential factor(&sgr;0)supports tunneling mechanism. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365366

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We have theoretically studied giant dipoles associated with transitions between magneto-electric subbands in a quantum wire subjected to a transverse magnetic field. The strengths of these dipoles and their resonant frequencies can be varied with the magnetic field which then allows one to tune the emission wavelength of these transitions. The large magnitude of the dipole moments also leads to a strong second-harmonic component of the dielectric susceptibility that can be utilized for nonlinear optical applications such as second-harmonic generation, limiting, mixing, optical switching, etc.©1997 American Institute of Physics institute.

ISSN:0021-8979    

DOI:10.1063/1.365378

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Highly crystalline nanoclusters of hexagonal (2Hpolytype)MoS2and several of its isomorphous Mo and W chalcogenides have been synthesized with excellent control over cluster size down to ∼2 nm. These clusters exhibit highly structured, bandlike optical absorption and photoluminescence spectra which can be understood in terms of the band-structures for the bulk crystals. Key results of this work include: (1) strong quantum confinement effects with blue shifts in some of the absorption features relative to bulk crystals as large as 4 eV for clusters ∼2.5 nm in size, thereby allowing great tailorability of the optical properties; (2) the quasiparticle (or excitonic) nature of the optical response is preserved down to clusters ≲2.5 nm in size which are only two unit cells thick; (3) the demonstration of the strong influence of dimensionality on the magnitude of the quantum confinement. Specifically, three-dimensional confinement of the carriers produces energy shifts which are over an order of magnitude larger than those due to one-dimensional (perpendicular to the layer planes) confinement emphasizing the two-dimensional nature of the structure and bonding; (4) the observation of large increases in the spin-orbit splittings at the top of the valence band at theKandMpoints of the Brillouin zone with decreasing cluster size, a feature that reflects quantum confinement as well as possible changes in the degree of hybridization of the electronic orbitals which make up the states at these points; and (5) the observation of photoluminescence due to both direct and surface recombination. Several of these features bode well for the potential of these materials for solar photocatalysis. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365367

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The effects of traps and shallow acceptors on the continuous-wave steady-state photoluminescence of GaAs-(Ga,Al)As quantum-well wires are studied at room temperature. The analysis is based on a quantum-mechanical calculation of the transition rates of radiative recombinations of excited-conduction electrons with free and bound (at acceptors) holes, and on a phenomenological treatment of the nonradiative rates associated with transitions involving conduction electrons falling into traps, and trapped electrons recombining with free holes. The various steady-state radiative and nonradiative e-h recombination lifetimes as function of the cw laser intensity are then obtained, as well as the dependence of the conduction-electron quasi-Fermi level (or chemical potential), and carrier densities on the laser intensity. We have also studied the laser-intensity dependence of various recombination efficiencies and of the integrated photoluminescence intensity. Finally, trap and impurity effects are shown to be quite important in a quantitative understanding of the room temperature steady-state photoluminescence of quantum-well wires. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365368

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We report spectroscopic ellipsometry (SE) studies on(Si)2(Ge)12,(Si)6(Ge)2,and(Si)12(Ge)2short period superlattices (SLs) whose optical response has not been reported yet. Multilayer calculations enabled us to determine the dielectric response of the superlattice layers. We report the clear observation of splitting of theE2peak in(Si)m(Ge)nsuperlattices contrary to the previous SE report that the separation was observed only in larger period SLs. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365369

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We present a study of p-doping influence on the Raman spectrum of thin SiGe quantum wells. We observe a laser wavelength dependence of the phonon frequencies. We show that the quantum confinement induces special effects which can be described by the renormalization of the phonon energy by the free charge continuum transitions. It is shown that the polarization of the laser and of the phonon can probe the confinement and the density of states of the quasi-free holes. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365370

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Strain effects on the enhancement of the luminescence capability of Ge/Si disordered superlattices (d-SLs) are investigated by comparing the photoluminescence (PL) properties of Ge/Si superlattices grown on Si substrate and on strain-free SiGe buffer layer as a substrate. The conduction valley,&Dgr;x,yor&Dgr;z,becomes the minimum energy state in the superlattices which is dependent on the strain and thus strained or strain-free substrate. In the case ofd-SLs on the Si substrate, there is no significant enhancement by artificially introduced disordering on the PL properties. On the contrary, remarkably enhanced photoluminescence is observed in thed-SLs grown on the SiGe buffer layer, showing thatd-SLs of SiGe material enhance the luminescence capability similar to the AlAs/GaAs and AlP/GaPd-SLs. The improvement of the PL properties by thed-SL grown on the SiGe buffer layer is discussed in the viewpoint of the directions of carrier localization and required for the carrier recombination. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365371

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

GaN andInxGa1−xNwere grown using a gas-source molecular-beam epitaxy method. Sapphire substrates were used for GaN growth. Ammonia(NH3)gas was used as the nitrogen source gas. As a result, GaN andInxGa1−xNcrystals with mirror surfaces were obtained using uncracked ammonia gas. Photoluminescence measurements ofInxGa1−xNand GaN were investigated. No deep-level emission was observed and only the band-edge emission spectra ofInxGa1−xNand Si-doped GaN crystals were observed. It was confirmed that, in the case of undoped GaN growth, the yellow luminescence disappeared upon increasing the V/III flux ratio. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365398

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We report on the results of small angle x-ray scattering and high resolution electron micrograph studies which reveal the structure of hydrothermally prepared porous silicon that emits blue light under photoexcitation. Our results constitute direct evidence that blue photoluminescence porous silicon includes high density silicon quantum dots. We show that these quantum dots are isolatedly dispersed in each amorphousSiO2linear column standing freely on the silicon substrate. The size of individual quantum dots is about 2 nm. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365399

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

a-SiC:H films with energy gap in the range 2.00–2.65 eV have been grown by plasma enhanced chemical vapor deposition in undiluted andH2dilutedSiH4+CH4gas mixtures, by making use of optimized deposition conditions. A complete picture of structural, compositional, optoelectronic, and defective properties for high quality films has been drawn for the first time. We show that the addition ofH2to the gas mixture leads to a different chemical composition of the deposited films; in particular, carbon incorporation is enhanced and a carbon fraction in the solid matrix up to C/(C+Si)≈0.45 can be obtained. These films have a higher mass density, a reduced microvoid and carbon cluster concentration, a better structural connectivity, and improved optoelectronic properties. For samples with optical gap below 2.4 eV, the reduced defect concentration ofH2diluted films results in an increase of the photoconductivity gain and the steady-state(&eegr;&mgr;&tgr;)ssvalues up to two orders of magnitude. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365400

出版商:AIP    

年代:1997

数据来源: AIP