摘要:

The problem of saturation of intersubband absorption and optical rectification in asymmetric quantum wells is discussed theoretically using the density matrix formalism. The effects of the electron‐electron interaction are taken into account. It is shown that near the resonance the spectral shapes of the nonlinear optical absorption coefficient and the nonlinear optical rectification coefficient are very similar. In the absence of the electron‐electron interaction the spectral responses are nearly Lorentzian. The electron‐electron interaction plays an important role in heavy doped systems. In structures with large spatial separation of the ground and the excited states wave functions, the depolarization effect is small and the absorption spectrum is affected mainly by the electrostatic Coulomb interaction. This interaction shifts the absorption peak to higher energy by an amount depending on the intensity of the incident light and leads to the significant distortion (asymmetry) of the line.  

ISSN:0021-8979    

DOI:10.1063/1.354339

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Transport of 1–10 keV electrons in solid media is investigated by means of a single scattering Monte Carlo scheme. Interaction of the incident electron with the target is modeled using bound atom elastic scattering cross sections, together with inelastic collision cross sections and excitation functions, for the weakly bound outer electrons, obtained from dielectric theory with a model dielectric function. Inelastic collisions of the incident particle with the tightly bound inner shell electrons of the target are sampled using semiclassical ionization cross sections. Representative backscattering and transmission calculations are presented and compared with experimental data.

ISSN:0021-8979    

DOI:10.1063/1.354340

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The neutral and charged species emitted by pulsed‐laser irradiation of polytetrafluoroethylene (PTFE) at 248 nm in vacuum have been examined. In particular, the species and properties of the emission products produced at typical fluence regimes used in the pulsed‐laser deposition of PTFE thin films have been characterized. The relative intensities of the major products as well as their dependence on laser fluence are presented, and a simple model is used to fit the observed fluence dependence. Evidence that the major neutral component, the monomer (C2F4), is formed from a thermally activated unzipping reaction is presented. The ionic species are derived from the neutral decomposition products, apparently ionized by electron collisions in the weak plasma generated at the target surface.

ISSN:0021-8979    

DOI:10.1063/1.354341

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Impurity doping in InP layer grown by metalorganic vapor‐phase epitaxy using tertiarybutylphosphine and organic doping sources is presented. Diethylzinc and diethylselenide were used as thep‐type andn‐type doping sources, respectively. Electrical properties and surface morphology of the impurity‐doped layers together with their growth condition dependence were investigated. Good controllability and reproducibility of the doping level were confirmed. The maximum doping levels of 1.8×1018and 1×1019cm−3were successfully attained forp‐InP andn‐InP, respectively. These results promise further safe metalorganic vapor‐phase epitaxy by using organic compounds for all precursors.

ISSN:0021-8979    

DOI:10.1063/1.354342

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The formation and the properties of fine silicon oxide particles in a hollow post‐type rf magnetron discharge in SiH4/O2/Ar gas mixtures were studied. ForP≳30 mTorr, primary fine particles (PFPs) with a diameter of about 20 nm are formed through homogeneous reactions. Their diameter increases with the system pressure. PFPs with sufficient negative charge are suspended in the plasma. They can further aggregate with other PFPs to form aggregated fine particles (AFP) with nearly spherical shape and larger diameter. The size of the AFP depends on the duration of the rf power. In the cw mode, AFPs gradually drift axially to both ends of the discharge system. The accumulation of AFPs at the end trap causes low‐frequency oscillation of the discharge. The films consisting of fine particles are loose and brittle. Infrared‐absorption spectrum shows that oxide formed in the homogeneous reaction has similar Si—O bond strain relaxation to that of the thermal oxides and the annealed oxide from chemical‐vapor deposition.

ISSN:0021-8979    

DOI:10.1063/1.354343

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Glass‐metal nanocomposite powders in the systems Fe/SiO2and Ni/SiO2have been prepared by the sol‐gel technique followed by reduction treatment. Bulk nanocomposites are then fabricated by hot pressing these powders. The metal particle diameters range from 8.9 to 14.8 nm. The materials show enhanced coercivities, e.g., a maximum of 82 Oe in the case of Ni/SiO2and a maximum of 474 Oe in the case of Fe/SiO2systems. The Mo¨ssbauer spectra of Fe/SiO2samples are comprised of a ferromagnetic component superposed on a superparamagnetic doublet.

ISSN:0021-8979    

DOI:10.1063/1.354344

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A transition from Ge segregation to trapping during high‐pressure oxidation of GexSi1−xalloys has been observed. The atomic fractionxof Ge was varied from 0.4% to 26%, and oxidations were performed at 740 °C under 102 atm of dry O2. It was observed that the effect of oxidation on the Ge distribution could be divided into three stages. In the initial stage of the oxidation, Ge was segregated from the growing oxide and accumulated in a Ge‐rich layer at the oxide/alloy interface. For alloys with high Ge content this initial stage was very short. In the second stage of oxidation, after a critical quantity of Ge had accumulated at the interface, there was a transition from segregation to trapping of Ge in the oxide. In the third stage, the critical amount of Ge remained segregated at the interface, and the final oxide layer was Ge free. A kinetic model based on a steady‐state equilibrium between the diffusive flux of Si across the Ge‐rich layer and the rate of Si consumption by the oxidation reaction predicts, with reasonable agreement, the critical quantity of segregated Ge for the onset of trapping.

ISSN:0021-8979    

DOI:10.1063/1.354345

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Indium‐tin‐oxide (ITO)/silicon oxide/mat‐texturedn‐Si junction solar cells having an energy conversion efficiency of 15% are fabricated by the spray pyrolysis method. Their characteristics and the junction properties are compared with the same junction solar cells having a flat Si surface. In cases where the ITO film is deposited on a hydrofluoric acid‐etched mat‐textured Si surface, the open circuit photovoltage (Voc) is low (405 mV). Scanning electron microscopy observation shows that high‐density dislocations are formed near the Si surface, and the temperature dependence of the current‐voltage characteristics suggests that the trap‐assisted multistep tunneling through the Si depletion layer is a dominant current flow mechanism. In cases where the ITO film is deposited on a thermal silicon oxide‐covered mat‐textured Si surface, the formation of the dislocations is suppressed, and consequentlyVocis increased to 485 mV. For this solar cell, a surface recombination current takes the dominant part of the dark current in the bias region below ∼250 mV, and a thermionic‐assisted tunneling current is dominant in the higher bias region. For a cell where the thermal silicon oxide‐covered mat‐textured Si surface is annealed at 800 °C under nitrogen before the deposition of the ITO film,Vocis further increased to 540 mV, and the energy conversion efficiency of 15% is achieved. In this case, the thermionic‐assisted tunneling current density is decreased by an increase in the barrier height due probably to a reduction in the density of the positive charge in the silicon oxide layer. The surface recombination current density is also reduced by the removal of interface states, leading to the improvement of the fill factor.

ISSN:0021-8979    

DOI:10.1063/1.354346

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Predictions of the three‐port model of the quantum theory of mixing are compared with measured results on 345 GHz superconductor‐insulator‐superconductor waveguide mixers. Single Nb‐Al2O3‐Nb tunnel junctions or two or four identical junctions in series are used as mixing elements. Two different waveguide mixerblocks, one with two tuners and another with one tuner, are used. In addition a single junction with integrated tuning stub is analyzed. Embedding impedances are obtained from fits to the pumpedI‐Vcurves for all three types of mixing elements. In all cases the dependence of mixer conversion and mixer noise on bias voltage, pump power, and embedding impedance is well described by the three‐port model. The measured mixer gain is lower than the calculated gain by a factor of 0.35–0.65, independent of the type of mixer. The use of an additional integrated tuning element does not change this factor. It is concluded that an excess noise power equivalent with a blackbody source of 40–65 K must be added to the mixer noise to account for the absolute value of the observed noise power.

ISSN:0021-8979    

DOI:10.1063/1.354347

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Detailed theoretical calculations of Auger and radiative recombination rates for an optimized InAs/InxGa1−xSb superlattice (SL) and bulk HgxCd1−xTe (MCT) show that 300 K background limited operation for a 60° field of view can be theoretically achieved up to 130 K for the 11 &mgr;m SL and up to 185 K for 5 &mgr;m MCT. The SL structure is theoretically superior to MCT for 11 &mgr;m operation. The converse is true at 5 &mgr;m.

ISSN:0021-8979    

DOI:10.1063/1.354348

出版商:AIP    

年代:1993

数据来源: AIP