摘要:

The in‐well ambipolar diffusion coefficients and carrier lifetimes in ordered all‐binary quantum wells, composed of (InAs)2(GaAs)5short‐period strained‐layer superlattices (SPSLSs) grown on [001]‐oriented GaAs substrates, are measured using picosecond optically induced transient grating and pump‐probe techniques. Quantum wells containing SPSLSs may be expected to exhibit higher in‐plane hole mobilities compared to InGaAs ternary alloy quantum wells because of a larger average indium content and reduced disorder scattering in the SPSLS structures. The results obtained in the SPSLSs are compared to those obtained in InGaAs alloy quantum wells of comparable well width and confinement energies. This indicates that, for a given SPSLS and its equivalent alloy, the ambipolar diffusion coefficients are comparable while the carrier lifetimes in the SPSLSs are longer. The longer carrier lifetimes in the SPSLSs suggest that these binary structures possess fewer nonradiative recombination centers than the alloys. The absence of a dramatic improvement in the transport properties, however, may indicate that imperfect interfaces in the SPSLSs may be offsetting their possible advantages, in spite of the fact that optical measurements in the SPSLSs indicate high quality and x‐ray and transmission electron microscopic measurements demonstrate the binary nature of these structures.

ISSN:0021-8979    

DOI:10.1063/1.355301

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Tunneling properties and their temperature variations of molecular bean epitaxy grown symmetric AlAs/GaAs/AlAs resonant tunneling diodes with thin barriers are studied theoretically and experimentally. The measured peak and valley current densities show strong dependences on temperature. A Monte Carlo simulation including impurity and optical‐phonon scatterings is developed for the calculation of the current‐voltage behavior of the double barrier structures. This approach reveals pronounced temperature dependent tunneling features which agree well with measured results.

ISSN:0021-8979    

DOI:10.1063/1.354795

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Improvement in photosensitivity, (&sgr;photo−&sgr;dark)/&sgr;dark, of chemically deposited CdSe thin films on annealing in air is discussed. The as‐prepared films of ∼0.5 &mgr;m thickness show photosensitivities of <10 under 600 W m−2illumination. Upon annealing the films in air for 1 h each at various temperatures their photosensitivity increases depending on the temperature of annealing: ∼10 (200 °C), ∼102(300 °C), ∼103(400 °C), and ∼107(450 °C). Air annealing at temperatures beyond 450 °C was found to cause degradation in the photosensitivity. The high photosensitivity is also accompanied by growth in photocurrent while maintaining a fast decay of ∼6 decades in <1 s after shutting off the illumination. Such a short decay time is unusual with chemically deposited photoconductive thin films. The results are explained on the basis of improvement in crystallinity and increase in chemisorption of oxygen upon annealing the films in air. X‐ray‐diffraction data and x‐ray photoelectron spectroscopy depth profiling of the annealed films are presented in support of this explanation.

ISSN:0021-8979    

DOI:10.1063/1.354796

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The variation of the Schottky barrier height in Ag, Au, and Al/n‐GaAs contacts has been investigated as a function of the annealing temperature of the GaAs substrate. Angle resolved x‐ray photoemission spectroscopy measurements show that the substrate surface changes from As‐rich into Ga‐rich over the applied annealing range. Two distinct barrier heights (about 0.85 and 0.65 eV) are found, depending on the metal and the annealing temperature. These values correspond to the Fermi level pinning positions expected for amphoteric defect reactions involving the AsGaand GaAsantisites, respectively. Changes in the barrier height are found at annealing temperatures associated with the removal of these defects.

ISSN:0021-8979    

DOI:10.1063/1.354797

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The surface potential of GaAs is strongly modified in the presence of a high‐energy electron beam due to the creation of electron‐hole pairs in the depletion region and the subsequent drift of the holes to the surface where they neutralize surface states. This effect is modeled in terms of a parameterK=A*T2/Ib(dE/dz)&eegr;, whereIbis the beam current density,A* is the effective Richardson constant,dE/dzis the beam energy loss per unit length, and &eegr;−1is the average energy required to create an electron‐hole pair. For the sample studied here, an 0.25‐&mgr;m layer withn&bartil;3×1017cm−3, we obtain a valueK&bartil;(7.5±0.8)×104cm atT=296 K andIb=0.33 &mgr;A/cm2, which givesA*&bartil;0.44 A/cm2 K2. Although this value ofA* is much lower than the theoretical estimate of 8 A/cm2 K2, it is in good agreement with other recent results.

ISSN:0021-8979    

DOI:10.1063/1.354798

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Capacitance‐voltage (C‐V) profiling has been used to measure the apparent carrier concentration profiles in Si/Si1−xGex/Si structures for a range of Ge percentages. Using Kroemers analysis, good agreement has been found between theoretical valence band offsets and those determined from the experimental data. The validity of Kroemers analysis has been assessed in the presence of traps using aC‐Vsimulation program. Under certain circumstances, large errors occur in the extracted valence band offset due to distortion of the apparent carrier concentration profile by traps. It is proposed that the experimental data presented here falls into a regime where minimal distortion is to be expected and is reflected by the accuracy of the extracted valence band offsets.

ISSN:0021-8979    

DOI:10.1063/1.354799

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Effects of oxygen partial pressure and substrate temperature as well as the Rb content in the target on theinsitugrowth of Ba1−xRbxBiO3(BRBO) films prepared by laser ablation are studied. The optimal target composition for growing the superconducting films in the present study is Ba:Rb:Bi=0.6:0.8:1.0. Oxygen partial pressure plays an important role in controlling the doping of Rb into the BRBO phase and hence the growth of the superconducting films. For the Ba0.6Rb0.8BiO3target the superconducting films can be grown at 410–485 °C in 1–50 mTorr of O2and the optimal pressure is about 20 mTorr, under which Rb is readily doped into the BRBO phase. Meanwhile the best film withTc(onset) of 27.5 K andTc(zero) of 25 K is grown at 475 °C in 20 mTorr of O2. The (100) preferred orientation is present in most of the superconducting films, while the (110) orientation is dominant in the films grown in 30–40 mTorr of O2at 430–450 °C which are about 100 °C higher than those reported previously. For the superconducting films thicker than 0.6 &mgr;m the cracking is commonly observed and the lower the growth temperature the greater the cracking. The degradation of theTcof the BRBO film on exposure to air (moisture) can be attributed to the surface contamination which may also be one of the causes for the semiconductive behavior of electrical resistivity in the normal state.

ISSN:0021-8979    

DOI:10.1063/1.354800

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Biaxially aligned YBa2Cu3O7−x(YBCO) thin films were produced on polycrystalline Ni‐based alloy, by using biaxial yttria‐stabilized‐zirconia (YSZ) intermediate layers formed by off‐normal ion‐beam‐assisted deposition. Most explicit in‐plane alignment was obtained when the YSZ layer formed with the beam‐incident angle of 55° from substrate normal.Jc‐Bcharacteristics and angular dependence ofJcon the magnetic field were measured. 5.0×105and 5.5×104A/cm2were obtained at 77 K with 0 and 8 T, respectively. The distribution of misorientation angles of in‐planeaandbaxes between YBCO grains was evaluated by both x‐ray pole figure measurement and planar observations of transmission electron microscopy. 50% of the grains had the misorientation angles restricted within ±5°. From the image of dislocations, the elastic strains at grain boundaries were estimated to be relaxed with lower misorientation angle. The high‐Jcproperties are understood to be obtained by the current paths through low‐angle grain boundaries.

ISSN:0021-8979    

DOI:10.1063/1.354801

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

This article formulates some wave phenomena in elastic superconductors, in which the effect of interaction of elastic waves with electromagnetic fields are taken into account. The effect of normal conduction current is also taken into consideration. It is shown that, due to the effect of dynamic motion, transverse magnetic waves may be induced by a transverse elastic wave in an elastic superconductor. Magnetic surface waves may also be induced by the Rayleigh elastic surface wave in the superconductor. The magnitude of the magnetic induction field associated with the induced magnetic wave can be significant at a certain excitation frequency. Furthermore, by considering the coupling effect between the elastic wave and the electromagnetic field, a study is given to investigate electromagnetoelastic surface waves in a semi‐infinite elastic superconductor with the aid of the theoretical formulation given in the article. It is shown that the characteristics of the coupled wave in a superconductor can be quite different from that in a perfect conductor due to the presence of the Meissner effect in the superconductor.

ISSN:0021-8979    

DOI:10.1063/1.354802

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Recent results concerning synthetic, structural, and magnetic work on 3,4,9,10‐perylenetetracarboxylic acid complexes of copper (II) are discussed. To date, the Cu(II)/PTCA/dien system has yielded two compositionally different crystalline products. Compound 1, [Cu3(dien)3(PTCA)(H2O)3](ClO4)22H2O represents the extremely rare case of three different molecular complexes of Cu(II), bound to the same ligands but with different stoichiometries, in the same compound. The magnetic lattice is comprised of three molecular substructures: complex1ais a monomeric cation, [Cu(dien)(H2O)2]2+; complex1bis a charge‐neutral dimer, [Cu2(dien)2‐(PTCA)(H2O)2]; and complex1ca charge‐neutral, infinite ladder‐like chain structure,∞1[Cu2(dien)2(PTCA)]n. Single crystal x‐ray analysis of compound2, [Cu2(dien)2(PTCA)(H2O)2]12H2O, reveals its structure simply consists of isolated, neutral dimers similar in structure to complex1b. Magnetic susceptibility data in the temperature range 1.7–300 K shows the dimeric unit in2to be a weakly exchange‐coupled antiferromagnetic dimer (−0.6 cm−1) and suggests that exchange interactions are indeed propagated through the PTCA anion over a Cu.....Cu separation of 15.7 A˚. This result, in addition to the expected Curie paramagnetism of monomer1a, affords the ladder‐like chain1cto be characterized as a chain of antiferromagnetically exchange‐coupled ferromagnetic dimers (+12 cm−1). Long range ordering is not observed down to 1.7 K. Compound1is the first example of a molecular‐based magnetic material that contains three different homonuclear magnetic molecules; a monomer, a dimer, and a ladder‐like chain.

ISSN:0021-8979    

DOI:10.1063/1.354774

出版商:AIP    

年代:1993

数据来源: AIP