摘要:

We present photoconductivity data on GaAs quantum wires grown on aV-grooved substrate by flow rate modulation epitaxy. They show that a moderate excitation power density, ∼1W/cm2,allows the observation of the absorption structure of a single GaAs quantum wire embedded in ap-i-ndiode. Furthermore, by increasing the number of active wires inside a diode, the photoconductivity signal is enhanced and additional details of the absorption structure are evidenced. And, finally, a rough quantitative agreement is obtained between the experimental absorption transitions and a simple calculation of the one-dimensional excitonic states using the envelope function approximation. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364410

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The acceptor-related photoluminescence of a GaAs–(Ga,Al)As superlattice, under the influence of a magnetic field applied parallel to the interfaces, is theoretically studied following a variational procedure within the effective-mass approximation. Electron and hole magnetic Landau levels and envelope wave functions were obtained by an expansion in terms of sine functions, whereas for the impurity levels the envelope functions were taken as products of sine and hydrogenic-like variational functions. Impurity binding energies and wave functions are obtained for acceptors at a general position in the superlattice and for different in-plane magnetic fields. Theoretical results corresponding to transitions from the conduction subband to states of acceptors (miniband-to-acceptore−A0transitions) at the edge and center positions of the GaAs quantum well compare well with available experimental data by Skromme &etal; [Phys. Rev. Lett.65, 2050 (1990)] on the magnetic-field dependence of the photoluminescence peak position of conduction miniband-to-acceptor transitions for different temperatures and values of the superlattice period. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364411

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The electronic properties of amorphous diamondlike carbon (DLC) films on silicon were examined for their capability for application to electronic devices. Metal-insulator-semiconductor (MIS) diodes andp-nheterojunction devices were created and characterized. The films used were grown using rf plasma assisted CVD of methane. They were grown onn-type silicon (100) wafers. Their structural composition was identified by Raman spectroscopy, ellipsometry, and x rays. They showed an insulating behavior and they were suitable for the creation of MIS devices. They showed extremely low internal conductivity due to defect currents. This conductivity showed a behavior of the formI∼T1/4. Then the density of trapping states at the silicon/carbon interface was measured using the conductance technique. The aim of investigating the nature and behavior of these states was to determine the magnitude of the density of states and try to find a way to reduce it. Thus optimization of the DLC/silicon interface could be made, for future reference to the construction of heterojunction devices containing the DLC/silicon configuration. The values of the density of states were of the order of1011–1012 cm−2 eV−1. Annealing had the effect of reducing the magnitude of theDit.The ion implantation technique was used to transform the insulating DLC films into semiconducting by adding boron ions as dopants to achievep-type conductivity. Thusp-nheterojunction devices were made, having silicon as one of the elements and DLC as the other element. Four different doses of boron were implanted, producing different conducting properties of the DLC films. Those devices performed like Schottky diodes for low boron doses and likep-ndiodes for high doses. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364412

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Capacitance–voltage (C–V) characteristics of aSi/C60heterojunction, i.e., a HF solution (7.3&percent; HF+30&percent;NH4F)treated Si surface interfaced solid crystal ofC60molecules, were measured while applying various ac frequencies. The entireC60thin film and near-interface region of the Si wafer behaved as a depletion region, with theC–Vcurve showing two distinct regions: one above and one below a threshold bias voltage. Below the threshold,C–Vcharacteristics were dependent on applied frequency, which suggests the presence of interface states that only affect capacitance at lower frequencies. This frequency dependence was analyzed by assuming a suitable equivalent circuit, and based on derived curve-fitting circuit parameters the interface states density was accordingly estimated to have a value as low as∼1011/cm2 eV.Such a small density indicates that only a few lattice defects occur within the interface of the HF-treated Si surface andC60crystal. Although no frequency dependence was observed above the threshold, theC–Vcharacteristics were found to be dependent on the width of the depletion region in Si. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364438

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Radio frequency glow discharge chemical vapor deposition has been used to deposit thin films ofa-Si1−xGex:Hwhich possess optoelectronic properties that are greatly improved over any yet reported in the range ofx⩾0.6.These films were deposited on the cathode (cathodic deposition) of an rf discharge. Their properties are assessed using a large variety of measurements and by comparison to the properties of alloys conventionally prepared on the anode (anodic deposition). Steady state photoconductivity measurements yield a quantum-efficiency-mobility-lifetime product, &eegr;&mgr;&tgr;, of (1–3)×10−7 cm2 V−1for1.00⩾x⩾0.75and(6–10)×10−8 cm2 V−1for0.75⩾x⩾0.50,and photocarrier grating measurements yield ambipolar diffusion lengths several times greater than previously obtained for alloys of largex.It is confirmed that the improvements in phototransport are not due to a shift in the Fermi level. In fact, results of recent measurements on lightly doped samples strongly suggest that for these cathodic alloys neither photocarrier is dominant[(&mgr;&tgr;)e≈(&mgr;&tgr;)h].The improvements are attributed in large part to the reduction of long range structural heterogeneity observed in x-ray scattering and electron microscopy, and partly to the reduction in midgap state density. In spite of the superior properties, an assessment of the data of the cathodic alloys suggests that alloying introduces mechanisms detrimental to transport which are not present ina-Si:Hora-Ge:H.The Urbach tail width is 42±2 meV for cathodica-Ge:Hand 45±2 meV for cathodica-Si1−xGex:Hand is constant withx.From differences in the band edges and tails we infer that the atomic bond ordering is different between the cathodic and anodic alloys. For a given composition the cathodic alloys have roughly an order of magnitude lower midgap state density than do the anodic alloys, and both midgap densities increase exponentially withx,consistent with defect creation models from which the lower midgap density can be attributed to a larger band gap and decreased valence band tail width. A photoluminescence peak is observed with an intensity roughly an order of magnitude greater than for the anodic alloys, and a significantly different peak energy. Section VII E provides an overview of the results and conclusions. The improved properties of these alloys have significant implications for current and future device applications. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364413

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We study the influence of lateral carrier diffusion on the properties ofIn0.35Ga0.65As/GaAsmultiple quantum well lasers by comparing theoretical and experimental results. A model including the carrier diffusion terms into the rate equations has been used to calculate the dc and small-signal lateral profiles for both unconfined and confined carriers in mesa waveguide devices. The theoretical results were compared with experimental results of the frequency dependence of the subthreshold electrical impedance and small-signal spontaneous emission, and with the measured threshold currents for lasers with different mesa widths. The comparison yielded an estimation for the nonradiative and radiative recombination coefficients, the ambipolar diffusion constant, and the external surface recombination velocity. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364414

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The real and imaginary parts of the surface impedance of YBCO superconductors have been studied at different microwave power levels. Using the relations for the critical current density and the grain boundary resistance, a relation for calculating the power dependence of the surface resistance has been obtained. Also, a relation to find the resonant frequency of a superconducting microstrip resonator at various input power levels has been derived. Measurements have been carried out on various microstrip resonators to study the variation of surface resistance and resonant frequency at different rf power levels. The experimental results are in good agreement with theoretical results. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364415

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The critical current densitiesJcof Chevrel phase wires with niobium as an antidiffusion barrier were measured in magnetic fields up to 24 T. At 20 T and 1.9 K,Jcreaches5.4×108A/m2and decreases slightly down to3.1×108A/m2at 24 T. A wire with a 20&percent; superconducting cross section has been successfully drawn and its overall critical current densityJcovexceeds 100A/mm2at 1.9 K up to a magnetic field slightly above 20 T. This demonstrates the ability of Chevrel phase wires to be used in high magnetic field applications. Moreover, some parts of the coil have certainly higherJc,sinceJcis very often limited by a thermal excursion of the entire coil. The effective upper critical field&mgr;0Hc2*,deduced from the magnetic field dependence ofJc,is too low compared to the expected bulk value, indicating that superconducting properties at the grain boundaries are still degraded. If the bulk&mgr;0Hc2can be restored at the grain surfaces,Jcovshould be higher than 100A/mm2up to at least 30 T. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364416

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

By extracting the layer-charges inYBa2Cu3Oxas functions of dopant oxygen contentxfrom neutron diffraction data (using the bond-valence-sum method), we show that the charge-transfer hypothesis of high-temperature cuprate-plane superconductivity fails to describe the data. A similar failure occurs forLa2−&bgr;Sr&bgr;CuO4as a function of Sr dopant concentration, and was implicitly predicted (before the charge-transfer hypothesis was enunciated) by self-consistent local-density-approximation calculations of R. V. Kasowski, M.-H. Tsai, J. D. Dow, and M. T. Czyzyk [Physica C162, 1349 (1989)]. Both failures can be traced to difficulties the hypothesis has with (i) the rules of chemistry which require electronegative dopants such as oxygen to attract electrons, not holes, and (ii) the law of conservation of (neutral) charge in each unit cell. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364417

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A capacitively coupled interdigital probe technique is described whereby the electrical characteristics of thin films at arbitrary frequencies in the range from about 100 kHz to beyond 1 GHz may be studied. Patterning of the film, deposition of electrodes, and bonding of electrical leads are unnecessary, so that all measurements are nondestructive and noninvasive. Preliminary measurements of the superconducting transition in thin films of the high-temperature superconductorYBa2Cu3O7−&dgr;in magnetic fields up to 1.5 T are presented and discussed. They show a large field-dependent peak, attributable to tunneling effects, just below the onset temperature. Anomalies are also seen at higher temperatures, providing evidence of a structural phase transition. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364363

出版商:AIP    

年代:1997

数据来源: AIP