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28 Aug 1989

Volume 55, Issue 9, pp. 813-921

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Time‐resolved measurements of tunneling between double quantum wells in In0.53Ga0.47As/InP

M. G. W. Alexander, W. W. Rühle, R. Sauer, and W. T. Tsang

Appl. Phys. Lett. 55, 885 (1989); http://dx.doi.org/10.1063/1.101615 (3 pages) | Cited 29 times

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Tunneling of electrons between double quantum wells is investigated by time‐resolved photoluminescence in the picosecond regime. The samples contain two In0.53Ga0.47As quantum wells with different widths, separated by various InP barriers. At low excitation density, the luminescence decay time of the narrower quantum well depends strongly on the thickness of the barrier, revealing the lifetimes to be tunneling controlled. A semiclassical model explains the observed nonresonant tunneling escape times. With increasing density, the luminescence decay time of the narrower quantum well strongly increases and finally saturates due to effective mass filtering, which leads to a lineup of the electron levels in both wells and resonant tunneling.
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73.40.Gk Tunneling
78.55.Cr III-V semiconductors
73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
78.47.-p Spectroscopy of solid state dynamics

Calculations of the electric field dependent far‐infrared absorption spectra in InAs/AlGaSb quantum wells

S. Hong, J. P. Loehr, J. E. Oh, P. K. Bhattacharya, and J. Singh

Appl. Phys. Lett. 55, 888 (1989); http://dx.doi.org/10.1063/1.101616 (3 pages) | Cited 5 times

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Excitonic and band‐to‐band absorption spectra are calculated for vertical incident radiation for the InAs/AlGaSb multiple quantum well structures. Due to the special band lineup of this heterostructure, the absorption spectra can be tailored to respond in far infrared. The electric field dependence of the spectra shows blue shift and enhanced absorption in contrast to the situation in type I quantum wells. Applications to far infrared detectors are discussed.
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78.66.Fd III-V semiconductors
78.66.Hf II-VI semiconductors
85.60.Gz Photodetectors (including infrared and CCD detectors)
73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
71.35.-y Excitons and related phenomena

Tailoring the intersubband absorption in quantum wells

W. Trzeciakowski and B. D. McCombe

Appl. Phys. Lett. 55, 891 (1989); http://dx.doi.org/10.1063/1.101617 (3 pages) | Cited 27 times

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Optical transitions between quantum well subbands have large oscillator strengths and narrow linewidths so that they can be used for the detection of infrared light. Here we show that the intersubband separation can be varied over a wide energy range by depositing a thin barrier layer of AlGaAs in the middle of a GaAs well. In a sufficiently narrow well it should be possible to push the subbands out of the well, i.e., perform the transition from the quasi‐two‐dimensional case to the quasi‐three‐dimensional case.
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78.66.Fd III-V semiconductors
78.66.Hf II-VI semiconductors
73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems

(InAs)3(GaAs)1 superlattice channel field‐effect transistor grown by molecular beam epitaxy

Naoki Nishiyama, Hiroshi Yano, Shigeru Nakajima, and Hideki Hayashi

Appl. Phys. Lett. 55, 894 (1989); http://dx.doi.org/10.1063/1.101618 (2 pages) | Cited 5 times

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A metal‐insulator‐semiconductor field‐effect transistor (MISFET) using the InAs‐GaAs superlattice as a channel layer has been successfully demonstrated for the first time. Device structure was grown by molecular beam epitaxy. The MISFET with 1.0 μm gate length exhibits a maximum external dc transconductance of 212 mS/mm at Vds =3.0 V and Vg =−1.5 V. This value is high among 1.0 μm gate length devices.
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85.30.Tv Field effect devices
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
73.61.Ey III-V semiconductors
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties

Novel method of patterning YBaCuO superconducting thin films

Q. Y. Ma, E. S. Yang, G. V. Treyz, and Chin‐An Chang

Appl. Phys. Lett. 55, 896 (1989); http://dx.doi.org/10.1063/1.102448 (3 pages) | Cited 24 times

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A unique method of patterning YBaCuO thin films based on the inhibition of superconductivity by Si‐YBaCuO intermixing has been developed. In the experiment, a thin Si film was first evaporated on a MgO substrate and subsequently patterned using laser direct‐write etching. Multilayered YBaCuO thin films were then deposited by e‐beam evaporation and annealed in a rapid thermal annealing system for 30–90 s at 980 °C. The YBaCuO film deposited on the silicon regions became insulating. Auger depth profiling measurements indicate that Si‐YBaCuO intermixing had occurred in these areas. Between the insulating regions, narrow YBaCuO superconducting lines were formed. For both 10‐μm‐wide, 1‐mm‐long and 2.5‐μm‐wide, 80‐μm‐long lines, the Tc was observed above 76 K. The critical current density of the lines was measured to be 300 A/cm2 at 75 K. This patterning technique may be useful for fabrication of high Tc superconducting interconnects and devices.
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74.78.-w Superconducting films and low-dimensional structures
85.25.Qc Superconducting surface acoustic wave devices and other superconducting devices
74.70.-b Superconducting materials other than cuprates
81.05.Je Ceramics and refractories (including borides, carbides, hydrides, nitrides, oxides, and silicides)
81.65.-b Surface treatments

Oxygen content control for as‐deposited YBa2Cu3Ox thin films by oxygen pressure during rapid cooling following laser deposition

M. Ohkubo, T. Kachi, T. Hioki, and J. Kawamoto

Appl. Phys. Lett. 55, 899 (1989); http://dx.doi.org/10.1063/1.102449 (3 pages) | Cited 18 times

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As‐deposited epitaxial YBa2Cu3Ox thin films have been prepared by ArF excimer laser deposition followed by a rapid cooling with controlled oxygen pressure. The best film after the rapid cooling has a Tc with a zero resistance of 88 K and a Jc (77 K) of 5.5×105 A/cm2. It has been observed that the c‐lattice parameter decreases, that is, the oxygen content x increases from ∼6 to 7, with increasing oxygen pressure during the rapid cooling. The relation between x and oxygen pressure is almost consistent with that for a powder sample being in a thermal equilibrium. We conclude that the oxygen content in the as‐deposited films is determined by the thermal equilibrium oxygen content during the cooling process.
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74.78.-w Superconducting films and low-dimensional structures
74.70.-b Superconducting materials other than cuprates
81.15.Kk Vapor phase epitaxy; growth from vapor phase
81.15.Jj Ion and electron beam-assisted deposition; ion plating

Epitaxial growth of superconducting YBa2Cu3O7−x thin films by reactive magnetron sputtering

G. C. Xiong and S. Z. Wang

Appl. Phys. Lett. 55, 902 (1989); http://dx.doi.org/10.1063/1.102450 (3 pages) | Cited 18 times

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Perfect epitaxial growth of superconducting YBa2Cu3O7−x thin films have been achieved on (100) SrTiO3, (110) SrTiO3, and (100) ZrO2 substrates using a modified planar dc magnetron sputtering system. The films exhibit zero resistances at 87–90 K with transition widths of about 2 K. The critical current density of 1.4×106 A/cm2 at 77 K so far has been measured on the (100) SrTiO3 substrate. The epitaxial orientation of the thin films was influenced by the substrate orientation, the substrate temperature, and the oxygen partial pressure. The quality of growth and the epitaxial orientation of the films were examined by electron channeling, x‐ray diffraction, and reflection high‐energy electron diffraction techniques. The preferential epitaxial orientation is discussed.
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74.78.-w Superconducting films and low-dimensional structures
81.15.Kk Vapor phase epitaxy; growth from vapor phase
81.15.Cd Deposition by sputtering
74.70.-b Superconducting materials other than cuprates

Offset criterion for determining superconductor critical current

J. W. Ekin

Appl. Phys. Lett. 55, 905 (1989); http://dx.doi.org/10.1063/1.102259 (3 pages) | Cited 38 times

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Critical‐current criteria based on electric field or resistivity can present a number of problems in defining critical current, especially for high Tc superconductors in the vicinity of the critical temperature or upper critical field. The resulting critical‐current density Jc can be quite arbitrary, since it depends strongly on criterion level at high fields and temperatures. These Jc definitions also create problems in distinguishing between superconductors and high‐conductivity normal metals such as copper. They can also bias Jc data when superconductors are compared that have different values of normal‐state resistivity. To minimize these problems, an intrinsic Jc criterion is proposed, which effectively separates superconducting and normal‐state properties. Based on the long‐standing concept of a flux‐flow resistivity, Jc is defined as the current where the tangent to the E‐J curve at a given electric field level extrapolates to zero electric field. This determines an offset Jc that minimizes the above problems. The criterion is particularly useful near Tc or near the effective upper critical field where the E‐J characteristic starts to approach ohmic behavior.
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74.25.Sv Critical currents
74.70.-b Superconducting materials other than cuprates
74.62.Bf Effects of material synthesis, crystal structure, and chemical composition
74.25.Uv Vortex phases (includes vortex lattices, vortex liquids, and vortex glasses)

Comparison of interplanar‐boson‐exchange models of high‐temperature superconductivity—possible experimental tests

R. C. Casella

Appl. Phys. Lett. 55, 908 (1989); http://dx.doi.org/10.1063/1.101619 (3 pages)

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Semiquantitative tests are considered of models, proposed earlier by this author, in which high Tc superconductivity follows from the exchange of unspecified intermediate bosons (IB) with ω≫ωphonon between fermionic pairs in different CuO2 layers. Earlier predictions of the dependence of Tc(n) on the number n of layers per cluster are extended to include the possibility that Tc(n=1)<∼10 K in the Tl 1201 and 2201 compounds. In common with other authors rapid saturation of Tc(n) with increasing n is found here. Nonetheless, the experimental value of Tc(n=1) in the Tl and Bi systems can play an important role in discriminating between the various models empirically.
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74.20.Mn Nonconventional mechanisms
74.20.Rp Pairing symmetries (other than s-wave)
74.70.-b Superconducting materials other than cuprates
74.78.Fk Multilayers, superlattices, heterostructures
74.20.-z Theories and models of superconducting state

Imaging of current‐induced superconducting‐resistive transitions by scanning electron microscopy in laser‐deposited superconducting thin films of Y1Ba2Cu3O7−x

A. Frenkel, E. Clausen, C. C. Chang, T. Venkatesan, P. S. D. Lin, X. D. Wu, A. Inam, and B. Lalevic

Appl. Phys. Lett. 55, 911 (1989); http://dx.doi.org/10.1063/1.101620 (3 pages) | Cited 14 times

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We have investigated the transport current‐induced resistive transitions in predominantly c‐axis oriented crystalline Y1Ba2Cu3O7−x superconducting thin films with scanning electron microscopic (SEM) imaging and its correlation with dc current‐voltage (I‐V) measurements. We find that there is a nonlinear gradual transition region in the I‐V curves that is caused by macroscopic effects (current ‘‘crowding’’, substrate defects, film thickness variation, etc.) and by microscopic dissipation effects. At a high current there is an abrupt transition to the normal state in the I‐V curve due to Joule heating. The SEM mapping provides a direct image of the above effects and is useful for characterization and study of fundamental transport properties of high Tc thin films.
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74.78.-w Superconducting films and low-dimensional structures
74.70.-b Superconducting materials other than cuprates
74.25.Sv Critical currents
74.62.Bf Effects of material synthesis, crystal structure, and chemical composition

Surface resistance of YBa2Cu3O7 films on SrTiO3 and LaGaO3 substrates

D. W. Cooke, E. R. Gray, R. J. Houlton, B. Rusnak, E. A. Meyer, J. G. Beery, D. R. Brown, F. H. Garzon, I. D. Raistrick, A. D. Rollet, and R. Bolmaro

Appl. Phys. Lett. 55, 914 (1989); http://dx.doi.org/10.1063/1.101621 (3 pages) | Cited 25 times

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Surface resistance measurements of films of YBa2Cu3O7 deposited onto single‐crystal substrates of LaGaO3 and SrTiO3 have been made at a frequency of 22 GHz. The measurements were made in either a copper or niobium cavity by replacing the end wall with the superconducting film. Typical surface resistance at 20 K are 1–2 mΩ for films on LaGaO3 and 6–8 mΩ for films on SrTiO3, as measured in the copper cavity. The LaGaO3 values lie within the sensitivity range of the Cu cavity (∼2 mΩ) and can only be considered upper limits. Similar measurements in a Nb superconducting cavity resulted in a surface resistance value of 0.2±0.1 mΩ at 4 K for the best LaGaO3‐based film. This value is more than an order of magnitude lower than Cu, and suggests that LaGaO3‐based films may offer immediate advantages in a number of applications.
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74.25.N- Response to electromagnetic fields
74.70.-b Superconducting materials other than cuprates
74.78.-w Superconducting films and low-dimensional structures
73.25.+i Surface conductivity and carrier phenomena

Heat treatment of wires on the basis of the high Tc superconductor YBa2Cu3Ox

P. Müller, M. Schubert, Ch. Rodig, G. Fuchs, and K. Fischer

Appl. Phys. Lett. 55, 917 (1989); http://dx.doi.org/10.1063/1.102451 (2 pages) | Cited 8 times

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Ag‐sheathed powder‐in‐tube wires on the basis of the high Tc superconductor YBa2Cu3Ox are produced by a drawing process. After cold working, the wires are not superconducting. By heat treatment at temperatures >900 °C, lattice defects are healed and the contact between the grains is improved. After this heat treatment the wires are slowly cooled in an O2 atmosphere in order to adjust the optimum O2 stoichiometry. The annealing temperature has a strong influence on the attainable critical current density. By annealing a wire with 420 ppm carbon content at 950 °C, jc values of 5020 A/cm2 at 4.2 K and 1130 A/cm2 at 77 K were achieved. Drastically reduced critical current densities in wires with high carbon content demonstrate the importance of a low carbon content for achieving high critical current densities in YBa2Cu3Ox .
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74.70.-b Superconducting materials other than cuprates
81.40.Gh Other heat and thermomechanical treatments
81.40.Rs Electrical and magnetic properties related to treatment conditions
74.25.Sv Critical currents

Observations of nanocrystals in thin TbFeCo films

Z. G. Li, David J. Smith, and K. Sickafus

Appl. Phys. Lett. 55, 919 (1989); http://dx.doi.org/10.1063/1.101622 (3 pages) | Cited 23 times

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Observations of thin films of amorphous TbFeCo by electron microscopy with a resolution limit of better than 0.2 nm invariably reveal the presence of very fine microcrystals (sizes ∼2–5 nm). Bright field imaging, in both plan view and in cross section, indicates that these nanocrystals have the crystal structure of α‐Fe or Co. In contrast, crystals of Tb metal (or Tb oxides) have not been observed. Similar observations of FeCo thin films, prepared under identical experimental conditions, reveal much larger crystals (typically 10–50 nm). It is concluded that the presence of the rare earth element Tb is crucial to the microstructure displayed by this kind of material.
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68.55.-a Thin film structure and morphology
75.50.Kj Amorphous and quasicrystalline magnetic materials
75.70.Ak Magnetic properties of monolayers and thin films
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