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19 Jul 1999

Volume 75, Issue 3, pp. 307-435

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p-type doping of Zn(Mg)BeSe epitaxial layers

E. Tournié and J.-P. Faurie

Appl. Phys. Lett. 75, 382 (1999); http://dx.doi.org/10.1063/1.124382 (3 pages) | Cited 9 times

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We have investigated the nitrogen-plasma p-type doping properties of both the ZnBeSe-ternary and the ZnMgBeSe-quaternary wide-bandgap alloys grown by molecular-beam epitaxy on (001) GaAs substrates. We show that net acceptor concentrations in the 1017 cm−3 range can be obtained for a bandgap as high as 3.2 eV in the ternary ZnBeSe alloy. In contrast, the bandgap of ZnMgBeSe quaternary alloys lattice matched onto GaAs should not exceed 2.97 eV to reach comparable doping levels. Our results reveal the detrimental influence of both ZnS and MgSe compounds on the p-type doping of wide-bandgap II–VI alloys. © 1999 American Institute of Physics.
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61.72.uj III-V and II-VI semiconductors
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.

Strain dependent gap nature of epitaxial β-FeSi2 in silicon by first principles calculations

Leo Miglio, V. Meregalli, and O. Jepsen

Appl. Phys. Lett. 75, 385 (1999); http://dx.doi.org/10.1063/1.124383 (3 pages) | Cited 30 times

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In this letter, we show that the gap nature in β-FeSi2 is turned from indirect to direct when a suitable strain field is induced in the structure. Such a lattice deformation corresponds to a full lattice matching for the epitaxial relationship β-FeSi2(110)//Si(111), which is one of the most common orientations occurring to β-FeSi2 precipitates in silicon. © 1999 American Institute of Physics.
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68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
73.20.Hb Impurity and defect levels; energy states of adsorbed species
64.75.-g Phase equilibria
71.15.-m Methods of electronic structure calculations

AlGaN/GaN heterostructures on insulating AlGaN nucleation layers

J. A. Smart, A. T. Schremer, N. G. Weimann, O. Ambacher, L. F. Eastman, and J. R. Shealy

Appl. Phys. Lett. 75, 388 (1999); http://dx.doi.org/10.1063/1.124384 (3 pages) | Cited 42 times

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A single temperature process using AlGaN nucleation layers has been developed that produces device-quality, GaN-based materials with bilayer step surfaces. The AlGaN nucleation layer is deposited by flow modulation organometallic vapor phase epitaxy at temperatures in excess of 1000 °C, where GaN and AlGaN films can be subsequently grown. We have optimized this process on both sapphire and SiC substrates, where the conditions for nucleation are found to be quite different. For growth on SiC, aluminum mole fractions ranging from 6% to 35% result in featureless surfaces. Optimizing the alloy composition and thickness of the nucleation layer on SiC allows the deposition of GaN buffer layers exceeding 5 μm without the formation of cracks. A minimum of 15% aluminum in the nucleation layer is required for smooth growth on sapphire substrates. High room temperature two-dimensional electron gas mobilities of 1575 and 1505 cm2/Vs with sheet charge densities of 1.0×1013 and 1.4×1013 cm−2 are observed in undoped AlGaN/GaN structures placed on insulating AlGaN nucleation layers on sapphire and SiC, respectively. © 1999 American Institute of Physics.
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73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
73.61.Ey III-V semiconductors
81.05.Ea III-V semiconductors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.15.Kk Vapor phase epitaxy; growth from vapor phase
85.30.Tv Field effect devices
73.50.Dn Low-field transport and mobility; piezoresistance
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Adjustment of the critical current in a Nb–InxGa1−xAs/InP Josephson contact by light exposure

Th. Schäpers, R. P. Müller, A. Kaluza, H. Hardtdegen, and H. Lüth

Appl. Phys. Lett. 75, 391 (1999); http://dx.doi.org/10.1063/1.124385 (3 pages) | Cited 4 times

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The critical current of a Nb–InGaAs/InP Josephson junction is increased stepwise by light exposure. Shubnikov–de Haas effect measurements under illumination show that the increase of the critical current originates from photogenerated electrons in the quantum well. A further enhancement of the critical current is gained under continuous illumination. © 1999 American Institute of Physics.
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74.50.+r Tunneling phenomena; Josephson effects
74.25.Sv Critical currents
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
72.40.+w Photoconduction and photovoltaic effects
73.61.Ey III-V semiconductors
74.78.-w Superconducting films and low-dimensional structures

Magnetostatic interdot coupling in two-dimensional magnetic dot arrays

K. Yu. Guslienko

Appl. Phys. Lett. 75, 394 (1999); http://dx.doi.org/10.1063/1.124386 (3 pages) | Cited 51 times

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In this letter the calculations of the magnetostatic coupling energy are done for the typical cylindrical dot arrays. The square lattice of submicron-size dots and rectangular “herringbone” lattice of nanoscaled dots are considered. The ground state of the square dot lattice is shown to be continuously degenerated noncollinear antiferromagnetic. For the herringbone dot lattice the uniaxial anisotropy induced by magnetostatic coupling is calculated. Some experimental results on the submicron dots and nanometer scaled Co dots are discussed. © 1999 American Institute of Physics.
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73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
75.50.Tt Fine-particle systems; nanocrystalline materials
75.10.Jm Quantized spin models, including quantum spin frustration

Effect of substrate-induced strain on the charge-ordering transition in Nd0.5Sr0.5MnO3 thin films

W. Prellier, Amlan Biswas, M. Rajeswari, T. Venkatesan, and R. L. Greene

Appl. Phys. Lett. 75, 397 (1999); http://dx.doi.org/10.1063/1.124387 (3 pages) | Cited 71 times

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We report the synthesis and characterization of Nd0.5Sr0.5MnO3 thin films grown by the pulsed laser deposition technique on [100]-oriented LaAlO3 substrates. X-ray diffraction (XRD) studies show that the films are [101] oriented, with a strained and quasi-relaxed component, the latter increasing with film thickness. We observe that transport properties are strongly dependent on the thickness of the films. Variable temperature XRD down to 100 K suggests that this is caused by substrate-induced strain on the films. © 1999 American Institute of Physics.
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75.70.Ak Magnetic properties of monolayers and thin films
68.55.-a Thin film structure and morphology

Enhanced tunnel magnetoresistance in Fe-doped Al2O3 barriers

R. Jansen and J. S. Moodera

Appl. Phys. Lett. 75, 400 (1999); http://dx.doi.org/10.1063/1.124388 (3 pages) | Cited 30 times

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It is shown that the tunnel magnetoresistance of a Co/Al2O3/Ni80Fe20 magnetic tunnel junction is enhanced by δ doping of the oxide barrier with Fe. Enhancements by a factor of up to 1.25 are observed for Fe layers less than a monolayer thick, positioned halfway in the Al2O3 insulator. The effect exists not only at low temperature, but persists up to room temperature, albeit slightly weakened. Also, the enhancement remains present under the application of voltages as large as 0.6 V. Possible explanations are discussed. © 1999 American Institute of Physics.
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73.40.Gk Tunneling
73.40.Rw Metal-insulator-metal structures
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.47.De Giant magnetoresistance
75.50.Cc Other ferromagnetic metals and alloys
75.50.Bb Fe and its alloys

Ion-beam patterning of magnetic films using stencil masks

B. D. Terris, L. Folks, D. Weller, J. E. E. Baglin, A. J. Kellock, H. Rothuizen, and P. Vettiger

Appl. Phys. Lett. 75, 403 (1999); http://dx.doi.org/10.1063/1.124389 (3 pages) | Cited 102 times

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Previously, ion-beam irradiation has been shown to locally alter the magnetic properties of thin Co/Pt multilayer films. In this work, we have used ion-beam irradiation through a silicon stencil mask having 1-μm-diam holes to pattern a magnetic film. Regularly spaced micrometer-sized regions of magnetically altered material have been produced over areas of a square millimeter in Co/Pt multilayers. These magnetic structures have been observed by magnetic force microscopy. The patterning technique is demonstrated with mask–sample spacing as large as 0.5 mm. In addition, smaller regions of magnetic contrast, down to 100 nm, were created by using two masks with partially overlapping micrometer holes. Such patterned magnetic films are of interest for application in high-density magnetic recording. © 1999 American Institute of Physics.
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75.50.Ss Magnetic recording materials
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.50.Cc Other ferromagnetic metals and alloys
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties

Alternating current loss in coplanar arrays of superconducting strips with bidirectional currents

Yasunori Mawatari and Hirofumi Yamasaki

Appl. Phys. Lett. 75, 406 (1999); http://dx.doi.org/10.1063/1.124390 (3 pages) | Cited 5 times

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Magnetic field, current density, and hysteretic alternating current loss power P in coplanar arrays of superconducting strip lines are analyzed on the bases of the critical state model. For a simplified model of a film-type fault current limiter, we consider a strip array in which multiple strip lines are periodically arranged in one plane and are carrying bidirectional currents. We investigate the effect of spacing between the strip lines on P. The P of the strip array is higher than that of a single strip, because the magnetic field in a strip is enhanced by currents in neighboring strips. © 1999 American Institute of Physics.
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84.40.Az Waveguides, transmission lines, striplines
85.25.-j Superconducting devices
84.30.Jc Power electronics; power supply circuits
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Scaling of ferroelectric properties in thin films

C. S. Ganpule, A. Stanishevsky, Q. Su, S. Aggarwal, J. Melngailis, E. Williams, and R. Ramesh

Appl. Phys. Lett. 75, 409 (1999); http://dx.doi.org/10.1063/1.124391 (3 pages) | Cited 96 times

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A fundamental issue in ferroic systems (ferromagnetic and ferroelectric) is the scaling of the order parameter (magnetization or polarization) with size. Specifically, in ferroelectric thin films, deviations in the polarization can occur due to: (i) competition between thermal vibrations and the correlation energy (which aligns the dipoles) and (ii) damage during fabrication. These deviations will have a profound impact on the performance of the next generation of high-density nonvolatile memories based on the spontaneous polarization. We have combined approaches, namely, focused ion-beam milling to define submicron capacitors and scanning force microscopy to examine the scaling of the fundamental ferroelectric response of these capacitors. We find that the capacitors exhibit ferroelectric properties for lateral dimensions down to at least 100 nm, suggesting that memories with densities in the range of 4–16 Gbits can be successfully fabricated. © 1999 American Institute of Physics.
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77.55.-g Dielectric thin films
77.80.-e Ferroelectricity and antiferroelectricity
77.22.Ej Polarization and depolarization
85.50.-n Dielectric, ferroelectric, and piezoelectric devices
84.32.Tt Capacitors
52.77.Bn Etching and cleaning
52.77.Dq Plasma-based ion implantation and deposition

Epitaxial ferroelectric Ba0.5Sr0.5TiO3 thin films for room-temperature tunable element applications

C. L. Chen, H. H. Feng, Z. Zhang, A. Brazdeikis, Z. J. Huang, W. K. Chu, C. W. Chu, F. A. Miranda, F. W. Van Keuls, R. R. Romanofsky, and Y. Liou

Appl. Phys. Lett. 75, 412 (1999); http://dx.doi.org/10.1063/1.124392 (3 pages) | Cited 102 times

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Perovskite Ba0.5Sr0.5TiO3 thin films have been synthesized on (001) LaAlO3 substrates by pulsed laser ablation. Extensive x-ray diffraction, rocking curve, and pole-figure studies suggest that the films are c-axis oriented and exhibit good in-plane relationship of 〈100〉BSTO//〈100〉LAO. Rutherford backscattering spectrometry studies indicate that the epitaxial films have excellent crystalline quality with an ion beam minimum yield χmin of only 2.6%. The dielectric property measurements by the interdigital technique at 1 MHz show room-temperature values of the relative dielectric constant, εr, and loss tangent, tan δ, of 1430 and 0.007 with no bias, and 960 and 0.001 with 35 V bias, respectively. The obtained data suggest that the as-grown Ba0.5Sr0.5TiO3 films can be used for development of room-temperature tunable microwave elements. © 1999 American Institute of Physics.
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77.55.-g Dielectric thin films
77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.22.Gm Dielectric loss and relaxation
84.40.-x Radiowave and microwave (including millimeter wave) technology
77.80.-e Ferroelectricity and antiferroelectricity
81.15.Fg Pulsed laser ablation deposition
82.80.Yc Rutherford backscattering (RBS), and other methods of chemical analysis
77.22.Ch Permittivity (dielectric function)

Isotopic study of metalorganic chemical vapor deposition of (Ba, Sr)TiO3 films on Pt

Y. Gao, T. Tran, and P. Alluri

Appl. Phys. Lett. 75, 415 (1999); http://dx.doi.org/10.1063/1.124393 (3 pages) | Cited 8 times

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Isotopic labeling experiments (18O2) have been carried out to understand the film-formation reactions in the metalorganic chemical vapor deposition of (Ba, Sr)TiO3 (BST) films. Time-of-flight secondary ion mass spectrometry reveals both M 18O and M 16O (M=Ba, Sr, Ti) in the BST films, indicating that the oxygen in the BST films originates from both the gas phase oxidants (18O), and the precursor ligands (16O). Use of a 50% 18O2-50% N2 16O mixture results in a reduction of 18O incorporation in the BST film, indicating direct involvement of N2O in the film-formation reactions. Addition of N2O in O2 also appears to improve film surface morphology and step coverage. © 1999 American Institute of Physics.
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77.55.-g Dielectric thin films
77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.35.B- Structure of clean surfaces (and surface reconstruction)
68.55.-a Thin film structure and morphology
79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces
82.80.Ms Mass spectrometry (including SIMS, multiphoton ionization and resonance ionization mass spectrometry, MALDI)

Role of lower valent substituent-oxygen vacancy complexes in polarization pinning in potassium-modified lead zirconate titanate

Qi Tan, Jianxing Li, and Dwight Viehland

Appl. Phys. Lett. 75, 418 (1999); http://dx.doi.org/10.1063/1.124394 (3 pages) | Cited 37 times

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The dependence of polarization switching on thermal histories has been investigated for K1+-modified lead zirconate titanate (PZT) ceramics by Sawyer–Tower polarization methods. It was found that double-loop-like polarization characteristics in the aged condition could be reversed to normal single loop polarization characteristics by quenching specimens from above the ferroelectric phase transition temperature. However, the P–E curves reversed back to double-loop-like characteristics after reaging specimens at elevated temperatures. Excess oxygen vacancies in La3+-modified PZT were not found to result in double-loop-like polarization hysteresis, whereas excess oxygen vacancies in K1+-modified PZT did. These results provide evidence for role of K1+-oxygen vacancy complexes in polarization pinning. © 1999 American Institute of Physics.
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77.80.Dj Domain structure; hysteresis
77.80.B- Phase transitions and Curie point
77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.22.Ej Polarization and depolarization
61.72.J- Point defects and defect clusters
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Control of Coulomb blockade characteristics with dot size and density in planar metallic multiple tunnel junctions

E. M. Ford and H. Ahmed

Appl. Phys. Lett. 75, 421 (1999); http://dx.doi.org/10.1063/1.124395 (3 pages) | Cited 9 times

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Coulomb gap width has been controlled in a planar metallic structure made by focused ion-beam deposition with landing energies in the range 10 eV–1 keV. Gold islands form by surface diffusion, with size and density determined by dose and landing energy. Increasing the dose reduces the Coulomb gap systematically as larger islands form and density is reduced. Lower ion landing energy increases the operating temperature despite an apparent increase in island size. Multiple tunnel junctions of lengths 10–200 nm have been fabricated and Coulomb blockade observed up to 200 K, with strong nonlinearities evident at room temperature. Single-electron transport through a single island has also been observed, exhibiting a periodic Coulomb staircase. © 1999 American Institute of Physics.
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73.23.Hk Coulomb blockade; single-electron tunneling
73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
73.61.At Metal and metallic alloys
68.35.Fx Diffusion; interface formation
68.35.B- Structure of clean surfaces (and surface reconstruction)

Evolution of Coulomb blockade spectra in parallel coupled quantum dots

A. S. Adourian, C. Livermore, R. M. Westervelt, K. L. Campman, and A. C. Gossard

Appl. Phys. Lett. 75, 424 (1999); http://dx.doi.org/10.1063/1.124396 (3 pages) | Cited 17 times

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We report low-temperature conductance measurements in the Coulomb blockade regime on two nominally identical tunnel-coupled quantum dots in parallel defined electrostatically in the two-dimensional gas of a GaAs/AlGaAs heterostructure. We find that the Coulomb blockade spectra of such devices exhibit two distinct sets of peaks, each of which behaves differently with varying interdot tunnel conductance and with temperature. The results conform to recent theories regarding the role of interdot quantum charge fluctuations, and provide evidence for the possible role of inelastic cotunneling between dots at finite interdot conductances. © 1999 American Institute of Physics.
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73.23.Hk Coulomb blockade; single-electron tunneling
73.61.Ey III-V semiconductors
73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
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Experimental studies of rubidium absolute polarization at high temperatures

S. Appelt, T. Ünlü, K. Zilles, N. J. Shah, S. Baer-Lang, and H. Halling

Appl. Phys. Lett. 75, 427 (1999); http://dx.doi.org/10.1063/1.124397 (3 pages) | Cited 6 times

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We report on measurements of the absolute rubidium (Rb) polarization, optically pumped by a high-power diode laser array, up to temperatures of 180 °C. The penetration of the pump laser light into a high-pressure cell has been studied experimentally and theoretically. The experimental results are compared to a model, which describes the local Rb polarization and optical pumping rate in the cell. © 1999 American Institute of Physics.
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32.80.Xx Level crossing and optical pumping
32.30.-r Atomic spectra
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Electric-field-induced ion migration in polymer-dispersed liquid- crystal films observed by near-field scanning optical microscopy

Erwen Mei and Daniel A. Higgins

Appl. Phys. Lett. 75, 430 (1999); http://dx.doi.org/10.1063/1.124398 (3 pages) | Cited 5 times

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Near-field scanning optical microscopy (NSOM) is used to study electric-field-induced ion migration in polymer-dispersed liquid-crystal (PDLC) films. An electric field is applied between the aluminum-coated NSOM probe and an optically transparent, conductive glass substrate, upon which the film is supported. Electric-field-induced reorientation of the liquid crystal beneath the probe is observed optically. PDLC films doped with tetraalkylammonium tetrafluoroborate salts show dramatically different reorientation dynamics than those prepared without ionic dopants. The behavior observed is attributed to the charging of double layers at the polymer/liquid-crystal interface that cancel the field within the “bulk” of the liquid crystal in time. Observation of subsequent liquid-crystal relaxation yields a measure of the local ion migration rate. © 1999 American Institute of Physics.
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61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order
68.15.+e Liquid thin films
66.10.C- Diffusion and thermal diffusion

Relations between interaction force and frequency shift in large-amplitude dynamic force microscopy

U. Dürig

Appl. Phys. Lett. 75, 433 (1999); http://dx.doi.org/10.1063/1.124399 (3 pages) | Cited 86 times

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Large-amplitude dynamic force microscopy based on measuring shifts of the resonance frequency of the force sensor has proved to be a powerful imaging tool. General expressions relating arbitrary interaction forces to resonance frequency shifts are derived using variational methods and Fourier expansion of the tip motion. For interactions with a range much shorter than the vibration amplitude, the frequency shift can be expressed in terms of a convolution product involving the interaction force and a weakly divergent kernel. The convolution can be inverted, thus enabling one to recover unequivocally interaction potentials and forces from measured frequency shift data. © 1999 American Institute of Physics.
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07.79.Lh Atomic force microscopes
68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)
68.37.Ps Atomic force microscopy (AFM)
68.37.Rt Magnetic force microscopy (MFM)
68.37.Uv Near-field scanning microscopy and spectroscopy
68.35.B- Structure of clean surfaces (and surface reconstruction)
02.30.Nw Fourier analysis
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