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11 Jan 1999

Volume 74, Issue 2, pp. 161-325

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Multiphoton-excited luminescence from diamond nanoparticles and an evolution to emission accompanying the laser vaporization process

Yu. D. Glinka, K.-W. Lin, and S. H. Lin

Appl. Phys. Lett. 74, 236 (1999); http://dx.doi.org/10.1063/1.122998 (3 pages) | Cited 1 time

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The photoluminescence (PL) from 100 nm diamond particles induced by infrared (IR) pulsed excitation [λexc = 1064 nm (1.16 eV), τ=10 ns] was observed. The laser light intensity dependence of the PL yield shows that this is the five-photon absorption process (the total energy of excitation 5.8 eV). The PL results from structure defects, impurities, and the graphite-like phase excited by a relaxation of the multiphoton (MP)—produced electron–hole pairs (the band gap of diamond Eg = 5.5 eV). By using the ultraviolet one-photon excitation [λexc = 354.7 nm (3.48 eV), τ=10 ns], the same PL bands were observed in support of the correctness of interpretation. The typical Swan bands of electronically excited C2 species dominate the spectra at the IR light intensity above the threshold for vaporization. It was concluded that the MP-excited PL precedes the emission accompanying the laser vaporization process. © 1999 American Institute of Physics.
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78.55.Ap Elemental semiconductors
78.66.Db Elemental semiconductors and insulators
78.66.Tr Fullerenes and related materials
78.66.Vs Fine-particle systems
81.05.ub Fullerenes and related materials
81.07.-b Nanoscale materials and structures: fabrication and characterization
64.70.Hz Solid-vapor transitions
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
81.05.Cy Elemental semiconductors

Nonlinear optical response of Ge nanocrystals in a silica matrix

A. Dowd, R. G. Elliman, M. Samoc, and B. Luther-Davies

Appl. Phys. Lett. 74, 239 (1999); http://dx.doi.org/10.1063/1.123267 (3 pages) | Cited 24 times

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Time-resolved degenerate-four-wave-mixing measurements were used to study the nonlinear optical response (intensity-dependent refractive index) of Ge nanocrystallites embedded in a silica matrix. Nanocrystals were fabricated by ion-implanting silica with 1.0 MeV Ge ions to fluences in the range from 0.6 to 3×1017 Ge cm−2, followed by annealing at 1100 °C for 60 min. For the highest fluence, this resulted in nanocrystals with a log-normal size distribution, having a geometric mean diameter of 3.0 nm and a dimensionless geometric standard deviation of 0.25. The intensity-dependent refractive index ∣n2∣ was measured at a wavelength of 800 nm and found to increase linearly with increasing Ge fluence. For the highest fluence, ∣n2∣ was determined to be in the range 2.7–6.9×10−13 cm−2 W−1, depending on the duration of the excitation pulse; values were consistently smaller for shorter pulse lengths. Relaxation of the nonlinear response was found to have two characteristic time constants, one <100 fs and the other ∼1 ps. © 1999 American Institute of Physics.
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78.66.Db Elemental semiconductors and insulators
61.46.-w Structure of nanoscale materials
42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
78.47.-p Spectroscopy of solid state dynamics
61.72.Cc Kinetics of defect formation and annealing

Electrochromic mechanism in a-WO3−y thin films

Se-Hee Lee, Hyeonsik M. Cheong, Ji-Guang Zhang, Angelo Mascarenhas, David K. Benson, and Satyen K. Deb

Appl. Phys. Lett. 74, 242 (1999); http://dx.doi.org/10.1063/1.123268 (3 pages) | Cited 59 times

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The electrochromic mechanism in amorphous tungsten oxide films is studied using Raman scattering measurements. The Raman spectra of as-deposited films show two strong peaks at 770 and 950 cm−1 due to vibrations of the W6+–O and W6+☒O bonds, respectively, and a weaker peak at 220 cm−1 that we attribute to the W4+–O bonds. When lithium or hydrogen ions and electrons are inserted, extra Raman peaks due to W5+–O and W5+☒O bonds appear at 330 and 450 cm−1, respectively. Comparison of the Raman spectra of sputtered isotopic a-W16O3−y and a-W18O3−y films confirms these assignments. We conclude that the as-deposited films contain mainly the W4+ and W6+ states, and the W5+ states are generated as a result of reduction of the W6+ states when lithium or hydrogen ions and electrons are inserted. We propose that the optical absorption in the colored films is caused by transitions between the W6+ and W5+, and W5+ and W4+ states. © 1999 American Institute of Physics.
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78.35.+c Brillouin and Rayleigh scattering; other light scattering
78.66.Nk Insulators
78.20.Jq Electro-optical effects
61.43.Er Other amorphous solids

Room-temperature deep-ultraviolet-stimulated emission from AlxGa1−xN thin films grown on sapphire

T. J. Schmidt, Yong-Hoon Cho, J. J. Song, and Wei Yang

Appl. Phys. Lett. 74, 245 (1999); http://dx.doi.org/10.1063/1.123269 (3 pages) | Cited 11 times

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Room-temperature deep-ultraviolet-stimulated emission (SE) has been observed from optically pumped metalorganic chemical vapor deposition grown AlxGa1−xN thin films. SE has been observed for Al concentrations as high as x = 0.26, with a resultant SE wavelength as low as 328 nm at room temperature. The results obtained for the AlxGa1−xN layers are compared with InxGa1−xN layers of comparable alloy concentration and GaN reference layers. The incorporation of Al into GaN is shown to result in AlxGa1−xN layers with similar high excitation-density emission behavior as GaN, in contrast to InxGa1−xN layers, which exhibit markedly different SE behavior. The observation of room-temperature SE from AlxGa1−xN layers of significant Al concentration illustrates the suitability of AlxGa1−xN based structures, not only for use in deep-ultraviolet detectors, but also as a potential source of deep-ultraviolet laser radiation. © 1999 American Institute of Physics.
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78.45.+h Stimulated emission
78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.05.Ea III-V semiconductors
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Zeeman splitting of shallow donors in GaN

Francisco Mireles and Sergio E. Ulloa

Appl. Phys. Lett. 74, 248 (1999); http://dx.doi.org/10.1063/1.123270 (3 pages) | Cited 13 times

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The Zeeman splitting of the donor spectra in cubic and hexagonal GaN is studied using an effective mass theory approach. Soft-core pseudopotentials were used to describe the chemical shift of the different substitutional dopants. The donor ground states calculated range from 29.5 to 33.7 meV, with typically 1 meV higher binding in the hexagonal phase. Carbon is found to produce the largest donor binding energy. The ionization levels and excited states are in excellent agreement with Hall and optical measurements, and suggest the presence of residual C in recent experiments. © 1999 American Institute of Physics.
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71.55.Eq III-V semiconductors
71.18.+y Fermi surface: calculations and measurements; effective mass, g factor
78.20.Ls Magneto-optical effects
77.22.Ch Permittivity (dielectric function)

Negative capacitance in forward biased hydrogenated amorphous silicon p+-i-n+ diodes

F. Lemmi and N. M. Johnson

Appl. Phys. Lett. 74, 251 (1999); http://dx.doi.org/10.1063/1.123271 (3 pages) | Cited 26 times

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We present experimental results and a physical explanation for the negative values of the small-signal capacitance of forward biased hydrogenated amorphous silicon (a-Si:H) p+-i-n+ diodes. The device capacitance varies with the probe signal frequency. In the low frequency range the measured capacitance shows large negative values if the diode is sufficiently forward biased. Higher forward bias voltages shift this effect to higher frequencies. Time domain response of the device to a small voltage step and a Fourier transform of the resulting transient current provide a verification of the results. The time domain data also suggest the physical mechanism responsible for the overall dispersive behavior: separate and delayed trapping processes for holes and electrons at the interfaces with the two doped layers. © 1999 American Institute of Physics.
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85.30.Kk Junction diodes
85.60.Dw Photodiodes; phototransistors; photoresistors
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths

Enhancement of the quantum-confined stark effect utilizing asymmetric quantum well structures

R. K. Gug and W. E. Hagston

Appl. Phys. Lett. 74, 254 (1999); http://dx.doi.org/10.1063/1.123272 (3 pages) | Cited 5 times

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The potential profile of a rectangular quantum well is optimized so as to maximize the quantum confined stark effect. Digitization of this profile gives a structure which can be grown experimentally. As an example, a well of width 100 Å in Al0.4Ga0.6As/GaAs is optimized giving a structure which is both asymmetric and double quantum well like. The associated redshift is increased at large fields by a factor of more than 4 compared with the original structure, and the associated oscillator strengths are a much more sensitive function of the applied field. This example illustrates the utility of such optimized structures for device applications. © 1999 American Institute of Physics.
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78.20.Jq Electro-optical effects
78.66.Fd III-V semiconductors

Evidence of discrete interface traps on thermally grown thin silicon oxide films

Jin Cai and Chih-Tang Sah

Appl. Phys. Lett. 74, 257 (1999); http://dx.doi.org/10.1063/1.123273 (3 pages) | Cited 5 times

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A very-low density (<1010 cm−2) residual discrete energy-level interface trap at 228 meV from the Si midgap is detected on crystalline silicon covered by thermally grown oxide. Three signatures pointing to discreteness are delineated in the experimental data of the gate voltage modulation of the dc steady-state electron-hole recombination rate. Using the Shockley-Read–Hall recombination kinetics, the energy level is shown to have a linewidth less than 20 meV. From the present and previous data, the dopant impurity origin of this residual interface trap species is speculated rather than the amphoteric silicon dangling bonds. © 1999 American Institute of Physics.
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73.20.Hb Impurity and defect levels; energy states of adsorbed species
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
73.61.Ng Insulators
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths

Modeling of the temperature dependence of the field-effect mobility in thin film devices of conjugated oligomers

J. H. Schön and B. Batlogg

Appl. Phys. Lett. 74, 260 (1999); http://dx.doi.org/10.1063/1.123274 (3 pages) | Cited 31 times

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A simple model is proposed for charge carrier transport across grain boundaries with an acceptor-like trap level. Potential wells between the grains are formed due to negatively charged grain boundaries. Based on this model, a variety of temperature dependencies of the charge carrier mobility can be described. Using realistic parameters, this model reproduces very well the measured temperature dependencies of the field-effect mobility in polycrystalline pentacene and oligothiophene thin film devices. Therefore, it seems to be difficult to investigate the intrinsic material properties of organic semiconductors using only polycrystalline field-effect devices, since they may be masked by the effects of traps and grain boundaries. © 1999 American Institute of Physics.
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85.30.Tv Field effect devices
73.50.Dn Low-field transport and mobility; piezoresistance
61.72.Mm Grain and twin boundaries
73.61.Ph Polymers; organic compounds
85.30.De Semiconductor-device characterization, design, and modeling

Exciton localization and the Stokes’ shift in InGaN epilayers

R. W. Martin, P. G. Middleton, K. P. O’Donnell, and W. Van der Stricht

Appl. Phys. Lett. 74, 263 (1999); http://dx.doi.org/10.1063/1.123275 (3 pages) | Cited 111 times

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We report a comparative study of the emission and absorption spectra of a range of commercial InGaN light-emitting diodes and high-quality epilayers. A working definition of the form of the absorption edge for alloys is proposed, which allows a unique definition of the Stokes’ shift. A linear dependence of the Stokes’ shift on the emission peak energy is then demonstrated for InGaN using experimental spectra of both diode and epilayer samples, supplemented by data from the literature. In addition, the broadening of the absorption edge is shown to increase as the emission peak energy decreases. These results are discussed in terms of the localization of excitons at highly indium-rich quantum dots within a phase-segregated alloy. © 1999 American Institute of Physics.
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78.66.Fd III-V semiconductors
78.55.Cr III-V semiconductors
85.60.Jb Light-emitting devices
71.35.Cc Intrinsic properties of excitons; optical absorption spectra
78.40.Fy Semiconductors
73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
78.60.Fi Electroluminescence

Threshold photoemission analysis of the surface reactions of triethylgallium and trimethylgallium on GaAs(100): A promising technique for kinetics studies

N. Viguier and F. Maury

Appl. Phys. Lett. 74, 266 (1999); http://dx.doi.org/10.1063/1.123276 (3 pages) | Cited 2 times

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The decomposition of triethylgallium and trimethylgallium on a As-stabilized GaAs(100) surface has been analyzed by near threshold photoemission. For T ⩽ 200 °C, the negative photocurrent change induced by adsorption of these electron acceptor molecules is directly related to the coverage. The adsorbed diethylgallium species decompose very rapidly above 240 °C to adsorbed Ga atoms, resulting in a positive photocurrent change due to the lower work function of a Ga surface. The activation energy and preexponential factor of this decomposition reaction were derived independently in the temperature range 200–240 °C from the time dependence of the photocurrent emitted from the surface upon exposure to triethylgallium at saturated coverage. © 1999 American Institute of Physics.
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82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.)
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
79.60.Dp Adsorbed layers and thin films
68.08.-p Liquid-solid interfaces
68.43.-h Chemisorption/physisorption: adsorbates on surfaces
82.20.Pm Rate constants, reaction cross sections, and activation energies

Germanium “quantum dots” embedded in silicon: Quantitative study of self-alignment and coarsening

O. Kienzle, F. Ernst, M. Rühle, O. G. Schmidt, and K. Eberl

Appl. Phys. Lett. 74, 269 (1999); http://dx.doi.org/10.1063/1.123277 (3 pages) | Cited 59 times

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We report on experiments aiming to produce Ge quantum dots embedded in Si. Employing cross-sectional transmission electron microscopy, we have studied the misfit stress-induced self-alignment of islands belonging to consecutive Stranski–Krastanov layers of Ge buried in Si by molecular beam epitaxy. Quantitative evaluation of the micrographs has revealed the critical Si interlayer thickness below which the island positions in successive Ge layers begin to correlate. Moreover, we have quantitatively analyzed the influence of the Si interlayer thickness on the coarsening of the Ge islands from one buried Ge layer to the next. © 1999 American Institute of Physics.
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68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
81.05.Cy Elemental semiconductors
68.37.Hk Scanning electron microscopy (SEM) (including EBIC)
68.37.Lp Transmission electron microscopy (TEM)
68.55.-a Thin film structure and morphology

Quantification of scanning capacitance microscopy imaging of the pn junction through electrical simulation

M. L. O’Malley, G. L. Timp, S. V. Moccio, J. P. Garno, and R. N. Kleiman

Appl. Phys. Lett. 74, 272 (1999); http://dx.doi.org/10.1063/1.123278 (3 pages) | Cited 38 times

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Determining the cross-sectional doping profile of very small metal–oxide–semiconductor field effect transistors and specifically the direct measurement of their channel length is necessary for true channel engineering to be possible. Scanning capacitance microscopy (SCM) has generated unprecedented images of the cross-sectional doping profiles of very small transistors. The bias voltage dependence of these images has motivated us to investigate the SCM technique in greater detail. Using electrical simulations, we have focused on the pn junction to establish the qualitative and quantitative relationship between the bias voltage and the pn junction location. The ability to confidently interpret the images produced with SCM will allow us to improve simulation models, trouble-shoot process flow, and determine the effective channel length of semiconductor devices. © 1999 American Institute of Physics.  
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85.30.Tv Field effect devices
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
85.30.De Semiconductor-device characterization, design, and modeling
61.72.S- Impurities in crystals

Low-resistance Ta/Ti Ohmic contacts for p-type GaN

Masaaki Suzuki, T. Kawakami, T. Arai, S. Kobayashi, Yasuo Koide, T. Uemura, N. Shibata, and Masanori Murakami

Appl. Phys. Lett. 74, 275 (1999); http://dx.doi.org/10.1063/1.123279 (3 pages) | Cited 45 times

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Although extensive efforts have continued to develop Ohmic contacts for p-type GaN, which have specific contact resistance (ρc) lower than that (ρc ∼ 10−2 Ω cm2) of conventional Ni/Au contacts, to the best of our knowledge no breakthrough has been reported in open literature. We demonstrated that bilayered Ta/Ti contacts have a ρc value of around 3×10−5 Ω cm2 for p-type GaN with a hole concentration of 7×1017 cm−3. This contact has resistance low enough to manufacture blue laser diodes, but deterioration of the ρc value during room-temperature storage is the key issue. © 1999 American Institute of Physics.
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73.40.Ns Metal-nonmetal contacts
73.40.Cg Contact resistance, contact potential
42.55.Px Semiconductor lasers; laser diodes

Detection of copper contamination in silicon by surface photovoltage diffusion length measurements

Worth B. Henley, Deepak A. Ramappa, and Lubek Jastrezbski

Appl. Phys. Lett. 74, 278 (1999); http://dx.doi.org/10.1063/1.123280 (3 pages) | Cited 26 times

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Surface photovoltage minority carrier lifetime/diffusion length analysis of copper contaminated silicon was performed. It was observed that copper and copper associated defects degrade minority carrier lifetime more in n-type than in p-type silicon. This finding is explained by analysis of copper related defect levels identified by other deep level transient spectroscopy studies. In copper contaminated p-type silicon, an optical or thermal activation procedure significantly degrades the diffusion length. A process similar to that of Fe–B in p-type silicon is proposed. The activation process dissociates the Cu–Cu pairs, a weak recombination center in p-type silicon, and the copper forms extended substitutional defects in silicon, which have much greater recombination activity. No recovery of diffusion length was observed following such an activation procedure. The difference in copper and iron diffusion length recovery properties after activation can be used to differentiate iron contamination from copper contamination. © 1999 American Institute of Physics.
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71.55.Cn Elemental semiconductors
72.40.+w Photoconduction and photovoltaic effects
73.25.+i Surface conductivity and carrier phenomena
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
72.80.Cw Elemental semiconductors
61.72.S- Impurities in crystals

Local coordination of Ga impurity in hydrogenated amorphous germanium studied by extended x-ray absorption fine-structure spectroscopy

G. Dalba, P. Fornasini, R. Grisenti, F. Rocca, D. Comedi, and I. Chambouleyron

Appl. Phys. Lett. 74, 281 (1999); http://dx.doi.org/10.1063/1.122999 (3 pages) | Cited 5 times

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The local structure of Ga-doped a-Ge:H films has been investigated by extended x-ray absorption fine-structure (EXAFS) fluorescence for impurity concentrations ranging from 1.5×1018 atoms cm−3 to 4.5×1020 atoms cm−3. The mean-coordination number of Ga atoms changes from around 4 (1.5×1018–1.5×1019 cm−3) to below 3 (1.5×1020–4.5×1020 cm−3) with rising concentration. The change from fourfold to threefold coordination occurs in a rather narrow impurity concentration range. The variance of the distance distribution function decreases with increasing Ga content, suggesting that well-ordered sites are present at high-impurity concentration. From EXAFS phase analysis the first Ga–Ge shell distance has been found to be 0.03 Å larger in the amorphous network than in Ga-doped crystalline Ge. © 1999 American Institute of Physics.
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68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
61.43.Dq Amorphous semiconductors, metals, and alloys
78.70.Dm X-ray absorption spectra

Majority- and minority-carrier traps in Te-doped AlInP

Y. R. Wu, W. J. Sung, and W. I. Lee

Appl. Phys. Lett. 74, 284 (1999); http://dx.doi.org/10.1063/1.123000 (3 pages) | Cited 2 times

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The properties of deep levels found in Te-doped AlInP grown by metal–organic chemical vapor deposition have been studied. By using pn-junction structure, both minority- and majority-carrier traps can be observed. Two deep levels are found in Te-doped AlInP: one majority-carrier trap and one minority-carrier trap. The activation energies of majority- and minority-carrier traps are 0.24±0.05 and 0.25±0.03 eV, respectively. The majority-carrier trap is uniformly distributed, indicating that this level belongs to some kind of bulk defect. © 1999 American Institute of Physics.
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71.55.Eq III-V semiconductors
73.61.Ey III-V semiconductors
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
73.20.Hb Impurity and defect levels; energy states of adsorbed species

Electron mobility in modulation-doped AlGaN–GaN heterostructures

R. Gaska, M. S. Shur, A. D. Bykhovski, A. O. Orlov, and G. L. Snider

Appl. Phys. Lett. 74, 287 (1999); http://dx.doi.org/10.1063/1.123001 (3 pages) | Cited 70 times

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We report on the measurements of the electron mobility in modulation-doped Al0.2Ga0.8N–GaN heterostructures grown on sapphire, conducting 6H–SiC, and insulating 4H–SiC substrates as a function of the sheet electron density, ns, at the heterointerface in a wide temperature range. The mobility increases with an increase in ns up to approximately 1×1013 cm−2 and decreases with a further increase in ns. This is explained by the electron spillover at high values of ns from the two-dimensional states at the AlGaN/GaN heterointerface into the delocalized states in the doped GaN channel. The maximum electron Hall mobility in excess of 2000 cm2/V s at room temperature and 11 000 cm2/V s at 4.2 K was measured in the heterostructures grown on 6H–SiC at the values of ns close to 1013 cm−2 and 7×1012 cm−2, respectively. © 1999 American Institute of Physics.
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73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
73.20.At Surface states, band structure, electron density of states
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Tunneling magnetoresistance at up to 270 K in La0.8Sr0.2MnO3/SrTiO3/La0.8Sr0.2MnO3 junctions with 1.6-nm-thick barriers

Takeshi Obata, Takashi Manako, Yuichi Shimakawa, and Yoshimi Kubo

Appl. Phys. Lett. 74, 290 (1999); http://dx.doi.org/10.1063/1.123002 (3 pages) | Cited 68 times

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Magnetic tunneling junctions are fabricated from epitaxially grown La0.8Sr0.2MnO3/SrTiO3/La0.8Sr0.2MnO3 trilayers. A large tunneling magnetoresistance of 150% is observed for a junction with a thin barrier layer (1.6 nm) under a low switching field (<10 Oe) at 5 K. A small tunneling magnetoresistance is observed even at 270 K, which is close to the ferromagnetic Curie temperature (290 K) of the La0.8Sr0.2MnO3 film. The large magnetoresistance and high operating temperature are attributed to the sufficiently thin and uniform barrier layer of SrTiO3. © 1999 American Institute of Physics.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.47.Gk Colossal magnetoresistance
75.47.De Giant magnetoresistance
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
75.50.Dd Nonmetallic ferromagnetic materials
73.40.Gk Tunneling

Highly sensitive magnetometers based on YBa2Cu3O7 Josephson junction arrays

S. Krey, O. Brügmann, and M. Schilling

Appl. Phys. Lett. 74, 293 (1999); http://dx.doi.org/10.1063/1.123003 (3 pages) | Cited 5 times

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The dependence of the critical current of a Josephson junction on the magnetic flux in the junction area can be used for the sensitive detection of external magnetic fields. In contrast to superconducting quantum interference devices, also the measurement of absolute magnetic fields is possible. To increase the transfer function ∂V/∂B, we use serial arrays of up to 105 Josephson junctions between flux-concentrating areas on 24° SrTiO3 bicrystal substrates. We investigate the scaling properties of the critical current Ic, the normal state resistance Rn and the flux density noise math in dependence on the number of Josephson junctions in the serial array. By the use of a magnetic field modulation scheme, the 1/f noise from critical current fluctuations in the junctions can be suppressed. At 77 K, we achieve a white noise level of math = 1.2 pT/math for a 105-junction array. © 1999 American Institute of Physics.
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07.55.Ge Magnetometers for magnetic field measurements
85.25.Cp Josephson devices
74.78.-w Superconducting films and low-dimensional structures
74.72.-h Cuprate superconductors
74.25.Sv Critical currents
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The ferroelectric properties of c-axis oriented Pb5Ge3O11 thin films prepared by metalorganic chemical vapor deposition

Tingkai Li, Fengyan Zhang, and Sheng Teng Hsu

Appl. Phys. Lett. 74, 296 (1999); http://dx.doi.org/10.1063/1.123955 (3 pages) | Cited 11 times

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C-axis oriented ferroelectric Pb5Ge3O11 thin films were prepared on Pt/Ir-coated Si wafers by metalorganic chemical vapor deposition (MOCVD) and rapid thermal process annealing techniques. The films were specular and crack free and showed complete crystallization with c-axis orientation for growth temperatures between 500 and 550 °C. Good ferroelectric properties were obtained for a 150-nm-thick film with Pt/Ir electrodes: the remanent polarization (2Pr) and coercive field (2Ec) values were about 3.8 μC/cm2 and 93 kV/cm, respectively. The films also showed excellent fatigue characteristics: no fatigue was observed up to 1×109 switching cycles. The leakage current increased with increasing applied voltage, and is about 3.6×10−7 A/cm2 at 100 kV/cm. The dielectric constant showed behavior similar to most ferroelectric materials in that the dielectric constant changed with applied voltages. The maximum dielectric constant is about 45. High-quality MOCVD Pb5Ge3O11 films can be used for single transistor ferroelectric memory devices. © 1999 American Institute of Physics.
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77.55.-g Dielectric thin films
77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.
68.55.-a Thin film structure and morphology
77.22.Ch Permittivity (dielectric function)
61.72.Cc Kinetics of defect formation and annealing
77.22.Ej Polarization and depolarization
61.50.-f Structure of bulk crystals
77.80.Fm Switching phenomena

Origin of anomalous polarization offsets in compositionally graded Pb(Zr, Ti)O3 thin films

Mark Brazier, M. McElfresh, and Said Mansour

Appl. Phys. Lett. 74, 299 (1999); http://dx.doi.org/10.1063/1.123004 (3 pages) | Cited 34 times

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Ferroelectric films with composition gradients normal to the substrate have recently been reported to exhibit anomalously large polarization offsets when hysteresis loops are driven with an ac voltage and measured using a Sawyer–Tower (ST) circuit. These offsets have been reported in graded Pb(Zr, Ti)O3 (over 400 μC/cm2) and graded (Ba, Sr)TiO3 (30 μC/cm2) films. In this work, it was found that the offset observed in graded Pb(Zr, Ti)O3 films can be attributed to development of a dc voltage across the ac-voltage driven film, rather than a polarization offset. Recognition of this result reduces these previously reported offset values by a factor of Cs/Cref, where Cref and Cs are the reference and sample capacitances, respectively, used in the ST circuit. These dc-voltage offsets still represent a phenomenon which may lead to novel device applications. © 1999 American Institute of Physics.
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77.80.Dj Domain structure; hysteresis
77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.55.-g Dielectric thin films
77.22.Ej Polarization and depolarization

Frequency-dependent interface capacitance of Al–Al2O3–Al tunnel junctions

K. T. McCarthy, S. B. Arnason, and A. F. Hebard

Appl. Phys. Lett. 74, 302 (1999); http://dx.doi.org/10.1063/1.123005 (3 pages) | Cited 20 times

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Four-terminal ac impedance measurements have been used to characterize Al–Al2O3–Al tunnel-junction capacitors over the frequency range of 10 Hz–100 kHz. The insulating barriers are thin enough to assure that the response can be modeled by a frequency-dependent interface capacitance in parallel with a frequency-independent tunnel junction resistor R0. The data reveal no sign of loss peaks down to 10 Hz and the impedance curves for a single junction, annealed to give different tunnel-junction resistance, collapse onto a single curve when R0 is used as a scaling parameter. The loss mechanism is ascribed to interface traps and is found to give an unusual asymptotic phase angle response when the real and imaginary parts of the complex capacitance are plotted against each other. © 1999 American Institute of Physics.
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73.40.Rw Metal-insulator-metal structures
73.20.Hb Impurity and defect levels; energy states of adsorbed species
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A metal-free, full-color stacked organic light-emitting device

G. Gu, G. Parthasarathy, and S. R. Forrest

Appl. Phys. Lett. 74, 305 (1999); http://dx.doi.org/10.1063/1.123006 (3 pages) | Cited 38 times

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We report the demonstration of a transparent, completely metal-free, full-color stacked organic light-emitting device (SOLED). The SOLED emits light from both top and bottom (substrate) surfaces with total external quantum efficiencies of 0.65%, 1.3%, and 2.2% for the green, blue, and red stacked subpixels, respectively. The respective top emission quantum efficiencies for the three subpixels are 0.23%, 0.63%, and 1.6%. The angular dependence of emission colors due to microcavity effects is weak when viewed from the top device surface. This metal-free SOLED is from 21% to 50% transparent over the entire visible spectral range. Capability for top emission makes this device suitable for integration with electronic components in active matrix display backplanes. © 1999 American Institute of Physics.
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85.60.Jb Light-emitting devices
85.60.Pg Display systems
78.66.Qn Polymers; organic compounds
42.70.Jk Polymers and organics
78.40.Me Organic compounds and polymers

Visible light-emitting devices with Schottky contacts on an ultrathin amorphous silicon layer containing silicon nanocrystals

S. Fujita and N. Sugiyama

Appl. Phys. Lett. 74, 308 (1999); http://dx.doi.org/10.1063/1.123007 (3 pages) | Cited 33 times

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We have fabricated light-emitting diodes (LEDs) with Schottky contacts on a single ultrathin amorphous silicon (Si) layer containing Si nanocrystals formed by simple techniques as used for standard Si devices. Orange electroluminescence (EL) from these LEDs could be seen with the naked eye at room temperature when a reverse bias voltage was applied. The EL spectrum has a major peak with a photon energy of 1.9 eV and a minor peak with a photon energy of 2.2 eV. The operation voltage is reduced to 4.0−4.5 V, which is low enough to be applied to a standard Si transistor. © 1999 American Institute of Physics.
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85.60.Jb Light-emitting devices
78.60.Fi Electroluminescence
78.66.Db Elemental semiconductors and insulators
78.66.Jg Amorphous semiconductors; glasses
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