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20 Dec 1999

Volume 75, Issue 25, pp. 3905-4030

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Temperature- and oxygen partial pressure-dependent electrical conductivity in nanoporous rutile and anatase

Th. Dittrich, J. Weidmann, F. Koch, I. Uhlendorf, and I. Lauermann

Appl. Phys. Lett. 75, 3980 (1999); http://dx.doi.org/10.1063/1.125513 (3 pages) | Cited 30 times

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The dependence of the electrical conductivity (σ) of sintered nanoporous TiO2 (rutile and anatase) layers on temperature (T) and partial pressure of oxygen (pO2) has been studied by impedance spectroscopy for T up to 450 °C and pO2 up to 10−5 mbar. The diameter of the TiO2 nanoparticles was changed between 5 and 60 nm. σ is thermally activated with EA = 0.85 eV independent of the absolute value of σ and depends on pO2 by power law for pO2<1–10 mbar. The electrical properties of reduced nanoporous TiO2 are determined by surface chemical reactions which lead to the formation of shallow donor and deep trap states. © 1999 American Institute of Physics.
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72.20.-i Conductivity phenomena in semiconductors and insulators
72.80.Ga Transition-metal compounds
71.55.Ht Other nonmetals
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)

Detection of Cr impurities in GaN by room temperature cathodoluminescence spectroscopy

M. Toth and M. R. Phillips

Appl. Phys. Lett. 75, 3983 (1999); http://dx.doi.org/10.1063/1.125528 (3 pages) | Cited 7 times

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Trace levels of Cr impurities in epitaxial GaN grown on sapphire substrates were investigated using cathodoluminescence (CL) spectroscopy. CL emissions characteristic of Cr in an octahedral crystal field were observed from β−Ga2O3 overlayers produced on GaN by post-growth thermal annealing in dry O2. Cr luminescence was also observed from the sapphire substrates, a likely source of the Cr contaminant. The presented results illustrate the use of β−Ga2O3 overlayers as high sensitivity indicators of the presence of Cr in GaN. © 1999 American Institute of Physics.
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68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
61.72.S- Impurities in crystals
78.60.Hk Cathodoluminescence, ionoluminescence
82.80.Ms Mass spectrometry (including SIMS, multiphoton ionization and resonance ionization mass spectrometry, MALDI)
78.66.Fd III-V semiconductors
71.70.Ch Crystal and ligand fields
61.72.Cc Kinetics of defect formation and annealing

Air-bridge-structured silicon nanowire and anomalous conductivity

Hideo Fujii, Seigo Kanemaru, Takashi Matsukawa, and Junji Itoh

Appl. Phys. Lett. 75, 3986 (1999); http://dx.doi.org/10.1063/1.125514 (3 pages) | Cited 16 times

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An air-bridge-structured silicon nanowire was made by micromachining a silicon-on-insulator (SOI) substrate and electrically characterized. The nanowire was isolated from the substrate by an air gap and typically 20–100 nm in diameter and 300–600 nm in length. Current–voltage characteristics of these wires were anomalous electric conductivity such as negative resistance and hysteresis at room temperature. Charge accumulation into surface states is considered a dominant characteristic. © 1999 American Institute of Physics.
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73.61.Cw Elemental semiconductors
73.50.Fq High-field and nonlinear effects
81.07.-b Nanoscale materials and structures: fabrication and characterization
81.05.Cy Elemental semiconductors
73.20.At Surface states, band structure, electron density of states

Room-temperature electroluminescence from erbium-doped porous silicon

Herman A. Lopez and Philippe M. Fauchet

Appl. Phys. Lett. 75, 3989 (1999); http://dx.doi.org/10.1063/1.125515 (3 pages) | Cited 17 times

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We demonstrate stable room-temperature electroluminescence (EL) at 1.54 μm from erbium-doped porous silicon devices under both forward- and reverse-bias conditions. Erbium was infiltrated in the pores ( ⩽ 1019 cm−3) by cathodic electrochemical migration of the ions followed by high-temperature annealing (950–1100 °C) in an oxygen and nitrogen environment. The devices exhibit an exponential EL dependence in both bias conditions as a function of input power. In reverse bias, the external quantum efficiency reaches 0.01%. The EL intensity decreases by a factor of 24 for reverse bias and 2.6 for forward bias when the temperature increases from 240 to 300 K. The different device characteristics in forward and reverse biases suggest that different excitation mechanisms are responsible for EL. © 1999 American Institute of Physics.
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78.60.Fi Electroluminescence
85.60.Jb Light-emitting devices
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Nanocomposite CoPt:C films for extremely high-density recording

M. Yu, Y. Liu, A. Moser, D. Weller, and D. J. Sellmyer

Appl. Phys. Lett. 75, 3992 (1999); http://dx.doi.org/10.1063/1.125516 (3 pages) | Cited 92 times

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Nanocomposite CoPt:C films were investigated as potential media for extremely high-density recording. An annealing temperature of over 600 °C is necessary to form nanocomposite CoPt:C films consisting of C matrix and fct CoPt nanocrystallites with grain sizes of 8–20 nm and coercivities of 3–12 kOe. Coercivity and grain size increase with increasing annealing temperature and decreasing C concentration and they are insensitive to film thickness. The average activation volumes are about 0.9×10−18 cm3. The properties of these nanocomposite CoPt:C films can be tailored to satisfy the thermal stability, coercivity, and media noise requirements for extremely high-density recording. © 1999 American Institute of Physics.
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75.50.Ss Magnetic recording materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
68.60.Dv Thermal stability; thermal effects
61.46.-w Structure of nanoscale materials
75.50.Kj Amorphous and quasicrystalline magnetic materials
75.70.Ak Magnetic properties of monolayers and thin films
81.40.Gh Other heat and thermomechanical treatments
68.55.-a Thin film structure and morphology

Exchange-bias systems with compensated interfaces

Miguel Kiwi, José Mejía-López, Ruben D. Portugal, and Ricardo Ramírez

Appl. Phys. Lett. 75, 3995 (1999); http://dx.doi.org/10.1063/1.125517 (3 pages) | Cited 52 times

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When a ferromagnetic metal (F) is in contact with an antiferromagnet (AF), often a shift of the hysteresis loop away from its normal, symmetric position around H = 0, to HE ≠ 0 does occur. This phenomenon is known as exchange bias (EB). We put forward an analytic model, for compensated AF interfaces, based on the AF interface freezing into a metastable canted spin configuration. The EB energy is reversibly stored in a spring-like magnet, or incomplete domain wall, in the F slab. Our theory yields the right values of HE and its F thickness dependence HE∝tF−1. It also predicts the F layer by layer magnetization profile. © 1999 American Institute of Physics.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Et Exchange and superexchange interactions
75.25.-j Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source x-ray scattering, etc.)
75.30.Gw Magnetic anisotropy
75.60.Ch Domain walls and domain structure
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Low-dielectric-constant cross-linking polymers: Film electrets with excellent charge stability

Reinhard Schwödiauer, Gerhard S. Neugschwandtner, Simona Bauer-Gogonea, Siegfried Bauer, and Werner Wirges

Appl. Phys. Lett. 75, 3998 (1999); http://dx.doi.org/10.1063/1.125518 (3 pages) | Cited 19 times

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The cross-linking low-dielectric-constant polymers benzocyclobutene (BCB) and perfluorocyclobutene (PFCB) are identified as film electrets with excellent charge stability, comparable to the members of the polytetrafluoroethylene family. BCB and PFCB films can be easily prepared on substrates by spin coating. The onset of molecular motion at the high- and low-temperature glass-like transitions is revealed by dielectric dilatometry. BCB and PFCB electret films exhibit remarkable stability regarding both negative and positive charging. They enlarge the family of charge electrets and may thus become interesting for miniaturized electret devices. © 1999 American Institute of Physics.
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77.84.Jd Polymers; organic compounds
77.22.Ej Polarization and depolarization
61.41.+e Polymers, elastomers, and plastics
65.40.De Thermal expansion; thermomechanical effects
64.70.P- Glass transitions of specific systems
64.70.Q- Theory and modeling of the glass transition
77.55.-g Dielectric thin films
77.22.Ch Permittivity (dielectric function)

Evidence of aluminum silicate formation during chemical vapor deposition of amorphous Al2O3 thin films on Si(100)

T. M. Klein, D. Niu, W. S. Epling, W. Li, D. M. Maher, C. C. Hobbs, R. I. Hegde, I. J. R. Baumvol, and G. N. Parsons

Appl. Phys. Lett. 75, 4001 (1999); http://dx.doi.org/10.1063/1.125519 (3 pages) | Cited 104 times

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Using narrow nuclear reaction resonance profiling, aluminum profiles are obtained in ∼3.5 nm Al2O3 films deposited by low temperature (<400 °C) chemical vapor deposition on Si(100). Narrow nuclear resonance and Auger depth profiles show similar Al profiles for thicker (∼18 nm) films. The Al profile obtained on the thin film is consistent with a thin aluminum silicate layer, consisting of Al–O–Si bond units, between the silicon and Al2O3 layer. Transmission electron microscopy shows evidence for a two-layer structure in Si/Al2O3/Al stacks, and x-ray photoelectron spectroscopy shows a peak in the Si 2p region near 102 eV, consistent with Al–O–Si units. The silicate layer is speculated to result from reactions between silicon and hydroxyl groups formed on the surface during oxidation of the adsorbed precursor. © 1999 American Institute of Physics.
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81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.35.Fx Diffusion; interface formation
68.55.-a Thin film structure and morphology
79.60.Jv Interfaces; heterostructures; nanostructures
79.20.Fv Electron impact: Auger emission
82.80.-d Chemical analysis and related physical methods of analysis
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Reconstruction of the charge collection probability in a semiconductor device from the derivative of collection efficiency data

C. Donolato

Appl. Phys. Lett. 75, 4004 (1999); http://dx.doi.org/10.1063/1.125530 (3 pages) | Cited 3 times

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A method is presented for analyzing the charge collection efficiency data that are obtained by irradiating a semiconductor device with an electron beam at different energies, and measuring the induced current. The procedure uses the numerical derivative of the function Rη(R) (η is the collection efficiency and R the electron range), and allows a direct reconstruction of the depth distribution of the charge collection probability in the device. Examples of application of the method to simulated data and published measurements are described. © 1999 American Institute of Physics.
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85.30.De Semiconductor-device characterization, design, and modeling
06.60.Mr Testing and inspecting procedures
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping

Metal–insulator–metal injector for ballistic electron emission spectroscopy

R. Heer, D. Rakoczy, G. Ploner, G. Strasser, E. Gornik, and J. Smoliner

Appl. Phys. Lett. 75, 4007 (1999); http://dx.doi.org/10.1063/1.125520 (3 pages) | Cited 3 times

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We introduce a solid-state version of ballistic electron emission microscopy/spectroscopy (BEEM/BEES) on GaAs–AlGaAs heterostructures using a metal–insulator–metal (MIM) injector structure that replaces the tip of the scanning tunneling microscope (STM). In the present work, the MIM injector is realized by an Al–Al2O3–Al tunnel junction yielding an easy-to-fabricate three-terminal device for ballistic electron spectroscopy. The device principle is applied to several GaAs–AlGaAs structures. The barrier heights obtained from the onsets of the ballistic current spectra are in good agreement with self-consistent calculations as well as earlier experimental results achieved with STM-based BEES. © 1999 American Institute of Physics.
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68.37.Vj Field emission and field-ion microscopy
73.40.Rw Metal-insulator-metal structures
79.70.+q Field emission, ionization, evaporation, and desorption
73.20.-r Electron states at surfaces and interfaces
73.40.Gk Tunneling
73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions

Electron drift mobility and electroluminescent efficiency of tris(8-hydroxyquinolinolato) aluminum

B. J. Chen, W. Y. Lai, Z. Q. Gao, C. S. Lee, S. T. Lee, and W. A. Gambling

Appl. Phys. Lett. 75, 4010 (1999); http://dx.doi.org/10.1063/1.125521 (3 pages) | Cited 58 times

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The electron drift mobility in films of tris(8-hydroxyquinolinolato) aluminum (Alq) deposited at different rates (0.2, 0.4, and 0.7 nm/s) on silicon has been determined by the time-of-flight technique. It has been found that the drift mobility of electrons in Alq increased by about two orders of magnitude as the deposition rate decreased from 0.7 to 0.2 nm/s. Further, the electron drift mobility in all Alq samples increased linearly with the square root of the applied electric field. Electroluminescent devices with a structure of indium tin oxide/α-naphthylphenylbiphenyl amine (NPB, 90 nm)/Alq (90 nm)/Mg:Ag were fabricated at different Alq deposition rates. The device efficiency was found to increase with increasing electron mobility in Alq. As the electron is the minority carrier in the present device, an increase in electron mobility in Alq would thus lead to an increase in device efficiency. © 1999 American Institute of Physics.
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78.60.Fi Electroluminescence
78.66.Qn Polymers; organic compounds
73.61.Ph Polymers; organic compounds
73.50.Dn Low-field transport and mobility; piezoresistance
85.60.Jb Light-emitting devices
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy

Low-noise submillimeter-wave NbTiN superconducting tunnel junction mixers

Jonathan Kawamura, Jian Chen, David Miller, Jacob Kooi, Jonas Zmuidzinas, Bruce Bumble, Henry G. LeDuc, and Jeff A. Stern

Appl. Phys. Lett. 75, 4013 (1999); http://dx.doi.org/10.1063/1.125522 (3 pages) | Cited 36 times

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We have developed a low-noise 850 GHz superconductor–insulator–superconductor quasiparticle mixer with NbTiN thin-film microstrip tuning circuits and hybrid Nb/AlN/NbTiN tunnel junctions. The mixer uses a quasioptical configuration with a planar twin-slot antenna feeding a two-junction tuning circuit. At 798 GHz, we measured an uncorrected double-sideband receiver noise temperature of TRX = 260 K at 4.2 K bath temperature. This mixer outperforms current Nb SIS mixers by a factor of nearly 2 near 800 GHz. The high-gap frequency and low loss at 800 GHz make NbTiN an attractive material with which to fabricate tuning circuits for SIS mixers. NbTiN mixers can potentially operate up to the gap frequency, 2Δ/h ∼ 1.2 THz. © 1999 American Institute of Physics.
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85.25.Pb Superconducting infrared, submillimeter and millimeter wave detectors
84.30.Qi Modulators and demodulators; discriminators, comparators, mixers, limiters, and compressors
84.40.Az Waveguides, transmission lines, striplines
07.57.Kp Bolometers; infrared, submillimeter wave, microwave, and radiowave receivers and detectors
85.25.Cp Josephson devices

Observation of deep traps responsible for current collapse in GaN metal–semiconductor field-effect transistors

P. B. Klein, J. A. Freitas, S. C. Binari, and A. E. Wickenden

Appl. Phys. Lett. 75, 4016 (1999); http://dx.doi.org/10.1063/1.125523 (3 pages) | Cited 67 times

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Deep traps responsible for current collapse phenomena in GaN metal–semiconductor field-effect transistors have been detected using a spectroscopic technique that employs the optical reversibility of current collapse to determine the photoionization spectra of the traps involved. In the n-channel device investigated, the two electron traps observed were found to be very deep and strongly coupled to the lattice. Photoionization thresholds for these traps were determined at 1.8 and at 2.85 eV. Both also appear to be the same traps recently associated with persistent photoconductivity effects in GaN. © 1999 American Institute of Physics.
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85.30.Tv Field effect devices
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
71.55.Eq III-V semiconductors
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Critical issues for the growth of high-quality (Al,Ga)N/GaN and GaN/(In,Ga)N heterostructures on SiC(0001) by molecular-beam epitaxy

O. Brandt, R. Muralidharan, P. Waltereit, A. Thamm, A. Trampert, H. von Kiedrowski, and K. H. Ploog

Appl. Phys. Lett. 75, 4019 (1999); http://dx.doi.org/10.1063/1.125524 (3 pages) | Cited 51 times

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We identify and discuss the essential strategies for the growth of (Al,Ga)N/GaN and GaN/(In,Ga)N heterostructures on SiC(0001) by both plasma-assisted and reactive molecular-beam epitaxy. Substrate preparation, nucleation, and growth conditions are optimized for simultaneously satisfying the requirements of high structural, morphological, optical, and electrical quality. The results demonstrate that molecular-beam epitaxy is a competitive technique for the growth of group-III nitrides. © 1999 American Institute of Physics.
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81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
81.05.Ea III-V semiconductors
68.55.-a Thin film structure and morphology
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
73.61.Ey III-V semiconductors
78.66.Fd III-V semiconductors

Scanning apertureless microscopy below the diffraction limit: Comparisons between theory and experiment

C. J. Hill, P. M. Bridger, G. S. Picus, and T. C. McGill

Appl. Phys. Lett. 75, 4022 (1999); http://dx.doi.org/10.1063/1.125525 (3 pages) | Cited 6 times

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The exact nature of the signal in scanning apertureless microscopy techniques is the subject of much debate. We have sought to resolve this controversy by carrying out simulations and experiments on the same structures. Simulations of a model of tip–sample coupling are shown to exhibit features that are in agreement with experimental observations at dimensions below the diffraction limit. The simulation of the optical imaging process is carried out using atomic force microscope data as a topographical template and a tip–sample dipole coupling model as the source of optical signal. The simulations show a number of key fingerprints including a dependence on the polarization of the external laser source, the size of the tip, and index of refraction of the sample being imaged. The experimental results are found to be in agreement with many of the features of the simulations. We conclude that the results of the dipole coupling theory agree qualitatively with experimental data and that apertureless microscopy measures optical properties, not just topography. © 1999 American Institute of Physics.
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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
07.79.Lh Atomic force microscopes
68.35.B- Structure of clean surfaces (and surface reconstruction)

WS2 nanotubes as tips in scanning probe microscopy

A. Rothschild, S. R. Cohen, and R. Tenne

Appl. Phys. Lett. 75, 4025 (1999); http://dx.doi.org/10.1063/1.125526 (3 pages) | Cited 39 times

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WS2 nanotubes a few microns long were attached to microfabricated Si tips and tested afterwards in an atomic force microscope by imaging a “replica” of high aspect ratio, i.e., deep and narrow grooves. These WS2 nanotube tips provide a considerable improvement in image quality for such structures when compared with commercial ultrasharp Si tips. The nanotube tip apex shape was extracted by blind reconstruction from an image of Ti spikes, showing a smooth cylindrical profile up to the end. © 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)
07.05.Pj Image processing
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Comment on “Contact resistance of carbon nanotubes” [Appl. Phys. Lett. 74, 2122 (1999)]

Paul Delaney and Massimiliano Di Ventra

Appl. Phys. Lett. 75, 4028 (1999); http://dx.doi.org/10.1063/1.125527 (2 pages) | Cited 9 times

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© 1999 American Institute of Physics.
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73.40.Ns Metal-nonmetal contacts
73.40.Cg Contact resistance, contact potential
73.61.Wp Fullerenes and related materials
71.20.Tx Fullerenes and related materials; intercalation compounds
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Response to “Comment on ‘Contact resistance of carbon nanotubes’ ” [Appl. Phys. Lett. 75, 4028 (1999)]

J. Tersoff

Appl. Phys. Lett. 75, 4030 (1999); http://dx.doi.org/10.1063/1.125529 (1 page) | Cited 1 time

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© 1999 American Institute of Physics
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73.40.Ns Metal-nonmetal contacts
73.40.Cg Contact resistance, contact potential
71.20.Tx Fullerenes and related materials; intercalation compounds
73.61.Wp Fullerenes and related materials
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