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See detailResonant microwave absorption determination of characteristic magnetic length in magnetic-field-annealed Vitroperm
Basheed, G. A.; Kaul, S. N.; Michels, Andreas UL

in Applied Physics Letters (2007), 91

The first direct resonant microwave absorption determination of the thermal renormalization of exchange stiffness, average magnetic anisotropy constant, and characteristic magnetic length in “field ... [more ▼]

The first direct resonant microwave absorption determination of the thermal renormalization of exchange stiffness, average magnetic anisotropy constant, and characteristic magnetic length in “field-annealed” Vitroperm samples with an initial magnetic permeability of μi = 20 000 and 150 000 has been presented and discussed. [less ▲]

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See detailRaman imaging of doping domains in graphene on SiO(2)
Stampfer, C.; Molitor, F.; Graf, D. et al

in Applied Physics Letters (2007), 91(24),

We present spatially resolved Raman images of the G and 2D lines of single-layer graphene flakes. The spatial fluctuations of G and 2D lines are correlated and are thus shown to be affiliated with local ... [more ▼]

We present spatially resolved Raman images of the G and 2D lines of single-layer graphene flakes. The spatial fluctuations of G and 2D lines are correlated and are thus shown to be affiliated with local doping domains. We investigate the position of the 2D line-the most significant Raman peak to identify single-layer graphene-as a function of charging up to vertical bar n vertical bar approximate to 4x10(12) cm(-2). Contrary to the G line which exhibits a strong and symmetric stiffening with respect to electron and hole doping, the 2D line shows a weak and slightly asymmetric stiffening for low doping. Additionally, the linewidth of the 2D line is, in contrast to the G line, doping independent making this quantity a reliable measure for identifying single-layer graphene. (C) 2007 American Institute of Physics. [less ▲]

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See detailTraveling-wave photomixer with recessed interdigitated contacts on low-temperature-grown GaAs
Mikulics, M.; Michael, E. A.; Schieder, R. et al

in Applied Physics Letters (2006), 88(4), 0411181-0411183

We have fabricated and characterized novel traveling-wave photomixers with recessed interdigitated metal-semiconductor-metal MSM contacts based on low-temperature-grown GaAs. The new recessed MSM geometry ... [more ▼]

We have fabricated and characterized novel traveling-wave photomixers with recessed interdigitated metal-semiconductor-metal MSM contacts based on low-temperature-grown GaAs. The new recessed MSM geometry led to an improved electric-field distribution inside the photomixer structure and resulted in an up-to-100% increase in the output power of continuously operated devices, compared to conventional MSM devices with standard surface electrodes fabricated on an identical material. The recessed electrode structure also resulted in lower saturation of output power at higher input powers, enabling it to take advantage of higher input powers. [less ▲]

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See detailTraveling-wave photomixers fabricated on high energy nitrogen-ion-implanted GaAs
Mikulics, M.; Michael, E. A.; Marso, Michel UL et al

in Applied Physics Letters (2006), 89(7), 071103

The authors report on fabrication and measurement of traveling-wave photomixers based on high energy and low dose nitrogen-ion-implanted GaAs. They used 3 MeV energy to implant N+ ions into GaAs ... [more ▼]

The authors report on fabrication and measurement of traveling-wave photomixers based on high energy and low dose nitrogen-ion-implanted GaAs. They used 3 MeV energy to implant N+ ions into GaAs substrates with an ion concentration dose of 3 1012 cm−2. The N+-implanted GaAs photomixers exhibit improvements in the output power in comparison with their counterparts, photomixers fabricated on low-temperature-grown GaAs. The maximal output power was 2.64 W at 850 GHz. No saturation of the output power with increased bias voltage and optical input power was observed. These characteristics make N+-implanted GaAs the material of choice for efficient high power sources of terahertz radiation. [less ▲]

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See detailGaAs photodetectors prepared by high-energy and high-dose nitrogen implantation
Mikulics, Martin; Marso, Michel UL; Mantl, Siegfried et al

in Applied Physics Letters (2006), 89

The authors report on the fabrication and characterization of photodetectors based on nitrogen-ion-implanted GaAs and the annealing dynamics in these devices. An energy of 400 keV was used to implant N ... [more ▼]

The authors report on the fabrication and characterization of photodetectors based on nitrogen-ion-implanted GaAs and the annealing dynamics in these devices. An energy of 400 keV was used to implant N ions in a GaAs substrate at an ion concentration of 1 1016 cm−2. Dark current measurements as well as measurements under illumination show that the material properties rapidly change during the annealing process. Photodetectors based on nitrogen-implanted GaAs materials with annealing temperatures up to 400 °C exhibit a subpicosecond carrier lifetime up to 0.6 ps. These properties make nitrogen-ion-implanted GaAs an ideal material for ultrafast photodetectors, as alternative to low-temperature-grown GaAs. [less ▲]

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See detailUltrafast metal-semiconductor-metal photodetectors on low-temperature-grown GaN
Mikulics, M.; Marso, Michel UL; Javorka, P. et al

in Applied Physics Letters (2005), 86(21), 211110

We have fabricated and characterized ultrafast metal-semiconductor-metal photodetectors based on low-temperature-grown sLTd GaN. The photodetector devices exhibit up to 200 kV/cm electric breakdown fields ... [more ▼]

We have fabricated and characterized ultrafast metal-semiconductor-metal photodetectors based on low-temperature-grown sLTd GaN. The photodetector devices exhibit up to 200 kV/cm electric breakdown fields and subpicosecond carrier lifetime. We recorded as short as 1.4-ps-wide electrical transients using 360-nm-wavelength and 100-fs-duration laser pulses, that is corresponding to the carrier lifetime of 720 fs in our LT GaN material. [less ▲]

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See detailLarge-area traveling-wave photonic mixers for increased continuous terahertz power
Michael, E. A.; Vowinkel, B.; Schieder, R. et al

in Applied Physics Letters (2005), 86(11), 111120-111123

A large-aperture design for terahertz traveling-wave photomixers, continuously pumped free space by two detuned diode lasers, is proposed and experimentally verified for devices based on low-temperature ... [more ▼]

A large-aperture design for terahertz traveling-wave photomixers, continuously pumped free space by two detuned diode lasers, is proposed and experimentally verified for devices based on low-temperature-grown GaAs sLT-GaAsd. It combines the advantages of conventional interdigitated small-area structures and traveling-wave devices. An output power of 1 µW at the mixing frequency of 1 THz was measured in initial testing, which meets local oscillator power requirements for superconducting heterodyne mixer devices. [less ▲]

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See detailPhotomixers fabricated on nitrogen-ion-implanted GaAs
Mikulics, M.; Marso, Michel UL; Cámara Mayorga, I. et al

in Applied Physics Letters (2005), 87(4), 41106-1-3

We report on fabrication and measurement of photomixers based on nitrogen-ion-implanted GaAs. We used energies of 500 keV, 700 keV, and 880 keV to implant N+ ions into GaAs substrates with an ion ... [more ▼]

We report on fabrication and measurement of photomixers based on nitrogen-ion-implanted GaAs. We used energies of 500 keV, 700 keV, and 880 keV to implant N+ ions into GaAs substrates with an ion concentration of 3 1012 cm−2. The resulting material exhibited 110 fs carrier lifetime due to implantation-induced defects. Our photomixers were fabricated as metal-semiconductor-metal devices, placed at the feed point of a broadband antenna. Optoelectronic measurements were performed in the wavelength range between 350 nm and 950 nm. In comparison to their counterparts photomixers fabricated on low-temperature-grown GaAs the N+-implanted GaAs photomixers exhibit improvements on both the output power and responsivity. A maximal responsivity of above 100 mA/W was achieved and we did not observe any dependence of the mixer cut-off frequency on the bias voltage. These characteristics make N+-implanted GaAs the material of choice for efficient optoelectronic photomixers. [less ▲]

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See detailGrowth and properties of GaN and AlN layers on silver substrates
Mikulics, Martin; Kočan, Martin; Rizzi, Angela et al

in Applied Physics Letters (2005), 87

We report on the preparation and properties of GaN and AlN layers grown by molecular-beam epitaxy on silver metal substrates. X-ray diffraction rocking curves show polycrystalline character of GaN with ... [more ▼]

We report on the preparation and properties of GaN and AlN layers grown by molecular-beam epitaxy on silver metal substrates. X-ray diffraction rocking curves show polycrystalline character of GaN with high preferential GaN 11-22 orientation. An intermetallic phase of Ga3Ag is found at the GaN/Ag interface. On the other hand, AlN layers exhibit a monocrystalline structure with a growth direction of 0002 . Schottky diodes prepared on GaN layers show good rectifying behavior and relatively low leakage current 10−3 A/cm2 . These results indicate that the III-nitride growth on metallic substrates might be used for low-cost and large-area electronic and photonic devices. [less ▲]

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See detailHigh-power SiO2/AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors
Kordoš, P.; Heidelberger, G.; Bernát, J. et al

in Applied Physics Letters (2005), 87(14), 143501-143504

We report on SiO2 /AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors MOSHFETs , which exhibit a 6.7 W/mm power density at 7 GHz. Unpassivated and SiO2-passivated heterostructure ... [more ▼]

We report on SiO2 /AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors MOSHFETs , which exhibit a 6.7 W/mm power density at 7 GHz. Unpassivated and SiO2-passivated heterostructure field-effect transistors HFETs were also investigated for comparison. Deposited 12 nm thick SiO2 yielded an increase of the sheet carrier density from 7.6 .10^12 to 9.2 .10^12 cm−2 and a subsequent increase of the static drain saturation current from 0.75 to 1.09 A/mm. The small-signal rf characterization of the MOSHFETs showed an extrinsic current gain cutoff frequency fT of 24 GHz and a maximum frequency of oscillation fmax of 40 GHz. The output power of 6.7 W/mm of the MOSHFETs measured at 7 GHz is about two times larger than that of HFETs. The results obtained demonstrate the suitability of GaN-based MOSHFETs for high-power electronics. [less ▲]

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See detailNeutron scattering and modeling of dipole-field-induced spin disorder in Nanoperm
Vecchini, C.; Moze, O.; Suzuki, K. et al

in Applied Physics Letters (2005), 87

We present magnetic-field-dependent small-angle neutron scattering data for the ferromagnetic nanocomposite Nanoperm (Fe89Zr7B3Cu1). The spin-misalignment scattering in the approach-to-saturation regime ... [more ▼]

We present magnetic-field-dependent small-angle neutron scattering data for the ferromagnetic nanocomposite Nanoperm (Fe89Zr7B3Cu1). The spin-misalignment scattering in the approach-to-saturation regime unexpectedly reveals pronounced lobes of high intensity at angles ±30−40° relative to the magnetic-field axis. Based on numerical calculations, the four-fold angular symmetry of the scattering pattern can be explained in terms of local spin misalignment, which originates from dipolar stray fields due to the mismatch of the saturation-magnetization values between the bcc Fe particles and the amorphous magnetic matrix. [less ▲]

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See detailInfluence of a carrier supply layer on carrier density and drift mobility of AlGaN/GaN/SiC high-electron-mobility transistors
Marso, Michel UL; Bernát, J.; Javorka, P. et al

in Applied Physics Letters (2004), 84 (2004)

Sheet carrier concentration and low-field drift mobility of intentionally undoped and modulation-doped AlGaN/GaN heterostructures on SiC substrate were evaluated by capacitance-voltage and channel ... [more ▼]

Sheet carrier concentration and low-field drift mobility of intentionally undoped and modulation-doped AlGaN/GaN heterostructures on SiC substrate were evaluated by capacitance-voltage and channel conductivity measurements. Sheet carrier concentration and average mobility at 0 V gate bias correspond to standard Hall results. Sheet carrier density increases from 6.831012 cm22 for the undoped sample up to 131013 cm22 for the device with the highest doping concentration, while the mobility decreases from 1800 to 1620 cm2/V s. The local mobility, on the other hand, depends only on the actual sheet carrier density and is not influenced by the doping concentration of the carrier supply layer. It reaches a maximum value of 2100 cm2/Vs at a carrier density of 331012 cm22. [less ▲]

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See detailInvestigation of buffer traps in an AlGaN/GaN/Si high electron mobility transistor by backgating current deep level transient spectroscopy
Marso, Michel UL; Wolter, M.; Javorka, P. et al

in Applied Physics Letters (2003), 82

The influence of a substrate voltage on the dc characteristics of an AlGaN/GaN high electron mobility transistor ~HEMT! on silicon ~111! substrate is profited to investigate traps that are located between ... [more ▼]

The influence of a substrate voltage on the dc characteristics of an AlGaN/GaN high electron mobility transistor ~HEMT! on silicon ~111! substrate is profited to investigate traps that are located between the substrate and the two-dimensional electron gas channel. The transient of the drain current after applying a negative substrate voltage is evaluated in the temperature range from 30 to 100 °C. With this method, known as backgating current deep level transient spectroscopy, majority carrier traps with activation energy of 200 meV as well as minority carrier traps at 370 meV are identified. The experiments are performed on completed HEMTs, allowing the investigation of the influence of device fabrication technology. [less ▲]

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See detailUltrafast and highly sensitive photodetectors fabricated on high-energy nitrogen-implanted GaAs,
Mikulics, M.; Marso, Michel UL; Kordoš, P. et al

in Applied Physics Letters (2003), 83

We have fabricated and tested metal–semiconductor–metal ~MSM! photodetectors based on nitrogen-ion-implanted GaAs. Nitrogen ions with energy of 700 and 880 keV, respectively, were implanted into epitaxial ... [more ▼]

We have fabricated and tested metal–semiconductor–metal ~MSM! photodetectors based on nitrogen-ion-implanted GaAs. Nitrogen ions with energy of 700 and 880 keV, respectively, were implanted into epitaxial GaAs films at an ion concentration of 331012 cm22. Ti/Au MSM photodetectors with 1-um-wide fingers were fabricated on top of the implanted GaAs. In comparison to low-temperature-grown GaAs photodetectors, produced in parallel in identical MSM geometry, the 880 keV N1-implanted photodetectors exhibited almost two orders of magnitude lower dark current ~10 nA at 1 V bias! and the responsivity more than doubled ~.20 mA/W at 20 V bias!. Illumination with 100-fs-wide, 810 nm wavelength laser pulses, generated ;2.5-ps-wide photoresponse signals with amplitudes as high as 2 V. The 2.5 ps relaxation time was the same for both the ion-implanted and low-temperature-grown devices and was limited by the MSM capacitance time constant. [less ▲]

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See detailConductivity and Hall effect measurements on intentionally undoped and doped AlGaN/GaN heterostructures before and after passivation,
Bernat, J.; Javorka, P.; Marso, Michel UL et al

in Applied Physics Letters (2003), 83((2003)), 5455

Conductivity and Hall effect measurements were performed before and after Si3N4 passivation of intentionally undoped and doped AlGaN/GaN heterostructures on Si and SiC substrates. An increase of the sheet ... [more ▼]

Conductivity and Hall effect measurements were performed before and after Si3N4 passivation of intentionally undoped and doped AlGaN/GaN heterostructures on Si and SiC substrates. An increase of the sheet carrier density ~up to ;30%! and a slight decrease of the electron mobility ~less than 10%! are found in all samples after passivation. The passivation induced sheet carrier density is 1.5– 231012 cm22 in undoped samples and only 0.731012 cm22 in 5–1031018 cm23 doped samples. The decrease of the electron mobility after passivation is slightly lower in highly doped samples. The channel conductivity in both types of unpassivated samples on Si and SiC substrates increases with an increase in doping density. After passivation, a well-resolved increase of channel conductivity is observed in the undoped or lightly doped samples and nearly the same channel conductivity results in the highly doped samples. [less ▲]

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See detailConduction in nonstoichiometric molecular-beam epitaxial GaAs grown above the critical thickness
Kordoš, P.; Marso, Michel UL; Luysberg, M.

in Applied Physics Letters (1998), 72

Different conduction behavior is observed in nonstoichiometric ~NS! molecular-beam epitaxial GaAs grown at 200 °C below and above the critical thickness. In the low-field Ohmic region only the ... [more ▼]

Different conduction behavior is observed in nonstoichiometric ~NS! molecular-beam epitaxial GaAs grown at 200 °C below and above the critical thickness. In the low-field Ohmic region only the monocrystalline part of the layer contributes to the room-temperature resistivity, but at higher temperatures the resistivity scales with the total layer thickness. In NS GaAs grown above the critical thickness, a superlinear J –V^n (n=2 – 3) dependence is found at intermediate fields. The prebreakdown voltage is proportional to the total thickness. This indicates that different defects control the electrical properties of the polycrystalline and monocrystalline parts of the NS GaAs. These results can be useful in the design of NS GaAs based devices, which operate at higher temperature and/or higher electric fields. [less ▲]

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See detailSpace-charge controlled conduction in low-temperature-grown molecular-beam epitaxial GaAs,
Kordoš, P.; Marso, Michel UL; Förster, A. et al

in Applied Physics Letters (1997), 71

Current transport in low-temperature (LT) molecular-beam epitaxial GaAs grown at 200-300 °C on an n+ GaAs substrate is studied by means of current–voltage–temperature characteristics. The resistivity of ... [more ▼]

Current transport in low-temperature (LT) molecular-beam epitaxial GaAs grown at 200-300 °C on an n+ GaAs substrate is studied by means of current–voltage–temperature characteristics. The resistivity of LT GaAs at low electric fields is rho>108 Ohm cm, much higher than resulting from van der Pauw measurements. It is found that the measured resistivity decreases with increasing the LT GaAs thickness. This is explained by space-charge effect in the vicinity of n+/LT GaAs junction and subsequent suppression of hopping conduction in the high-field junction region. [less ▲]

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See detailInP/InGaAs Photodetector Based on a High Electron Mobility Transistor Layer Structure: Its Response at 1.3 µm wavelength
Horstmann, M.; Marso, Michel UL; Fox, A. et al

in Applied Physics Letters (1995), 67(1995), 106-108

We report on the investigation of the room-temperature optoelectronic behavior of a metal–semiconductor–metal two-dimensional electron gas photodiode based on the two-dimensional electron gas of a high ... [more ▼]

We report on the investigation of the room-temperature optoelectronic behavior of a metal–semiconductor–metal two-dimensional electron gas photodiode based on the two-dimensional electron gas of a high electron mobility transistor structure. The photodetector is fabricated in the InP/InGaAs material system, without use of Al-containing layers. Optoelectronic measurements on a device with a finger spacing of 3 mm show a full width at half-maximum (FWHM) of the pulse response of <60 ps, which is the resolution limit of our measurement equipment. Low-temperature measurements at 40 K with electro-optical sampling at a wavelength of 890 nm show a FWHM of 1 ps. [less ▲]

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