References of "Lüth, H"
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See detailTemperature-dependence of the phase-coherence length in InN nanowires
Blömers, Ch UL; Schäpers, Th; Richter, T. UL et al

in Applied Physics Letters (2008), 92(13), 132101-132103

We report on low-temperature magnetotransport measurements on InN nanowires, grown by plasma-assisted molecular beam epitaxy. The characteristic fluctuation pattern observed in the conductance was ... [more ▼]

We report on low-temperature magnetotransport measurements on InN nanowires, grown by plasma-assisted molecular beam epitaxy. The characteristic fluctuation pattern observed in the conductance was employed to obtain information on phase-coherent transport. By analyzing the root mean square and the correlation field of the conductance fluctuations at various temperatures, the phase-coherence length was determined. [less ▲]

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See detailPhase-coherent transport in InN nanowires of various sizes
Blömers, Ch UL; Schäpers, T.; Richter, T. UL et al

in Physical Review (2008), B 77(20),

We investigate phase-coherent transport in InN nanowires of various diameters and lengths. The nanowires were grown by means of plasma-assisted molecular beam epitaxy. Information on the phase-coherent ... [more ▼]

We investigate phase-coherent transport in InN nanowires of various diameters and lengths. The nanowires were grown by means of plasma-assisted molecular beam epitaxy. Information on the phase-coherent transport is gained by analyzing the characteristic fluctuation pattern in the magnetoconductance. For a magnetic field oriented parallel to the wire axis, we found that the correlation field mainly depends on the wire cross section, while the fluctuation amplitude is governed by the wire length. In contrast, if the magnetic field is perpendicularly oriented, for wires longer than approximately 200 nm, the correlation field is limited by the phase coherence length. Further insight into the orientation dependence of the correlation field is gained by measuring the conductance fluctuations at various tilt angles of the magnetic field. [less ▲]

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See detailInvestigation on Localized States in GaN Nanowires
Polenta, L. UL; Rossi, M.; Cavallini, Anna et al

in ACS Nano (2008), 2

GaN nanowires with diameters ranging between 50 and 500 nm were investigated by electrical and photoinduced current techniques to determine the influence of their size on the opto-electronic behavior of ... [more ▼]

GaN nanowires with diameters ranging between 50 and 500 nm were investigated by electrical and photoinduced current techniques to determine the influence of their size on the opto-electronic behavior of nanodevices. The conductivity, photoconductivity, and persistent photoconductivity behavior of GaN nanowires are observed to strongly depend on the wire diameter. In particular, by spectral photoconductivity measurements, three main sub-band-gap optoelectronic transitions were detected, ascribed to the localized states giving rise to the characteristic blue, green, and yellow bands of GaN. Photoconductivity with below-band-gap excitation varies orders of magnitude with the wire diameter, similarly to that observed for near-band-edge excitation. Moreover, yellow-band-related signal shows a superlinear behavior with respect to the band-edge signal, offering new information for the modeling of the carrier recombination mechanism along the nanowires. The photoconductivity results agree well with a model which takes into account a uniform distribution of the localized states inside the wire and their direct recombination with the electrons in the conduction band. [less ▲]

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See detailDoping Concentration of GaN Nanowires Determined by Opto-Electrical Measurements
Richter, T. UL; Lüth, H.; Meijers, R. et al

in Nano Letters (2008), 8(9), 3056-3059

The influence of n-doping on the electrical transport properties of GaN nanowires is investigated by photoconductivity measurements on wires with different diameters. The electrical transport in nanowires ... [more ▼]

The influence of n-doping on the electrical transport properties of GaN nanowires is investigated by photoconductivity measurements on wires with different diameters. The electrical transport in nanowires is extremely sensitive to the wire diameter because of the size dependent barrier for surface recombination. This effect is used to determine the doping level of the nanowires and to complete and consolidate our previously developed surface recombination model for GaN nanowires. [less ▲]

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See detailAcoustic charge transport in GaN nanowires
Ebbecke, J.; Maisch, S.; Wixforth, A. et al

in Nanotechnology (2008), 19(27), 275708-5

We present acoustic charge transport in GaN nanowires (GaN NWs). The GaN NWs were grown by molecular beam epitaxy (MBE) on silicon(111) substrates. The nanowires were removed from the silicon substrate ... [more ▼]

We present acoustic charge transport in GaN nanowires (GaN NWs). The GaN NWs were grown by molecular beam epitaxy (MBE) on silicon(111) substrates. The nanowires were removed from the silicon substrate, aligned using surface acoustic waves (SAWs) on the piezoelectric substrate LiNbO3 and finally contacted by electron beam lithography. Then, a SAW was used to create an acoustoelectric current in the GaN NWs which was detected as a function of radio-frequency (RF) wave frequency and its power. The presented method and our experimental findings open up a route towards new acoustic charge transport nanostructuredevices in a wide bandgap material such as GaN. [less ▲]

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See detailTerahertz photonic mixers as local oscillators for hot electron bolometer and superconductor-insulator-superconductor astronomical receivers, ,
Cámara Mayorga, I.; Muñoz Pradas, P.; Michael, E. A. et al

in Journal of Applied Physics (2006), 100

A pump experiment of two astronomical heterodyne receivers, a superconductorinsulator- superconductor SIS receiver at 450 GHz and a hot-electron-bolometer HEB receiver at 750 GHz, is reported. A low ... [more ▼]

A pump experiment of two astronomical heterodyne receivers, a superconductorinsulator- superconductor SIS receiver at 450 GHz and a hot-electron-bolometer HEB receiver at 750 GHz, is reported. A low-temperature-grown GaAs metal-semiconductor-metal photonic local oscillator LO was illuminated by two near infrared semiconductor lasers, generating a beat frequency in the submillimeter range. I-V junction characteristics for different LO pump power levels demonstrate that the power delivered by the photomixer is sufficient to pump a SIS and a HEB mixer. SIS receiver noise temperatures were compared using a conventional solid-state LO anda photonic LO. In both cases, the best receiver noise temperature was identical Tsys=170 K . [less ▲]

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See detailComparative study on unpassivated and passivated AlGaN/GaN HFETs and MOSHFETs
Heidelberger, G.; Bernát, J.; Gregušová, D. et al

in Physica Status Solidi A. Applications and Materials Science (2006), 203(7), 1876-1881

In this comparative study we investigate AlGaN/GaN-based unpassivated and passivated HFETs and MOSHFETs with regards to DC-, RF-, and power-performance. For optimal comparability, all devices emanate from ... [more ▼]

In this comparative study we investigate AlGaN/GaN-based unpassivated and passivated HFETs and MOSHFETs with regards to DC-, RF-, and power-performance. For optimal comparability, all devices emanate from the same wafer consisting of a SiC-substrate, a 3 μm GaN- and a 30 nm Al0.28Ga0.72N-layer. Devices are processed simultaneously to a large extend. Passivated devices are coated with a 10 nm thick SiO2-layer between the electrodes, MOSHFETs contain a 10 nm thick SiO2-layer serving as gate-insulator underneath the gate and as conventional passivation-layer between the electrodes. Unpassivated devices serve as reference. We present empirical evidence that MOSHFETs outperform both the conventional and the passivated HFETs with respect to DC-, RF-, and power-performance, and we point out the different mechanisms responsible for the behaviour of the devices. [less ▲]

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See detailGaN-nanowhiskers: MBE-growth conditions and optical properties
Meijers, R.; Richter, T.; Calarco, R. et al

in Journal of Crystal Growth (2006), 289(1), 381-386

Morphology and optical properties of GaN nanowhiskers grown by molecular beam epitaxy(MBE) have been studied in correlation with growth parameters. It was shown that the growth parameters can be tuned ... [more ▼]

Morphology and optical properties of GaN nanowhiskers grown by molecular beam epitaxy(MBE) have been studied in correlation with growth parameters. It was shown that the growth parameters can be tuned such that uniform, well separated and high-quality nanowhiskers are obtained. Using an optimized ramp of Ga beam equivalent pressure (BEP) during the growth, the tapering or coalescence of nanowhiskers can be suppressed. By increasing the growth temperature the density of nanowhiskers is reduced, but the crystalline quality is improved as can be concluded from cathodoluminescence (CL) results. [less ▲]

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See detailAn AlGaN/GaN two-color photodetector based on AlGaN/GaN/SiC HEMT layer structure
Marso, Michel UL; Fox, A.; Heidelberger, G. et al

in Physica Status Solidi C. Current Topics in Solid State Physics (2006), 3(6), 2261-2264

The optoelectronic properties of MSM diodes that are based on an AlGaN/GaN HEMT layer system are investigated as function of the applied bias. For low voltages, the two-dimensional electron gas acts as a ... [more ▼]

The optoelectronic properties of MSM diodes that are based on an AlGaN/GaN HEMT layer system are investigated as function of the applied bias. For low voltages, the two-dimensional electron gas acts as a barrier for the depletion layer. Therefore, only the upper AlGaN barrier layer contributes to the photocurrent. In the high voltage regime the depletion region penetrates the GaN buffer that adds the spectral responsivity of GaN to the pure AlGaN behaviour in the low voltage regime. The ratio of the responsivities at 350 nm and 300 nm wavelength can be switched from 0.01 at 2.5 V to 0.8 at 4 V bias. This property makes the MSM-2DEG a candidate for use as two-color photodetector. Device fabrication uses standard HEMT processing steps, allowing integration in HEMT circuits without the need of sophisticated growth or etching techniques. [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 detailImpact of surface treatment under the gate on the current collapse of unpassivated AlGaN/GaN heterostructure field-effect transistors
Kordoš, P.; Bernát, J.; Gregušová, D. et al

in Semiconductor Science & Technology (2006), 21 (2006)

Unpassivated GaN/AlGaN/GaN/SiC heterostructure field-effect transistors were fabricated on intentionally undoped and 5 × 1018 cm−3 modulationdoped material structures. The influence of surface treatment ... [more ▼]

Unpassivated GaN/AlGaN/GaN/SiC heterostructure field-effect transistors were fabricated on intentionally undoped and 5 × 1018 cm−3 modulationdoped material structures. The influence of surface treatment before gate metallization on the gate leakage and drain current collapse of the devices was observed. In the case of a short HCl treatment (∼5 s), a relatively small gate leakage (<10−6A mm−1 at −6 V gate bias) but large current collapse (∼30% after applying 5 μs wide pulses) were measured. On the other hand, devices with a longer surface treatment (15–20 s) showed an increased gate leakage (>10−4A mm−1) and a simultaneously negligible current collapse (<5%). This effect is qualitatively similar in devices prepared on the undoped and doped heterostructures. It is assumed that a thin interfacial oxide layer under the gate might be responsible for a lower leakage current and a larger current collapse of the devices. [less ▲]

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See detailTechnology related issues regarding fabrication of AlGaN/GaN-based MOSHFETs with GdScO3 as dielectric,
Heidelberger, G.; Roeckerath, M.; Steins, R. et al

in Proc. 6th Intern. Conf. Advanced Semicon. Dev. & Microsystems ASDAM’06 (2006)

Starting out from our well established process for AlGaN/GaN HFETs we discuss ways to enrich the process in order to fabricate Metal-Oxide-Semiconductor HFETs (MOSHFETs) with a Gadolinium Scandate (GdScO3 ... [more ▼]

Starting out from our well established process for AlGaN/GaN HFETs we discuss ways to enrich the process in order to fabricate Metal-Oxide-Semiconductor HFETs (MOSHFETs) with a Gadolinium Scandate (GdScO3) insulation layer. In particular, adequate processing orders, various etching procedures and possible drawbacks of the GdScO3 deposition process on ohmic contacts are discussed. Making use of the gained knowledge we fabricated GdScO3-MOSHFETs for the first time. Compared to a conventional HFET the new device shows a higher saturation drain current and a lower gate leakage current. Nevertheless, the potential insulating properties of GdScO3 are not fully exploited yet and further optimization of the deposition process is needed. [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 detailComparison of AlGaN/GaN MSM varactor diodes based on HFET and MOSHFET layer structures
Marso, Michel UL; Fox, A.; Heidelberger, G. et al

in Proc. 6th Intern. Conf. Advanced Semicon. Dev. & Microsystems ASDAM’06 (2006)

In this comparative study we investigate the performance of AlGaN/GaN based MSM varactor diodes based on HFET and MOSHFET layer systems. Device fabrication uses standard HFET fabrication technology ... [more ▼]

In this comparative study we investigate the performance of AlGaN/GaN based MSM varactor diodes based on HFET and MOSHFET layer systems. Device fabrication uses standard HFET fabrication technology, allowing easy integration in MMICs. Devices with different electrode geometries are characterized by DC and by S-parameter measurements up to 50 GHz. The HFET based varactors show capacitance ratios up to 14. The MOSHFET based devices, on the other hand, exhibit lower capacitance ratios and poorer stability because of theinsulation layer between electrodes and semiconductor. [less ▲]

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See detailComparison of AlGaN/GaN MSM Varactor Diodes Based on HFET and MOSHFET Layer Structures
Marso, Michel UL; Fox, A.; Heidelberger, G. et al

in IEEE Electron Device Letters (2006), 27

In this letter, the performance of AlGaN/GaN-based metal–semiconductor–metal (MSM) varactor diodes based on HFET and MOSHFET layer systems is investigated. Passivated HFET MSM devices are coated with a 10 ... [more ▼]

In this letter, the performance of AlGaN/GaN-based metal–semiconductor–metal (MSM) varactor diodes based on HFET and MOSHFET layer systems is investigated. Passivated HFET MSM devices are coated with a 10-nm-thick SiO2 layer between the electrodes; in MOSHFET-based diodes, this layer is also used as an insulator underneath the gate. Device fabrication uses standard HFET fabrication technology, allowing easy integration in monolithic microwave integrated circuits. Devices with different electrode geometry are characterized by direct current and by S-parameter measurements up to 50 GHz. The HFET-based varactors show capacitance ratios up to 14 and cutoff frequencies up to 74 GHz. The MOSHFET-based devices, on the other hand, show lower capacitance ratios and poorer stability because of the insulation layer between electrodes and semiconductor. [less ▲]

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See detailUltrafast and Highly-Sensitive Photodetectors with Recessed Electrodes Fabricated on Low-Temperature-Grown GaAs
Mikulics, M.; Wu, S.; Marso, Michel UL et al

in IEEE Photonics Technology Letters (2006), 18 (2006)(5-6), 820-822

We have fabricated and characterized ultrafast metal–semiconductor–metal (MSM) photodetectors with recessed electrodes, based on low-temperature-grown GaAs. The new recessed-electrode MSM geometry led to ... [more ▼]

We have fabricated and characterized ultrafast metal–semiconductor–metal (MSM) photodetectors with recessed electrodes, based on low-temperature-grown GaAs. The new recessed-electrode MSM geometry led to an improved electric-field distribution inside the photodetector structure and resulted in a 25% breakdown voltage and sensitivity increase with simultaneous four-fold reduction of capacitance, as compared to the identical MSM devices with planar electrodes. Time-resolved studies performed using 100-fs-duration laser pulses showed that recessed-electrode MSMs exhibited 1.0-ps-wide photoresponse transients with no slow after-pulse tails and their photoresponse time was 0.9 ps. The improved transient photoresponse parameters are the main advantages of the recessed-electrode geometry. [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 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 detailSiO2 /AlGaN/GaN MOSHFET with 0.7 µm gate-length and fmax / fT of 40/24 GHz
Bernát, J.; Gregusová, D.; Heidelberger, G. et al

in Electronics Letters (2005), 41(11), 667-668

The performance of SiO2/AlGaN/GaN MOSHFETs is described. The C–V measurements showed slight increase in sheet carrier density after applying 12 nm-thick PECVD SiO2. The devices exhibited gate leakage ... [more ▼]

The performance of SiO2/AlGaN/GaN MOSHFETs is described. The C–V measurements showed slight increase in sheet carrier density after applying 12 nm-thick PECVD SiO2. The devices exhibited gate leakage current of 5 .10 ^-10 A/mm. Small-signal RF characterisation of 0.7 mm gate length devices yielded an fT of 24 GHz and an fmax of 40 GHz, which are comparable to those typical for state-of-the-art AlGaN/GaN HFETs. [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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