Acoustic charge transport in GaN nanowires

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 ▲]

Phase-coherent transport in InN nanowires of various sizes

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 ▲]

Doping Concentration of GaN Nanowires Determined by Opto-Electrical Measurements

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 ▲]

Impact of surface treatment under the gate on the current collapse of unpassivated AlGaN/GaN heterostructure field-effect transistors

in Semiconductor Science and 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 ▲]

Comparison of AlGaN/GaN MSM Varactor Diodes Based on HFET and MOSHFET Layer Structures

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 ▲]

An AlGaN/GaN two-color photodetector based on AlGaN/GaN/SiC HEMT layer structure

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 ▲]

Technology related issues regarding fabrication of AlGaN/GaN-based MOSHFETs with GdScO3 as dielectric,

in Proceedings of 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 ▲]

Traveling-wave photomixer with recessed interdigitated contacts on low-temperature-grown GaAs

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 ▲]

Ultrafast Low-Temperature-Grown Epitaxial GaAs Photodetectors Transferred on Flexible Plastic Substrates

in IEEE Photonics Technology Letters (2005), 17(8), 1725-1727

We demonstrate low-temperature (LT)-grown GaAs photodetectors transferred on flexible polyethylene terephthalate (PET) plastic substrates. The LT-GaAs layer was patterned into 20 20 m2 chips, which after ... [more ▼]

We demonstrate low-temperature (LT)-grown GaAs photodetectors transferred on flexible polyethylene terephthalate (PET) plastic substrates. The LT-GaAs layer was patterned into 20 20 m2 chips, which after placing on the PET substrates were integrated with coplanar strip transmission lines. The devices exhibit low dark currents ( 2 10 8 A), subpicosecond photoresponse time, and signal amplitudes up to 0.9 V at the bias voltage of 80 V and under laser beam excitation power of 8 mW at 810-nm wavelength. At the highest bias ( 80 V) level, an increase of the response time (up to 1.3 ps) was observed and attributed to the influence of heating effects due to low thermal conductivity of PET. Our LT-GaAs-on-PET photodetectors withstand hundredfold mechanical bending of the substrate and are intended for applications in hybrid optoelectronic circuits fabricated on noncrystalline substrates, in terahertz imaging, and in biology-related current-excitation tests. [less ▲]

SiO2 /AlGaN/GaN MOSHFET with 0.7 µm gate-length and fmax / fT of 40/24 GHz

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 ▲]