A. Soltani

193nm deep-ultraviolet solar-blind cubic boron nitride based photodetectors

Deep-ultraviolet (DUV) solar-blind photodetectors based on high-quality cubic boron nitride (cBN) films with a metal/semiconductor/metal configuration were fabricated. The design of interdigitated circular electrodes enables high homogeneity of electric field between pads. The DUV photodetectors present a peak responsivity at 180nm with a very sharp cutoff wavelength at 193nm and a visible rejection ratio (180 versus 250nm) of more than four orders of magnitude. The characteristics of the photodetectors present extremely low dark current, high breakdown voltage, and high responsivity, suggesting that cBN films are very promising for DUV sensing.

AlGaN-GaN HEMTs on Si with power density performance of 1.9 W/mm at 10 GHz

AlGaN-GaN high electron mobility transistors (HEMTs) on silicon substrate are fabricated. The device with a gate length of 0.3-/spl mu/m and a total gate periphery of 300 /spl mu/m, exhibits a maximum drain current density of 925 mA/mm at V/sub GS/=0 V and V/sub DS/=5 V with an extrinsic transconductance (g/sub m/) of about 250 mS/mm. At 10 GHz, an output power density of 1.9 W/mm associated to a power-added efficiency of 18% and a linear gain of 16 dB are achieved at a drain bias of 30 V. To our knowledge, these power results represent the highest output power density ever reported at this frequency on GaN HEMT grown on silicon substrates.

Effects of Self-Heating on Performance Degradation in AlGaN/GaN-Based Devices

A self-consistent electrothermal transport model that couples electrical and thermal transport equations is established and applied to AlGaN/GaN device structures grown on the following three different substrate materials: 1) SiC; 2) Si; and 3) sapphire. Both the resultant I-V characteristics and surface temperatures are compared to experimental I -V measurements and Raman spectroscopy temperature measurements. The very consistent agreement between measurements and simulations confirms the validity of the model and its numerical rendition. The results explain why the current saturation in measured I-V characteristics occurs at a much lower electric field than that for the saturation of electron drift velocity. The marked difference in saturated current levels for AlGaN/GaN structures on SiC, Si, and sapphire substrates is directly related to the different self-heating levels that resulted from the different biasing conditions and the distinctive substrate materials.

Magnetoelectric effect near spin reorientation transition in giant magnetostrictive-aluminum nitride thin film structure

Hybrid giant magnetostrictive-piezoelectric film/film structures exhibiting magnetoelectric (ME) effect associated with a magnetic instability of the spin reorientation transition type are presented. We first present the theoretical study of a clamped beam actuator composed of a piezoelectric layer on a substrate actuated by a magnetostrictive layer. The actuator is a polished 50 μm thick 18×5 mm2 silicon substrate coated by an electrode, aluminum nitride, and magnetostrictive nanostructured layer. A ME coefficient of 30 V Oe−1 cm−1 at a 35 KHz longitudinal resonance was measured. Nonlinear excitation of this mode showed a “nonlinear” dynamic ME coefficient of 4 V Oe−1 cm−1.

Investigation of the negative differential resistance reproducibility in AlN/GaN double-barrier resonant tunnelling diodes

AlN/GaN double-barrier resonant tunnelling diodes were grown by molecular beam epitaxy on GaN/sapphire template and processed into mesa diameters from 2 μm to 4 μm. The current-voltage characteristics were carried out in direct current operation and under-high vacuum. A sharp negative differential resistance (NDR) was detected in the forward bias at 120 K. The NDR was observed for the mesa size of 2 μm at 4 V with a peak-to-valley current ratio of 3.5. The measurement conditions were chosen to make NDR reproducible more than 50 times and apparent in both scan voltage directions after electrical treatment.

Characterization of AlN metal-semiconductor-metal diodes in the spectral range of 44–360nm: Photoemission assessments

The absolute responsivity of a metal-semiconductor-metal (MSM) photodiode based on high quality AlN material has been tested from the vacuum ultraviolet (vuv) to the near UV wavelength range (44–360nm). The metal finger Schottky contacts have been processed to 2μm in width with spacing between the contacts of 4μm. In the vuv wavelength region, the measurement methodology is described in order to distinguish the contribution of the photoemission current from the internal diode signal. In the wavelength range of interest, AlN MSM is sensitive and stable under brief vuv irradiation. The MSM shows a 200∕360nm rejection ratio of more than four orders of magnitude and demonstrates the advantages of wide band gap material based detectors in terms of high rejection ratio and high output signal for vuv solar observation missions.

Power Performance of AlGaN/GaN High-Electron-Mobility Transistors on (110) Silicon Substrate at 40 GHz

This letter reports the first millimeter-wave power demonstration of AlGaN/GaN high-electron-mobility transistors grown on a (110) silicon substrate. Owing to an AlN/AlGaN stress-mitigating stack and in spite of the twofold surface symmetry of Si (110), it is possible to obtain crack-free GaN layers for the fabrication of millimeter-wave power devices with high performance. The device exhibits a maximum dc drain current density of 1.55 A/mm at VGS = 0 V with an extrinsic transconductance of 476 mS/mm. An extrinsic current gain cutoff frequency of 81 GHz and a maximum oscillation frequency of 106 GHz are deduced from Sij parameters. At 40 GHz, an output power density of 3.3 W/mm associated with a power-added efficiency of 20.1% and a linear power gain of 10.6 dB is obtained.

New developments on diamond photodetector for VUV solar observations

A new large-size metal–semiconductor–metal photoconductor device of 4.6 mm in diameter based on diamond material has been reprocessed and characterized in the vacuum-ultraviolet (VUV) wavelength range. The metal finger contacts have been processed to 2 µm in width with spacing between the contacts of 5 µm for a bias voltage of 5 V. The responsivity, stability, linearity and homogeneity have been tested. Solutions and progresses on diamond processing are identified and are reported. In the VUV wavelength range of interest, the diamond photodetector is sensitive with a maximum response of 48 mA W−1 at 210 nm with a corresponding external quantum efficiency of 42%, homogenous and stable under short irradiation. It indicates a 200–400 nm rejection ratio of more than four orders of magnitude and demonstrates the advantages of diamond-based detectors in terms of high rejection ratio and high output signal for VUV solar observation missions.

Hexagonal boron nitride nanowalls: physical vapour deposition, 2D/3D morphology and spectroscopic analysis

Hexagonal boron nitride nanowalls were synthesized using reactive radio-frequency magnetron sputtering in combination with a hexagonal BN target. The nanowall formation is purely governed by addition of hydrogen to the nitrogen/argon gas mixture, and leads to a decreased incorporation of carbon and oxygen impurities. The surface morphology is assessed with scanning electron microscopy, while stoichiometry and reduced impurity content of the material was evidenced using Rutherford backscattering spectroscopy. Transmission electron microscopy confirms the hexagonal nature of the nanowalls, whose luminescent properties are studied with cathodoluminescence spectroscopy, shedding more light on the location and nature of the excitonic emission and crystalline quality of the h-BN nanowalls.

AlGaN/GaN HEMTs on (001) Silicon Substrate With Power Density Performance of 2.9 W/mm at 10 GHz

AlGaN/GaN High Electron Mobility Transistors (HEMT) on a (001)-oriented silicon (Si) substrate are fabricated. The device with a gate length of 300 nm and a total gate periphery of 300 μm exhibits a maximum dc drain current density of 600 mA/mm at VGS = 0 V with an extrinsic transconductance (gm) of about 200 mS/mm. An extrinsic current gain cutoff frequency (ft) of 37 GHz and a maximum oscillation frequency (fmax) of 55 GHz are deduced from S-parameter measurements. At 10 GHz, an output power density of 2.9 W/mm associated to a power-added efficiency (PAE) of 20% and a linear gain of 7 dB are obtained at VDS = 30 V and VGS = -2 V. To our knowledge, these power results represent the highest output power density ever reported at this frequency on GaN HEMT grown on (001) Si substrate.