Rolf Aidam

Imaging standoff detection of explosives using widely tunable midinfrared quantum cascade lasers

The use of a tunable midinfrared external cavity quantum cascade laser for the standoff detection of explosives at medium distances between 2 and 5 m is presented. For the collection of the diffusely backscattered light, a high-performance infrared imager was used. Illumination and wavelength tuning of the laser source was synchronized with the image acquisition, establishing a hyperspectral data cube. Sampling of the backscattered radiation from the test samples was performed in a noncooperative geometry at angles of incidence far away from specular reflection. We show sensitive detection of traces of trinitrotoluene and pentaerythritol tetranitrate on real-world materials, such as standard car paint, polyacrylics from backpacks, and jeans fabric. Concentrations corresponding to fingerprints were detected, while concepts for false alarm suppression due to cross-contaminations were presented.

GaN-Based Submicrometer HEMTs With Lattice-Matched InAlGaN Barrier Grown by MBE

GaN-based high electron mobility transistors (HEMTs) with a nearly strain-free high-Al-content quaternary barrier and electron mobilities up to 1590 cm2/Vs have been grown on 4H-SiC using molecular beam epitaxy (MBE). The processed devices with 150-nm gate length exhibit a high dc performance with a maximum current density of 2.3 A/mm and an extrinsic transconductance up to 675 mS/mm that is among the highest values reported until now for any III-N transistor. We further present, to our knowledge, the first power measurements at 10 GHz of MBE-grown GaN HEMTs with nearly lattice-matched InAlGaN barrier achieving 47% power-added efficiency at 10 V and an output power density of 5.6 W/mm at 30-V bias.

Influence of the surface potential on electrical properties of AlxGa1−xN/GaN heterostructures with different Al-content: Effect of growth method

The influence of the growth method on the surface potential and thus on the sheet carrier concentration of GaN capped AlxGa1−xN/GaN heterostructures was evaluated. Nominally undoped low pressure metal-organic vapor-phase (MOVPE) and plasma-assisted molecular beam epitaxial (PA-MBE) grown structures with an Al-content between 12% and 30% yield carrier concentrations from 3.6×1012 to 1.2×1013 cm−2. A difference of the concentrations for a fixed Al-content was found between the different epitaxial techniques. This result indicates unambiguously different surface potentials determined quantitatively from the carrier concentration, and is verified in addition by the results of photoreflectance spectroscopy. The GaN surface potentials of MOVPE and PA-MBE grown samples amounts to (0.26±0.04) and (0.61±0.10) eV irrespective of the Al-content of the barrier layer. After device fabrication, we find that due to the identical surface potential defined by the Ni Schottky gate, the threshold voltage for a given Al-cont...

Compositional variation of nearly lattice-matched InAlGaN alloys for high electron mobility transistors

Quaternary InAlGaN semiconductors with AlN mole fractions between 40% and 81% and respective InN contents of 7% and 19% including InAlN as a ternary border case have been grown by plasma-assisted molecular beam epitaxy. The electron mobility in InAlGaN-based heterostructures increases with Ga concentration in the barrier up to 1460 cm2/V s at a sheet electron density of 1.9×1013 cm−2. An advanced spacer comprising an AlN/GaN/AlN triple-layer sequence is inserted between the GaN-buffer and the InAlGaN-barrier. Transistors with thin quaternary barrier show a large current density of 2.3 A/mm and an excellent transconductance of 675 mS/mm.

onHigh-peak-power strain-compensated GaInAs/AlInAs quantum cascade lasers (λ∼4.6 μm) based on a slightly diagonal active region design

Employing a “slightly diagonal” active region design for the quantum cascade lasers compared to a reference sample based on the conventional vertical transition design [R. Kohler et al., Appl. Phys. Lett. 76, 1092 (2000)], we have improved the maximum operation temperature, room-temperature maximum peak power per facet, and room-temperature slope efficiency from 320 K, 200 mW, and 570 mW/A to higher than 360 K, 3.2 W, and 2200 mW/A, respectively, for the device size of 16 μm×3 mm with as-cleaved facets operated in pulsed mode.

High-power high-brightness ridge-waveguide tapered diode lasers at 14xx nm

High-power spatially single-mode diode lasers at 1.4 - 1.5 μm wavelength are of interest as pump lasers for Raman and rare-earth doped fiber amplifiers as well as for material processing and for Light Detection and Ranging (LIDAR) at eye-safe wavelengths. A cost-efficient way to realize high-power high-brightness devices is the tapered resonator concept. We demonstrate InGaAsP/InP based diode lasers with compressively strained quantum wells and wavelengths around 1480 nm which were grown by solid source MBE. From broad area lasers with variations in quantum well number and waveguide layer thickness, parameters for the logarithmic gain model are deduced. With their implementation in 2-dimensional BPM simulations, an optimized resonator geometry is derived. Devices employ a 500 μm ridge section followed by a 2000 μm taper section with 6° angle. Continuous-wave (cw) output powers reach more than 1.5 W. Beam quality is characterized in terms of near field and far field distribution, M2, and astigmatism. An excellent agreement is found between measurement and simulation. For narrow-linewidth operation, devices are provided with anti-reflection coatings on both facets and spectrally stabilized with an external grating. We achieve 0.7 W single mode power and a side mode suppression ratio (SMSR) of 42 dB. Reliability is tested in terms of facet stability and lifetime. Pulsed measurements reveal a power stability up to more than 5 MW/cm2. From cw aging tests at 1 W output power, lifetimes of about 6,000 h are extrapolated.

Long wavelength emitting GaInN quantum wells on metamorphic GaInN buffer layers with enlarged in-plane lattice parameter

Metamorphic (i.e., linear composition graded) GaInN buffer layers with an increased in-plane lattice parameter, grown by plasma-assisted molecular beam epitaxy, were used as templates for metal organic vapor phase epitaxy (MOVPE) grown GaInN/GaInN quantum wells (QWs), emitting in the green to red spectral region. A composition pulling effect was observed allowing considerable higher growth temperatures for the QWs for a given In composition. The internal quantum efficiency (IQE) of the QWs was determined by temperature and excitation power density dependent photoluminescence (PL) spectroscopy. An increase in IQE by a factor of two was found for green emitting QWs grown on metamorphic GaInN buffer compared to reference samples grown on standard GaN buffer layers. The ratio of room temperature to low temperature intensity PL of the red emitting QWs were found to be comparable to the PL efficiency of green emitting QWs, both grown on metamorphic GaInN buffers. The excitation density and well width dependence of the IQE indicate a reduction of the quantum confined Stark effect upon growth on GaInN buffer layers with increased in-plane lattice parameter.

450 GHz amplifier MMIC in 50 nm metamorphic HEMT technology

We present a passivated 50 nm gate length metamorphic high electron mobility transistor (mHEMT) technology optimized for the successful fabrication of submillimeter-wave MMICs. A BCB based planarization process is used for placing a second 450 nm wide gate head, which is defined by optical lithography, on top of a 50 nm e-beam written T-gate. Due to the very low intrinsic resistances of the realized mHEMT devices an extrinsic maximum transconduction gm,max of 2100 mS/mm was achieved together with an maximum drain current ID,max of 1300 mA/mm. Furthermore, transit frequencies fT and fmax of 370 and 670 GHz were extrapolated. The fmax extrapolation is based on measured S-parameters up to 220 GHz and compared with the small signal model used for circuit design on the 50 nm mHEMT process. The presented transistor technology was used to fabricate a four-stage common source amplifier circuit in grounded coplanar waveguide topology demonstrating a linear gain of 13 dB at 450 GHz. Assuming matching losses of 1.5 dB per stage within the MMIC the measured circuit gain of 3.3 dB per stage is in good agreement with the 4.6 dB transistor gain predicted by the small signal model.

Strain control of AlGaN/GaN high electron mobility transistor structures on silicon (111) by plasma assisted molecular beam epitaxy

This paper reports on the use of plasma assisted molecular beam epitaxy of AlGaN/GaN-based high electron mobility transistor structures grown on 4 in. Si (111) substrates. In situ measurements of wafer curvature during growth proved to be a very powerful method to analyze the buffer layer’s thickness dependent strain. The Ga/N ratio at the beginning of growth of the GaN buffer layer is the critical parameter to control the compressive strain of the entire grown structure. An engineered amount of compressive strain must be designed into the structure to perfectly compensate for the tensile strain caused by differences in the thermal expansion coefficient between the epi-layer and substrate during sample cool down from growth temperatures. A maximum film thickness of 4.2 μm was achieved without the formation of any cracks and a negligible bow of the wafers below 10 μm. Measurement of the as-grown wafers revealed depth profiles of the charge carrier concentration comparable to values achieved on SiC substrat...

InP DHBT Based IC Technology for over 80 Gbit/s Data Communications

In this paper, we report a manufacturable InP DHBT technology, suitable for medium scale mixed-signal and monolithic microwave integrated circuits. The InGaAs/InP DHBTs were grown by MBE and fabricated using conventional process techniques. Devices with an emitter junction area of 4.8μm 2 exhibited peak cutoff frequency (f T ) and maximum oscillation frequency (f MAX ) values of 265 and 305 GHz, respectively, and a breakdown voltage (BV CEo ) of over 5 V. Using this technology, a set of mixed-signal IC building blocks for > 80Gbit/s fibre optical links, including distributed amplifiers (DA), voltage controlled oscillators (VCO), and multiplexers (MUX), have been successfully fabricated and operated at 80 Gbit/s and beyond.