Ivan Yahniuk

Pressure- and temperature-driven phase transitions in HgTe quantum wells

We present theoretical investigations of pressure- and temperature-driven phase transitions in HgTequantum wells grown on a CdTe buffer. Using the eight-band k·p Hamiltonian we calculate evolution of energy-band structure at different quantum well widths with hydrostatic pressure up to 20 kbars and temperature ranging up to 300 K. In particular, we show that, in addition to temperature, tuning of hydrostatic pressure allows us to drive transitions between semimetal, band insulator, and topological insulator phases. Our realistic band-structure calculations reveal that the band inversion under hydrostatic pressure and temperature may be accompanied by nonlocal overlapping between conduction and valence bands. The pressure and temperature phase diagrams are presented.

Terahertz 3D printed diffractive lens matrices for field-effect transistor detector focal plane arrays

We present the concept, the fabrication processes and the experimental results for materials and optics that can be used for terahertz field-effect transistor detector focal plane arrays. More specifically, we propose 3D printed arrays of a new type - diffractive multi-zone lenses of which the performance is superior to that of previously used mono-zone diffractive or refractive elements and evaluate them with GaN/AlGaN field-effect transistor terahertz detectors. Experiments performed in the 300-GHz atmospheric window show that the lens arrays offer both a good efficiency and good uniformity, and may improve the signal-to-noise ratio of the terahertz field-effect transistor detectors by more than one order of magnitude. In practice, we tested 3 × 12 lens linear arrays with printed circuit board THz detector arrays used in postal security scanners and observed significant signal-to-noise improvements. Our results clearly show that the proposed technology provides a way to produce cost-effective, reproducible, flat optics for large-size field-effect transistor THz-detector focal plane arrays.

MBE grown GaN/AlGaN lateral Schottky barrier diodes for high frequency applications

The authors report the results of studies of lateral and vertical Schottky barrier diodes (SBD) based on molecular beam epitaxy (MBE) grown GaN/AlGaN heterostructures with two dimensional electron gas (2DEG) present at the heterostructure interface. The epilayers were grown by plasma assisted molecular beam epitaxy under metal-rich conditions on commercially available freestanding (0001) GaN substrates. Here, the authors present detailed results for two structures with electron sheet density (N2DEG) of 4.6 × 1012 and 1 × 1013 cm−2 and room temperature mobility of μ2DEG = 1925 cm2/V s and μ2DEG = 1760 cm2/V s, respectively. The processing of lateral and vertical Schottky barrier diodes was performed by laser writer using shallow mesas and Ni/Au metallization for Schottky barriers. The direct current electrical tests performed on lateral and vertical Schottky barrier diodes yielded a turn-on voltage of 0.6 and 1 V, respectively. For both cases, the reverse current registered in the experiment was very low a...

Low frequency noise in two-dimensional lateral GaN/AlGaN Schottky diodes

Schottky diodes with Ni/Au contact to the side of the two dimensional channel in GaN/AlGaN system were fabricated and studied. This kind of lateral heterodimensional diodes demonstrated the ideality factor n = 1.2–1.25 and apparent barrier height φb = (0.59–0.63) eV. The noise measurements within the frequencies range from 1 Hz to 50 kHz showed that the diodes demonstrated the superposition of 1/f and generation recombination noise. In spite of extremely small area of lateral Schottky diodes, the amplitude of noise was similar or even smaller than that for AlGaN and GaN Schottky diodes with the regular contact. This makes GaN-based lateral Schottky diodes to be very promising devices for RF and terahertz applications.

Lateral Schottky barrier diodes based on GaN/AlGaN 2DEG for sub-THz detection

This work presents results of investigations of lateral and vertical Schottky diodes based on Molecular Beam Epitaxy (MBE) grown GaN/AlGaN heterostructures. We have used plasma assisted molecular beam epitaxy in metal rich conditions on freestanding GaN commercial substrates and GaN/sapphire heterosubstrates. The optimized technological procedures have been used to avoid parasitic conduction channels through adjacent epitaxial layers, regrowth interface (RI) or conductive substrates. The investigated 2 dimensional electron gas (2DEG) epistructures were characterized using room temperature Hall measurements. The device processing was performed by using the laser writer technique and shallow mesa etching for electrical insulation of Schottky barrier diodes (SBDs). Our electrical measurements and first detection experiments performed in sub-THz confirm high quality of epitaxial layers, processing and possibility to use lateral SBD as high frequency (HF) detectors.

Imaging and Gas Spectroscopy for Health Protection in Sub-THz Frequency Range

An overview of main results concerning THz detection related to plasma nonlinearities in nanometer field effect transistors is presented. In particular the physical limits of the responsivity, speed and the dynamic range of these detectors are discussed. As a conclusion, we will present applications of the FET THz detectors for construction of focal plane arrays. These arrays, together with in purpose developed diffractive 3D printed optics lead to construction of the demonstrators of the fast postal security imagers and nondestructive industrial quality control systems. We will show also first results of FET based imaging that uses for contrast not only usual THz radiation amplitude, but also the degree of its circular polarization. Sub-THz high resolution gas spectroscopy is shown to be a powerful means to diagnose various diseases via exhaled breath analysis.

Diffractive optics for GaN terahertz detectors arrays

Fabrication and experimental tests of materials and optics for Terahertz range used in field effect transistor detectors arrays are described. The method using diffractive optics, fabricated in 3D printing technology, was used for tests with GaN/AlGaN based FETs sub-THz detectors working in 300 GHz atmospheric window. The lens arrays focus energy exactly on the detectors and additionally reduce mutual detector crosstalk. Finally they improve detectors signal to noise ratio by more than one order of magnitude. Moreover such lens arrays are cost-effective, easy reproducible and thin elements.

Electrically controlled wire-channel GaN/AlGaN transistor for terahertz plasma applications

We report on a design of fin-shaped channel GaN/AlGaN field-effect transistors developed for studying resonant terahertz plasma oscillations. Unlike common two dimensional FinFET transistor design, the gates were deposited only to the sides of the two dimensional electron gas channel, i.e., metal layers were not deposited on the top of the AlGaN. This side gate configuration allowed us to electrically control the conductivity of the channel by changing its width while keeping the carrier density and mobility virtually unchanged. Computer simulations and analytical model describe well the general shape of the characteristics. The side gate control of the channel width of these transistors allowed us to eliminate the so-called oblique plasma wave modes and paves the way towards future terahertz detectors and emitters using high quality factor plasma wave resonances.We report on a design of fin-shaped channel GaN/AlGaN field-effect transistors developed for studying resonant terahertz plasma oscillations. Unlike common two dimensional FinFET transistor design, the gates were deposited only to the sides of the two dimensional electron gas channel, i.e., metal layers were not deposited on the top of the AlGaN. This side gate configuration allowed us to electrically control the conductivity of the channel by changing its width while keeping the carrier density and mobility virtually unchanged. Computer simulations and analytical model describe well the general shape of the characteristics. The side gate control of the channel width of these transistors allowed us to eliminate the so-called oblique plasma wave modes and paves the way towards future terahertz detectors and emitters using high quality factor plasma wave resonances.

Noise limitations of GaN lateral Schottky diodes for THz applications

Schottky diodes are key elements of RF and terahertz electronics. The design of the lateral Schottky with the contact directly to the side of the electron 2D channel in GaN/AlGaN system provides an advantage of extremely small capacitance and series resistance. The lateral Schottky devices based on high quality GaN/AlGaN heterostructures were grown by molecular beam epitaxy. The current voltage characteristics of fabricated diodes demonstrated small ideality factor of n=1.2−1.25, apparent barrier height Φb=(0.59−0.63) eV, and high reverse breakdown voltage exceeding 90 V. At low frequencies below 50 kHz the diodes demonstrated the superposition of 1/f and generation recombination noise. The spectral noise density of current fluctuations at small currents was proportional to the current squared. At high currents this dependence tended to saturate, which can be attributed to the influence of the series resistance. In comparison with regular GaN-based Schottky diodes fabricated in this work and reported in other publications, lateral Schottky diodes demonstrated the low frequency noise of the same or even smaller amplitude. Low level of noise makes GaN/AlGaN lateral Schottky diodes to be very promising devices for RF and terahertz applications.

Terahertz imaging with GaAs and GaN plasma field effect transistors detectors

An overview of recent results concerning THz detection related to plasma nonlinearities in nanometer field effect transistors is presented. In particular, the research on the dynamic range of these detectors is described and two different technologies GaAs and GaN are compared. As a conclusion, we will show first real world applications of the plasma field effect transistors based THz detectors: demonstrators of the imagers (cameras and linear scanners) developed for fast postal security and for nondestructive industrial quality control.