Various designs of dielectric waveguides made of heterostructures with CdHgTe quantum wells grown by molecular beam epitaxy have been studied to generate stimulated emission in the 15–30 μm wavelength range. The reduction of radiation losses in optimized structures has made it possible to reduce the threshold intensity of the generation of stimulated emission to ~100 W/cm 2 . Modernized growth technology has ensured the reduction of the residual cadmium content in HgCdTe quantum wells to 2.5%, which has allowed us to increase the threshold energy of Auger recombination, as well as the maximum temperature for the observation of stimulated emission at interband transitions above 100 K. The results obtained are prerequisites for the implementation of coherent radiation sources exceeding in characteristics of lead–tin chalcogenide lasers used in the 15–30 μm spectral range.

Double-barrier GaAs/AlAs resonant tunneling diodes (RTDs) have become the promising elements for the development of sub-mm and THz emitters. We report on the fabrication of the RTD samples that were characterized via RF-reflectometry to determine the parameters of its equivalent circuit. By using numerical simulation we show that the coplanar transmission line with the RTD under study provides an amplification up to 8 GHz. Keywords: Resonant tunneling diodes, active microstrip transmission lines, distributed emitters, diodes with double metal contacts.

Double-barrier GaAs/AlAs resonant tunneling diodes (RTDs) have become the promising elements for the development of sub-mm and THz emitters. We report on the fabrication of the RTD samples that were characterized via RF-reflectometry to determine the parameters of its equivalent circuit. By using numerical simulation we show that the coplanar transmission line with the RTD under study provides an amplification up to 8 GHz.

We report a detailed study of the bipolar persistent photoconductivity in an HgTe/CdHgTe double quantum well (DQW), which can be a perspective for studying topological states in these structures. Photoconductivity spectra measurements in the range of 1.1–3.1 eV as well as transport measurements under different illumination conditions were performed at T = 4.2 K. Based on the results, the processes occurring in the structure under illumination and leading to a change in the carrier concentration in the DQW have been established. They include interband generation in the CdTe cap layer and in the CdHgTe barrier layer and electron transitions from the spin-split band in the CdHgTe barrier layer to the conduction band in the CdTe cap layer. The presence of the CdTe cap layer and the appropriate cadmium fraction in the CdHgTe barrier layers have been shown to be the main factors determining the key features of the spectra. Finally, we suggest an effective method of controlling the conductivity type of HgTe/CdHgTe structures using light with different wavelengths.

Residual-photoconductivity spectra (RPS) are studied for HgTe/CdHgTe quantum-well heterostructures of n- and p-type conduction at T = 4.2 K. RPS is shown to be both positive (an increase in the carrier concentration in the quantum well) and negative depending on the illumination wavelength. The RPS maxima in the sample with n-type conduction in general correspond to the RPS minima in the p-type samples and vice versa. It is found for p-type samples that illumination at specific wavelengths leads to the “freezing” of free carriers in the quantum well (QW) but not to a change in the conduction type. This fact indicates the important role of the built-in electric field in the RPS mechanism; this field is “switched-off” upon QW neutralization.

The Landau level spectroscopy technique has been used to explore the electronic structure of the valence band in a series of p-type HgTe/HgCdTe quantum wells with both normal and inverted ordering of bands. We find that the standard axial-symmetric 4-band Kane model, which is nowadays widely applied in physics of HgTe-based topological materials, does not fully account for the complex magneto-optical response observed in our experiments—notably, for the unexpected avoided crossings of excitations and for the appearance of transitions that are electric-dipole forbidden within this model. Nevertheless, reasonable agreement with experiments is achieved when the standard model is expanded to include effects of bulk and interface inversion asymmetries. These remove the axial symmetry, and among other, profoundly modify the shape of valence bands.

HgTe quantum wells (QWs) are two-dimensional semiconductor systems that change their properties at the critical thickness d c , corresponding to the band inversion and topological phase transition. The motivation of this work was to study magnetotransport properties of HgTe QWs with thickness approaching d c , and examine them as potential candidates for quantum Hall effect (QHE) resistance standards. We show that in the case of d > d c (inverted QWs), the quantization is influenced by coexistence of topological helical edge states and QHE chiral states. However, at d ≈ d c , where QW states exhibit a graphene-like band structure, an accurate Hall resistance quantization in low magnetic fields ( B ≤ 1.4 T) and at relatively high temperatures ( T ≥ 1.3 K) may be achieved. We observe wider and more robust quantized QHE plateaus for holes, which suggests—in accordance with the “charge reservoir” model—a pinning of the Fermi level in the valence band region. Our analysis exhibits advantages and drawbacks of HgTe QWs for quantum metrology applications, as compared to graphene and GaAs counterparts.

Spectra of persistent photoconductivity (PPC) in HgTe/CdHgTe quantum well (QW) heterostructures of both n- and p-types have been investigated at T = 4.2 K. PPC is shown to be either positive (increase of carrier concentration in QW) or negative depending on a wavelength of the illumination. As a general trend, PPC maxima in n-type sample correspond to PPC minima in p-type samples and vice versa. It is discovered that in p-type samples the illumination with certain wavelengths results in the freezing out of free carriers in QWs but not in the conversion of the conductivity type. The latter indicates a significant role in the PPC mechanism of the built-in electric field that is switching off at the QW neutralization.

We report on the clear evidence of massless Dirac fermions in two-dimensional system based on III-V semiconductors. Using a gated Hall bar made on a three-layer InAs/GaSb/InAs quantum well, we restore the Landau level fan chart by magnetotransport and unequivocally demonstrate a gapless state in our sample. Measurements of cyclotron resonance at different electron concentrations directly indicate a linear band crossing at the G point of the Brillouin zone. Analysis of experimental data within an analytical Dirac-like Hamiltonian allows us not only to determine the velocity (v(F) = 1.8x10(5) m/s) of massless Dirac fermions, but also to demonstrate a significant nonlinear dispersion at high energies.

The effects of the residual photoconductivity in HgTe/CdHgTe (013) double quantum-well heterostructures are studied at T = 4.2 K. It is shown that the residual photoconductivity in this system has a bipolar character, i.e., both positive and negative persistent photoconductivity is present depending on the illumination wavelength.

AbstractThe electron cyclotron resonance spectra in classical and quantizing magnetic fields in asymmetric heterostructures with HgCdTe/CdHgTe quantum wells with selective barrier doping are investigated. Self-consistent calculations of the energy spectra at B = 0 and Landau levels in the framework of the 8-band Kane model using the Hartree approximation are made. The strong (~10%) splitting of the cyclotron resonance line observed in weak fields is attributed to the Rashba effect in samples with inverted and normal band structures. The evolution of absorption lines upon a variation in the magnetic field is investigated up to 34 T, when the magnetic quantization already dominates over Rashba splitting.

The electron cyclotron resonance spectra in classical and quantizing magnetic fields in asymmetric heterostructures with HgCdTe/CdHgTe quantum wells with selective barrier doping are investigated. Self-consistent calculations of the energy spectra at B = 0 and Landau levels in the framework of the 8-band Kane model using the Hartree approximation are made. The strong (~10%) splitting of the cyclotron resonance line observed in weak fields is attributed to the Rashba effect in samples with inverted and normal band structures. The evolution of absorption lines upon a variation in the magnetic field is investigated up to 34 T, when the magnetic quantization already dominates over Rashba splitting.

We report the studies of high-quality HgTe/(Cd,Hg)Te quantum wells (QWs) with a width close to the critical one d_c, corresponding to the topological phase transition and graphene like band structure in view of their applications for Quantum Hall Effect (QHE) resistance standards. We show that in the case of inverted band ordering, the coexistence of conducting topological helical edge states together with QHE chiral states degrades the precision of the resistance quantization. By experimental and theoretical studies we demonstrate how one may reach very favorable conditions for the QHE resistance standards: low magnetic fields allowing to use permanent magnets ( B ≤ 1.4T) and simultaneously realtively high teperatures (liquid helium, T ≥ 1.3K). This way we show that HgTe QW based QHE resistance standards may replace their graphene and GaAs counterparts and pave the way towards large scale fabrication and applications of QHE metrology devices.

AbstractThe effects of the residual photoconductivity in HgTe/CdHgTe (013) double quantum-well heterostructures are studied at T = 4.2 K. It is shown that the residual photoconductivity in this system has a bipolar character, i.e., both positive and negative persistent photoconductivity is present depending on the illumination wavelength.

The band structure of three-layer symmetric InAs/GaSb/InAs quantum wells confined between AlSb barriers is analyzed theoretically. It is shown that, depending on the thicknesses of the InAs and GaSb layers, a normal band structure, a gapless state with a Dirac cone at the center of the Brillouin zone, or inverted band structure (two-dimensional topological insulator) can be realized in this system. Measurements of the cyclotron resonance in structures with gapless band spectra carried out for different electron concentrations confirm the existence of massless Dirac fermions in InAs/GaSb/InAs quantum wells.

Теоретически исследована зонная структура в трехслойных симметричных квантовых ямах InAs/GaSb/InAs, ограниченных барьерами AlSb. Показано, что в зависимости от соотношения толщин слоев InAs и GaSb в системе может реализовываться нормальная зонная структура, бесщелевое состояние с дираковским конусом в центре зоны Бриллюэна и инвертированная зонная структура (двумерный топологический изолятор). Экспериментальные исследования циклотронного резонанса в образцах с бесщелевым зонным спектром, выполненные при различных значениях концентрации электронов, подтверждают существование безмассовых дираковских фермионов в квантовых ямах InAs/GaSb/InAs. DOI: 10.21883/FTP.2017.01.8244

В квантовых ямах HgTe/CdHgTe шириной 8 нм с концентрацией электронов (1.7-13)·1011 см-2 проведены исследования осцилляций Шубникова-де-Гааза в диапазоне температур от 1.6 до 40 K. Из анализа температурной зависимости амплитуды осцилляций при целочисленных факторах заполнения определены значения щелей между уровнями Ландау и квантовое время рассеяния. Экспериментальные значения щелей находятся в хорошем согласии с результатами одноэлектронных расчетов энергий уровней в рамках 8-зонной модели Кейна. Полученные экспериментальные значения ширины плотности состояний свидетельствуют о сильном экранировании обменного взаимодействия в квантовых ямах HgTe/CdHgTe. DOI: 10.21883/FTP.2017.12.45175.38

Shubnikov-de Haas oscillations are studied in 8-nm-wide HgTe/CdHgTe quantum wells with an electron concentration of (1.7–13) × 10 11 cm –2 in the temperature range from 1.6 to 40 K. The gaps between Landau levels and the quantum relaxation time are determined from the temperature dependence of the oscillation amplitude at integer filling factors. The experimental gap values are found to be in good agreement with the results of the single-particle calculation of the level energies using the 8-band Kane model. The experimental widths of the density of states are indicative of profound screening of the exchange interaction in HgTe/CdHgTe quantum wells.

We report on the temperature-dependent magneto absorption and magnetotransport spectroscopy of HgTe/CdHgTe quantum wells above the critical well thickness d(c). Our results, obtained in magnetic fields up to 16 T and temperature range from 1.7 to 150 K, clearly indicate a change in the band-gap energy with temperature. A topological phase transition between quantum spin Hall and trivial insulator states, revealing appearance of single-valley Dirac fermions at T = 27 and 90 K for 6.5 and 8 nm QWs respectively, was clearly observed in our magnetospectroscopy measurements.