Showing papers by "Nikolay N. Mikhailov published in 2013"

Nonlocal resistance and its fluctuations in microstructures of band-inverted HgTe/(Hg,Cd)Te quantum wells

Abstract: We investigate experimentally transport in gated microsctructures containing a band-inverted HgTe/Hg${}_{03}$Cd${}_{07}$Te quantum well Measurements of nonlocal resistances using many contacts prove that in the depletion regime the current is carried by the edge channels, as expected for a two-dimensional topological insulator However, high and nonquantized values of channel resistances show that the topological protection length (ie, the distance on which the carriers in helical edge channels propagate without backscattering) is much shorter than the channel length, which is $\ensuremath{\sim}$100 $\ensuremath{\mu}$m The weak temperature dependence of the resistance and the presence of temperature dependent reproducible quasiperiodic resistance fluctuations can be qualitatively explained by the presence of charge puddles in the well, to which the electrons from the edge channels are tunnel-coupled

Spectra and kinetics of THz photoconductivity in narrow-gap Hg1–xCdxTe (x < 0.2) epitaxial films

Abstract: Investigation into far infrared photoconductivity (PC) in narrow-gap epitaxial bulk Hg1–xCdxTe (x < 0.2) films grown by molecular beam epitaxy and chemical vapor deposition techniques is presented. A broadband of photosensitivity in terahertz region is found at 4.2 and 77 K. Some long-wavelength peculiarities of spectra are discovered and their origins are discussed. We also study PC relaxation process with nanosecond time resolution. It is found that carrier relaxation is non-radiative and measured lifetimes show that some of the structures are applicable for detecting in very long-wavelength infrared range.

Weak localization of Dirac fermions in HgTe quantum wells

Abstract: Weak localization in a system of gapless two-dimensional Dirac fermions in HgTe quantum wells with thickness d = 6.6 nm, which corresponds to the transition from a normal to an inverted spectrum, has been investigated experimentally. A negative logarithmic correction to the conductivity of the system has been observed both at the Dirac point and in the vicinity of this point. The anomalous magnetoresistance of two-dimensional Dirac fermions is positive. This indicates that weak localization in the system of two-dimensional Dirac fermions occurs owing to localization and interaction effects in the presence of rapid spin relaxation.

Specific features of the spectra and relaxation kinetics of long-wavelength photoconductivity in narrow-gap HgCdTe epitaxial films and heterostructures with quantum wells

Abstract: The spectra and relaxation kinetics of interband photoconductivity are investigated in narrow-gap Hg1 − xCdxTe epitaxial films with x = 0.19–0.23 and in structures with HgCdTe-based quantum wells (QWs), having an interband-transition energy in the range of 30–90 meV, grown by molecular-beam epitaxy on GaAs (013) substrates. A long-wavelength sensitivity band caused by impurities or defects is found in the spectra of the structures with quantum wells in addition to the interband photoconductivity. It is shown that the lifetimes of nonequilibrium carriers in the structures with QWs is less than in bulk samples at the same optical-transition energy. From the measured carrier lifetimes, the ampere-watt responsivity and the equivalent noise power for a film with x = 0.19 at a wavelength of 19 μm are estimated. When investigating the relaxation kinetics of the photoconductivity at 4.2 K in high excitation regime, it is revealed that radiative recombination is dominant over other mechanisms of nonequilibrium-carrier recombination.

Analysis of the photoluminescence spectra of Cd x Hg 1–x te heteroepitaxial structures with potential and quantum wells grown by molecularbeam epitaxy

Abstract: A theoretical model for description of the band diagram and the photoluminescence spectra of heteroepitaxial structures (HES) based on Cd x Hg1–x Te (MCT) with potential and quantum wells (QW) grown by molecularbeam epitaxy (MBE) is developed. A special feature of the model is that the model takes into account the dependence of electron affinity on the MCT composition and temperature as well as the dependence of the electron concentration in intrinsic MCT and hole concentration in vacancy-doped MCT. The calculations are made and the results are compared with the experimental photoluminescence spectra of various structures with QW reported in the literature.

The effect of electron-hole scattering on transport properties of a 2D semimetal in the HgTe quantum well

Abstract: The influence of e-h scattering on the conductivity and magnetotransport of 2D semimetallic HgTe is studied both theoretically and experimentally. The presence of e-h scattering leads to the friction between electrons and holes resulting in a large temperature-dependent contribution to the transport coefficients. The coefficient of friction between electrons and holes is determined. The comparison of experimental data with the theory shows that the interaction between electrons and holes based on the long-range Coulomb potential strongly underestimates the e-h friction. The experimental results are in agreement with the model of strong short-range e-h interaction.

Manifestation of a semimetallic state in cyclotron resonance in low-symmetry HgTe-based quantum wells

Abstract: Cyclotron-resonance measurements in 21-nm-thick HgTe/CdHgTe quantum wells of different crystallographic orientations have been performed. It has been found that, in contrast to the structures with the (001) orientation of the quantum-well plane, (013)-oriented quantum wells are semimetallic and their absorption spectra exhibit both electron and hole cyclotron-resonance lines. The simultaneous presence of the two types of charge carriers originates from an overlap between the upper heavy-hole quantum-confinement subbands hh1 and hh2. This overlap is caused by the strong interaction of these subbands with the Dyakonov-Khaetskii interface state. Calculations carried out using the eight-band kp-Hamiltonian indicate that, for known values of the band-structure parameters, the overlap between hh2 and hh1 subbands does not occur; this result is in agreement with the cyclotron-resonance data for (001)-oriented structures. The enhanced interaction between heavy-hole and interface states owing to the existence of steps at low-symmetry heterointerfaces may be the mechanism responsible for the appearance of an overlap between subbands in HgTe quantum wells with orientation different from (001).

Magnetoabsorption in narrow-gap HgCdTe epitaxial layers in the terahertz range

Abstract: The magnetoabsorption and photoconductivity spectra are investigated in the terahertz (THz) range at a temperature of T = 4.2 K for n-Hg1 − x Cd x Te bulk epitaxial layers of various compositions (both semiconductor and semimetallic) grown by molecular-beam epitaxy. Within the framework of the Kane 8 · 8 model, the electron and hole Landau levels are calculated. It is shown that, in contrast to the results of previous investigations, all observed resonance lines are related to transitions between the Landau levels of free carriers (the cyclotron resonance in the conduction band and the transitions between heavy-hole and electron Landau levels), which is evidence of the high purity and structural perfection of the samples. The possibility of using zero-gap Hg1 − x Cd x Te solid solutions as THz photodetectors tunable by magnetic field is shown.

The photoelectrical properties of MIS structures based on heteroepitaxial n-Hg1–xCdxTe (x = 0.21–0.23)

Abstract: The dependences of photo-emf of metal-insular-semiconductor structures based on n-Hg1–xCdxTe (x = 0.21–0.23) on bias voltage, frequency, and temperature are experimentally studied. The photoelectrical characteristics of the metal-insulator-semiconductor structures based on epitaxial n-Hg1–xCdxTe (x = 0.21–0.23), including those taking into account the presence of near-surface graded-gap layers having an increased content of CdTe at various temperatures are calculated. The calculated dependences are compared with experiment and it is shown that the differential resistance of the spatial charge region for the metal-insulatorsemiconductor structures based on n-Hg1–xCdxTe (x = 0.21–0.23) without a graded-gap layer is restricted by tunneling through deep levels, whereas the differential resistance of the same structures with graded-gap layers – by the charge-carrier diffusion.

High-temperature photoluminescence of CdHgTe solid solutions grown by molecular-beam epitaxy

Abstract: The spectra of the high-temperature (up to 300 K) photoluminescence of the heteroepitaxial structures based on the CdHgTe solid solutions that are grown using molecular-beam epitaxy and emit at room temperature in the wavelength interval λ = 1.5–4.3 μm are analyzed. It is demonstrated that the observed photoluminescence of the CdHgTe narrow-gap semiconductor at high temperatures and the specific features of the high-temperature spectra can be interpreted using the disorder of the solid solution as in the case of the solid solutions of the group-III nitrides.

Two-dimensional semimetal in wide HgTe quantum wells: Charge-carrier energy spectrum and magnetotransport

Abstract: The magnetoresistivity and the Hall and Shubnikov-de Haas effects in heterostructures with a single 20.2-nm-wide quantum well made from the gapless semiconductor HgTe are studied experimentally. The measurements are performed on gated samples over a wide range of electron and hole densities. The data obtained are used to reconstruct the energy spectrum of electrons and holes in the vicinity of the extrema of the quantum-confinement subbands. It is shown that the charge-carrier dispersion relation in the investigated systems differs from that calculated within the framework of the conventional kp model.

Cyclotron resonance in HgCdTe-based heterostructures in strong magnetic fields

Abstract: Cyclotron resonance (CR) study of HgCdTe-based quantum wells with both inverted and normal band structures in quantizing magnetic fields was performed In normal-band sample in addition to electron CR transitions a strong absorption line presumably related to impurity transitions was discovered Anticrossing of electron and hole Landau levels was observed in inverted-band sample In semimetallic HgTe quantum wells with inverted band structure, a hole CR line was observed for the first The obtained results were interpreted within the Kane 8 × 8 model, the valence band offset of CdTe and HgTe, and the Kane parameter EP being adjusted

HgCdTe structures for dual-band photodetectors operating in the 3–5 and 8–12 µm spectral ranges

Abstract: A bilayer cadmium-mercury-tellurium (CMT) heterostructure was designed consisting of photosensitive layers of compositions x CdTe = 0.29–0.32 and x CdTe = 0.220–0.230, sensitive in the spectral ranges of 3–5 and 8–12 µm, a barrier layer between them, and wide-band variable-gap layers on the heterojunction and the surface grown on a GaAs substrate with ZnTe and CdTe buffer layers. The molecular beam epitaxial (MBE) growth of the heteroepitaxial structure (HES) was controlled by real time ellipsometry. After the growth, the composition distribution throughout the thickness was measured by reflection spectra with layer-by-layer chemical etching. There is good agreement between the results of composition measurements using ellipsometry and reflection spectra. P-type conductivity of bilayer MBE CMT HESs was obtained after thermal annealing at 220–240 °C in an inert gas (helium) for 24 h. The concentration of holes in the photosensitive layers is (4–10) ·1015 cm−3 and (8–20) · 1015 cm−3 at 78 K.

Relaxation of a radiation-damaged layer formed during ion-beam milling of CdHgTe solid solutions

Abstract: We have studied the relaxation of electrical properties in radiation-damaged layers formed during ion-beam milling (IBM) of cadmium mercury telluride (CdHgTe) solid solutions. It is established that the initial concentration of donor centers formed due to radiation-induced defects and the dynamics of their decomposition in the course of aging (relaxation) are determined by the composition of the solid solution. The mobility of charge carriers in radiation-damaged CdHgTe layers upon relaxation also depends on the solid solution composition.

AN INVESTIGATION INTO THE ADMITTANCE OF MIS- STRUCTURES BASED ON MBE HgCdTe WITH QUANTUM WELLS

Abstract: The results of investigations into the complex admittance of the MIS-structures based on heteroepitaxial MBE Hg1–xCdxTe with quantum wells (QW) in the test-signal frequency range 1 kHz – 2 МHz at temperatures 8– 300 K are reported. The thickness of single HgTe QWs was 5.6 and 7.1 nm, the content in the 35-nm thick barrier layers – 0.65 and 0.62, respectively.