L. S. Bovkun

Long-wavelength injection lasers based on Pb1–x Sn x Se alloys and their use in solid-state spectroscopy

Diffusion lasers based on Pb1–xSnxSe alloys for a wide spectral range (7–40 μm) are developed. The emission spectra of these lasers in the temperature range of 18–80 K are investigated. It is shown that the laser-emission wavelength can be widely tuned by varying the operation temperature, which allows a spectral range of 7–26 THz to be covered. The possibility of using these lasers for the spectroscopy of solids, and, in particular, for the magnetooptical spectroscopy of narrow-gap semiconductor structures based on HgCdTe, is demonstrated.

Terahertz injection lasers based on PbSnSe alloy with an emission wavelength up to 46.5 μm

Diffusion injection lasers based on Pb1 – xSnxSe alloy, emitting in a wide spectral range of 10–46.5 μm depending on the composition and temperatures are fabricated. A technology for growing high-quality single crystals from the vapor phase under conditions of free growth is developed. The dependences of the total emission intensity on the pump current and the emission spectra of injection lasers based on Pb1 – xSnxSe are studied. In these samples, lasing of long-wavelength radiation to a record wavelength of 46.5 μm is achieved.

Exchange enhancement of the electron g -factor in a two-dimensional semimetal in HgTe quantum wells

The exchange enhancement of the electron g-factor in perpendicular magnetic fields to 12 T in HgTe/CdHgTe quantum wells 20 nm wide with a semimetal band structure is studied. The electron effective mass and g-factor at the Fermi level are determined by analyzing the temperature dependence of the amplitude of Shubnikov–de Haas oscillation in weak fields and near odd Landau-level filling factors ν ≤ 9. The experimental values are compared with theoretical calculations performed in the one-electron approximation using the eight-band kp Hamiltonian. The found dependence of g-factor enhancement on the electron concentration is explained by changes in the contributions of hole- and electron-like states to exchange corrections to the Landau-level energies in the conduction band.

Cyclotron resonance of dirac fermions in InAs/GaSb/InAs quantum wells

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.

Magnetooptical Studies and Stimulated Emission in Narrow Gap HgTe/CdHgTe Structures in the Very Long Wavelength Infrared Range

We investigate the prospects of HgTe/HgCdTe quantum wells for long-wavelength interband lasers (λ = 15–30 μm). The properties of stimulated emission (SE) and magnetoabsorbtion data of QWs structures with wide-gap HgCdTe dielectric waveguide provide an insight on dominating non-radiative carrier recombination mechanism. It is shown that the carrier heating under intense optical pumping is the main factor limiting the SE wavelength and intensity, since the Auger recombination is greatly enhanced when carriers populate high energy states in the valence band.

On the band spectrum in p -type HgTe/CdHgTe heterostructures and its transformation under temperature variation

The magnetoabsorption and interband photoconductivity spectra of HgTe/CdHgTe quantum wells exhibiting p-type conductivity are studied at different temperatures. It is shown that, for a sample with a normal band structure, the long-wavelength edge of the spectra shifts to higher energies with temperature increase, indicating an increase of the band gap in the quantum well. For a sample with an inverted band structure, it is for the first time found that the long-wavelength cut-off shifts to lower energies due to the topological phase transition from the inverted band structure to the normal structure with temperature increase. The experimental data are in agreement with the results of theoretical band-structure calculations based on the Kane model.

Activation conductivity in HgTe/CdHgTe quantum wells at integer Landau level filling factors: Role of the random potential

Shubnikov-de Haas oscillations are studied in 8-nm-wide HgTe/CdHgTe quantum wells with an electron concentration of (1.7–13) × 1011 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.

Magnetospectroscopy of double HgTe/CdHgTe quantum wells

The magnetoabsorption spectra in double HgTe/CdHgTe quantum wells (QWs) with normal and inverted band structures are investigated. The Landau levels in symmetric QWs with a rectangular potential profile are calculated based on the Kane 8 × 8 model. The presence of a tunnel-transparent barrier is shown to lead to the splitting of states and “doubling” of the main magnetoabsorption lines. At a QW width close to the critical one the presence of band inversion and the emergence of a gapless band structure, similar to bilayer graphene, are shown for a structure with a single QW. The shift of magnetoabsorption lines as the carrier concentration changes due to the persistent photoconductivity effect associated with a change in the potential profile because of trap charge exchange is detected. This opens up the possibility for controlling topological phase transitions in such structures.

THz lasers based on narrow-gap semiconductors

We report recent results on THz lasing in PbSnSe and HgCdTe. The wavelength of 46.3 microns was achieved in a PbSnSe laser with diffusion p-n junction. In HgCdTe waveguide structure with quantum wells (QWs) we demonstrate twofold increase in stimulated emission (SE) wavelength compared to previous works. The threshold pumping intensity is as low as 0.12 kW/cm2 for the SE wavelength of 9.5 microns. Molecular beam epitaxy of HgCdTe allows reproducible growth of thin QWs that can provide the “symmetrical” energy-momentum laws in conduction and valence bands, like in bulk PbSnSe, and thus suppress Auger recombination. Further prospects concerning THz lasers based on HgCdTe QWs are discussed.