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Temperature evolution of the high-resolution far-infrared transmission spectra of 6H-SiC single crystal slab is experimentally examined in a temperature range of 5–320 K. Temperature dependences of the phonon lifetimes for long-lived phonons, namely the IR active folded transverse acoustic (FTA) phonon doublet, are determined. These dependences are interpreted in the framework of a theoretical model developed using first principle anharmonic lattice dynamics. It is shown that three-phonon anharmonic processes dominate in the FTA phonon decay at temperatures above 160 K resulting in a rapid decrease of the phonon lifetimes with increasing temperature. Contribution of four-phonon anharmonic processes is negligibly small at temperatures below 320 K.

Terahertz time-domain spectroscopy and Fourier-transform infrared spectroscopy were developed as the method for the investigation of high-frequency characteristics of two-dimensional electron gas and GaN:C buffer layers in AlGaN/AlN/GaN heterostructures grown on a semi-insulating SiC substrate. The reflectance and transmittance spectra of the selected heterostructure layers were studied after the top layers were removed by a reactive ion etching. Results were numerically analyzed using the transfer matrix method taking into account the high-frequency electron conductivity via a Drude model and complex dielectric permittivity of each epitaxial layer via a one-phonon-resonance approximation. Good agreement between the experiment and theory was achieved revealing the temperature dependent electron effective mass in AlGaN/AlN/GaN high electron mobility transistor structures and the small damping factors of optical phonons due to high crystal quality of the epitaxial layers fabricated on the SiC substrate.

We report on the study of far-infrared transmission in silicon carbide of 6H polytype in the region of IR active folded transverse acoustic (FTA) phonon doublet. Joint analysis of the transmittance spectra obtained at low and high resolutions is performed to determine spectral dependences of the refractive index and absorption coefficient. A dual Lorentz oscillator model is used to simulate the FTA phonon contributions to the dielectric function. The oscillator strengths and phonon lifetimes for both the components of the FTA phonon doublet in 6H-SiC are determined. At temperature decrease from 300 to 5 K, the FTA phonon lifetimes increase more than six times, while the oscillator strengths remain the same.

Emission of terahertz radiation from a AlGaN/GaN/Al $_{\mathbf {2}} \mathbf {O} _{\mathbf {3\, }}$ heterostructure with a surface metal grating is investigated under conditions of 2D electron heating in a lateral electric field. The studies are performed under essentially nonequilibrium conditions when the effective temperatures of 2D electrons and 2D plasmons are several times higher than the lattice temperature. This makes it possible to observe and explore high-quality peaks of intense THz radiation corresponding to 2D plasmon resonance.

The weakest mode of the infrared active folded phonons in 4H–SiC, folded transverse acoustic (FTA) mode, has been experimentally studied. The lifetime of the FTA phonons has been determined. The lifetime is 58 ± 4 ps at room temperature and increases to 140 ± 7 ps when the temperature is decreased to 5 K. These results were obtained by the experimental study of far-infrared transmission of the SiC crystal and its theoretical simulation using an oscillator model for the dielectric function around the phonon resonance. The FTA phonon lifetime was a fitting parameter of the model when analyzing the irregular interference pattern in the transmission spectrum measured with a high resolution (0.2 cm−1). Oscillator strength of the FTA phonon resonance has also been determined. It is equal to (6.0 ± 0.5 ) × 10 − 4 and does not depend on temperature.

We report on studies of electrically excited non-equilibrium 2D electrons and 2D plasmons in an AlGaN/GaN nanostructure. Optical access to 2D plasmons is provided by means of a metal grating fabricated at the nanostructure surface, while the properties of 2D electrons are examined in the samples without metal grating. The paper focuses on the creation of highly non-equilibrium conditions when the effective temperature of 2D electrons is much higher than the crystal lattice temperature. Such conditions are realized by applying short electrical pulses with a low repetition frequency. A method has been developed for independently determining the temperature of hot electrons and the temperature of the crystal lattice under an applied electric field. It has been shown that under highly non-equilibrium conditions the spectral density of terahertz electroluminescence of 2D plasmons can significantly exceed that of 2D electrons at a certain frequency.

Terahertz radiation emission from an electrically excited AlGaN/GaN heterostructure with a surface metal grating was studied under conditions of two-dimensional (2D) electron heating by the lateral electric field. Intensive peaks related to nonequilibrium 2D plasmons were revealed in the terahertz emission spectra with up to 4 times selective amplification of the radiation emission in the vicinity of 2D plasmon resonance. This selective emission was shown to be frequency-controllable by the grating period. Exact spectral positions of the 2D plasmon resonances were preliminarily experimentally detected with the help of equilibrium transmission spectra measured at various temperatures. The resonance positions are in a satisfactory agreement with the results of theoretical simulation of the transmission spectra performed using a rigorous solution of Maxwell’s equations. The effective temperature of hot 2D electrons was determined by means of I– V characteristics and their analysis using the power balance equation. It was shown that for a given electric field, the effective temperature of nonequilibrium 2D plasmons is close to the hot 2D electron temperature. The work may have applications in GaN-based electrically pumped emitters of terahertz radiation.Terahertz radiation emission from an electrically excited AlGaN/GaN heterostructure with a surface metal grating was studied under conditions of two-dimensional (2D) electron heating by the lateral electric field. Intensive peaks related to nonequilibrium 2D plasmons were revealed in the terahertz emission spectra with up to 4 times selective amplification of the radiation emission in the vicinity of 2D plasmon resonance. This selective emission was shown to be frequency-controllable by the grating period. Exact spectral positions of the 2D plasmon resonances were preliminarily experimentally detected with the help of equilibrium transmission spectra measured at various temperatures. The resonance positions are in a satisfactory agreement with the results of theoretical simulation of the transmission spectra performed using a rigorous solution of Maxwell’s equations. The effective temperature of hot 2D electrons was determined by means of I– V characteristics and their analysis using the power balance ...

We report on experimental studies of the surface plasmon-phonon polariton excitations in heavily doped GaAs epitaxial layers. Reflection and emission of radiation in the frequency range of 2-19 THz were investigated for samples with surface-relief grating, as well as for samples with planar surface. The reflectivity spectrum for p-polarized radiation measured for the sample with the surface-relief grating demonstrates a set of resonances attributed to excitations of different surface plasmon-phonon polariton modes. The observed resonances lie beyond the limits of the Reststrahlen band. Terahertz radiation emission from the samples was studied in nonequilibrium conditions under the pulsed electric field excitation. Two contributions to the spectral density of the terahertz radiation have been revealed, the first being due to bulk plasmon-phonon polaritons (PPhPs), while the second originating from the surface PPhPs. A field dependence of the effective temperature of the bulk PPhPs has been established. Polarization dependence of the terahertz radiation related to surface PPhPs has been experimentally examined for the first time.

Optical properties of a heavily-doped GaAs epitaxial layer with a regular grating at its surface have been experimentally investigated in the terahertz spectral range. Reflectivity spectra for the layer with a profiled surface drastically differ from those for the as-grown epilayer with a planar surface. For s-polarized radiation, this difference is totally caused by the electromagnetic wave diffraction at the grating. For p-polarized radiation, additional resonant dips arise due to excitation of surface plasmon-phonon polaritons. Terahertz radiation emission under significant electron heating in an applied pulsed electric field has also been studied. Polarization measurements revealed pronounced peaks related to surface plasmon-phonon polariton resonances of the first and second order in the emission spectra.

The circular photon drag effect is observed in a bulk semiconductor. The photocurrent caused by a transfer of both translational and angular momenta of light to charge carriers is detected in tellurium in the mid-infrared frequency range. Dependencies of the photocurrent on the light polarization and on the incidence angle agree with the symmetry analysis of the circular photon drag effect. Microscopic models of the effect are developed for both intra- and inter-subband optical absorption in the valence band of tellurium. The shift contribution to the circular photon drag current is calculated. An observed decrease of the circular photon drag current with increase of the photon energy is explained by the theory for inter-subband optical transitions. Theoretical estimates of the circular photon drag current agree with the experimental data., Comment: 8 pages, 4 figures

Optical transmission and reflection for a single GaN/AlGaN heterojunction grown on sapphire have been investigated with high and low spectral resolution in the spectral range of 8–47 meV. For comparison, the same spectra have been measured for the sapphire substrate. Refractive index dispersion has been determined for sapphire from the spectra measured with high resolution. Then the data on 2D electron absorption in the GaN/AlGaN heterojunction were obtained from the low resolution spectra. The spectra of terahertz emission from the GaN/AlGaN heterojunction under 2D electron heating in strong electric field have been measured for the first time.

Far infrared photoconductivity spectra due to excitation of shallow acceptors in strained multiquantum well Ge/Ge1−xSix (x≈0.1) heterostuctures are investigated. It is shown that these spectra are shifted toward longer wavelengths in the far infrared region compared with those of bulk p-Ge, owing to “built-in” strain and size quantization, which lead to splitting of the light-and heavy-hole subbands in the Ge layers. Shallow acceptor spectra are calculated variationally for bulk germanium under uniaxial tension, which is “equivalent” to the strained Ge layers in the heterostructures. Although this method is only appropriate for wide quantum wells (dGe≈800 A), the calculations are shown to qualitatively account for photoconductivity spectra obtained from narrower wells (dGe≈200 A) as well.

The impurity photoconductivity spectra of Ge/Ge1−xSix strained heterostructures with quantum wells are investigated. It is established that the built-in deformation in quantum-size Ge layers substantially changes the spectrum of shallow acceptors, shifting it into the long-wavelength region of the far-IR range. In strong magnetic fields the photoconductivity lines are observed to split and shift as a function of the field. This makes it possible to carry out a classification of the transitions.

Variation of absorption of terahertz radiation in lateral electric field was investigated in GaN epitaxial layers. Different behaviour of the absorption modulation in electric field was observed for radiation polarized along electric field and perpendicular to it. Joint analysis of optical and transport measurements let us obtain field dependencies of mobility, electron concentration and absorption cross-section. For terahertz radiation polarized perpendicular to the electric field, results are in accordance with Drude model of free electron absorption. Another polarization demonstrates significant deviation that is yet to be studied more thoroughly.

Surface plasmon polaritons are investigated in heavily doped n-GaN epitaxial layers. The grating etched on the surface of the epitaxial layer is used to convert photons into the surface plasmon polaritons and vice versa. The spectral study of reflection demonstrates the possibility of nonequilibrium surface plasmon polaritons excitation due to terahertz radiation scattering on the grating. Terahertz electroluminescence is investigated under lateral electric field. The luminescence spectrum demonstrates a significant contribution of nonequilibrium surface plasmon polariton scattering to terahertz radiation emission.

Surface plasmon polaritons in n-GaN are investigated. Experiments on optical reflection demonstrate the possibility of surface plasmon polariton excitation on the surface of the grating structure using terahertz radiation. Study of terahertz radiation emission under lateral electric field shows that the scattering of hot surface plasmon polaritons on the grating makes a significant contribution to the radiation intensity. In the certain spectral range this emission mechanism prevails.

Cyclotron resonance of 2D holes in high-mobility undoped multi-quantum-well Ge/GeSi heterostructure has been studied in both 'classical' and quantizing magnetic fields. Effects of hole heating on 2D hole cyclotron resonance has been investigated. The calculations of 2D hole Landau levels in rectangular quantum well have been performed allowing to interpret the evolution of CR spectra in going from 'classical' to 'quantum' range.

The paper deals with the first investigations of the 2D hot hole effects in multilayer heterostructures Ge/Ge1−xSix aimed at the realization of dynamical heating and intraband population inversion of carriers in strong electric fields. Ge/Ge1−xSix heterostructures (x ≈ 0.1, dGE ≈ 200 A) were grown on Ge(lll) substrates by CVD technique. In these heterostructures the quantum wells in the valence band are realized in thin germanium layers which are uniformly compressed in the plane of the structure because of the mismatch of lattice periods of Ge and GeSi. The deformation may be represented as a result of hydrostatic compression and equivalent uniaxial tension of Ge layers; the latter is responsible for the valence band splitting. Earlier we have observed in undoped heterostructures the CR line of photoexcited 2D holes (m c ≈ 0.07m 0) in 2-mm wavelength range at liquid helium temperature1. The halfwidth of CR line corresponds to the high value of the hole mobility μ ≈ 105 cm2/V·s thus testifying the quality of the heterostructures.

Last years strained heterostructures on base Ge and Si are of considerable interest due to wide range of properties which can be changed through band-structure engineering. We deal with heterostructures Ge/Ge1-xSix (x ≈ 0.1, dGe ≤ 200A) which were grown on moderately doped Ge (111) substrates by CVD techique. In these heterostructures quantum wells in the valence band are realized in germanium layers while the energy structure of conductance band is not quite clear yet. Theoretical calculations of the energy spectra give the negligible value of the conductance band offset (it seems to be the reason of the absence of the electron effects in the our experiments) while in the valence band the depth of the quantum well is about 100 meV. Ge layers are uniformly compressed in the plane of the structure because of the mismatch of lattice periods of Ge and Ge/Ge1-xSix This deformation may be represented as a result of hydrostatic compression and equivalent uniaxial tension PeqUiv of Ge layers; the latter is responsible for the valence band splitting. Earlier we have observed the cyclotron resonance (CR) absorption line of high mobility light 2D holes in split valence band in Ge layers [1] and revealed the electric field effects on the hole systems [2]. In this paper we report the further investigation of hot hole CR, that give mush information about the heating and energy spectrum of 2D holes . Also hot hole far infrared emission from selectively doped heterostructures was observed. The results of this experiment are shortly discussed.

Hot hole p-Ge masers and lasers operating in millimiter and far IR ranges (up to 100 μm) belong to a new group of tunable semiconductor generators discovered in the early ’80s(1). They are proposed to be used in solid state spectroscopy since their discovery. These generators may be preferable to other sources of the same frequency since it is possible to set the sourcer of radiation near by the sample and provide ultralow background radiation conditions in low temperature experiments.The performance of the tunable spectrometer with hote hole p-Ge emitter is demostrated by its application to studies of cyclotron resonance in strained multi-quantum-well (MQW) heterostructures (HS) Ge/Ge0.87Si0 13 and investigations of shallow impurities in these heterostructures.

Cyclotron resonance of photoexcited carriers in Ge/Ge1- x Si x multilayer strained undoped heterostructures (HSs) was investigated for the first time. Cyclotron resonance line of positive charge carriers with the effective mass m c≈0.07m 0 corresponding to that of holes in the upper subband in the strained quantum wells in Ge layers was observed in the absorption and mm-photoconductivity spectra. The persistent photoconductivity is shown to arise after interband illumination owing to the free holes remaining in the sample.