Amplification of electromagnetic waves at odd harmonics of the cyclotron resonance of heavy holes with negative effective masses in semiconducting diamond

It is shown theoretically that the absorption coefficient for circularly polarized electromagnetic waves at the cyclotron resonance of heavy holes with negative effective masses in diamond in parallel electric and magnetic fields oriented along the [001] crystal axis takes negative values at the frequency of any of the n+1 harmonics (n=0,4,8, etc.) for the right (electron) polarization and at the frequency of any of the n−1 harmonics (n=4,8,12, etc.) for the left (hole) polarization. In an electric field E≈10⁴ V/cm and magnetic fields H=30–80 kOe, at lattice temperatures of 77–100 K, and for a hole concentration of (3–5)×10¹⁵ cm⁻³, the absorption coefficient for an electromagnetic wave at the third harmonic ω ₃=3ω=2.5×10¹² s⁻¹ (wavelength λ₃=0.92 mm) is as high as η ₃=(−7)–(−30) cm⁻¹. Fiz. Tekh. Poluprovodn. 32, 504–508 (April 1998)     

Interaction of surface acoustic waves with a two-dimensional electron gas in the presence of spin splitting of the Landau bands

The absorption and variation of the velocity of a surface acoustic wave of frequency f=30 MHz interacting with two-dimensional electrons are investigated in GaAs/AlGaAs heterostructures with an electron density n=(1.3–2.8)×10¹¹cm² at T=1.5–4.2K in magnetic fields up to 7 T. Characteristic features associated with spin splitting of the Landau level are observed. The effective g factor and the width of the spin-split Landau bands are determined: g*≅5 and A=0.6 meV. The greater width of the orbital-split Landau bands (2 meV) relative to the spin-split bands is attributed to different shielding of the random fluctuation potential of charged impurities by 2D electrons. The mechanisms of the nonlinearities manifested in the dependence of the absorption and the velocity increment of the SAW on the SAW power in the presence of spin splitting of the Landau levels are investigated. Fiz. Tekh. Poluprovodn. 33, 979–985 (August 1999)     

Intraband light absorption in quasi-two-dimensional systems in external electric and magnetic fields

Intraband light absorption in parabolic quantum wells is studied with an electric field directed along the spatial quantization axis and a magnetic field parallel to the plane of the size-confined system. In such a geometry direct optical transitions between the quantum-well levels are possible, the peak light absorption coefficient reaches large values (∼3×10² cm⁻¹), and the frequency of the absorption maximum depends on magnetic field strength. It is shown for the normal incidence of electromagnetic waves that the level of absorption decreases with increasing electric field strength and that it is incorrect to confine the calculations to the Born approximation in strong magnetic fields. Fiz. Tekh. Poluprovodn. 33, 828–831 (July 1999)     

Energy spectrum of acceptors in the semimagnetic semiconductors p-Hg1−x MnxTe in the spin-glass region

The far-IR transmission and photoconductivity in the semimagnetic alloys p-Hg_1−x Mn_xTe (x=0.20–0.22) at temperatures 2–7 K were investigated at fixed frequencies of optically pumped molecular lasers in the region 49–311 μm. We report the observation of photoexcitation of acceptors from the ground into excited states under conditions when the direct interaction of the magnetic moments of the manganese ions becomes substantial and results in the formation of a spin-glass phase. It was found that the internal field produced by the spontaneous and external polarizations of the magnetic moments of the Mn²⁺ ions in the spin-glass temperature range influences the energy spectrum of acceptors in a magnetic field. Fiz. Tekh. Poluprovodn. 32, 450–452 (April 1998)     

Absorption of a strong electromagnetic wave by electrons in a superlattice in a quantizing electric field

Intraminiband absorption of light by electrons in a quantum superlattice in a quantizing electric field is investigated theoretically taking into account the electron-phonon interaction. It is assumed that the interaction with optical dispersion-free phonons makes the main contribution to electron scattering. It is shown that the point ω=ω ₀ (ω is the light frequency, and ω ₀ is the optical phonon frequency) conditionally divides the ω dependence of the absorption into two parts: ω<ω ₀, the region of exponentially weak absorption and ω>ω ₀, the region of “strong” absorption. An electric field shifts the region of strong absorption in the red direction of the spectrum. Fiz. Tekh. Poluprovodn. 33, 1355–1358 (November 1999)     

Hole boil-off and the magnetoresitance of the semimagnetic semiconductor Hg1−x MnxTe1−y Sey

Galvanomagnetic effects were investigated in gapless and narrow-gap semiconductors of the form Hg_1−x Mn_xTe_1−y Se_y with x=0.03–0.11, y=0.01–0.10 (−150<ɛ _g <190)meV and acceptor concentration 5.4×10¹⁶<N _A <4.3×10¹⁸ cm⁻³. In magnetic fields H=5–50 kOe and at T=1.3–4.2 K, the observed hole concentration p=1/eR was found to increase by a substantial factor (of up to 500). This was accompanied by a fall in the longitudinal (ρ _zz ) and transverse (ρ _xx ) magnetoresistivities. The hole “boil-off” is assumed to be a consequence of the existence at H=0 of a bound magnetic polaron and the delocalization of carriers when these states are destroyed by the external magnetic field. The anomalous ratio of longitudinal-to-transverse resistivities (ρ _zz >ρ _xx ), observed at liquid-helium temperatures and in magnetic fields H>10 kOe, is explained in terms of the properties of the energy spectrum of the valence band of semimagnetic semiconductors in quantizing magnetic fields. Fiz. Tekh. Poluprovodn. 31, 1198–1205 (October 1997)     

Photoelectric and photomagnetic properties of the gapless semiconductor CdxHg1−x Te in the infrared and millimeter spectral regions when an energy gap is opened

The magnetic-field and strain dependences of the photoelectric, photomagnetic, and photothermomagnetic characteristics are determined for the gapless semiconductor Cd_xHg_1−x Te (x=0.04–0.16), in which an energy gap is opened by external effects. In excitation by infrared radiation the photosignal is observed to increase sharply with an increase in the applied magnetic field or uniaxial elastic deformation. For radiation in the millimeter spectral range the photoresponse exhibits giant oscillations associated with the variation of the electron density. This phenomenon is confirmed by the field dependence of the photomagnetic Hall effect. It is shown that the photothermomagnetic effect is a differential signal relative to the photoconductivity signal. Fiz. Tekh. Poluprovodn. 31, 35–42 (January 1997)     

Indirect electronic transitions in semiconductors occurring as a result of scattering of charge carriers by dislocations in a quantizing magnetic field

The light absorption due to indirect electronic transitions in a semiconductor in a quantizing magnetic field is calculated under the assumption that an edge dislocation plays the role of a third body. The characteristic light frequency and magnetic field dependences of the absorption coefficient are determined for the mechanism considered. Fiz. Tekh. Poluprovodn. 32, 453–454 (April 1998)     

Optical properties of crystals of the solid solutions (InSb)1−x (CdTe)x

The reflection and absorption spectra of crystals of the solid solutions (InSb)_1−x (CdTe)_x in the wavelength interval 2.5–25 μm were measured within the limits of solubility of CdTe in InSb (x⩽0.05) at room temperature. Analysis of the experimental results confirmed the applicability of the Kane theory for all compositions investigated. The variation of the optical band gap ɛ _g ^opt and the effective mass m _c at the Fermi level as a function of composition was determined. It is shown that the minimum values m _c=0.8×10⁻² m ₀ and ɛ _g ^opt =0.07 eV are reached for x=0.02–0.03. Information about the predominant mechanism of scattering for each alloy is obtained from the absorption curves in the region of absorption by free charge carriers. X-Ray crystallographic investigations were performed and the change Δa(x) in the lattice constant of the solid solutions relative to pure InSb was determined. It is shown that the behavior of m _c(x) and ɛ _g ^opt is uniquely determined by Δa(x). In turn, Δa(x) is determined by the complicated character of the interaction of the dopants with one another and with the InSb lattice. Fiz. Tekh. Poluprovodn. 32, 303–306 (March 1998)     

Nuclear magnetic resonance spectra of 119Sn and 125Te in SnTe and SnTe:Mn

A study is reported of the nuclear magnetic resonance spectra of ¹¹⁹Sn and ¹²⁵Te in SnTe with hole concentrations p ₇₇=1.42×10²⁰−2.3×10²¹ cm⁻³ and in SnTe:Mn (N _Mn=0.5 and 5 at. %, p ₇₇=8×10²⁰ cm⁻³) at T=4.2–300 K. Considerable broadening of NMR lines due to hyperfine magnetic interactions between nuclear and electron spins was observed in SnTe with p ₇₇>2ײ⁰ cm⁻³. Asymmetric broadening of the resonance lines was observed in the rhombohedral phase of SnTe and SnTe:Mn. The temperature dependence of the NMR line width of ¹²⁵Te in SnTe:Mn is in agreement with the magnetic phase diagram for N _Mn=5 at. %. The superparamagnetic phase of SnTe:Mn is formed at T=20±2 K and the ferromagnetic phase is formed at T=4.2 K. Fiz. Tekh. Poluprovodn. 31, 1187–1191 (October 1997)     

Determining surface recombination rates in epitaxial layers of n-CdxHg1−x Te from measurements of the planar magnetoresistance and relaxation times for nonequilibrium charge carriers

The dependence of the planar magnetoresistance on magnetic field has been measured for epitaxial layers of n-Cd_xHg_1−x (x=0.211, 0.22) at 300 and 77 K. The 77 K measurements were made in electric fields below and above the threshold field for avalanche impact ionization. The measurement results for the planar magnetoresistance and relaxation time of nonequilibrium charge carriers are used to determine surface recombination rates. Fiz. Tekh. Poluprovodn. 32, 1076–1078 (September 1998)     

Application of Continuously Frequency-Tunable 0.4 THz Gyrotron to Dynamic Nuclear Polarization for 600 MHz Solid-State NMR

In this paper we present results that demonstrate the utility of a continuously frequency-tunable 0.4 THz-gyrotron in a dynamic nuclear polarization (DNP)-enhanced solid-state NMR (SSNMR) spectroscopy at one of the highest magnetic fields, B ₀ = 14.1 T (600 MHz for ¹H Larmor frequency). Our gyrotron called FU CW VI generates sub-mm wave at a frequency near 0.4 THz with an output power of 4–25 W and a tunability over a range of more than 1 GHz by sweeping the magnetic field at the gyrotron cavity. We observed overall down shifting of the central frequency by up to ~1 GHz at high radiation duty factors and beam current, presumably due to the cavity thermal expansion by a heating, but the tunable range was not significantly changed. The frequency tunability facilitated the optimization of the DNP resonance condition without time-consuming field-sweep of the high-resolution NMR magnet, and enabled us to observe substantial enhancement of the SSNMR signal (ε _DNP = 12 at 90 K).     

Luminescence and electrical properties of InGaN/AlGaN/GaN light emitting diodes with multiple quantum wells

Luminescence spectra of light-emitting diodes based on InGaN/AlGaN/GaN heterostructures with multiple quantum wells are studied for currents in the range J=0.15 μA-150 mA. The comparatively high quantum efficiency for low J(J _max=0.5–1 mA) is a consequence of a low probability for the nonradiative tunnel current. The current-voltage characteristics J(V) are studied for J=10⁻¹²–10⁻¹ A; they are approximated by the function V=ϕk+mkT· [1n(J/J ₀)+(J/J ₁)^0.5] + J · R _s. The portion of V∞(J/J ₁)^0.5 and measurements of the dynamic capacitance indicate that i-layers adjacent to the active layer play an important role. The spectra are described by a model with a two-dimensional density of states with exponential tails in multiple quantum wells. The rise in T with increasing J is determined from the short-wavelength decay of the spectrum of the blue diodes: T=360–370 K for J=80–100 mA. An emission band is observed at 2.7–2.8 eV from green diodes at high J; this band may be explained by phase separation with different amounts of In in the InGaN. Fiz. Tekh. Poluprovodn. 33, 445–450 (April 1999)     

Photovoltaic effect in an asymmetric GaAs/AlGaAs nanostructure produced as a result of laser excitation

Photocurrent has been observed in a GaAs/GaAlAs structure with three asymmetric quantum wells in a magnetic field H parallel to the surface of sample irradiated with a quasicontinuous-wave laser with λ=1.065 μm. The current flows in the plane of the layers in a direction perpendicular to the magnetic field. The magnitude of the current increases with H, and when the magnetic field is switched, the sign of the photocurrent changes. The effect is explained on the basis of a model with asymmetric electronic wave functions in a magnetic field. Fiz. Tekh. Poluprovodn. 31, 872–874 (July 1997)     

The magnetoplasma effect in single crystals of antimony at T⩾80 K

Transmission spectra of infrared laser radiation (λ=10.6 μm) passed through samples consisting of two symmetric halves of an antimony single crystal separated by a small gap are investigated in pulsed magnetic fields B⩽20 T at temperatures T⩾80 K. The magnetoplasma effect was observed for the magnetic induction B≈15 T, with change in the transmission close to 100%. The magnetoplasma relaxation time has been determined. The possibility of using such objects as IR optical valves with response time not worse than 10⁻⁴ s is demonstrated. Fiz. Tekh. Poluprovodn. 32 1318–1319 (November 1998)     

Frenkel’-Poole effect for boron impurity in silicon in strong warming electric fields

A method based on measurement of the thermally stimulated conductivity of a weakly compensated semiconductor, which is doped with a deep impurity and which contains an impurity component that is shallower than the main component, has been developed for investigating the Frenkel’-Poole effect. The results of an investigation of the thermally stimulated conductivity of Si:Ga samples with gallium density N _A =(2–3)×10¹⁸ cm⁻³ and low accompanying impurity content (⩽10¹³ cm⁻³) are reported. The conductivity was measured after extrinsic photoexcitation of samples heated at a rate β=0.6 K/s in the temperature range T=4.2–24 K in electric fields E=20–1000 V/cm. It is shown that the maximum on the curves of the thermally stimulated conductivity is due to the thermally stimulated emptying of the boron impurity and shifts to lower values of T as E increases. The decrease of the ionization energy of impurity B in an electric field, which turns out to be somewhat weaker than the field according to the Frenkel’-Poole model for singly charged Coulomb centers, is found from the shift of the maximum. Fiz. Tekh. Poluprovodn. 31, 777–780 (July 1997)     

Quasi-gapless semiconductor: p-type indium arsenide

Data on the Hall coefficient R and the resistivity ρ as functions of the magnetic field (to H=12 kOe) and electric field (to E=25 V/cm) in a heavily doped, compensated semiconductor p-type InAs with a density of excess acceptors N _ext=(N _A −N _D )⋍10¹⁶ cm⁻³ and a ratio N _A /N _D ⋍0.9 are used to calculate the characteristic carrier parameters at hydrostatic pressures up to P=1.5 Pa and temperatures T=77.6 and 300 K. It is found that a deep acceptor band is situated in the tail of the density of states of the conduction band, and a state of the heavily doped, fully compensated semiconductor type is established at low temperatures. Fiz. Tekh. Poluprovodn. 33, 36–38 (January 1999)     

Propagation of a surface acoustic wave in a layered system containing a two-dimensional conducting layer

A surface acoustic wave can propagate in a piezoelectric crystal above which lies a two-dimensional conducting layer. The electric fields excited by the wave in the crystal penetrate into the two-dimensional layer and excite dissipative currents in it, which results in absorption of the wave and change of its velocity. These characteristics are calculated for different configurations of the layered system taking into account not only the surface conductivity but also surface diffusion. When the layer has an elastic contact with the crystal, for the configuration allowance is made not only for the piezoelectric but also the deformation interaction of the sound wave with the electrons. Fiz. Tekh. Poluprovodn. 31, 478–482 (April 1997)     

Optical spectroscopy of excitonic states in zinc diarsenide

The luminescence and transmission of zinc diarsenide single crystals near the fundamental absorption edge have been investigated in the temperature range 4.2–300 K. Intense luminescence and absorption lines at 1.0384, 1.0488, and 1.0507 eV, referring to the ground state (n=1) and excited states (n=2, n=3) of a free exciton were observed at low temperatures. The free-exciton binding energy was found to be ∼13.9 meV on the basis of the hydrogen-like model and the direct band gap was found to be 1.0523, 1.0459, and 0.9795 eV at 4.2, 78, and 300 K, respectively. Fiz. Tekh. Poluprovodn. 31, 1029–1032 (September 1997)     

Evolution of static negative differential conductivity in Ga1−x AlxAs as a function of the transverse magnetic field and the composition of the solid solution

The authors investigate the behavior of the current-voltage characteristic of electrons under conditions of dynamic intervalley transfer in strong E⊥H fields in a solid solution Ga_1−x Al_xAs as the energy gap Δɛ between the lower and upper valleys is gradually diminished. It is shown that the static negative differential conductivity is particularly sensitive to variation of the magnetic field H for small gaps Δɛ. Increasing H suppresses the static negative differential conductivity in this case. This technique can be used to eliminate low-frequency Gunn oscillations while simultaneously preserving the dynamic negative differential conductivity, which is suppressed in the submillimeter spectral range. Fiz. Tekh. Poluprovodn. 31, 987–988 (August 1997)