Electron transport characteristics were measured at room temperature applying nanosecond duration electrical pulses on two-dimensional electron gas (2DEG) channel in AlGaN/GaN heterostructures grown on SiC substrate. Crystal lattice self-heating effects were minimised by usage of nanosecond duration pulses being as short as 2 ns. We were able to reach highest current densities of 1.24 A/mm and 1.80 A/mm for different type of AlGaN/AlN/GaN and AlGaN/GaN heterostructures under the electric field of 57 kV/cm and 69 kV/cm, respectively. Even stronger electric field usage was allowed by the pulse source employed (up to 75 kV/cm for channel length of 12.5 m), however, that was not needed as the electric current near saturation was observed. The three-dimensional (3D) electron channels in GaN epilayer on GaN substrate were also investigated under pulsed electric field reaching up to 216 kV/cm. The electron drift velocity was found from experimental charge current density characteristics assuming that Hall electron density does not change in the sample under high electric fields. Maximum drift velocities for the 2DEG channels in AlGaN/GaN and AlGaN/AlN/GaN samples were found to be up to 1.2 × 10 ^7 cm/s at 69 kV/cm and 1.1 × 10 ^7 cm/s at 57 kV/cm, respectively, while, the highest value of ∼ 2.6 × 10 ^7 cm/s at 216 kV/cm was obtained for the 3D GaN channels. In the region of high electric fields the electron drift velocity in 2DEG channel was smaller than that in 3D channel as the result of stronger hot-phonon effect.

We demonstrate the high voltage switch based on a Mn-doped GaN semiconductor involving sub-bandgap photo-excitation by means of Nd:YAG laser emitting in the second harmonic. Induced bandtails of Mn were found to be photoconductive starting from the 700 nm wavelength threshold. Turn-on time of the high voltage switch was probed with short 10 ps duration laser pulses revealing the voltage pulse duration and rise time values can be as short as 1 ns and 100 ps, respectively. Relaxation of the switch photoconductivity exhibited a 2 ns decay time at low-intensity laser pulses, while at the highest intensities, it increased up to 14 ns value due to the saturation of the intermediate band or nonradiative defects. The resistance of on state was regulated up to ten orders of magnitude under the change of the laser pulse intensity, which allowed the switch to wire kilovolt voltage pulses on the load of 50 $\Omega $ impedance. The developed high-voltage switch with performance in nanosecond time scale is beneficial for applications where high repetition rate operation is required.

Electro-optical modulation of a terahertz beam by drifting space-charge domains in n-GaN epilayers under pulsed electric field excitation was found and investigated at a temperature of 77 K. The free charge carrier contribution was observed as the attenuation of terahertz (THz) transmission whose value independently on THz beam polarization increased with the electric field, in the presence of drifting space-charge domains, up to 10%. The electro-optical contribution, on the other hand, was sensitive to beam polarization and demonstrated a nonlinear increase in THz transmission up to 50% under the external electric field up to 1.6 kV/cm, while higher field values led to an electrical breakdown of samples operating in the presence of drifting space-charge domain.

Gallium nitride semiconductor was investigated in pulsed electric field, demonstrating interaction of terahertz radiation with space-charge domains propagating with a speed of acoustic waves. The modulation of terahertz optical transmission was investigated in two polarizations, indicating the differences for terahertz radiation polarized along and orthogonally to the electric field.

Parametric generation of oscillations and waves is a paradigm, which is known to be realized in various physical systems. Unique properties of quantum semiconductor superlattices allow us to investigate high-frequency phenomena induced by the Bragg reflections and negative differential velocity of the miniband electrons. Effects of parametric gain in the superlattices at different strengths of dissipation have been earlier discussed in a number of theoretical works, but their experimental demonstrations are so far absent. Here, we report on the first observation of the dissipative parametric generation in a subcritically doped GaAs/AlGaAs superlattice subjected to a dc bias and a microwave pump. We argue that the dissipative parametric mechanism originates from a periodic variation of the negative differential velocity. It enforces excitation of slow electrostatic waves in the superlattice that provide a significant enhancement of the gain coefficient. This work paves the way for a development of a miniature solid-state parametric generator of GHz-THz frequencies operating at room temperature.

Terahertz (THz) spectra of electrically driven two-dimensional (2D) plasmons in grating-gated AlGaN/GaN high-electron mobility transistor (HEMT) structure were investigated in emission and transmission geometry at the temperature of 80 K. The 2D plasmon resonances were observed in the frequency range of 1-3 THz. The resonance position and intensity were found to be related to the grating period and the bias voltage applied to the transistor terminals.

Formation and drift of space-charge domains with velocity of sound were experimentally observed in charge current traces of a high-quality lightly doped GaN semiconductor under pulsed electric fields at room and liquid nitrogen temperatures. A GaN epilayer was developed on an Ammono GaN substrate to achieve the electron density and low-field mobility values of 1.06 × 1016 cm−3 and 1021 cm2/V s (at 300 K) and 0.21 × 1016 cm−3 and 2652 cm2/V s (at 77 K), respectively. The formation of moving space-charge domains was observed only in samples with the lengths of 1 mm and longer arising at the critical electric fields starting from ∼0.4 and ∼0.8 kV/cm at 77 and 300 K, respectively. Basic electron transport parameters were found investigating short samples with the lengths of 65 μm and shorter in a wide range of electric fields up to 150 kV/cm, at which the thermal material breakdown occurred. The critical length of a sample for space-charge domain formation was estimated considering the acoustoelectric effects in analysis of pulsed current–voltage characteristics, thus defining the constraints for the design of GaN-based power electronic devices.

The n-type GaN epilayers with low electron density were developed on a native substrate using the metalorganic vapour phase epitaxy method and investigated under pulsed electric fields until material breakdown and optically in the spectrum range from 0.1 THz to 60 THz at two temperatures of 77 K and 300 K. The epilayers demonstrated the low-field electron mobility and density values reaching up to 1021 cm2/V·s and 1.06 × 1016 cm−3 (at 300 K) and 2652 cm2/V·s and 0.21 × 1016 cm−3 (at 77 K), respectively. Maximum injected electric power value till the damage of the GaN epilayer was found to be up to 1.8 GW/cm3 and 5.1 GW/cm3 at 77 K and 300 K, respectively. The results indicate new practical possibilities of the GaN material controlled by an external electric field.

Cadmium telluride (CdTe) semiconductors are used in thin-film photovoltaics, detectors, and other optoelectronic applications. For all technologies, higher efficiency and sensitivity are achieved with reduced charge carrier recombination. In this study, we use state-of-the-art CdTe single crystals and electro-optical measurements to develop a detailed understanding of recombination rate dependence on excitation and temperature in CdTe. We study recombination and carrier dynamics in high-resistivity (undoped) and arsenic (As)-doped CdTe by employing absorption, the Hall effect, time-resolved photoluminescence, and pump-probe in the 80–600 K temperature range. We report extraordinarily long lifetimes (30 µs) at low temperatures in bulk undoped CdTe. Temperature dependencies of carrier density and mobility reveal ionization of the main acceptors and donors as well as dominant scattering by ionized impurities. We also distinguish different recombination defects. In particular, shallow As Te and deep V Cd −As Cd acceptors were responsible for p-type conductivity. AX donors were responsible for electron capture, while nonradiative recombination centers (V Cd −As Te , As 2 precipitates), and native defects (V Cd −Te Cd ) were found to be dominant in p-type and n-type CdTe, respectively. Bimolecular and surface recombination rate temperature dependencies were also revealed, with bimolecular coefficient T −3/2 temperature dependence and 170 meV effective surface barrier, leading to an increase in surface recombination velocity at high temperatures and excitations. The results of this study allowed us to conclude that enhanced crucible rotation growth of As-doped CdTe is advantageous to As activation, leading to longer lifetimes and larger mobilities and open-circuit voltages due to lower absorption and trapping.

A cavityless dissipative parametric gain in quantum GaAs/AlGaAs superlattice is demonstrated and discussed. Using a waveguide-based setup under the biasing of dc and microwave pump, emission lines, corresponding to the simultaneous multiphoton processes in the superlattice, were observed. It was shown that the incident electromagnetic wave experiences transformation into a slow longitudinal electrostatic wave inside the superlattice, ensuring hence the gain levels exceeding 1000 cm(-1).

Photo-detection in the near-infrared is commonly performed by Ge or InGaAs-based photodetectors. Further extension of the detection range to the mid-infrared region can be performed by germanium-tin (GeSn) material which shows promising characteristics and is fully compatible with silicon electronics as can be directly grown on silicon substrates. In this study, we focused on the optoelectronic properties of the photodiodes prepared by using 200 nm thick Ge0.95Sn0.05 epitaxial layers on Ge/n-Si substrate with aluminium contacts. Photodiodes were formed on non-irradiated and Nd:YAG laser irradiated Ge0.95Sn0.05 layers. The samples were irradiated by pulsed Nd:YAG laser with 61.5?259.2 MW/cm2 intensity. The photodiodes were characterized by using short laser pulses with the wavelength in 2.0?2.6 ?m range. The laser-irradiated diode was found more sensitive in the long wavelength range due to laser induced Sn atoms redistribution providing formation of graded bandgap structure. Sub-millisecond photocurrent relaxation in the diodes revealed their suitability for image sensors. Our findings open the perspective for improving the photo-sensitivity of GeSn alloys in the mid-infrared by pulsed laser processing.

In this work we present a study of spatial and temperature dependencies of carrier lifetime and photoluminescence in pseudomorphic Ge0.95Sn0.05 layer on silicon by applying contactless time-resolved differential absorption and photoluminescence techniques. The observed small lifetime and photoluminescence intensity spatial variations (similar to 10%) indicate for good material homogeneity. Differential transmission kinetics exhibit fast bleaching and slow absorption components related to thermalization (few hundreds of picoseconds) and surface recombination (few tens of nanoseconds) processes. Temperature dependent photo-luminescence measurements indicate activation of carriers from L to G valley, governing dominant direct photoluminescence emission band at 1850 nm at 300 K. These direct transitions explain more efficient and blue-shifted by 150 nm photoluminescence emission at room temperature. At low temperatures direct emission is related to non-thermalized carriers, while indirect one is caused by depopulation of light hole band with temperature.

We report three approaches to development of electrically-pumped THz emitters based on III-Nitride structures. The first approach entails the investigation of two-dimensional (2D) plasmons in grating-gated AlGaN/GaN heterostructures performed at temperatures above liquid nitrogen by means of THz time-domain spectroscopy (TDS). Comparative analysis of the experimental data revealed the considerable phase shift of transmitted THz pulses at the resonant frequencies of collective oscillations of the grating-gated 2D electron gas (2DEG). The use of 2D plasmons is proposed for the development of tunable-frequency THz emitters with electrical control of the emission frequency, beam wave front and directivity. Another approach is based on the THz electroluminescence of shallow impurities such as oxygen and silicon in the standard AlGaN/GaN high electron mobility transistor (HEMT) structures. The surface plasmonphonon polaritons (SPPhP) in n-GaN grating are also considered for the development of electrically-pumped THz sources under thermal and electrical excitation of directive (coherent) radiation. We note that emission frequency can be governed either by gate voltage (2D plasmons) or structural design (SPPhP). All discussed methods are compared in terms of achieved quality factor, operating temperature and emitted power.

Photoexcited carrier dynamics in 1014–1018 cm−3 density range was investigated by using complementary optical and microwave techniques. Bulk lifetime decrease from 4 μs to 460 ns with excitation and its increase with temperature were observed in bulk n-type 4H-SiC crystal. The latter data and modeling of excitation-dependent hole capture with subsequent electron capture provided the trap position at EV + 0.19 eV and electron (hole) lifetimes of 360 ns (100 ns), correspondingly, at high excitation conditions. Lifetime increase with temperature was observed due to the trap thermal activation and reduction of electron capture cross section. The trap origin was discussed in terms of silicon vacancy related and titanium point defects. Photoluminescence spectra in 24–300 K range revealed bound exciton luminescence and nitrogen-aluminum donor-acceptor pair band, which time-resolved and excitation dependent dynamics was analyzed.

Nonuniformly compensated InSb is studied in stationary and pulsed dc-electric fields under terahertz radiation illumination in temperature range of 8-300 K. It was found that at low temperatures the sample conductance grows with applied electric field nonlinearly due to increased scattering of ionized impurities and due to activation of random fluctuation potential. We demonstrate that the terahertz irradiation leads the conductance of the sample also to increase. With the change in excitation frequency from 0.762 THz to 1.63 THZ the modulation of the sample resistance becomes lower indicating the predominance of hot electron governed effects in non-uniform crystal due to electron mean energy and drift velocity relaxation. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Hot electron transport governed enhancement of light diffraction efficiency on transient grating has been observed in dcbiased semiinsulating InP:Fe crystal at below the energy gap excitation by picosecond laser pulses. Numerical analysis of dcfield-induced changes of free-carrier and electrooptic nonlinearities has been performed and allowed to attribute the observed effect to formation of a high-field domain grating, leading to dominant refractive index modulation by the quadratic electrooptic nonlinearity.