Electron transport and detection of terahertz radiation in a GaN/AlGaN submicrometer field-effect transistor

Electron transport and photoresponse in the terahertz range in a GaN/AlGaN field-effect transistor with the submicrometer gate (0.25 μm) and two-dimensional electron gas in the channel (the electron concentration n _s = 5 × 10¹² cm^?2) were studied at 4.2 K. The charge-carrier mobility in the transistor’s channel μ = 3500 cm²/(V s) was determined from the dependence of the conductance on magnetic field. It is found that the dependence of photovoltage at the radiation frequency f = 574 GHz on the gate voltage (i.e., on the concentration of two-dimensional electrons) features a characteristic maximum, which is related to a resonance response of the subgate plasma in the transistor channel.     

Detection of terahertz radiation by resonant tunneling nanoheterostructures

The phenomenon of terahertz radiation detection by resonant tunneling structures (RTSs) has been studied. The calculations of the changes ΔI₀ in the direct current (DC) component I₀ under the action of an alternating electric field were carried out by the solution of a nonstationary Schrodinger equation with a time-periodic electric field based on the Floquet mode expansion of the wave functions. The dependences of the DC component I₀ in resonant tunneling structures on the frequency ν and AC signal amplitude V_ac have been built. It is shown that the ΔI₀ value in triple-barrier RTSs at resonance frequency hv ≈ E_r2–E_r1 (E_r1 and E_r2 are the energies of the size-quantized levels) can exceed a low-frequency value by more than an order of magnitude. The parameters of the structures have been optmized, in order to use them in the terahertz radiation detectors in the anbsence of an external bias. The possibility of tuning the resonance frequency in the terahertz range by changing the DC bias has been shown.     

Generation of Submillimeter-Wave Radiation with GaAs Tunnett Diodes and InP Gunn Devices in a Second or Higher Harmonic Mode

Efficient second-harmonic power extraction was demonstrated recently with GaAs tunnel injection transit-time (TUNNETT) diodes up to 235 GHz and with InP Gunn devices up to 325 GHz. This paper discusses the latest theoretical and experimental results from second-harmonic power extraction at submillimeter-wave frequencies and explores the potential of using power extraction at higher harmonic frequencies to generate continuous-wave radiation with significant power levels at frequencies above 325 GHz. Initial experimental results include output power levels of more than 50 μW at 356 GHz from a GaAs TUNNETT diode in a third-harmonic mode and at least 0.2–5 μW in the frequency range 400–560 GHz from InP Gunn devices in a third or higher harmonic mode. The spectral output of these submillimeter-wave sources was analyzed with a simple Fourier-transform terahertz spectrometer and, up to 426 GHz, with a spectrum analyzer and appropriate harmonic mixers. Initial experimental results from a GaAs/AlAs superlattice electronic device at D-band (110–170 GHz) and J-band (170–325 GHz) frequencies are also included.     

Specific features of light current-voltage characteristics of <Emphasis Type="Italic">p-i-n</Emphasis> structures based on amorphous silicon in the case of the tunnel-drift mechanism of dark current transport

Current-voltage (I–V) characteristics of p-i-n structures based on amorphous silicon (α-Si:H) with small hole diffusion lengths (shorter than the thickness of the i-layer of a p-i-n structure) have been experimentally studied with and without illumination. It is shown that forward I–V characteristics of structures of this kind can be described by a dependence inherent in diodes, with a diode ideality factor two-three times the maximum value of 2, theoretically predicted for generation-recombination currents in p-n junctions. The dark current is always substantially lower than the photocurrent in a cell biased with a voltage approximately equal to the opencircuit voltage of the photocell. Dark currents cannot contribute to the I–V characteristic under illumination. The photocurrent decreases with increasing photovoltage at a bias lower than the open-circuit voltage because of a decrease in the collection coefficient and the increasingly important role of back diffusion of electrons into the p-contact, rather than as a result of the dark injection. In the case of biases exceeding the open-circuit voltage, back diffusion becomes the predominant component of the current.     

Interaction of charge carriers with the localized magnetic moments of manganese atoms in <Emphasis Type="Italic">p</Emphasis>-GaInAsSb/<Emphasis Type="Italic">p</Emphasis>-InAs:Mn heterostructures

Transport properties of p-Ga_1?xIn_xAs_ySb_1?y/p-InAs:Mn heterostructures with undoped layers of solid solutions similar in composition to GaSb (x?0.22) grown by liquid-phase epitaxy on substrates with a Mn concentration of (5–7)×10¹⁸ cm^?3 are studied. It is ascertained that there is an electron channel at the interface (from the InAs side). The anomalous Hall effect and negative magnetoresistance are observed at relatively high temperatures (77–200) K. These phenomena can be attributed to the s-d-exchange interaction between Mn ions of the substrate and s electrons of the two-dimensional channel. The effective magnetic moment of Mn ions was evaluated as μ=200µ_B at T=77 K.     

Extending Nyberg construction on Galois fields of odd characteristic

As is known, the Nyberg design S-boxes possess the cryptographic properties valuable for practical application. Up to date this construction has been considered only for fields of characteristic 2. This paper presents an extension of the Nyberg construction to the fields of odd characteristic. The notion of nonlinearity distance of p-function is introduced, and the affine ternary code is built. The Nyberg design S-boxes with fields characteristic p = 3 for all lengths N ≤ 243 are built. The nonlinearity distances are calculated, and it is shown that with an increase of S-box length, these distances increase essentially faster as compared to the fields of characteristic p = 2.     

Higher azimuthal electromagnetic oscillations of <Emphasis Type="Italic">HE</Emphasis>-and <Emphasis Type="Italic">EH</Emphasis>-types in disc dielectric resonator

Features of higher azimuthal electromagnetic oscillations of “whispering” mode of HE-and EH-types in dielectric resonator of millimeter waveband are researched. Comparison of resonance characteristics of HE-and EH-types oscillations in such resonator is carried out.     

Simulation of high-power 8-mm band avalanche-oscillator diodes

The results of computer simulation of 8-mm band high-power avalanche-oscillator diodes (AOD) based on abrupt reverse biased p–n junctions with constant voltage have been presented. It is shown that AOD synchronously generate two oscillations in p- and n-regions of p–n junction, respectively. A technique is proposed for determining the parameters that ensure the diode operation in the mode of coherent oscillations. It is shown that the diode output power in this operation mode increases at the expense of summing-up the electron and hole components. The dynamic range of output power and the electronic efficiency are also determined.     

Superlattice conductivity under the action of a nonlinear electromagnetic wave

The influence exerted by a nonlinear electromagnetic wave on the dc conductivity of a superlattice is analyzed. An essentially nonlinear current-voltage (I-V) characteristic is obtained. A portion of negative absolute conductivity appears in the I-V characteristic at certain nonlinear-wave parameters. A distinction is made between the given case and that of a monochromatic wave, when a large number of such portions may exist. For typical superlattice parameters, some of the negative absolute conductivity must appear at a nonlinear wave field strength E₀≈1.8×10³ V/cm.     

Special features of the Fourier spectra of magnetoresistance oscillations in a heavily doped heterostructure

The Fourier spectra of oscillations of magnetoresistance in the heavily doped Al _x Ga_{1 ? x} As:Si/GaAs heterostructure, with filling of the two size-quantization subbands, the ground subband E _m and the excited subband E _p , are studied. In the concept of the probabilities of intrasubband and intersubband transitions involving the Landau levels, the origin of harmonics at the frequencies F _m ± F _p in the oscillations of magnetoresistance are interpreted. The ratio between the peak amplitudes A _m and A _p at the frequencies F _m and F _p of the harmonics is close to unity, with equal probabilities of the relations A _m /A _p > 1 and A _m /A _p < 1. At A _m /A _p > 1, the damping of the Landau quantization is controlled by the Coulomb potential, whereas at A _m /A _p < 1 it is controlled by relaxation of two-dimensional electrons due to the heterointerfacial roughness.     

Simulating jumps of the output power of a quasi-active microwave limiter based on <Emphasis Type="Italic">p</Emphasis>-i-<Emphasis Type="Italic">n</Emphasis> diodes

A program for time-domain simulation of a GaAs integrated power limiter based on two p-i-n diodes is developed. The system is simulated via numerical solution the problem with the initial conditions for a nonlinear system of equations that describes the electric circuit of the limiter. For a limiter with a 1.8 μm-thick i-layer and a frequency of 9.4 GHz, the dependence of the amplitude of the fundamental harmonic of the current passing through the diode on the amplitude of the fundamental harmonic of the voltage across this diode is shown to be S-shaped. This shape of the dependence for the detecting diode is the cause of jumps of the output power. Results of the simulation of a limiter with a 1.2-μm-thick i-layer for the frequencies 3.0, 9.4, and 25.0 GHz are presented. At the frequency 25 GHz, the amplitude modulation of the output signal is observed.     

ESR studies of nanocrystalline silicon films obtained by pulsed laser ablation of silicon targets

Nanocrystalline silicon films formed using laser ablation of silicon targets were studied using electron spin resonance. The measurements were performed in the X band with modulation of the magnetic field at a frequency of ～100 kHz at temperatures of 300 and 77 K. Two types of spectra were observed. The first type of spectra is related to the high concentration of dangling silicon bonds in Si nanocrystals and SiO_x sheaths of nanocrystals and are inherent in nanocrystalline silicon (nc-Si) films that do not exhibit photoluminescence in the visible region of the spectrum. The second type of spectra is related to the presence of E′ centers, nonbridging oxygen hole centers (NBOHC), and peroxide radicals and is characteristic of films with photoluminescence in the visible region of the spectrum, which indicates that high-barrier SiO₂ layers exist in these films. An increase in the photoluminescence intensity and a decrease in the signal of electron spin resonance were observed in porous nc-Si films exposed to atmospheric air for a long time.     

Effects of spatial reproduction as a result of the interference of electron waves in two-dimensional semiconductor nanostructures with parabolic quantum wells

Effects of spatial nonuniformity for the probability flux density j_x(x, z) or for the density of the quantum-mechanical current ej_x(x, z), where e is the elementary charge) are studied. These effects arise in two-dimensional semiconductor nanostructures that consist of thin rectangular and wide parabolic quantum wells that are consecutively arranged in the direction of the electron-wave propagation (the x axis) and are oriented along the dimensional-quantization axis z. A nonuniform distribution of j_x(x, z) arises as a result of interference of electron waves that propagate simultaneously in a wide quantum well over different quantum-dimensional subbands. Particular attention is paid to the effects of the spatial reproduction of electron waves in the nanostructures under consideration. It is shown that the transverse distribution j_x(0, z) existing at the entry to the wide quantum well is reproduced to a certain accuracy at a distance of X₁ from the entry. In addition, the initial distribution j_x(0, z) is reproduced periodically in the sections X_q = qX₁ (coefficients q are integers). The results of numerical calculations of magnitudes of these effects in the structures that are symmetric with respect to the z axis are reported; a modification of the effects under consideration in asymmetric nanostructures is considered.     

Resonance tunneling and nonlinear current in heterobarriers with complex law of carrier dispersion

New effects in resonance electron tunneling in a GaAs/Al_xGa_1?xAs/GaAs single-barrier heterostructure are analyzed with applied electric bias taken into account. Γ-X mixing of electron states at the interfaces is responsible for the Fano resonance in the barrier transmission coefficient. Trajectories of the Fano resonances and their interplay with the Breit-Wigner resonances in an electric field are studied. The current-voltage characteristic of the heterobarrier is calculated. A knowledge of the differential conductance makes it possible to obtain the Fano resonance profile and determine its parameters.     

Development of Millimeter Wave Fabry-Pérot Resonator for Simultaneous Electron-Spin and Nuclear Magnetic Resonance Measurement

We report a Fabry-Pérot resonator with spherical and flat mirrors to allow simultaneous electron-spin resonance (ESR) and nuclear magnetic resonance (NMR) measurements that could be used for double magnetic resonance (DoMR). In order to perform simultaneous ESR and NMR measurements, the flat mirror must reflect millimeter wavelength electromagnetic waves and the resonator must have a high Q value (Q?>?3000) for ESR frequencies, while the mirror must simultaneously let NMR frequencies pass through. This requirement can be achieved by exploiting the difference of skin depth for the two frequencies, since skin depth is inversely proportional to the square root of the frequency. In consideration of the skin depth, the optimum conditions for conducting ESR and NMR using a gold thin film are explored by examining the relation between the Q value and the film thickness. A flat mirror with a gold thin film was fabricated by sputtering gold on an epoxy plate. We also installed a Helmholtz radio frequency coil for NMR and tested the system both at room and low temperatures with an optimally thick gold film. As a result, signals were obtained at 0.18 K for ESR and at 1.3 K for NMR. A flat-mirrored resonator with a thin gold film surface is an effective way to locate NMR coils closer to the sample being examined with DoMR.     

Resonant Γ-<Emphasis Type="Italic">X</Emphasis> tunneling in single-barrier GaAs/AlAs/GaAs heterostructures

The electron transport through single-barrier GaAs/AlAs/GaAs heterostructures is studied. This transport is caused by resonant tunneling between the two-dimensional states related to the Γ valley of the GaAs conduction band and various two-or zero-dimensional donor states related to the lower X valleys of the AlAs conduction band. The resonant electron tunneling both via various two-dimensional states related to the X_z and X_xy valleys in AlAs (the X_z and X_xy states) and via related states of Si donors X _z ^D and X _xy ^D was observed. This circumstance made it possible to determine the binding energies of these states (E_B(X _z ^D )≈50 meV and E_B(X _xy ^D )≈70 meV, respectively) directly from the results of identification of resonance features in transport characteristics. An analysis of the structure of experimental resonances corresponding to tunneling between the Γ and X Landau levels in a magnetic field made it possible to determine the transverse effective mass in the X valleys of AlAs (m_t=(0.2±0.02)m₀). An additional fine structure of donor resonances is observed in experimental transport characteristics. This fine structure is caused by resonant tunneling of electrons through the states of the donors that are located in various atomic layers of the AlAs barrier (in the growth direction) and therefore have different binding energies.     

Analysis and calculation of current-flow amplitude spectrum of a microwave mixer diode at poly-harmonic excitation

A new approximating function in the form of a fifth-order polynomial is obtained for the current-voltage characteristic of a mixer diode with Schottky barrier and with compensation of potential barrier of p-n-junction. Mathematical expressions are set up and relative levels calculated for the amplitude spectrum of harmonic and combinative components of the current in a mixer diode excited by four independent signals with different frequencies. The conditions for minimum of the combinative components are formulated for a wide dynamic range of input signals.     

Temperature-Dependent Refractive Index of Quartz at Terahertz Frequencies

Characterisation of materials often requires the use of a substrate to support the sample being investigated. For optical characterisation at terahertz frequencies, quartz is commonly used owing to its high transmission and low absorption at these frequencies. Knowledge of the complex refractive index of quartz is required for analysis of time-domain terahertz spectroscopy and optical pump terahertz probe spectroscopy for samples on a quartz substrate. Here, we present the refractive index and extinction coefficient for α-quartz between 0.5 THz and 5.5 THz (17–183 cm^??1) taken at 10, 40, 80, 120, 160, 200 and 300 K. Quartz shows excellent transmission and is thus an ideal optical substrate over the THz band, apart from the region 3.9 ± 0.1 THz owing to a spectral feature originating from the lowest energy optical phonon modes. We also present the experimentally measured polariton dispersion of α-quartz over this frequency range.     

Symmetric stripline duplexer

The compact duplexer using the symmetric stripline and containing the fifth-order comb filters in transmit channel TX (2300–2370 MHz) and receive channel RX (2510–2580 MHz) is constructed, and the characteristics of obtained duplexer are measured. This duplexer is built using dielectric material Al₂O₃ (Alumina, polikor) with high thermal conductivity that makes it possible to use the operating power of 10Wat small dimensions 57-11.8-4 mm. The losses in bandpass of TX and RX filters did not exceed 3 dB during the filter attenuation at adjacent channel frequencies of no less than 60 dB. It is shown that the selection of the width of metallized strip at the base of quarter-wave resonators makes it possible to change the duplexer width for attaining the required value. The circuit of coupling of resonators with loads used in this design made it possible to obtain a sufficiently high level of isolation from each other for RX and TX channels of duplexer. This level does not exceed 60 dB. The finite thickness of internal conductors of stripline amounting to 16 μm is taken into account while building the duplexer that results in good agreement between the simulation and measurement results.