Studies on the high-frequency properties of 〈111〉, 〈110〉 and 〈100〉 oriented GaAs IMPATT diodes

High-frequency analysis has been carried out to predict the rf performance of 111, 110 and 100 oriented p ⁺ nn ⁺, n ⁺ pp ⁺ (single drift region) and n ⁺ npp ⁺ (double drift region) GaAs IMPATT diodes for opertion at 35 and 60 GHz. The microwave performance is observed to be highly sensitive to crystal orientation in case of p ⁺ nn ⁺ and n ⁺ npp ⁺ diodes whereas orientation of the substrate has negligible effect on n ⁺ pp ⁺ avalanche diodes. The calculation shows that 111 oriented GaAs IMPATT diode would provide the largest magnitude of negative resistance and negative conductance for both SDR p ⁺ nn ⁺ and DDR n ⁺ npp ⁺ diodes which indicates that high microwave power with high conversion efficiency can be realised from these 111 oriented GaAs devices. This result can be explained from the experimental data of electron and hole ionization rates for different orientations in GaAs.     

Invisibility cloaking in light‐scattering media

A major aim of researchers working in the field of optics and photonics is to mold the flow of light in optical structures and devices. In the regime of ballistic light propagation, transformation optics has given a certain boost, for which optical invisibility cloaking devices are striking examples. Our capability to mold the flow of light in the regime of diffuse light propagation in light‐scattering media has fallen behind—while diffuse light from clouds, white wallpaper, computer monitors, and light‐emitting diodes is literally all around us every day. In this review, we summarize progress in steering the flow of diffuse light in turbid media which was triggered by the mathematical analogy between electrostatics, magnetostatics, stationary heat conduction, and stationary light diffusion. We give an extensive tutorial introduction to the mathematics of the diffusion equation for light and its solutions, present an overview on the current experimental state‐of‐the‐art of simple core–shell invisibility cloaking, and compare these experiments with diffusion theory as well as with more advanced modelling based on Monte Carlo simulations. The latter approach enables spanning the bridge from diffusive to ballistic light propagation.

     

A novel design approach for the epitaxial layer for 4H-SiC and 6H-SiC power bipolar devices

The paper evaluates the optimal design of the low-doped base region inside power diodes and other bipolar devices. It is demonstrated theoretically that a low-doped base region of P⁺N⁻N⁺ diodes can provide a high breakdown voltage and an optimal on-resistance . A simple, accurate and CPU timesaving approach is presented to extract an optimal value for the base region width, W_B, and its doping concentration, N_D. The paper details an analytical relation between W_B and N_D, and gives a method for quantifying the trade-off between their values for a given breakdown voltage and for obtaining the minimal on-resistance. Analytical results are confronted with experimental results for 4H-SiC- and 6H-SiC-based diodes.     

Spin Relaxation in the quantized Hall regime in the presence of disorder

We study the spin relaxation (SR) of a two-dimensional electron gas in the quantized Hall regime and discuss the role of spatial inhomogeneity effects on the relaxation. The results are obtained for small filling factors ν?1) or when the filling factor is close to an integer. In either case, SR times are essentially determined by a smooth random potential. For small n, we predict a “magneto-confinement” resonance manifested in the enhancement of the SR rate when the Zeeman energy is close to the spacing of confinement sublevels in the low-energy wing of the disorder-broadened Landau level. In the resonant region, the B-dependence of the SR time has a peculiar nonmonotonic shape. If ν?2n+1, the SR is going nonexponentially. Under typical conditions, the calculated SR times range from 10^?8 to 10^?6 s.     

Statistical mechanics of probabilistic cellular automata

We investigate the behavior of discrete-time probabilistic cellular automata (PCA), which are Markov processes on spin configurations on ad-dimensional lattice, from a rigorous statistical mechanics point of view. In particular, we exploit, whenever possible, the correspondence between stationary measures on the space-time histories of PCAs on ^d and translation-invariant Gibbs states for a related Hamiltonian on ( ^d+1). This leads to a simple large-deviation formula for the space-time histories of the PCA and a proof that in a high-temperature regime the stationary states of the PCA are Gibbsian. We also obtain results about entropy, fluctuations, and correlation inequalities, and demonstrate uniqueness of the invariant state and exponential decay of correlations in a high-noise regime. We discuss phase transitions in the low-noise (or low-temperature) regime and review Toom's proof of nonergodicity of a certain class of PCAs.     

Quantum oscillations in confined and degenerate Fermi gases. II. The phase diagram and applications of half-vicinity model

For part I see DOI: 10.1016/j.physleta.2018.02.006. Size and density dependent quantum oscillations appear in Fermi gases under strong confinement and degeneracy conditions. We provide a universal recipe that explicitly separates oscillatory regime from non-oscillatory (stationary) one. A phase diagram representing stationary and oscillatory regimes on degeneracy-confinement space is proposed. Analytical expressions of phase transition interfaces are derived. The critical point, which separates entirely stationary and oscillatory regions, is determined and its dependencies on aspect ratios are examined for anisometric domains. Accuracy of the half-vicinity model and the phase diagram are verified through the quantum oscillations in electronic heat capacity and its ratio to entropy.     

Analysis of integrated thyristor switching-off by a reverse gate pulse current

To increase the maximum power current density of an integrated n⁺p'Nn'p⁺-type thyristor during switching-off by a current pulse in the control circuit, the injection of electrons from the n⁺ emitter should be interrupted before the recovery of the collector p'N junction. This has been done using a rapidly increasing reverse gate current pulse with an amplitude equal to the amplitude of the power switched-off current. After the interruption of the emitter injection, the remaining current through the device is the current of holes extracted from the collector region via the gate electrode. Like in insulated gate bipolar transistors (IGBTs), the physical mechanism that limits the maximum density of the switch-off current is the dynamic avalanche breakdown, which is initiated by the holes extracted through the space charge region of the collector p'N junctions.     

Random time averaged diffusivities for Lévy walks

We investigate a Lévy walk alternating between velocities ±v ₀ with opposite sign. The sojourn time probability distribution at large times is a power law lacking its mean or second moment. The first case corresponds to a ballistic regime where the ensemble averaged mean squared displacement (MSD) at large times is ?x ²? ∝ t ², the latter to enhanced diffusion with ?x ²? ∝ t ^ν, 1 < ν < 2. The correlation function and the time averaged MSD are calculated. In the ballistic case, the deviations of the time averaged MSD from a purely ballistic behavior are shown to be distributed according to a Mittag-Leffler density function. In the enhanced diffusion regime, the fluctuations of the time averages MSD vanish at large times, yet very slowly. In both cases we quantify the discrepancy between the time averaged and ensemble averaged MSDs.     

Origin of the defect states at ZnS/Si interfaces

Electrical characterisation of silicon surfaces contaminated by a zinc-sulphide overlayer has been carried out by forming Schottky diodes on the silicon after the ZnS has been etched off. The techniques include current-voltage, capacitance-voltage, and deep-level transieni spectroscopy. The Schottky diodes show clear memory of the presence of the ZnS overlayer and the electrical characteristics are far from ideal. Five deep levels in the sub-surface region of the silicon are detected, corresponding to the Zn⁺, Zn⁺⁺, S^–, S^–– states and probably to a Zn–B complex (p-type). Diffusion of the zinc and sulphur into the silicon is therefore confirmed and this diffusion is thought to create a compensated layer at the interface. These impurity states control the electrical characteristics of the surface in these diodes.     

Random response time of thin avalanche photodiodes

The avalanche built-up time using random response time model for avalanche photodiode (APD) is presented. A random response time model considers the randomness of times at which the primary and secondary carriers exit the multiplication region. The dead-space effect is included in our model to demonstrate its effect on response time of APDs especially for the thin devices. Our results show that feedback impact ionisation process and dead-space prolong the response time in APDs. The time response of homojunction InP p⁺-i-n⁺ diodes with the multiplication region of 0.281, 0.582 and 1.243 m are calculated.     

Powerful diode nanosecond current opening switch made of <Emphasis Type="Italic">p</Emphasis>-silicon (<Emphasis Type="Italic">p</Emphasis>-SOS)

The nanosecond semiconductor diode-based opening switch (SOS-diode) capable of switching currents with densities up to several tens of kiloamperes per cubic centimeter represents a p⁺p’Nn⁺ silicon structure fabricated by the deep simultaneous diffusion doping (to about 200 μm) of n-Si by Al and B from one side and P from the other. In the SOS mode, first a short pulse of forward current passes through the diode and then a fast-growing pulse of reverse voltage is applied. A resulting pulse of reverse current carries away injected holes and thereby forms a plasma front in the p’ layer, which moves toward the p’N junction. When the hole concentration in the flow exceeds the dopant concentration in the p’ layer, a space charge region arises in this layer, the resistivity of the diode increases sharply, and the current switches to a load connected parallel to the diode. Early results concerning an alternative configuration of the SOS diode are presented. Here, the diode was made by the rapid simultaneous diffusion of B and P from the opposite sides of a p-Si wafer to a depth of 60-80 μm. If a short pulse of forward current is passed through such a p⁺pn⁺ structure and a pulse of reverse voltage is then applied, a plasma front arising in the p⁺ region moves toward the p⁺p interface through the heavily doped (i.e., low-resistivity) p⁺ region. Having crossed this interface, the front passes into a low-doped region, where the hole concentration in the flow becomes much higher than the dopant concentration and a space charge region causing the current to pass to the load forms at once. It is shown experimentally that, all other things being the same, the time of current breaking in the p-SOS-diode is roughly twice as short as in the conventional n-SOS-diode, switched currents are considerably lower, and the fabrication technique of p-SOS-diodes is much simpler. Ways of optimizing the design of the semiconductor structure of the p-SOS-diode to further raise the speed are outlined.     

Gallium arsenide avalanche-type S-diode based on a n+-π-ν-n structure

The results are presented of an investigation of the electrical characteristics of avalanche S-diodes based on a new type of structure (⁺---n), which is obtained by diffusing iron in GaAs with n = 1.10¹⁷ cm^–3. The diodes have regions of S-type negative differential resistance (NDR), for both bias polarities and have a number of other features by comparison with diodes based on the --n structures with fused contacts. We discuss the mechanism for the formation of the NDR region of the voltage-current characteristics of the diode.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 54–58, April, 1986.     

Systematically too low values of the cranking model collective inertia parameters

Deformed Nilsson and Woods-Saxon potentials were employed for generating single particle states used henceforth for calculating the inertia tensor (cranking model and monopole pairing) and the collective energy surfaces (Strutinsky method). The deformation was parametrized in terms of quadrupole and hexadecapole degrees of freedom. The classical energy expression obtained from the inertia tensor and energy surfaces was quantized and the resulting stationary Schrödinger equation was solved using the approximate method. The secondI ^π=0 ₂ ⁺ collective level energies were calculated for the Rare Earth and Actinide nuclei and the results compared with the experimental data. The vibrational level energies agree with the experimental ones much better for spherical nuclei for both single particle potentials; the discrepancies for deformed nuclei overestimate the experimental results by roughly a factor of two. It is argued that coupling of the axially symmetric quadrupole degrees of freedom to non-axial and hexadecapole ones does not affect the conclusions about systematically too low mass parameter values. The alternative explanation of the systematic deviations from the 0 ₂ ⁺ level energies could be a systematically too high stiffness of the energy surfaces obtained with the Strutinsky method.     

Transition to ballistic regime for heat transport in helium II

The size-dependent and flux-dependent effective thermal conductivity of narrow capillaries filled with superfluid helium is analyzed from a thermodynamic continuum perspective. The classical Landau evaluation of the effective thermal conductivity of quiescent superfluid, or the Gorter–Mellinck regime of turbulent superfluids, is extended to describe the transition to ballistic regime in narrow channels wherein the radius R is comparable to (or smaller than) the phonon mean-free path ?   in superfluid helium. To do so, we start from an extended equation for the heat flux incorporating non-local terms, and take into consideration a heat slip flow along the walls of the tube. This leads from an effective thermal conductivity proportional to R²$R^2$ (Landau regime) to another one proportional to R? (ballistic regime). We consider two kinds of flows: along cylindrical pipes and along two infinite parallel plates.     

Hot carrier effects in double injection phenomena

Carrier heating is shown to be responsible for unusualI(V) characteristics observed in small-sizep ⁺ nn ⁺ silicon on sapphire (SOS) devices. The classical quadratic law of the semiconductor regime becomes linear for high fields. The influence of dimensions and doping is experimentally checked and a model, based on the regional approximation method, is proposed. The key role is assumed by the hot-carrier region, growing from the cathode, where the electron and hole mobilities are field dependent: µ~E^–. A full agreement with the experiment in SOS is found for=0.5. The operating of hot carrierp ⁺ nn ⁺ devices can be described concretely with usual formalism by using the concept of effective carrier mobilities, which depend on the applied voltage.     

Structural changes during Ar-ion irradiation of laser-deposited Fe/Ag multilayers

During 350 keV Ar⁺ irradiation at 77K, laser-deposited Fe–Ag multilayers first show stress relaxation and demixing processes at the interfaces followed by grain coarsening and a supersaturation of the bcc -Fe phase with Ag due to ballistic mixing. At high fluence, the fcc -Fe(Ag) phase (a=3.65 Å) is formed, which can be explained by either chemically guided ballistic short-range relocations or by the occurrence of thermal spikes, where all atoms possess sufficient energy to allow collective structural rearrangements, but only during such a short time that a decomposition due to long-range diffusion is suppressed.     

Tunneling and injection in ferromagnetic structures InGaAs/GaAs/(Ga,Mn)As and InGaAs/<Emphasis Type="Italic">n</Emphasis> <Superscript>+</Superscript>-GaAs/(Ga,Mn)As

The spin light-emitting diodes based on InGaAs/GaAs heterostructures with a quantum well and an injector in the form of a (Ga,Mn)As ferromagnetic layer have been studied. It has been demonstrated that the efficiency of electron spin injection in the structure with a (Ga,Mn)As/n⁺-GaAs tunneling barrier can be controlled by varying the parameters of n⁺-GaAs. The spin injection control mechanisms associated with the thermal activation and tunneling of carriers have been discussed.     

The Structure of Radiative Tunnel Recombination Sites in Emulsion Microcrystals of AgBr(I)

To identify the structure of emissive tunnel recombination sites in the emulsion microcrystals of silver bromide AgBr(I) with iodine contaminations and to determine the role of an emulsion medium in their formation, the temperature dependence of the luminescence spectra in the range from 77 to 120 K, the kinetics of the growth of the maximum luminescence intensity value at λ ≈ 560 nm, and the luminescence flash spectrum stimulated by the infrared light are investigated. Two types of the AgBr_{1 – x}(I _x ) (x = 0.03) microcrystals—namely, obtained in an aqueous solution and on a gelatin substrate—are used in the studies. It is established that the emissive tunnel recombination sites with a luminescence maximum at λ ≈ 560 nm in AgBr_{1 – x}(I _x ) (x = 0.03) are the {(I _a ^- I _a ^- )Ag _i ⁺ } donor–acceptor complexes with the I _a ^- iodine ions located in neighbor anionic sites of the AgBr(I) crystal lattice, next to which the Ag _i ⁺ interstitial silver ion is positioned. With an increase in the temperature, the {(I _a ^- I _a ^- )Ag _i ⁺ } sites undergo structural transformation into the {(I _a ^- I _a ^- )Ag_in⁺} sites, where n = 2, 3, …. Moreover, the {(I _a ^- I _a ^- )Ag _in ⁺ } sites (n = 2) after the capture of an electron and hole also provide the tunnel recombination with a luminescence maximum at λ ≈ 720 nm. The influence of an emulsion medium consists in that gelatin interacts with the surface electron-localization sites, i.e., the interstitial silver ions Ag _in ⁺ , n = 1, 2, and forms the complexes {Ag _in ⁰ G⁺} (n = 1, 2) with them. The latter are deeper electron traps with a small capture cross section as compared to the Ag _in ⁺ sites (n = 1, 2) and that manifest themselves in that the kinetics of the luminescence growth in AgBr(I) to a stationary level at λ ≈ 560 nm is characterized by the presence of “flash firing.” At the same time, the luminescence flash stimulated by IR light, for which the Ag _in ⁺ (n = 1, 2) electron-localization sites are responsible, is absent. It is supposed that the electrons localized on the {Ag _in ⁺ G⁺} complexes (n = 2) retain the capability for emissive tunnel recombination with holes localized on paired iodine sites with a luminescence maximum at λ ≈ 750 nm.     

Nonlinear dynamics in wurtzite InN diodes under terahertz radiation

We carry out a theoretical study of nonlinear dynamics in terahertz-driven n⁺nn⁺ wurtzite InN diodes by using time-dependent drift diffusion equations. A cooperative nonlinear oscillatory mode appears due to the negative differential mobility effect, which is the unique feature of wurtzite InN aroused by its strong nonparabolicity of the Γ₁ valley. The appearance of different nonlinear oscillatory modes, including periodic and chaotic states, is attributed to the competition between the self-sustained oscillation and the external driving oscillation. The transitions between the periodic and chaotic states are carefully investigated using chaos-detecting methods, such as the bifurcation diagram, the Fourier spectrum and the first return map. The resulting bifurcation diagram displays an interesting and complex transition picture with the driving amplitude as the control parameter.     

Saturation behavior of extrinsic photoconductivity in GaP light emitting diodes at high infrared intensities

Saturation of extrinsic photoconductivity in GaP:N(Zn, Te) diodes could be achieved by excitation with a TEA-CO₂-laser. At wavelengths in the 10 m range intensities of several 100 kW/cm² being near the damage threshold were applied. Carrier lifetimes of 60 ps at 4.2 K and 200 ps at 77 K could be estimated. The only conceivable mechanism explaining these short time constants is the capture of infrared excited holes by ionized shallow acceptors in the highly compensated p-side of the diode.