High-field Domain Research Articles

Formation Process of High-Field Domain in Superlattices Observed by Photoluminescence Spectra Branch

We report photoluminescence Stark-shift behaviors under various optical excitation intensity for a GaAs/AlAs superlattice. The study clarifies the formation process of the high-field domain by analyzing photoluminescence (PL) spectra. We observed changes in splitting feature of the PL spectra, by varying excitation carrier density near resonance voltages between the Γ states. This clarifies how the inner electric field in superlattices varies by increasing carrier density.

Domain formation in a one-dimensional superlattice

The dynamics of the temporal formation of high electric field domains in a one-dimensional superlattice miniband subject to inelastic phonon scattering is analyzed here. Our formulation is based on the derivation of equations of motion for generating functions of electron momentum fluctuations which take account of electron scattering by acoustic phonons, facilitating the determination of analytical expressions for both the electron drift velocity and the diffusion coefficient as functions of the applied bias voltage. The speed of domain formation and the minimum carrier concentration required are determined explicitly.

Origin of current instability in GaAs/AlGaAs heterostructures

We propose a mechanism to explain the electric instability often observed in modulation-doped heterostructures GaAs/AlGaAs when current is passed along the heterostructure layers. The instability is caused by hot electron transport in AlGaAs layer that is not only heavily doped, but also strongly compensated due to the presence of DX-centers. This layer contains a large-scale random potential of significant magnitude, which strongly affects electron transport. The heating of electrons in the percolation cluster net and electron transfer from the cluster into the random potential wells result in the appearance of latent negative differential conductivity causing the current instability. When the instability gives rise to the formation of a high electric field domain, one of the domain walls blocks the current flow through the two-dimensional electron gas. Experimental results supporting this mechanism are given.

Thermal breakdown, bistability, and complex high-frequency current oscillations due to carrier heating in superlattices

The impact of electron heating on vertical electrical transport in superlattices is shown to cause an S-shaped current–voltage characteristic in addition to the conventional N type occurring at lower fields. Our calculations are supported by experimental data. The combination of S- and N-type instabilities leads to a modified structure of the high-field domains associated with self-generated GHz oscillations.

SEM-動画システムによる半絶縁性GaAsの遅いドメインの観察

SEM-動画システムによる半絶縁性GaAsの遅いドメインの観察

Delay Time Measurements of NiS2—xSex-Based Switches

Delay time measurements in M|NiS 2-x Se x |M structures (M is a metal), exhibiting electrical switching with N-shaped V-I characteristics, are presented. The time was found to depend strongly on the applied voltage, tending to infinity at a voltage V near dc threshold voltage, and sharply decreasing as the pulse amplitude rises. For sufficiently high overvoltages the delay time falls in the range from about 10 μs to 1 ms. The formation of high electric field domains was observed. The possible influence of nonthermal, electronic effects on switching and on the metal-insulator transition in NiS 2-x Se x is discussed.

Magnetic Control of Electric-Field Domains in Semiconductor Superlattices

Applying magnetic fields up to $B\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}29\mathrm{T}$, we investigate the formation of electric-field domains in Landau-quantized semiconductor superlattices. Surprisingly, the electric-field distribution remains determined by the subband spacing rather than by the cyclotron energy. At the domain boundary, however, strong inter-Landau-level tunneling occurs, which leads to a magnetically tunable number of extra resonances in the current-voltage curve. In addition, we discuss a novel magnetic-field assisted high-field domain also stabilized by inter-Landau-level tunneling.

Photorefractive Gunn effect

We present and numerically solve a model of the photorefractive Gunn effect. We find that high-field domains can be triggered by phase-locked interference fringes, as has been recently predicted on the basis of linear stability considerations. Since the Gunn effect is intrinsically nonlinear, we find that such considerations give at best order-of-magnitude estimations of the parameters critical to the photorefractive Gunn effect. The response of the system is much more complex, including multiple wave shedding from the injecting contact, wave suppression, and chaos with spatial structure.

Formation mechanism of electric field domains

Hydrostatic pressure measurements are used to investigate the formation mechanism of electric field domains in doped weakly-coupled GaAs/AlAs superlattices. For the first plateau-like region in the I–V curve, two kinds of sequential resonant tunnelling are observed. For P<2 kbar the high-field domain is formed by the Γ–Γ process, while for P>2 kbar the high-field domain is formed by the Γ–X process. For the second plateau-like region, the high-field domain is attributed to Γ–X sequential resonant tunnelling.

Sequential resonant tunnelling through Landau levels in GaAs/AlAs superlattices

Electron transport in heavily-doped GaAs/AlAs superlattices in parallel electric and magnetic fields is reported. The current–voltage (I–V) characteristic exhibited the feature of negative differential velocity (NDV) and high electric field domain effect at different biases. Under strong magnetic fields, sequential resonant tunnelling through Landau levels in the negative differential velocity regime is observed, which are manifested as oscillations in the conductance–voltage characteristics.

High-gain and oscillatory transconductance by InGaAs/InAlAs multiple-quantum-well emitter bipolar transistor

We report an InGaAs/InAlAs multiple-quantum-well (MQW) emitter bipolar transistor prepared by molecular beam epitaxy. There are three distinct operating regimes to be observed in the studied structure. At small input base currents, low field band-type conduction provides the output current. The device exhibits a small gain and works as a normal transistor. With further increase in the base current, the high field starts appearing in the MQW superlattice. We observe that both the output current and transconductance exhibit an oscillatory behaviour in terms of sequential resonant-tunnelling through an expanding high field MQW domain. Beyond the condition of expansion of high field domain, the electron current increases rapidly by tunnelling through the triangular barrier and emitting over the base layer. The MQW superlattice now works as a barrier to hole minority carriers within this region. We obtain a common-emitter current gain as high as 240 with a small offset voltage of about 80 mV.

Nonlinear and oscillatory electronic transport in superlattices as a probe of structural imperfections

Abstract We present a theoretical analysis of the correlations between the macroscopic global transport properties and microscopic imperfections of a superlattice. High-field transport and domain formation is modelled using a microscopic quantum transport model which includes interface roughness and impurity scattering and describes resonant and nonresonant tunnelling processes in weakly coupled multiple quantum wells without adjustable parameters. Our analysis can be used on one hand to identify random fluctuations of the structural parameters in real samples like deviations from the perfect periodicity due to varying barrier widths, or doping densities in the individual wells. On the other hand, we demonstrate that the current–voltage characteristics can be tailored in a wide range to exhibit specific features like sharply rising steps at given voltages, arbitrarily modulated current maxima of multistable branches, or self-generated current oscillations.

Switching and control of electric field configurations along multi-quantum well structures

The phenomenon of high-field domain formation and expansion along a superlattice/multi-quantum well system has been known since the first pioneering works on these structures. Experimentally it has usually been observed by monitoring negative differential resistance oscillations in the electric characteristics (I-V curves) of the samples. It is due to the fact that a uniform field distribution becomes unstable with respect to the formation of high and low electric field domains. When increasing the applied field, one can observe the progressive inducement of additional quantum well periods into the high-field domain, at voltages correlated with the alignment energies and with the periods of the oscillations in the I-V characteristics. Recently, optical tools were used to probe, in a more direct way, the field distributions along such systems. These experiments can be performed using interband or intersubband spectroscopies. The behaviour of electric field configurations along superlattices/multi-quantum well structures is strongly linked to the availability of free-space charge. Heavy doping or strong illumination providing this charge will induce stable field configurations. A supply of charge which is less than needed may result in a spatial oscillation of space charge among different sites. The regime of lowest charge supply can produce transient field domains resulting in spontaneous current oscillations reminiscent of transferred electron devices and Gunn diodes; however, the negative differential resistance driving the effect originates from low-dimensional transport properties rather than from bulk inter-valley transfer. In addition to its importance for transport studies, the presence of space-charge accumulation at definite sites along a superlattice/multi-quantum well system, together with the presence of different field domains, raise the possibility of spatial control, localization and/or modulation of optical nonlinear effects along such structures. We review previous studies, which used intersubband spectroscopy to study field domain switching, and present new results on how to use photoluminescence quenching of interband transitions, under the application of an applied field, to further investigate domain formation and expansion in multi-stack samples.

High-field domain formation in selectively doped N-type GaAs/AlGaAs multiple quantum well structures

We have studied the sequential tunneling of electrons in selectively doped GaAs/AlGaAs tight-binding multiple quantum wells (MQWs) using two structures with different doping profiles; one with Si doping in the AlGaAs barriers and the other with Si doping in the GaAs wells. In both cases, the current–voltage (I–V) measurements showed two series of current oscillations which persisted up to 200 K. TheI–Vcharacteristics also showed a hysteresis effect at increasing and decreasing voltage sweep. The voltage separations between the adjacent current oscillations in theI–Vcharacteristics of the structures with ohmic and Schottky contact metallization give a good measure of the energy difference between the excited states and the ground state in the quantum well, with good agreement with theoretical calculations. The process of sequential electron tunneling through the MQW structures is discussed in terms of the creation and extension of high-field domains (HFDs) due to the applied bias.

An Algorithm to Develop Lumped Model for Gunn-Diode Dynamics

A nonlinear lumped model can be developed for Gunn-Diodes to describe the diffusion effects as the domain travels from cathode to anode of a Gunn-Diode. The model describes the domain extinction and nucleation phenomena. It allows the user to specify arbitrary nonlinear drift velocity V(E) and nonlinear diffusion D(E).The model simulates arbitrary Gunn-Diode circuits operating in any matured high field domain or in the LSA mode.Here we have constructed an algorithm to lead to development of this model

Current Self-Oscillations in Undoped, Photoexcited GaAs/AlAs Type-I Superlattices

Self-sustained current oscillations have been observed in undoped, photoexcited GaAs/AlAs type-I superlattices with different well and barrier thicknesses. The oscillations originate from a periodic motion of a space charge layer forming the boundary between low- and high-field regions of electric-field domains. When undamped photocurrent oscillations are detected, the high-field domain is found to originate from Γ–X resonant transfer. This observation implies that through the Γ–X resonance the recombination lifetime for electrons and holes is prolonged so that undamped photocurrent oscillations in the MHz regime can even be observed in undoped type-I superlattices.

Sequential resonant tunnelling through Landau levels in GaAs/AlAs superlattices

Electron transport in heavily doped GaAs/AlAs superlattices in parallel electric and magnetic fields is reported. The current - voltage characteristics exhibit features of negative differential velocity at the position of the lowest narrow miniband and high electric field domains at high biases. Under strong magnetic field, sequential resonant tunnelling through Landau levels in the negative differential velocity regime is observed, which is manifested as oscillations in the conductance - voltage characteristics.

Dynamics of Light-Triggered Gunn-Domain Structure: Domain Multiplication and Current Oscillations

In the present work we analyze the evolution of a periodical high-field domain structure, triggered by picosecond or nanosecond light pulses in biased GaAs and InP bulk crystals. We applied a full-hydrodynamic model as well as the Monte Carlo technique to calculate hot carrier transport of nonuniformly heated and spatially modulated carrier plasma. Time-resolved spatial profiles of carrier and internal electric field distribution revealed Gunn-domain splitting and its multiplication to new carrier and field gratings in microwave field. In dc field, coherent drift and fusion of Gunn-domain structure lead to oscillations of the total current which frequency depends on light intensity and modulation.

Negative differential conductance in strained Si point contacts and wires

The observation of negative differential conductance (NDC) in simple nanostructure geometries fabricated on high-mobility Si/SiGe strained-layer heterostructures is reported. The NDC is observed in the drain characteristic of etch-defined “point contacts” with lithographic width and length of 0.22, and 0.12 μm, respectively. Current peak-to-valley ratios as large as 2.0 are observed at T=1.3 K. The NDC is also observed in “wire” geometries as long as 19 μm, and can persist to temperatures as high as 83 K, with a minimum in the differential conductance observable at 103 K. The NDC in long wires is accompanied by the formation of a high-field domain at the drain end of the wire. The effect is only observed in laterally constricted geometries, but is not a result of quantum confinement or impurity-related trapping. We suggest that the NDC and the attendant domain formation are caused by phonon emission by hot electrons within the constricted geometry.

High-field Domain Formation Conditions in Semiconductor Multiple Quantum Well Sequential Resonant Tunneling Structures

We investigated the tunneling current-voltage ( I–V ) characteristics of Al0.3Ga0.7As/GaAs multiple quantum well (MQW) diodes with various electron densities, N s, and scattering rates, Γ. Clear periodic negative differential resistances (NDRs) due to the formation of high-field domains were observed for the samples with low N s and Γ, while such NDRs were completely absent in the first plateau region of the I–V characteristics of the samples with high N s and Γ. It was found that such formation/destruction of the high-field domains in MQW diodes is controlled by the interplay of the tunneling rate and the electron supply functions between adjacent coupled quantum wells. The condition for the high-field domain formation is clarified as a function of N s and Γ.