Resonant inelastic tunneling using multiple metallic quantum wells.

The quantum mechanical phenomenon of tunneling is of interest from fundamental as well as device points of views. Semiconductor microstructures, and in particular asymmetric double quantum well structures (a-DQWS), provide ideal systems for investigating tunneling phenomena, especially when an electric field is applied to bring various electronic levels into resonance. We first discuss various resonant and nonresonant tunneling processes for electrons and holes, including both coherent and incoherent resonant tunneling and then review recent extensive investigations of resonant and non-resonant tunneling rates of electrons and holes in asymmetric double quantum well structures using time resolved spectroscopy. We also show that cw and ultrashort pulse experiments probe different physics and that it is difficult to obtain quantitative information about resonances and resonant tunneling rates from cw experiments.

Sequential resonant and nonresonant tunneling have been investigated in doped GaAs/AlGaAs multiple quantum wells. Negative differential resistance attributable to high field domains expanding through the structure are observed. It is argued that the conductance mechanism is changing with increasing bias, from resonant ground-state tunneling in the low bias regime to nonresonant ground and ground to first exited state in the oscillating differential resistance regime. At higher bias the evolution from nonresonant to resonant tunneling is studied. Calculated tunneling currents are in good agreement with the measured currents both for on and off resonance.

We have demonstrated a new tunneling process: the direct creation of GaAs quantum well excitons through electron resonant tunneling. The two-particle nature of such a tunneling process makes it different from the ordinary one-particle (electron, hole, or exciton) tunneling process in resonant tunneling conditions and results in different $I$- $V$ characteristics. This resonant tunneling process may open a door toward electrically pumped excitonic cavity quantum electrodynamics and optoelectronic devices.

We review our comprehensive investigations of electron and hole tunneling times between quantum wells separated by a single barrier. These studies show strong electronic and phonon resonances, strong influence of collisions and relaxation on tunneling times and provide new insights into nonresonant and resonant tunneling in semiconductor microstructures.

In this work, we have investigated magneto-transport and polarization resolved photoluminescence of a GaAs/AlGaAs resonant tunneling diode with Si delta-doping at the center of the quantum well under a magnetic field parallel to the tunnel current. Three resonant peaks were observed in the current-voltage characteristics curve (J(V)) which were associated to donor-assisted resonant tunneling, electron resonant tunneling and to phonon-­assisted resonant tunneling. The optical emission from GaAs contact layers shows evidence of highly spin-polarized two-dimensional electron and hole gases which affect the spin-polarization of carriers in the well. The quantum well photoluminescence shows strong circular polarization degrees with values up to 85% under 15T at the donor assisted resonant tunneling peak voltage. Our results can be exploited for future development of voltage-controlled spintronics devices.

The band gap in photonic materials with periodic spatial modulation of refractive index greater than unity can actually be regarded as a potential barrier for photons. Similar to semiconductor quantum well systems due to the electronic band-gap mismatch, a photonic quantum well can be constructed by sandwiching a uniform medium between two photonic barriers. The transmission and reflection coefficients of light through the photonic quantum well are calculated by a modal expansion method with an R-matrix propagation algorithm. Resonance tunneling through the photonic quantum well structure is observed by varying either the well width or the frequency of incident light. Resonance peaks are found within the band-gap region, and indicate the existence of photon virtual states in the well.

A strong effect of a quantum well (QW) incorporated into a quantum dot (QD) structure on the density of states of the system and the efficiency of carrier transfer from the barrier material to QDs is revealed in InAs/GaAs–InGaAs/GaAs dot-well, tunnel-injection structures. When tuning the QW states in resonance with excited QD states, the carrier flux can be effectively controlled by varying the spacer thickness or barrier height. Enhanced carrier tunneling between QW and QD states is observed by means of photoluminescence excitation spectroscopy for reduced spacer thicknesses. Our results demonstrate that resonant coherent electron tunneling is substantially faster for the second than for the first QW subband and results in the formation of hybrid electronic states delocalized across the QW/QD interface.

Several techniques were proposed to achieve solid state spin filtering such as magnetic tunnel junctions comprised of half-metallic compounds or solid state Stern-Gerlach apparatus. Another alternative consists in using spin-dependent resonant tunneling through magnetically active quantum wells. Recent advances in molecular beam epitaxial growth made it possible to fabricate exotic heterostructures comprised of magnetic films or buried layers (ErAs, GaxMn1-xAs) integrated with conventional semiconductors (GaAs) and to explore quantum transport in these heterostructures. It is particularly interesting to study spin-dependent resonant tunneling in double-barrier resonant tunneling diodes (RTD) with magnetic elements such as GaAs/AlAs/ErAs/AlAs/ErAs/AlAs/GaAs, GaxMn1-xAs/AlAs/GaAs/AlAs/GaAs, and GaAs/AlAs/GaxMn1-xAs/AlAs/GaAs. We present the results of our theoretical studies and computer simulations of transmission coefficients and current-voltage characteristics of resonant tunneling diodes based on these double-barrier structures. Resonant tunneling of holes (GaxMn1-xAs-based RTDs) is considered. Our approach is based on k·p perturbation theory with exchange splitting effects taken into account. We analyze exchange splitting of different resonant channels as a function of magnetization as well as spin polarization of the transmitted current as a function of bias. We found that resonant tunneling I – V characteristics of the double-barrier magnetic hererostructures strongly depend on the doping level in the emitter as well as on the orientation of the magnetization.

The effect of the periodicity over the transmission property in semiconductor superlattices is theoretically investigated. It is found that the increasing the number of periods in GaAs/AlGaAs multiple quantum wells (MQW) introduces additional resonant states. it is also noted that the asymmetry of periodic potential profile is one key parameter in reaching high values of the transmission coefficient. These results can provide a new method to obtain filters in the submillimeter range.

Resonant tunneling through a single quantum well of GaAs has been observed. The current singularity and negative resistance region are dramatically improved over previous results, and detecting and mixing have been carried out at frequencies as high as 2.5 THz. Resonant tunneling features are visible in the conductance-voltage curve at room temperature and become quite pronounced in the I-V curves at low temperature. The high-frequency results, measured with far IR lasers, prove that the charge transport is faster than about 10−13 s. It may now be possible to construct practical nonlinear devices using quantum wells at millimeter and submillimeter wavelengths.

The Breit–Wigner formula is exploited to calculate the tunneling transmission probability affected by longitudinal optical phonon scattering in multiple quantum wells composed of Ga1−xAlxAs compounds. A quantitative result shows that the resonance electron energy and the integrated total transmission probability through the potential barriers are not changed, while the transmission peak is reduced and the linewidth is broadened.

Theoretical and experimental studies of photoemission from Al quantum wells on Si( [formula omitted

In this chapter, we consider a three-dimensional waveguide that, far from the coordinate origin, coincides with a cylinder G containing the axis x. The cross-section of G is a two-dimensional domain (of an arbitrary form) with smooth boundary. The waveguide has two narrows of small diameter \(\varepsilon \). The waveguide part between the narrows plays the role of a resonator and there can arise conditions for electron resonant tunneling. We take \(\varepsilon \) as small parameter and obtain asymptotic formulas for characteristics of the resonant tunneling as \(\varepsilon \rightarrow 0\).

An asymmetric double quantum wells (QWs) structure with resonant tunneling is suggested to achieve high efficient four wave mixing (FWM). We analytically demonstrate that the resonant tunneling can induce high efficient mixing wave in such a semiconductor structure with a low light pump wave. In particular, the FWM conversion efficiency can be enhanced dramatically in the vicinity of the center frequency. This interesting scheme may be used to generate coherent long-wavelength radiation in solid-state system.

A new type of field emission resonant tunneling diode has been proposed and investigated both theoretically and experimentally. The diode is based on an Si–SiOx–Si multilayer cathode containing an SiOx layer as the input potential barrier, an Si layer as the quantum well, and a vacuum layer as the output potential barrier of a double barrier quantum structure. The calculation predicted the existence of the resonant maxima (three or four depending on the input barrier height) of the current density–electric field dependencies. Frequency dependencies of the diode microwave impedance pointed to the existence of negative conductance resulting from resonant tunneling through an energy level in the quantum well (QW) at electron transit angle values in the interval from zero up to near 2π/3. Also found is a peak of negative conductance on the frequency dependence with the greatest peak value ∼57 S/cm at a frequency of 0.63 THz. The maximum upper frequency of the negative conductance band of more than 2 THz at a transit angle near 0.45π is reached when resonant tunneling occurs through the third resonant level in the QW. Experimental results confirmed the existence of the resonance peak in the investigated resonant-tunneling structure.