Discrete electroluminescence lines in sub-micron p-i-n resonant tunnelling diodes

We have fabricated GaAs/AlAs p-i-n double barrier resonant tunnelling diodes with active lateral dimensions down to 0.25μn² using optical lithography and wet etching. Many devices have been investigated and systematic variations in the quantum well emission have been observed as the device size is decreased. We observe a red shift of the quantum well recombination lines. In addition a new line is observed at lower energy in the spectra of the smallest devices. The quantum well luminescence efficiency is found to be constant down to the smallest device size.     

PT-symmetry breaking in resonant tunneling heterostructures

We present fermionic model based on symmetric resonant tunneling heterostructure, which demonstrates spontaneous symmetry breaking in respect to combined operations of space inversion (P) and time reversal (T). PT-symmetry breaking manifests itself in resonance coalescence (collapse of resonances). We show that resonant energies are determined by eigenvalues of auxiliary pseudo-Hermitian PT-invariant Hamiltonian.     

Spin-dependent resonant tunneling in ZnSe/ZnMnSe heterostructures

Using the transfer matrix method and the effective-mass approximation, the effect of resonant states on spin transport is studied in ZnSe/ZnMnSe/ZnSe/ZnMnSe/ZnSe structures under the influence of both electric and magnetic fields. The numerical results show that the ZnMnSe layers, which act as spin filters, polarize the electric currents. Variation of thickness of the central ZnSe layer shifts the resonant levels and exhibits an oscillatory behavior in spin current densities. It is also shown that the spin polarization of the tunneling current in geometrical asymmetry of the heterostructure where two ZnMnSe layers have different Mn concentrations, depends strongly on the thickness and the applied bias.     

Zeeman splitting of single semiconductor impurities in resonant tunneling heterostructures

Zeeman splitting of the ground state of single impurities in the quantum wells of resonant tunneling heterostructures is reported. We determine the absolute magnitude of the effective magnetic spin splitting factorg* for a single impurity in a 44 Å Al_0.27Ga_0.73As/GaAs/Al_0.27Ga_0.73As quantum well to be 0.28±0.02. This system also allows for independent measurement of the electron tunneling rates through the two potential barriers and estimation of the occupation probability of the impurity state in the quantum well.     

Interband resonant tunneling in superconductor heterostructures in a quantizing magnetic field

The resonant tunneling of electrons through quasistationary levels in the valence band of a quantum well in double-barrier structures based on III–V materials with type-II heterojunctions is considered in a quantizing magnetic field directed perpendicularly to the interfaces. The transmission coefficients of the tunnel structure for transitions from states corresponding to different Landau levels are calculated using the Kane model. It is shown that transitions with a unit change in the Landau level index n as a result of mixing of the wave functions of states with opposite spin orientations are possible on the interfaces due to spin-orbit coupling. The probability of such transitions can be comparable to the probability of transitions without a change in the Landau level index for InAs/AlGaSb/GaSb resonant-tunneling structures. Fiz. Tverd. Tela (St. Petersburg) 40, 2121–2126 (November 1998)     

Nonlinear quantum mode of terahertz electromagnetic wave amplification in resonant tunneling heterostructures

An analysis of high-frequency properties of the resonant tunneling diode (RTD) in a strong microwave electromagnetic field showed that the high-frequency current response increasing with the microwave power significantly more rapidly saturates out in the case of classical amplification mode, than in the case of “quantum” amplification mode. This makes the “quantum” mode even more attractive in comparison with the classical mode from the viewpoint of the possibility of amplification and generation in the range of subterahertz and terahertz frequencies and offers new opportunities to advance towards these frequencies.     

Time dependence of resonant tunneling in heterostructures with a three-trough energy spectrum

The time dependence of resonant electron tunneling inGaAs andAlAs quantum heterostructures is studied for a three-trough model. From an analysis of transmitted and reflected wave phases, the spectra of tunneling and reflection times are obtained. The tunneling of a wave packet through a two-barrier heterostructure is modeled by numerical solution of the nonstationary Schrödinger equation.     

Testing of resonant tunneling double barrier heterostructures by BEEM/BEES

This work shows how ballistic electron emission microscopy and spectroscopy (BEEM/BEES) can be used for study of the tunneling double barrier heterostructures. From positions of resonant levels follows that roughness at the interfaces is present. The variation of the thickness of the well atributed to the existed roughness is estimated using simple calculation. Presented at the 1st Czech-Chinese Workshop “Advanced Materials for Optoelectronics”, Prague, Czech Republic, June 13–17, 1998. This work is partly supported by the Grant Agency of the Czech Republic under grant No. 102/97/0427.     

Far infrared electroluminescence in cascade type-II heterostructures

Far infrared radiation from type-II heterostructures is observed. The samples used for the observations have the form of ten-period cascade structures, each period containing InAs and AlGaAsSb quantum wells separated by a variband barrier. The results of studying the vertical transport and the electroluminescence testify that the radiation is caused by transitions between the electron and hole states existing in the adjacent quantum wells.     

Monolithically integrated chirp-managed laser based on a resonant tunneling filter

We have proposed a monolithically integrated chirp-managed laser (CML) that consists of a directly modulated single-mode DFB laser and an optical spectrum reshaper (OSR) filter based on a double-slanted-trench resonant tunneling structure (DST-RTS). Slanted trenches facilitates the occurrence of resonant tunneling effect, which produces a steep-edge narrow-band OSR filter, and meanwhile directing most of the reflected waves out of the laser cavity, consequently eliminating the need of an isolator. Characteristics of the DST-RTS filter have been investigated and simulation results show that the proposed 25 Gbps 1.55 μm CML can send signal over 22 km standard single-mode fiber for bit error rate of 10^?12.     

Multiple wavelength generation using aperiodically poled lithium niobate

We present a simple method to design aperiodically poled crystals to obtain multiple nonlinear optical processes simultaneously. We use this technique to design and make an aperiodically poled lithium niobate crystal with which we obtained eight distinct sum-frequency generation processes, and another sample with which we obtained four distinct optical parametric generation processes. We also show that the peak effective nonlinearities of aperiodic crystals designed with this technique are larger and their widths are narrower than what can be obtained with a crystal of the same length with separate sections with well-defined poling periodicities.     

Two-color laser wavelength effect on intense terahertz generation in air

Near-IR femtosecond lasers have been proposed to produce high-field terahertz radiation in the air via the laser-plasma interaction, but the physical mechanism still needs to be further explored. In this work, we theoretically investigate the effect of the two-color laser wavelength on the terahertz generation in the air based on a transient photocurrent model.We show that the long wavelength laser excitation can greatly enhance the terahertz amplitude for a given total laser intensity. Furthermore, we utilize a local current model to illustrate the enhancement mechanism. Our analysis shows that the terahertz amplitude is determined by the superposition of contributions from individual ionization events, and for the long wavelength laser excitation, the electron production concentrates in a few ionization events and acquires the larger drift velocities, which results in the stronger terahertz radiation generation. These results will be very helpful for understanding the terahertz generation process and optimizing the terahertz output.     

Resonant tunneling in double superlattice barrier heterostructures

We have investigated resonant tunneling in double barrier heterostructures in which the tunnel barriers have been replaced by short period superlattices, and have shown for the first time quantum well confinement in a single quantum well bounded by superlattices. These results also demonstrate the first utilization of short period binary superlattices as effective tunnel barriers to replace the conventional Al_xGa_1−xAs barriers. The superlattice structure does not exhibit the asymmetry around zero bias in the electrical characteristics normally observed in the conventional Al_xGa_1−xAs barrier structures, suggestive of reduced roughness at the inverted interface by superlattice smoothing. The superlattice barrier also exhibits an anomalously low barrier height. The performance of this symmetric superlattice structure is compared with an intentionally constructed asymmetric double barrier superlattice structure, which exhibits pronounced asymmetry in the electrical characteristics. The observed behavior supports the view that resonant enhancement occurs in the quantum well.     

Subband current in resonant tunneling diode

An accumulation layer is formed on the emitter side of a biased resonant tunneling diode (RTD) leading to a similar subband structure as in the ordinary MOS-system. Electrons occupying the subbands can tunnel through the RTD-structure and give rise to a significant contribution to the diode current. We calculate the subband current from our semiclassical transport model developed earlier for the ordinary tunneling current. The model includes quantum interference and bulk scattering by utilizing an optical approximation for the coherent part of the wave function. The subband current turns out to be of the same order of magnitude as the ordinary tunneling current component. It is shifted to higher voltages and therefore it increases the valley current. In order to reduce the subband current and improve the peak-to-valley current ratio (PVCR), we propose a novel RTD-structure with a grading in front of the emitter barrier. The purpose of the grading is to suppress the formation of the accumulation layer and thereby decrease the valley current. Calculations show that PVCR increases by a factor of two using a proper design of the grading.