A. Makosa

Deep-level defects at lattice-mismatched interfaces in GaAs-based heterojunctions

Electrical properties of lattice-mismatch-induced defects in GaAs/GaAsSb and GaAs/InGaAs heterojunctions have been studied by means of an electron-beam-induced current (EBIC) in a scanning electron microscope and deep-level transient spectroscopy (DLTS). DLTS measurements, carried out with p-n junctions formed at the interfaces, revealed one electron trap and two hole traps induced by the lattice mismatch. The electron trap, at about Ec-0.68 eV, has been attributed to electron states associated with threading dislocations in the ternary compound. By comparing the concentration of this trap, revealed by DLTS, with EBIC results on the diffusion length, obtained for heterojunctions with different lattice mismatches, it is inferred that the minority-carrier lifetime is controlled by dislocations in the epilayer region close to the interface. Two new hole traps have been ascribed to defects associated with the lattice-mismatched interface of the heterostructures.

Remnant magnetoresistance in ferromagnetic (Ga,Mn)As nanostructures

Magneto-resistive nanostructures have been investigated. The structures were fabricated by electron beam lithography patterning and chemical etching from thin epitaxial layers of the ferromagnetic semiconductor (Ga,Mn)As, in shape of three nanowires joined in one point and forming three-terminal devices, in which an electric current can be driven through any of the three pairs of nanowires. In these devices, a novel magneto-resistive memory effect has been demonstrated, related to a rearrangement of magnetic domain walls between different pairs of nanowires in the device consisting in that its zero-field resistance depends on the direction of previously applied magnetic field. The nanostructures can thus work as two-state devices providing basic elements of nonvolatile memory cells. (Less)

Capture kinetics at deep-level defects in MBE-grown CdTe layers

The results of deep-level transient spectroscopy (DLTS) investigations in n-type CdTe layers grown by the molecular-beam epitaxy (MBE) technique on lattice-mismatched GaAs substrates are described. Three electron traps and one hole trap, at rather low concentrations of the order of 1013 cm−3, have been revealed in the DLTS spectra measured under various bias conditions of Schottky diodes prepared on the as-grown CdTe layers. One of the electron traps has been attributed to electron states of dislocations on the ground of the logarithmic capture kinetics for capture of electrons into the trap states. The other three traps, displaying exponential capture kinetics, have been attributed to native point defects produced during the epitaxial growth of CdTe. The microscopic nature of the defects responsible for the traps is discussed taking into account recent results of first-principles calculations of the properties of dominant native defects in CdTe.

Anisotropic electric-field-enhanced electron emission from deep-level defects in GaAs

Strongly anisotropic electric-field enhancements of the thermal emission rates of electrons from the EL3 and EL5 deep-level defects in n-type GaAs crystal have been revealed with the double-correlation deep-level transient spectroscopy. The results, analysed by taking into account both the Poole?Frenkel and phonon-assisted tunnel effects, evidence a strong coupling of the defects to the lattice vibronic modes. The defect potential anisotropy of EL3 is consistent with the defect identification as an off-centre substitutional oxygen on the arsenic site. The revealed surprising break-down of the emission-rate enhancement, for the electric field applied along the 1?0?0 crystallographic direction, is interpreted as resulting from a possible reorientation of the EL3 defect, owing to a jump of the oxygen ion into a neighbouring lattice site, driven by a strong electric field. On the other hand, a close pair divacancy complex is suggested to be responsible for the EL5 defect.

Voltage-controlled interference of ballistic electrons in an open quantum dot

Abstract We studied ballistic transport through a square quantum dot defined in AlGaAs/GaAs heterostructure by Schottky gates deposited on its surface. Electrons travelled between entry and exit openings in front and back walls of the dot. We have found that the conductance through the dot oscillates as a function of the potential difference between side gates of the dot. The effect is due to the voltage-controlled interference between electron trajectories which undergo single reflection from the opposite side walls and it is a counterpart of the electrostatic Aharonov–Bohm effect.

Quantum interference of electrons transmitted throughout a double-barrier resonant tunnelling structure under a perpendicular magnetic field

Recently, we have discovered a fine oscillatory structure (FOS) of the current flowing through resonant tunnelling devices consisting basically of two very thin AlAs barriers separated by a GaAs quantum well. In this work we have found two striking properties of FOS in a magnetic field applied normally to the current flow: (i) a phase shift of the FOS on the bias-voltage scale which is proportional to the squared induction of the magnetic field; (ii) rapid oscillatory-type fall-off in FOS amplitude with increasing magnetic field. We demonstrate, using a simple quantitative model, that both properties can be consistently explained in terms of quantum interference between electrons which escaped from the quantum well and those partly reflected at a potential step on the collector side of the device. The physical conditions necessary for the appearance of FOS are discussed.

Effect of the device size on the performance of resonant-tunneling diodes

Presents experimental results obtained with resonant-tunneling diodes fabricated from (GaAl)As/GaAs and (CdMg)Te/CdTe semiconductor heterostructures grown - respectively - by low-pressure metal-organic vapor-phase epitaxy and molecular-beam epitaxy methods. These results demonstrate, firstly, the appearance of intriguing oscillatory structure on the rising slope of the resonant peak in small-diameter (GaAl)As/GaAs-based devices and, secondly, the improvement of current-voltage characteristics of (CdMg)Te/CdTe-based resonant-tunneling diodes due to reduction of their cross-sectional area.

Ferromagnetic (Ga,Mn)As nanostructures for spintronic applications

Magneto-resistive, cross-like nanostructures have been designed and fabricated by electron-beam lithography patterning and chemical etching from thin epitaxial layers of the ferromagnetic semiconductor (Ga,Mn)As. The nanostructures, composed of two perpendicular nanostripes crossing in the middle of their length, represent four-terminal devices, in which an electric current can be driven through any of the two nanostripes. In these devices, a novel magneto-resistive memory effect, related to a rearrangement of magnetic domain walls in the central part of the device, has been demonstrated. It consists in that the zero-field resistance of a nanostripe depends on the direction of previously applied magnetic field. The nanostructures can thus work as two-state devices providing basic elements of nonvolatile memory cells.

Native defects in MBE-grown CdTe

Deep-level traps in both n- and p-type CdTe layers, grown by molecular-beam epitaxy on GaAs substrates, have been investigated by means of deep-level transient spectroscopy (DLTS). Four of the traps revealed in the DLTS spectra, which displayed exponential kinetics for capture of charge carriers into the trap states, have been assigned to native point defects: Cd interstitial, Cd vacancy, Te antisite defect and a complex formed of the Te antisite and Cd vacancy. Three further traps, displaying logarithmic capture kinetics, have been ascribed to electron states of treading dislocations generated at the mismatched interface with the substrate and propagated through the CdTe layer.

MAGNETIC FIELD INDUCED RESONANCE IN A DOUBLE-BARRIER RESONANT TUNNELING DIODE

Abstract The effect of a magnetic field in the singular enhancement of tunneling near threshold has been investigated in specifically designed double-barrier resonant tunneling structures. The experimental results prove that an additional increase of tunnel current is due to impurity-assisted tunneling. It is shown that a donor bound state arises as a result of dopant diffusion into the barrier. These states are identified as strongly localized DX centers.