Yu. V. Dubrovskii

Effect of channel doping on the low-frequency noise in GaN/AlGaN heterostructure field-effect transistors

We examined low-frequency noise in doped and undoped channel GaN/AlGaN/SiC heterostructure field-effect transistors with different Al content in the barrier. The observed noise spectra follow the 1/fγ law with 0.8⩽γ⩽1.2 for frequencies f up to 100 kHz. Our results indicate two orders of magnitude reduction in the input-referred noise spectral density in the undoped channel devices with respect to the noise density in the doped channel devices of comparable electric characteristics. Low temperature measurements reveal generation—recombination-type peaks in the spectra of the doped channel devices. Effects of the piezoelectric charges at the GaN/AlGaN interface are also discussed.

Resonant tunneling and photoluminescence spectroscopy in quantum wells containing self-assembled quantum dots

We investigate the optical and electrical properties of n-i-n GaAs/(AlGa)As double barrier resonant tunneling diodes (RTDs) in which a layer of InAs self-assembled quantum dots (QDs) is embedded in the center of the GaAs quantum well. A combination of photoluminescence (PL) and electrical measurements indicates that the electronic states and charge distribution in this type of RTD are strongly affected by the presence of the dots. Also, the dot PL properties depend strongly on bias, being affected by tunneling of majority (electrons) and minority (photocreated holes) carriers through the well. The measurements demonstrate nonlinear effects in the QD PL by means of resonant tunneling and the possibility of using the dot PL as a probe of carrier dynamics in RTDs.

Nonlinear electron transport in normally pinched-off quantum wire

Nonlinear electron transport in normally pinched-off quantum wires was studied. The wires were fabricated from AlGaAs/GaAs heterostructures with high-mobility two-dimensional electron gas by electron beam lithography and following wet etching. At certain critical source-drain voltage the samples exhibited a step rise of the conductance. The differential conductance of the open wires was noticeably lower than e2/h as far as only part of the source-drain voltage dropped between source contact and saddle point of the potential relief along the wire. The latter limited the electron flow injected to the wire. At high enough source-drain voltages the decrease of the differential conductance due to the real space transfer of electrons from the wire in GaAs to the doped AlGaAs layer was found. In this regime the sign of the differential magnetoconductance was changed with reversing the direction of the current in the wire or the magnetic field, when the magnetic field lies in the heterostructure plane and is directed perpendicular to the current. The dependence of the differential conductance on the magnetic field and its direction indicated that the real space transfer events were mainly mediated by the interface scattering.

Anisotropy of electronic wave functions in self-assembled InAs dots embedded in the center of a GaAs quantum well studied by magnetotunneling spectroscopy

We present an experimental study of electron wave functions in InAs/GaAs self-assembled quantum dots by magnetotunneling spectroscopy. The electronic wave functions have a biaxial symmetry in the growth plane, with axes corresponding to the main crystallographic directions in the growth plane. Moreover, we observed the in-plane anisotropy of the subbands of the quantum well.

Magnetic field variation of tunnelling gap between disordered two-dimensional electron systems

Abstract We have investigated tunnelling between disordered two-dimensional electron systems in a magnetic field parallel to the current. At liquid-helium temperatures, the high magnetic field creates a gap in the tunnelling density of states that depends linearly on magnetic field. The temperature dependence of the magnetic field variation of the equilibrium tunnelling conductance reveals features which could be interpreted as a manifestation of the insulator-quantum Hall-insulator transition.

Wavelet analysis and its application in tunneling and x-ray spectroscopy

The capabilities and limitations of wavelet analysis are demonstrated for the illustrative case of two practial applications: investigation of magneto-tunneling signals and the extended x-ray absorption fine structure (EXAFS) of uranium. A brief introduction to wavelet analysis is given. The term “window size” of the mother wavelet function is defined; this concept underlies one of the ideas for a criterion of optimal choice of the mother wavelet function. The capabilities of a software package developed by the authors are demonstrated; this software was used for all the calculations done in the course of this study.

Probing the electronic properties of disordered two-dimensional systems by means of resonant tunnelling

We investigate resonant tunnelling in GaAs/(AlGa)As double-barrier resonant-tunnelling diodes in which a single layer of InAs self-assembled quantum dots is embedded in the centre of the GaAs quantum well. The dots provide a well-defined and controllable source of disorder in the well and we use resonant tunnelling to study the effect of this disorder on the electronic properties of the well.

Suppression of the equilibrium tunneling current between slightly disordered two-dimensional electron systems with different electron concentrations in a high magnetic field

Tunneling between parallel two-dimensional electron gases (2DEG) in accumulation layers formed on both sides of the single doped AlGaAs barrier are examined in both zero and high magnetic field. Accumulation layers are separated from highly n-doped contact regions which freely supply electrons to the 2DEGs via 80 nm thick lightly n-doped spacer layers. Strongly oscillating current with magnetic field along the 2DEGs is absent in this arrangement. Without magnetic field resonant tunneling between 2DEGs with different as grown electron concentration could be settle by application of external voltage bias. High magnetic fields (ν<1) shift resonant tunneling to zero external bias and suppresses tunneling current, creating wide gap in the tunneling density of states at the Fermi level arisen from the in-plane Coulomb interaction in the 2DEGs.

Spatial mapping of the electron eigenfunctions in InAs self-assembled quantum dots by magnetotunnelling

We use magnetotunnelling spectroscopy as a non-invasive probe to produce two-dimensional spatial images of the probability density of an electron confined in a self-assembled semiconductor quantum dot. The images reveal clearly the elliptical symmetry of the ground state and the characteristic lobes of the higher-energy states.

Tunneling resonances in structures with a two-step barrier

Electron transport through an asymmetric heterostructure with a two-step barrier N+GaAs/N−GaAs/Al0.4Ga0.6As/Al0.03Ga0.97As/N−GaAs/N+GaAs was investigated. Features due to resonance tunneling both through a size-quantization level in a triangular quantum well, induced by an external electric field in the region of the bottom step of the barrier (Al0.03Ga0.97As layer), and through virtual levels in two quantum pseudowells of different width are observed in the tunneling current. The virtual levels form above the bottom step or above one of the spacers (N−GaAs layer) as a result of interference of electrons, in the first case on account of reflection from the Al0.4Ga0.6As barrier and a potential jump at the Al0.03Ga0.97As/N−GaAs interface and in the second case — from the Al0.4Ga0.6As barrier and the potential gradient at the N−GaAs/N+GaAs junction, reflection from which is likewise coherent.