We report on the role of bismuth as a surfactant in the growth
of InAs quantum dots (QDs) on GaAs (001) by metal organic
vapour phase epitaxy. Atomic force microscopy
investigations have shown that bismuth suppresses coalescence
of the InAs QDs and advances a more uniform size distribution. The photoluminescence spectra of the
Bi-assisted grown QDs present several narrow peaks from the ground
and the excited state transitions with full width at half
maximum (FWHM) as narrow as 25 meV (both at 77 and 300 K).
Due to such low values of the FWHM we were able to observe up
to two well resolved excited state transitions in the
photovoltage spectra measured by an electrolyte cell
technique. The lowest ground transition energies observed were
0.93 eV at 77 K and 0.875 eV at 300 K (emission wavelength
1.46 µm). So using Bi-assisted growth it is possible
to cover the 1.3 µm band, which is important for
optoelectronic applications in the InAs/GaAs material system.
Formation of such `deep' QDs without misfit dislocations was
explained by the formation of a graded-composition transient InGaAs
alloy layer at the GaAs/InAs hetero-interface as a result of
diffusion intermixing of the components. The proposed mechanism
for the effect of Bi on the QDs' morphology is that Bi
decreases the surface mobility of the In atoms on the growing
surface, preventing the coalescence of the QDs. Because of its rather
large covalent radius (compared with that of As), Bi is not
incorporated into the QDs' material segregating on the growing
surface.
The low-frequency noise in a nanometer-sized virtual memristor consisting of a contact of a conductive atomic force microscope (CAFM) probe to an yttria stabilized zirconia (YSZ) thin film deposited on a conductive substrate is investigated. YSZ is a promising material for the memristor application since it is featured by high oxygen ion mobility, and the oxygen vacancy concentration in YSZ can be controlled by varying the molar fraction of the stabilizing yttrium oxide. Due to the low diameter of the CAFM probe contact to the YSZ film (∼10 nm), we are able to measure the electric current flowing through an individual filament both in the low resistive state (LRS) and in the high resistive state (HRS) of the memristor. Probability density functions (Pdfs) and spectra of the CAFM probe current in both LRS and HRS are measured. The noise in the HRS is found to be featured by nearly the same Pdf and spectrum as the inner noise of the experimental setup. In the LRS, a flicker noise 1/fγ with γ ≈ 1.3 is observed in the low-frequency band (up to 8 kHz), which is attributed to the motion (drift/diffusion) of oxygen ions via oxygen vacancies in the filament. Activation energies of oxygen ion motion determined from the flicker noise spectra are distributed in the range of [0.52; 0.68] eV at 300 K. Knowing these values is of key importance for understanding the mechanisms of the resistive switching in YSZ based memristors as well as for the numerical simulations of memristor devices.
We observe a series of sharp resonant features in the tunnelling differential conductance of InAs quantum dots. We found that dissipative quantum tunnelling has a strong influence on the operation of nano-devices. Because of such tunnelling the current-voltage characteristics of tunnel contact created between atomic force microscope tip and a surface of InAs/GaAs quantum dots display many interesting peaks. We found that the number, position, and heights of these peaks are associated with the phonon modes involved. To describe the found effect we use a quasi-classical approximation. There the tunnelling current is related to a creation of a dilute instanton-anti-instanton gas. Our experimental data are well described with exactly solvable model where one charged particle is weakly interacting with two promoting phonon modes associated with external medium. We conclude that the characteristics of the tunnel nanoelectronic devices can thus be controlled by a proper choice of phonons existing in materials, which are involved.
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