S. A. Smagulova

Charge spectroscopy of Si nanocrystallites embedded in a SiO2 matrix

In the present work we have determined the electronic levels in systems of Si nanocrystallites (NCs) embedded in the insulating matrix of silicon dioxide, SiO2, by employing the charge deep-level transient spectroscopy (Q-DLTS) technique. We have clearly shown that these levels are associated with the NCs. Correspondingly, we suggest that the levels that we found are associated mainly with two quantum confinement energies, 0.14 and 0.19 eV. These energies are shown to be consistent with the corresponding theoretical estimates for the presently studied Si–NCs/SiO2 systems. The fact that these levels are almost fixed for the various samples studied suggests the importance of the bulk-surface coupling under quantum confinement conditions.In the present work we have determined the electronic levels in systems of Si nanocrystallites (NCs) embedded in the insulating matrix of silicon dioxide, SiO2, by employing the charge deep-level transient spectroscopy (Q-DLTS) technique. We have clearly shown that these levels are associated with the NCs. Correspondingly, we suggest that the levels that we found are associated mainly with two quantum confinement energies, 0.14 and 0.19 eV. These energies are shown to be consistent with the corresponding theoretical estimates for the presently studied Si–NCs/SiO2 systems. The fact that these levels are almost fixed for the various samples studied suggests the importance of the bulk-surface coupling under quantum confinement conditions.

Charge deep-level transient spectroscopy of SiO2 and Al2O3 layers with embedded Ge nanocrystals

The present study of deep level transient spectroscopy (DLTS) is focused on a comparison of the trap states in two types of Ge nanocrystallites (NCs)-insulator composites. The investigated systems were the dielectric matrices Al2O3 and SiO2 in which the Ge NCs were embedded. We have found couples of traps with related values of activation energies in both the Ge:Al2O3 and the Ge:SiO2 films. In the films with a relatively low Ge content (where only small NCs sized 3–5 nm could have been detected by means of Raman spectroscopy), we observed traps with an energy level ∼50 meV in the Ge:Al2O3 films and 120 and 50 meV in the Ge:SiO2 films. In both systems, we found that the electron traps have a small carrier capture cross-section (10−21–10−23 cm2). We have identified the levels of the traps to be the quantum confinement levels in the small Ge NCs. For samples of higher Ge contents, where the NC size reaches about 20 nm and where an appreciable portion of the dielectric matrix consists of amorphous Ge (α-Ge), ...

Resistive switching effect and traps in partially fluorinated graphene films

A reversible resistive switching effect is detected in films created from the suspension of partially fluorinated graphene (more specifically, small graphene or few-layer graphene quantum dots in a fluorographene matrix), which makes them promising for resistive memory applications. This paper is focused on the investigation and comparison of the traps and transport in such films in both the low- and high-resistance state. The appearing set of traps for holes and electrons in the band gap of fluorinated graphene is revealed in the films at the low-resistance state, and their parameters (activation energy and trap density) are defined using charge spectroscopy. The minimum relaxation time of nonequilibrium carriers from different traps is found to be about 700 ns, and the energy level position of corresponding traps from the conduction band of a silicon substrate equals 0.08 eV. It has also been demonstrated that the carrier transport in the low-resistance state is determined by the same traps, and they form conductive channels in the film. Transport and non-equilibrium recharging processes in the state of high resistance are found to occur above all by means of carrier tunneling through the potential barriers in the films.

Enhanced formation of Ge nanocrystals in Ge:SiO 2 layers by swift heavy ions

In this paper we report the ability of swift heavy Xe ions with an energy of 480MeV and a fluence of 10 12 cm −2 to enhance the formation of Ge nanocrystals within SiO2 layers with variable Ge contents. These Ge-SiO2 films were fabricated by the co-sputtering of Ge and quartz sources which followed various annealing procedures. In particular, we found that the irradiation of the Ge:SiO2 films with subsequent annealing at 500 ◦ C leads to the formation of a high concentration of nanocrystals (NCs) with a size of 2‐5nm, whereas without irradiation only amorphous inclusions were observed. This effect, as evidenced by Raman spectra, is enhanced by pre-irradiation at 550 ◦ C and post-irradiation annealing at 600 ◦ C, which also leads to the observation of room temperature visible photoluminescence. (Some figures may appear in colour only in the online journal)

Films fabricated from partially fluorinated graphene suspension: structural, electronic properties and negative differential resistance

The band structure and electric properties of films created from a partially fluorinated graphene suspension are analyzed in this paper. As may be inferred from the structural study, graphene islands (quantum dots) are formed in these films. Various types of negative differential resistance (NDR) and a step-like increase in the current are found for films created from the fluorinated graphene suspension. NDR resulting from the formation of the potential barrier system in the film and corresponding to the theoretical prediction is observed for a relatively low fluorination degree. The origin of the NDR varies with an increase in the fluorination degree of the suspension. The observation of NDR in the fluorinated films widens the range of application of such films, including as active device layers fabricated using 2D printed technologies on rigid and flexible substrates.

Graphene-based humidity sensors: the origin of alternating resistance change

The response of a graphene-based humidity sensor is considered as a function of film structures. Analysis of the resistance changes due to water molecule adsorption on the graphene or multi-layer graphene (MLG) surface is performed for films with different structures and resistivities from hundreds of ohms/sq to hundreds of kilo-ohms/sq. The results revealed possible increase, decrease and non-monotonous behavior of resistance with changes in film structure. Adsorption of water molecules at grain boundary defects is assumed to lead to an increase in film resistivity due to the donor property of water and the p-type conductivity of graphene. Another type of conductive center with a higher capture cross-section is realized in the case of water molecule adsorption at edge defects in MLG films (the formation of conductive chains with ionic conductivity). If these chains form a continuous network the film resistivity decreases. The result of the competition between the opposite effects of the conductivity compensation and formation of the water-based conductive chains depends on the film structure and determines the response of humidity sensors. Sensor sensitivity is found to increase when only one type of defect determines water adsorption (edge defects or grain boundary defects).

Deep-level spectroscopy studies of confinement levels in SiGe quantum wells

The recharging of quantum confinement levels in SiGe quantum wells (QWs) was studied by charge deep-level transient spectroscopy (Q-DLTS) for Si/SiGe/Si structures with different Ge contents in the SiGe layer. The set of levels were observed as the different slopes in the Arrhenius plots for the same Q-DLTS peak in different temperature ranges. These activation energies were compared to the energies of quantum confinement levels in the QW calculated in frames of six-band model taking into account spin-orbit interaction and attributed to a thermally activated tunneling of holes from the SiGe QW.

Hydrothermal Synthesis of Luminescent Carbon Dots from Glucose and Birch Bark Soot

Carbon dots were prepared by the method of hydrothermal synthesis from carbon precursors of glucose and birch bark soot in aqueous ammonia. The distribution of lateral sizes of carbon dots testifies their average size to be 10–12 nm for glucose and 20–22 nm for soot. Infrared absorption spectra indicate oxygen groups on the surfaces of obtained carbon dots. Aqueous suspensions with glucose-based carbon dots exhibit strong absorption in the visible region from 300 nm to 500 nm. Soot-based carbon dots demonstrate strong absorption in the ultraviolet region, but are transparent in the visible region. The luminescence spectra exhibit that carbon dots synthesized from glucose and soot are luminescent in the same spectral region, their wavelengths of radiation depend on the wavelengths of excitation, and the intensity of luminescence depends on the presence of oxygen groups on the surface of carbon dots. Carbon dots synthesized from glucose and soot have great prospects in terms of their application in biology and medicine.

Resonant tunneling in Si/SiGe/Si structures with a single quantum well under surface passivation

Si/SiGe/Si structures with single SiGe quantum wells (QWs) of different Ge content in the SiGe layer were studied at temperatures of 80 to 300 K. The structure surfaces were passivated with organic monolayers of 1 octadecene. The passivation removed the surface-defect charge resulting in a high hole population of SiGe QWs and made it possible to observe clearly the resonant tunneling of holes through confined levels in SiGe QW. The tunneling became evident as the steps on current–voltage characteristics. The energies of confined levels found from these data are consistent with calculated ones and with energies found from DLTS measurements. It was shown that the carrier emission from QWs is mainly caused by thermally activated tunneling at low biases and direct tunneling through the confined states at high biases.

Confinement Levels in Passivated SiGe/Si Quantum Well Structures

The set of quantum confinement levels in SiGe quantum wells (QW) was observed in the temperature range from 80 to 300 K by means of charge deep-level transient spectroscopy (Q-DLTS) and transport measurements. These observations proved possible due to a passivation of structure surface with organic monolayer deposition. Si/SiGe/Si structures with different Ge contents in SiGe layer were studied. The confined levels in passivated structures became apparent through DLTS measurements as various activation energies in temperature dependence of the rate of carrier emission from QW. It was found that the recharging of SiGe QWs and carrier emission accomplish due to thermo-stimulated tunneling. The steps in the current-voltage characteristics originated from direct tunneling via the confined states were found to determine the current flow at high fields.