D. Nesheva - Academia.edu (original) (raw)

Papers by D. Nesheva

Journal of Materials Science, 2020

Layers and devices utilizing semiconductor nanocrystals have been the subjects of intensive resea... more Layers and devices utilizing semiconductor nanocrystals have been the subjects of intensive research due to applications in opto-and microelectronic devices, solar cells, detectors, memories and in many more fields. We have shown previously that those nanocrystals in dielectric matrices undergo a substantial reformation during electron irradiation. The research of the interaction between semiconductor nanoclusters and irradiation is important for both the intentional modification of the structures and for understanding the stability of those devices under harsh, radiative conditions (e.g. space, nuclear, medical diagnosis, or similar applications). In the present research, we investigated the influence of neutron irradiation on substoichiometric silicon oxide. We investigated both homogeneous case and inhomogeneous case of matrices with silicon nanoclusters. We found that a fast neutron flux of 5.5 9 10 13 neutrons/cm 2 s and a fluence of 3.96 9 10 17 neutrons/cm 2 induce phase separation in the homogeneous films, whereas it decreases the volume fraction of the amorphous silicon phase caused by the reducing size of amorphous nanoclusters in the inhomogeneous films.

Newest Updates in Physical Science Research Vol. 3, 2021

NATO Science for Peace and Security Series A: Chemistry and Biology, 2011

ABSTRACT

The Journal of Physical Chemistry C, 2010

The transport properties of nonequilibrium (photoexcited) charge carriers in sonochemically synth... more The transport properties of nonequilibrium (photoexcited) charge carriers in sonochemically synthesized threedimensional (3D) assemblies of AgBiS 2 quantum dots (QDs) deposited as thin films were studied. To characterize the photoconduction of quantum-confined nanocrystals close packed in thin film form, both stationary and time-resolved experiments were performed. Besides by interband electronic transitions in the bulklike part of the nanocrystals, the photoresponse of nanocrystalline films was found to be also affected to a greater extent by the crystal boundary barrier height modulation upon illumination. The surface and bulk recombination velocities were found to be comparable. Good agreement was obtained between the band gap energy determined by analysis of the photoconductivity data measured by the constant field and the constant photocurrent method (∼1.18 eV). This value is in agreement with the optical spectroscopy data. It is higher than the optical band gap of a bulk specimen of this semiconductor, due to 3D confinement effects on the charge carrier motions within individual QDs. The nonequilibrium conductivity was found to relax exponentially with a time constant of 1.67 ms, which corresponds to average lifetime of minority charge carriers (holes).

The trap density N t and mobility-lifetime product μr in CdSe thin films are investigated combini... more The trap density N t and mobility-lifetime product μr in CdSe thin films are investigated combining data from thermally stimulated conductivity and photoconductivity measurements. A method of simultaneous fitting of thermally stimulated conductivity spectra is applied and four peaks at 193, 213, 230 and 258 K are resolved in the temperature range 80-270 K. Temperature dependences of the steady-state photoconductivity under excitation with strongly absorbed light (525 and 630 nm) are measured at photon fluxes of 5x10 13- - 5x10 15 cm -2 .s -1 and temperature dependences of the exponent y in the intensity dependence of the photoconductivity are obtained. The pr products and trap densities N t are calculated for both kinds of light excitation. It is shown that they depend on the wavelength of the exciting light, being 2-3 times larger for excitation with A=630 nm.

Acta Physica Polonica A, 2009

A series of ZnSe single layers having thickness between 30 nm and 1 µm was deposited on c-Si and ... more A series of ZnSe single layers having thickness between 30 nm and 1 µm was deposited on c-Si and glass substrates at room substrate temperature. Thermal evaporation of ZnSe powder in high vacuum has been applied. Moreover, SiO x /ZnSe periodic multilayers prepared by the same deposition technique and having ZnSe layer thickness of 2 and 4 nm have been studied. Raman spectra were measured at 295 K, using the 442 nm line of a He-Cd laser as well as different lines of the Ar + or Ar + /Kr + lasers. The observed Raman features have been related to multiple optical phonon (1LO to 4LO) light scattering and connected with the existence of randomly oriented crystalline ZnSe grains in both ZnSe single layers and ZnSe layers of the multilayers. Relatively large line width (≈ 15 cm −1) of the 1LO band has been observed and related to lattice distortion in the crystalline grains and existence of amorphous phase in the layers thinner than 100 nm. The Raman spectra measured on both ZnSe single layers and SiO x /ZnSe multilayers using the 488 nm line with a gradually increased laser beam power indicate an increased crystallinity at high irradiation levels.

Philosophical Magazine B, 1994

ABSTRACT The band and subband absorption in single CdSe layers and pnpn type Se/CdSe multilayers ... more ABSTRACT The band and subband absorption in single CdSe layers and pnpn type Se/CdSe multilayers (MLs) has been measured by the constant photocurrent method. From studies on the MLs' band absorption the conclusion is reached that the photoconductivity of the CdSe sublayers decreases with both decreasing thickness and evaporation rate. An increase of the Urbach edge parameter Eu from 57 to 80 meV has been obtained on decreasing the CdSe sublayer thickness or on increasing the deposition rate. A ‘shoulder’ has been observed in the MLs' absorption measured in the lowest energy region. Indirect transitions from the Se valence band to the CdSe conduction band have been proposed as a possible explanation of the shoulder absorption. The observed change in the shoulder position with changing CdSe sublayer thickness has been attributed to quantum size effects.

Semiconductor Science and Technology, 1997

ABSTRACT Multilayer amorphous selenium-based photoreceptors were prepared by thermal evaporation ... more ABSTRACT Multilayer amorphous selenium-based photoreceptors were prepared by thermal evaporation in a vacuum at . Amorphous multilayers of Se/CdSe, as well as a combination of a thin film and multilayers were used as photogeneration layers. `Pure' Se or were the transport layers. The xerographic properties of the photoreceptors were studied and some peculiarities of the charge transport in different designed photoreceptors are discussed. Xerographic photosensitivity at illumination with different wevelengths was determined and a comparison with the sensitivity of conventional photoreceptors is made. All photoreceptors are photosensitive in the whole visible region. The photoreceptors including are also sensitive in the near-infrared region. The xerographic properties of the studied photoreceptors remained unaltered in air at room temperature for more than 12 months.

Journal of Physics: Conference Series, 2017

First results on resistive switching in SiOx film containing crystalline silicon nanoparticles ar... more First results on resistive switching in SiOx film containing crystalline silicon nanoparticles are reported. SiOx layers (x = 1.15) with thickness of 50 nm were deposited on n-Si crystalline substrates and annealed for 60 min at 1000oC to grow crystalline nanoparticles. Part of the samples were annealed in an inert atmosphere, while the rest were subjected to a two-step (O2+N2/N2) annealing process. Current-voltage (I-V) characteristics were by applying positive or negative voltage to the top contact. For both types of samples the I-V characteristics were asymmetric with lower currents measured at negative voltage, especially in the case of two-step annealed samples. In most of the N2 annealed structures switching behavior high-low/low-high resistance state was observed in both polarities at voltages with amplitudes in the range (2 - 4) V. Uncontrolled switching low/high resistance was also seen, more frequently at positive voltages. In contrast, the two-step annealed samples showed stable behavior. The transition high-low resistance state was achieved by negative voltages in the (-2, -5) V range leading to an increase of the current by more than three orders of magnitude. The structures were reset to the high resistive state, by positive voltage in the range (3 - 4) V. Uncontrolled switching was not observed in the two-step annealed samples for both polarities and they showed higher reliability regarding the number of switching cycles.

Journal of Physics: Conference Series, 2021

ZnSe thin films with thickness of 30 nm and 50 nm were prepared on Corning 7059 glass substrates ... more ZnSe thin films with thickness of 30 nm and 50 nm were prepared on Corning 7059 glass substrates at room temperature by applying periodically interrupted physical vapour deposition of ZnSe and various deposition rates. All as-deposited films were annealed at 200°C and some of them were further annealed at 400°C. Results from spectroscopic ellipsometry have shown that the porosity of the films which is important for the ethanol sensitivity decreases with increasing deposition rate and annealing temperature. The porosity changes caused by annealing of the films at 400°C are accompanied with thickness decrease.

Glass Physics and Chemistry

The Raman spectra of glasses in the ternary (Ge2S3) x (As2S3)1−x and (GeS2) x (As2S3)1−x systems ... more The Raman spectra of glasses in the ternary (Ge2S3) x (As2S3)1−x and (GeS2) x (As2S3)1−x systems are reported. It is shown that the degree of disorder in the (Ge2S3) x (As2S3)1−x glasses increases with an increase in the Ge content. The differences in the spectra of glasses with the same molar content of As2S3 are discussed in terms of the changes occurring in the local structure due to deviations from the stoichiometry. The observed changes in the intensity of the boson peak indicate changes in the medium-range order structure of the glasses under study.

Chalcogenide glasses from the GeSe 2-GeTe-PbTe system are synthesized. The temperatures of glass-... more Chalcogenide glasses from the GeSe 2-GeTe-PbTe system are synthesized. The temperatures of glass-transition, crystallization and melting are determined using differential thermal analysis. On the basis of the obtained values the glass forming ability K G of the glasses is determined by Hruby's criterion. The maximum value of K G is obtained for composition (GeSe 2) 60 (GeTe) 20 (PbTe) 20. The minimal volume of the micro-voids V h and the energy for their formation E h are calculated. The values of V h and E h vary within 0.0219-0.0320 nm 3 and 15.89-17.31 kJ/mol, respectively. The influence of the composition on the investigated characteristics is analyzed.

Philosophical Magazine Part B, 1995

The constant photocurrent method and small-angle X-ray diffraction have been used in order to stu... more The constant photocurrent method and small-angle X-ray diffraction have been used in order to study the changes in the interfaces of Se/CdSe multilayers as well as in the structural disorder of CdSe sublayers after annealing. An improvement of the interface quality accompanied by a decrease of the Urbach edge parameter EU has been found after annealing the multilayers with sublayer

2010 27th International Conference on Microelectronics Proceedings, 2010

Semiconductor Science and Technology, 2008

ABSTRACT Thin films of CdSe with thicknesses of 50, 75 and 100 nm were prepared by physical vapou... more ABSTRACT Thin films of CdSe with thicknesses of 50, 75 and 100 nm were prepared by physical vapour deposition using a 'one-step' or a 'step-by-step' deposition approach. The influence of the deposition conditions and film thickness on the microstructure and electrical properties has been explored by means of atomic force microscopy (AFM), dark conductivity, photoconductivity and thermally stimulated conductivity measurements. The AFM results have shown that smaller grains are formed in thinner films and that the size decrease is enhanced when the step-by-step deposition approach is used. A significant increase of the dark current activation energy with decreasing layer thickness has been registered and identified with a Fermi-level shift. A decrease of dark conductivity and photoconductivity has been observed with decreasing layer thickness which has been connected with both a size-induced increase of the interface defect density and appearance of new 'faster' recombination centres. The observed decrease is stronger in step-by step layers, in which the density of incorporated oxygen is higher. The oxygen creates new acceptor defects and causes band bending at grain boundaries. From the thermally stimulated conductivity data two kinds of defect states have been resolved situated at about 0.55 eV and 0.67 eV below the conduction band. A simultaneous fitting method has been applied to determine the main trap parameters.