I. Bineva

Composition and structure of ZnxCd1?xSe single layers prepared by thermal evaporation of ZnSe and CdSe

Single layers of ZnxCd1?xSe with five different compositions and thickness of 400 nm have been prepared by thermal vacuum evaporation, through alloying of ultra thin ZnSe and CdSe films with equivalent thickness of 0.12, 0.25 or 0.37 nm. The deposition was carried out on rotating substrates kept at room temperature. The layer composition was varied by alloying ZnSe and CdSe films with different equivalent thicknesses. The film composition x = 0.39, 0.52, 0.59, 0.69 and 0.8 has been determined by Energy-Dispersive Spectroscopy and confirmed with Raman scattering data. The microstructure of ZnxCd1?xSe has been investigated by Atomic Force Microscopy and Raman scattering measurement. The Atomic Force Microscopy results have revealed that the layers are nanocrystalline and the grain size is ? 20 nm. The Raman scattering data have shown four replicas of the longitudinal optical phonons, thus confirming the conclusion for the layer crystallinity. The obtained results have shown that the applied deposition technique makes possible preparation of ternary nanocrystalline ZnxCd1-xSe layers with desired compositions..

High energy electron-beam irradiation effects in Si-SiOx structures

Homogeneous SiOx films (x=1.3, 200 nm and 1000 nm thick) and composite a-Si-SiOy films (y ~ 1.80) containing amorphous Si nanoparticles have been prepared on crystalline (c-Si) substrate. A part of the films was irradiated at temperature below 50°C by 20 MeV electrons with two different fluences (7.2x1014 and 1.44x1015 el.cm-2). Atomic force microscopy (AFM), Raman spectroscopy and capacitance (conductance) - voltage (C(G)-V) measurements on Al/c-Si/SiOx/Al or Al/c-Si/(a-Si-SiOy)/Al structures were used to get information about the irradiation induced changes in the surface morphology, the phase composition in the film bulk and at the Si-SiOx interface. The AFM results show that the electron irradiation decreases the film surface roughness of the films annealed at 250°C. The Raman scattering data imply appearance of amorphous silicon phase and some structural changes in the oxide matrix of the homogeneous SiOx films. In the composite films electron beam stimulated decrease of the defects at the a-Si/SiOy interface has been assumed. The initial C(G)-V results speak about electron induced formation of electrically active defects in the SiOy matrix of the composite films.

Effects of annealing atmosphere and substrate on the photoluminescence and Raman scattering from Si nanocrystals in a SiO2 matrix

Thin films of SiOx have been prepared on quartz or c-Si substrates by thermal evaporation of SiO in vacuum and post-annealed at 1373 K in an argon or hydrogen atmosphere. High-resolution electron microscopy has shown the existence of silicon nanocrystals in the annealed films, and this result has been confirmed by Raman scattering. Photoluminescence has been observed from annealed films and attributed to radiative recombination in Si nanocrystals. Its intensity is appreciably higher upon annealing in Ar than in H2. It is shown that substrates strongly affect the Raman scattering from Si nanocrystals in nc-Si–SiO2 thin films with low filling factors.

Application of Metal-Oxide-Semiconductor structures containing silicon nanocrystals in radiation dosimetry

Abstract This article makes a brief review of the most important results obtained by the authors and their collaborators during the last four years in the field of the development of metal-insulator-silicon structures with dielectric film containing silicon nanocrystals, which are suitable for applications in radiation dosimetry. The preparation of SiOx films is briefly discussed and the annealing conditions used for the growth of silicon nanocrystals are presented. A two-step annealing process for preparation of metal-oxide-semiconductor structures with three-layer gate dielectrics is described. Electron Microscopy investigations prove the Si nanocrystals growth, reveal the crystal spatial distribution in the gate dielectric and provide evidences for the formation of a top SiO2 layerwhen applying the two-step annealing. Two types of MOS structures with three region gate dielectricswere fabricated and characterized by high frequency capacitance/conductancevoltage (C/G-V) measurements. The effect of gamma and ultraviolet radiation on the flatband voltage of preliminary charged metal-oxide-semiconductor structures is investigated and discussed.

Annealing induced changes in ternary nanostructured ZnxCd1-xSe thin films: structure and morphology

Single layers of ZnxCd1-xSe with various compositions (x = 0.39, 0.59 and 0.8) were prepared by thermal vacuum evaporation at room substrate temperature. Consecutive deposition of small portions of ZnSe and CdSe with equivalent thickness of 0.12 or 0.37 nm was applied. X-ray diffraction and atomic force microscopy measurements were applied to explore the evolution of the crystal structure, microstructure, composition and surface morphology upon furnace annealing at 200 °C and 400 °C in an inert atmosphere. It has been found that as-deposited films were nanocrystalline with a grain size of around 10 nm and cubic structure. Upon annealing the size increased approximately three times and the cubic structure was preserved; no appearance of wurtzite phase was observed. It has been also ascertained that annealing caused significant reduction of the film surface roughness. Atomic force microscopy phase images revealed existence of a second phase on the surface of as-deposited films which disappeared after annealing. The effect of the preparation conditions on the film properties and annealing induced changes is discussed.

Formation of Si Nanocrystals in Thin SiO2 Films for Memory Device Applications

X-ray Diffraction and Reflectivity, Transmission Electron Microscopy and Atomic Force Microscopy were applied to study the effect of thermal annealing on the properties of thin SiOx films (~ 15 nm) prepared by thermal evaporation of SiO in vacuum. It has been shown that furnace annealing at 1000 oC causes phase separation and formation of uniformly distributed Si nanocrystals into a SiO2 matrix. Clockwise hysteresis has been observed in the C-V curves measured and explained by assuming charging and discharging of the NCs with carriers, which tunnel from the Si substrate.

MOS Structures Containing Si Nanocrystals for Applications in UV Dosimeters

Metal-Oxide-Semiconductor structures with semitransparent Au top electrode and containing Si nanocrystals in the gate dielectric are fabricated and studied. The structures can be charged negatively or positively by injecting or extracting electrons from the top electrode. Illumination with 395-400 nm, 10.4 mW UV light source causes discharge of previously charged structures with rate which varies between 2 mV/s and 12 mV/s. The discharge rate depends on the sign of the trapped charge, as well on the internal electric field in the gate dielectric.

Nanocrystalline Layers of CdSe Produced by Means of a Multilayer Approach

Multilayer structures are deposited by sequential physical vapor deposition of CdSe and a second amorphous (SiO x , GeS 2) or polycrystalline (ZnSe) semiconductor. The layer thickness of CdSe is varied between 1 and 10 nm; the layer thickness of the second material is either equal or twenty times greater then that of CdSe. High-resolution electron microscopy results prove the formation of CdSe nanocrystalline layers, which are continuous in the structures with equal layer thickness of the constituent materials. In the second kind of structures CdSe layers are discontinuous; particles with nearly spherical form are observed, whose spatial distribution followed the surface morphology of the previous film. The mechanism of nonepitaxial semiconductor nanocrystal growth on’ smooth’ and ‘rough’ surfaces is discussed. Quantum-size increase of the optical band gap of CdSe nanocrystals is observed in all multilayer structures prepared. It is ascribed to one-dimensional carrier confinement in the continuous CdSe layers and quasi three-dimensional one in the discontinuous layers.

Photoluminescence from 20 MeV electron beam irradiated homogeneous SiOx and composite Si-SiOx films

The effect of 20 MeV electron irradiation on the room temperature photoluminescence from homogeneous SiOx and composite Si-SiOx films, containing amorphous or crystalline Si nanoparticles, is studied. Layers with x = 1.5 and 1.7 and thickness of 200 nm were deposited on crystalline silicon substrates by thermal evaporation of SiO in vacuum. Film annealing in an inert atmosphere at 700 oC or 1000 oC for 60 min was applied to grow amorphous or crystalline silicon nanoparticles, respectively, in a SiOx matrix. Samples from all types of films were irradiated with 20 MeV electrons at close to room temperature and a fluence of 2.4x1014 el.cm-2. Photoluminescence was measured under excitation with the 488 nm line of an Ar+ laser. The electron irradiation causes a decrease of the integrated photoluminescence intensity in composite samples with initial x = 1.7 containing amorphous or crystalline nanoparticles and x = 1.5 samples with Si nanocrystals. The electron irradiation of x = 1.5 samples with amorphous nanoparticles slightly increases the photoluminescence intensity. The obtained results are discussed in terms of electron beam induced phase separation and Si nanoparticle size increase.

Long term ageing changes in structure and morphology of nanocrystalline ZnxCd1−xSe thin films

Long term morphological and structural changes of ZnxCd1-xSe (x=0.4, 0.6 and 0.8) thin films, obtained by solid state alloying of consecutively deposited submonolayers of ZnSe and CdSe, were investigated by means of Atomic Force Microscopy (AFM) and X-ray diffraction (XRD). The XRD investigations show typical solid solution patterns without any decomposition. A relaxation of the lattice is observed through the years, showing tendency towards thermodynamic equilibrium. A. non-uniform strain is observed for x= 0.4 and 0.8, which is proven by the AFM measurements to be due to structural defects. 2D fast Fourrier transform patterns are derived from the AFM images and grain structure evolution and various morphological changes occurring in the films with time are observed and discussed.