M. Sendova-Vassileva

Structural and Optical Properties of CVD Thin Tungsten Oxide Films

Thin amorphous and polycrystalline tungsten oxide films were prepared by chemical vapour deposition at atmospheric pressure from a metalorganic precursor - tungsten hexacarbonyl. The dependence of the structural and optical properties of tungsten oxide films on the technological conditions has been investigated by XRD, Raman spectroscopy and spectroscopic ellipsometry.

Structure and reversible lithium intercalation in a new P′3-phase: Na2/3Mn1−yFeyO2 (y = 0, 1/3, 2/3)

In this contribution, new data on the reversible Li+ intercalation in iron substituted sodium manganates are provided. Novel Na2/3Mn1−yFeyO2 (y = 0, 1/3 and 2/3) compounds with a P′3-type structure are prepared from freeze-dried citrate precursors at 500 °C. A new structural element is the development of three-layer oxygen stacking contrary to the well-known P2-type Na2/3MnO2 with a two-layer sequence. The effect of Fe additives on the structure of Na2/3MnO2 was examined by XRD powder diffraction and TEM analysis. The oxidation state and the distribution of transition metal ions in Na2/3Mn1−yFeyO2 were analysed using electron paramagnetic resonance spectroscopy. The lithium intercalation in Na2/3Mn1−yFeyO2 was investigated in two-electrode lithium cells of the type Li|LiPF6 (EC:DMC)|Na2/3Mn1−yFeyO2. The stability of the layered phases during lithium intercalation was studied by ex situ Raman spectroscopy. It was found that the intermediate Na2/3Mn2/3Fe1/3O2 composition is able to intercalate Li+ reversibly in high amounts. Details of the structure and its stability during the Li+ intercalation are discussed.

Structural, optical and electrical properties of V doped ZnO thin films deposited by r.f. magnetron sputtering

Structural, optical and electrical properties of V doped ZnO thin films deposited by r.f. magnetron co-sputtering on glass substrates at different temperature, Ts, between 150°C and 500°C are studied. The EDS analyses indicate that the average vanadium content in the films is in the range of 0.86–0.89 at. %. XRD spectra demonstrate preferential (002) crystallographic orientation with c-axis perpendicular to the substrate surface and grains sizes of the films about 21–29 nm. The band gap energy, Eg, values are in the range of 3.44–3.47 eV. The deposited V doped ZnO films have low resistivity − (2–8). 10−3 Ω cm. Raman spectra show vibrational phonons modes typical for ZnO. Comparison with the structural, optical and electrical properties of thin films ZnO and ZnO:Al is given.

Raman Spectroscopy Study on Na2/3Mn1-xFexO2 Oxides

The structural properties of sodium manganates and iron substituted sodium manganates with compositions Na2/3Mn1-xFexO2 (x=0, 1/3 and 2/3) were studied by Raman spectroscopy. The Raman spectroscopy allows distinguishing between layered phases with orthorhombic (Cmcm space group) and hexagonal (P63/mmc space group) distortion. It has been found that the crystal structure and the composition of Na2/3MnO2 display a strong dependence on the history of the thermal treatment. The orthorhombic distorted modification is stabilized at high temperatures (1000 oC). At lower quenching temperature, there is a phase separation into an orthorhombic and a hexagonal modification, concomitant with an increase in the oxidation state of Mn. When Fe substitutes for Mn, the hexagonal modification is stabilized. In order to understand the origin of the Raman spectra of Na2/3Mn1-xFexO2, we have used Na2/3Co2/3Mn1/3O2 as a standard for hexagonal structure, where Co3+ and Mn4+ are statistically distributed in the transition metal layers.

Polycrystalline silicon thin films obtained by metal-induced crystallization

The formation of high-quality polycrystalline silicon (poly-Si) on different substrates has important applications in the development of thin-film transistors, solar cells, image sensors, etc. In this study, we present the results of an investigation of poly-Si films on glass, formed by aluminum-induced crystallization. The process is based on the isothermal annealing for 3 h at 500 °C of co-sputtered Al+Si or sputtered a-Si films on glass, with and without thermally evaporated Al. The poly-Si films were investigated by Raman spectroscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy.

Preparation of wire structured ZnO films by electrochemical deposition

This article reports the synthesis of wire structured ZnO by electrochemical deposition on a glass substrate covered by thin film of ITO or SnO2 doped with F (SnO2:F) and on a carbon tissue. The influence of the structure of the transparent conductive films on the structural properties of the ZnO films obtained was studied by SEM and Raman spectroscopy. The results show that the films have a columnar structure with grain size of about 200-500 nm. The films deposited on the SnO2:F coated glass substrate have better structural properties. Pillar structure is observed on the surface of the fiber of the carbon tissue.

I‐V characteristics of n‐ZnO/poly‐Si thin film heterostructure

ZnO has been extensively studied for many different applications, such as gas sensors, transparent conductive electrodes in solar cells and light emitting diodes, etc., because of its chemical stability and sensitivity to different adsorbed gases, amenability to doping, large energy gap, nontoxicity and low cost. ZnO thin films have been applied for device engineering in different heterostructures, as well. In this work the I‐V characteristics of two thin film heterostructures n‐ZnO/p‐poly‐Si with ZnO films with different thicknesses, 80 and 300 nm are studied. The ZnO polycrystalline films are deposited by RF sputtering in an atmosphere of Ar on the surface of a p‐type poly‐Si layer. The poly‐Si films are prepared on glass substrate by the method of Aluminum Induced Crystallization (AIC) from precursor layers of Al and a‐Si:H. The Al and a‐Si:H precursor films are deposited by magnetron sputtering on glass substrate. The a‐Si:H film precursors contain 9 at.% hydrogen. The isotermal annealing of the struc...

Sm3+ photoluminescence in co-sputtered SiO2 thin films

Thin films of SiO2 containing Sm were deposited by magnetron co-sputtering. The concentration of Sm in the films was varied by changing the amount of Sm placed on the fused silica target. The samarium content in the films was established by Rutherford back-scattering measurements. Photoluminescence (PL) was excited with the 488-nm line of an Ar ion laser. It consists of several lines in the visible, which are assigned to transitions between multiplets of Sm3+. The concentration dependence of the PL intensity was studied. It passes through a maximum and decreases at higher Sm content. The films were annealed in vacuum at temperatures between 300 and 1000 °C. There was a strong increase in the PL intensity after annealing at 600 °C, and then it fell again after annealing at 1000 °C. The data are compared to spectra of Sm3+ in other matrices. The mechanisms of PL excitation and quenching are discussed.

Transport properties of structures containing a-Si : H : Er

The I–V characteristics of diode structures containing sputtered a-Si : H : Er films for potential LED applications have been studied. The a-Si : H : Er films were deposited on p+ Si substrates. On top of the film, ZnO : Al transparent conductive layers, 3 mm in diameter, were deposited. The back contacts were made of Al. The Er concentration in the films was varied between 0.05 and 0.16 at %, and the hydrogen concentration between 14 and 24 at %. It was found that the log–log plot of the forward I–V characteristic of a typical sample consisted of several regions with different slopes. On the basis of models put forward in the literature, it is proposed that the electron transport in the studied structure is governed by unipolar carrier injection, and the space charge limited currents are controled by two types of electron trap. The first are probably deep acceptor traps formed by dangling bonds, and the second are donor-like levels created by Er–O complexes. The energy positions of these traps are estimated.

Optical, film surface and photovoltaic properties of PTB7-Fx-based polymer-organic solar cells prepared in ambient conditions

Bulk heterojunction organic solar cells based on active layer of PTB7-Fx (x = 0–100), conjugated block copolymer incorporating benzo[1,2-b:4,5-b′]dithiophene and partly fluorinated thieno[3,4-b]thiophene units, and fullerene derivatives as phenyl C61 and phenyl C71 butyric acid methyl esters sandwiched between a transparent anode of indium tin oxide, a hole conducting layer of either poly(ethylene dioxythiophene) : polystyrene sulfonate or magnetron-sputtered molybdenum oxide and evaporated aluminum cathodes were fabricated. Polymer-organic thin films were prepared at 1:1.5 mass ratio of the donor:acceptor mixture and deposited from dichlorobenzene solution of different concentrations by spin coating in ambient conditions. To control the active layer morphology, the films were subjected either to post-deposition treatments (annealing) at different temperatures or incorporation of an additive such as diiodooctane. Optical transmission, film surface topography and thickness measurements were used to characterize the active layer. Test devices with the architecture described above were prepared and their current–voltage characteristics, quantum efficiency and impedance spectra were measured and used to compare the different active layers, architectures and deposition sequences.