Transport properties of nanocrystalline silicon and silicon–germanium

We report on the minority carrier lifetime, diffusion length and mobility in nanocrystalline Si and (Si,Ge) p⁺nn⁺ devices. The devices were fabricated on stainless steel using VHF plasma deposition techniques. Minority carrier lifetime was measured using junction reverse recovery techniques. The minority carrier lifetime was found to be well correlated with the inverse of defect density and increased with increasing measurement temperature. Simultaneous measurement of diffusion length and lifetime yielded values for hole mobility.     

Electrical and dielectric properties of glass system NaPO3–KHSO4

Glass samples have been prepared in the NaPO₃–KHSO₄ binary system with the classical melting, casting and annealing steps. Electrical and dielectrical properties of glass samples were studied. Measurements of DC and AC conductivity and complex electrical permittivity of xNaPO₃–(100 ? x)KHSO₄ glass system were carried out at temperatures ranging from room temperature to temperature located 15 °C below glass transition temperature T_g. Results showed that changes of NaPO₃ concentration considerably affect values of observed parameters. DC conductivity of glass increases as NaPO₃ concentration grows until concentration x = 60. However, beyond this value a sharp decrease of DC conductivity was observed. In addition relaxation times showed abrupt changes at concentration x = 60, corresponding to the lowest relaxation times at the temperature 90 °C.     

Electrical and optical properties of amorphous boron and amorphous concept for ß-rhombohedral boron

Measurements of the conductivity (σ), thermoelectric power (S) and thermal conductivity (κ) of amorphous boron are made over wide temperature ranges (T = 77–850 K for σ, T = 300–850 K for S and T = 80–1100 K for κ). The room temperature spectral dependencies of the reflection (R) and absorption (α) coefficients are determined for the wavelength intervals 2–25 μm and 1.3–25 μm respectively. The I–V characteristics are also studied and shown to be consistent with the Poole-Frenkel law.The value obtained for the thermal energy gap of amorphous boron (1.3 eV) is slightly smaller than that of crystal ß-rhombohedral boron (1.4 eV). The temperature dependence of the electrical conductivity can be satisfactorily described by the Mott law $\text{ln}\text{σ ≈ ?(T}{}_0\text{/T)}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$, where $\text{T}{}_0\text{? 10}{}^8\text{K}$. This gives an estimate, N ≈ 10¹⁸ cm^?3, for the concentration of trapping levels responsible for the hopping conduction. The value $\text{?}{}_0\text{? 9}$ is found from the spectral dependence of R while α has Urbach-like character $\text{? α ≈}\text{exp}\text{(}\text{h}\text{?}\text{ω/Δ)}$, where $\text{Δ ? 0.19}\text{eV}$.A comparison is made between amorphous boron and crystalline ß-rhombohedral boron. Because of the very complex crystal structure and the large dimensions of the unit cell of ß-boron, some of its physical properties could be qualitatively described on the basis of the so-called ‘amorphous concept’.     

New chiral metal-mesogenic nanosystems “silver - thiocholesterol” and their adsorption properties

ABSTRACT

New chiral matrices for thin film chromatography were obtained using hybrid metal-mesogenic nanosystems «silver – thiocholesterol» with different metal to ligand ratio, immobilized on silica gel particles. It was shown, that heteroatomic derivative of cholesterol – thiocholesterol and its composition with small silver nanoparticles formed in the system by the chemical reduction of silver ions possess liquid crystalline cholesteric mesophase. Molar ratio between thiocholesterol ligand molecules (L) and silver (Ag) insignificantly influenced on the size of silver nanoparticles formed in the system: for molar ratio Ag : L = 1:5 the main diameter of nanoparticles was equal to (2,7 ± 0,4) nm, for molar ratio Ag : L = 1:2 – (2, 2 ± 0,4) nm, for molar ratio Ag : L = 1:0,5 – (2,1 ± 0,6) nm. The new chiral matrices for thin film chromatography possess enantioselectivity related to optical isomers of 2,2′-diamino-1,1′-binaphtol (DABN) and trifluoroantranylethanol (TFAE). We have succeeded to select optical isomers of TFAE with selecting factor equal to 1,56.     

Electrical and dielectric properties of MnF2–ZnF2–NaPO3 glasses

Direct electrical conductivity and dependencies of complex electrical modulus vs. temperature and frequency have been measured on glasses from the MnF₂–ZnF₂–NaPO₃ system. These glasses are sensitive to atmospheric humidity and as a consequence, the electrical conductivity increases up to temperature of 50 °C. A hydrated layer is created by the effect of water and leads to the significant increase of the electrical conductivity in the case of 0MnF₂–20ZnF₂–80NaPO₃ glass. This behavior is governed by Arrhenius relation where the values of activation energy are increasing and values of the electrical conductivity are decreasing with the amount of MnF₂. Dielectric measurements show that a heterogeneous phase is formed in the bulk of glasses. This may be seen when plotting complex electrical modulus in the complex plane. The records made by the light microscope confirmed the occurrence of the other phase in the bulk of glasses.     

Electrochromic properties of nickel oxide and mixed Co/Ni oxide films prepared via sol–gel route

The Ni oxide and mixed Co/Ni oxide films were prepared by sol–gel dip coating method at optimum conditions. The XRD analysis reveals the pure and Co mixed nickel oxide films to be in amorphous state. The field emission SEM images reveal nanopore like structure for Ni oxide film and well defined grains with pores for Ni oxide films containing 5 wt.% of Co. Electrochromic properties have been studied using cyclic voltammetric (CV) and in situ spectro-electrochemical techniques. The pure and cobalt mixed (5 wt.%) Ni oxide films exhibit anodic/cathodic diffusion coefficient of 4.93 ± 0.14/3.74 ± 0.10 × 10^?10 cm²/s and 10.00 ± 0.24/7.60 ± 0.20 × 10^?10 cm²/s respectively after 300 cycles. The cobalt mixed (5 wt.%) Ni oxide films exhibit the bleached/coloured state transmission of 90.42/7.21% with a photopic constrast ratio of 12.54 and the colouration and bleaching time were 5.9 and 2.4 s respectively. The addition of cobalt beyond 5% leads to poor transparency and inhibited electrochromic switching character.     

Electrical and optical properties of the AsTeIn and GeSeIn chalcogenide systems

The influence of indium on the optical and properties of As_2−xTe_3−xxIn_2x, As_20−xTe_80−xIn_2x and Ge_20−xSe_80−xIn_2x is described. In Te-containing glasses the Fermi level is shifted by 0.05 eV and in Se-containing glasses by 0.2 eV towards the valence band.     

Optical and electrical properties of β-Ni (OH)2/reduced graphene oxide nanocomposite

Abstract

Structural and optical properties of β-Ni(OH)₂/reduced graphene oxide nanocomposite which was obtained by ultrasound dispersion of hydrothermally synthesized β-Ni(OH)₂ and reduced graphene oxide were investigated. Frequency dependencies of conductivity for initial components and composite materials were studied. Specific surface areas of reduced graphene oxide, β-Ni(OH)₂ and β-Ni(OH)₂/reduced graphene oxide are 402, 20 and 88 m²/g, respectively. It was established that initial ultrafine β-Ni(OH)₂ and β-Ni(OH)₂/reduced graphene oxide composite have a percolation electrical conductivity. The enlarging of β-Ni(OH)₂/reduced graphene oxide optical direct band gap energy compared to the initial components due to O/C contents changes was observed.     

Optoelectronic properties of hot-wire silicon layers deposited at 100 °C

Hot-wire chemical vapor deposition is employed for the deposition of amorphous and microcrystalline silicon layers at substrate temperature kept below 100 °C with the aid of active cooling of the substrate holder. The hydrogen dilution is varied in order to investigate films at the amorphous-to-microcrystalline transition. While the amorphous layers can be produced with a reasonably low defect density as deduced from subgap optical absorption spectra and a good photosensitivity, the microcrystalline layers are of a lesser quality, most probably due to a decrease of crystallinity during the film growth. In the amorphous growth regime, the Urbach energy values decrease with increasing hydrogen dilution, reaching a minimum of 67 meV just before the microcrystalline threshold. By varying the total gas pressure, the growth rate of the films is changed. The lowest deposition rate of this study (0.16 nm/s) produced the amorphous sample with the highest photoresponse (1 × 10⁶).     

Thermal,structure and morphological properties of lithium disilicate glasses doped with copper oxide and their glass–ceramic derivatives

This research aims to investigate and compare the structural and the morphological properties of both lithium disilicate glasses doped with copper oxide and their glass–ceramic derivatives. Density measurements were measured for all samples by Archimedes method at room temperature. Differential scanning calorimetric analysis was used to determine the glass transition temperature (T_g) and crystallization temperature (T_c) for all glasses. The glass transition temperature was observed to decrease with increasing CuO concentration indicating the formation of non-bridging oxygen bonds in the glass network. X-ray analysis patterns reveal the appearance of crystalline lithium metasilicate phase as the main phase within the glass–ceramic derivatives, and their crystallite sizes were observed to decrease as the CuO increased. Experimental infrared absorption data indicate the existence of characteristic vibrational bands due to structural building SiO₄ units in resemblance to the same vibrations observed from traditional crystalline silicates. Scanning electron microscopic investigations show the vitreous nature for lithium disilicate glasses and the distinct crystalline morphological features for the corresponding glass–ceramic derivatives.     

MBE growth and transport properties of silicon δ-doped GaAs/AlAs quantum well structures for terahertz frequency detection

We present the electrical characterization of n-type GaAs/AlAs multiple quantum well (MQW) structures designed for terahertz (THz) radiation sensing at cryogenic temperatures. The samples were grown by solid source molecular beam epitaxy (MBE) and were δ-doped with silicon atoms at each potential well center. Temperature dependent Hall effect data show that (i) the conduction in these planar doped structures is thermally activated below 180 K, (ii) the free carriers sheet densities are near the metal–insulator transition, and (iii) the low temperature mobility is controlled by ionized impurities scattering. The study of the magneto-transport properties at 1.3 K further indicates that only the fundamental electronic sub-band of the two-dimensional electronic gas is populated. The MQW samples were then processed into lateral mesa-shaped photodetectors to investigate their spectral response in the THz frequency range. The preliminary experimental results for the proposed detection scheme, which involves transitions in the confined shallow donor impurity states, are described.     

Magnetic properties and crystal structure of β-Ta

Polycrystalline samples of β-Ta with Frank-Kasper σ-phase structure prepared by electrolysis are studied. The atomic parameters are determined by the Rietveld method. The magnetic susceptibility is measured in the temperature range 4.2 < T < 273 K and the dependence of the magnetization on the magnetic field strength is measured at 4.2 and 77 K. The analysis of interatomic distances and the results of magnetic measurements indicate that clusters with a localized excess charge exist in β-Ta. The data obtained in this study suggest structural changes in β-Ta at T < 77 K.     

Microcrystalline silicon oxide (μc-SiOx:H) alloys: A versatile material for application in thin film silicon single and tandem junction solar cells

We report on the development and application of n-type hydrogenated microcrystalline silicon oxide (μc-SiO_x:H) alloys in single and tandem junction thin film silicon solar cells. Single junction microcrystalline silicon (μc-Si:H) solar cells are prepared in n-i-p deposition sequence where n-type μc-SiO_x:H films serve as window layers. In tandem solar cells, μc-SiO_x:H layers are placed between amorphous (a-Si:H) and μc-Si:H component cells, serving as an intermediate reflector. The requirements for μc-SiO_x:H layer depending on its application are discussed. Our results show that the optical, electrical and structural properties of μc-SiO_x:H can be conveniently tuned over a wide range to fulfil various requirements for applications in both types of cells. Additionally, the properties of μc-SiO_x:H layers appear to be substrate dependent, which should be taken into account when layers are utilized in cells. The advantages of low refractive index and high optical band gap allow to achieve high efficiencies of 9.2% and 12.6% for single junction and tandem solar cells, respectively.     

Vibrational spectra,electronic properties and effect of alkyl chain length on chemical stability of nematogens – A comparision using DFT and HF methods

The vibrational spectra of lower homologous series of nematogenic p-n-alkylbenzoic acids (nBAC) with 4 (4BAC) and 5 (5BAC) carbon atoms in the alkyl chain have been investigated using the Density Functional Becke3-Lee-Yang-Parr (B3LYP) level with the basis set 6–31++G(d.p) and Hartree Fock (HF) with the same basis set. The observed vibrational spectra has been resolved and assigned in detail for comparision with both the molecules. These results indicate that DFT and HF values are slightly different at both the level. A comparision of electronic properties such as HOMO (E_HOMO), LUMO (E_LUMO) energies, energy gap (E_g), ionization potential (I), electron affinity (A), electro negativity (χ), chemical hardness (η), electronic chemical potential (μ), electrophilicity index (ω), and softness (S) has been made. It has been observed that decrement occurred in the energy band gap value of isolated molecule with increment in alkyl chain length. This provides valuable information regarding the stability of liquid crystal materials.     

Energy of Pair Interaction of Molecular Systems: Excited State – Ground State of Molecules Pentyl – and Pentyloxy – Cyanobiphenyls

Abstract

The energies of pair interaction of molecular systems: first excited state and ground state of molecules pentyl - and pentyloxy - cyanobiphenyls was calculated and equilibrium configuration of these molecular systems was determinate. Pair orientated destabilization energies evaluation is given at the absorbtion and emission processes.     

Detailed study of surface and interface properties of μc-Si films

The properties of surfaces and interfaces of microcrystalline films deposited by radio-frequency plasma enhanced chemical vapor deposition (RF-PECVD) were studied by spectroscopic ellipsometry. The effect of the low-power reactive ion etching (RIE) on the properties of the films was investigated. The surface properties could be effectively improved using RIE to eliminate of the top porous part of the films, without deterioration of bulk layer properties. Ellipsometric measurements from both film and substrate sides were used for the study of the interface properties of the various samples deposited on fused silica substrates. We show that the crystalline fraction that determined from modeling of ellipsometric spectra measured from the film side could be overestimated.     

Electrical properties of glass and liquid semiconductor system (As2Te3)x(Tl2Se)1−x

We have measured the electrical conductivity and thermoelectric power of the pseudobinary system (As₂Te₃)_x(Tl₂Se)_1−x both in the glass and liquid states. The effect of the addition of excess Tl has also been examined. It has been found that the activation energy of electronic conduction is closely related to the average atomization energy of these materials.     

Evolution of mechanical properties and final textural properties of resorcinol–formaldehyde xerogels during ambient air drying

Porous carbon xerogels can be obtained by convective drying of resorcinol (R)–formaldehyde (F) hydrogels, followed by pyrolysis. Drying conditions have to be carefully controlled when crack-free monoliths with well-defined shape and size are required. The knowledge of the mechanical properties of the RF xerogels and their evolution with water content is essential to model their thermo-hygro-mechanical behavior during convective drying and avoid mechanical stresses leading to deformation and cracking of the sample. The shrinkage behavior and the mechanical properties of RF xerogels obtained with R/C ratio ranging from 300 to 1500 were investigated. R/C greatly influences the shrinkage and mechanical properties of the wet gel, on the one hand, and the mechanical and textural properties of the dried gel, on the other hand. The smaller the R/C, the higher the shrinkage, the stiffening, and the viscoelastic character of the xerogels. Water content has an influence on both the stiffness of the gels and the viscoelastic response. Generally, samples lose their mechanical viscous character and become more rigid when they are dried. Finally, mercury porosimetry measurements showed that the gels exhibit a marked lowering of their stiffness upon compression, interpreted as a result of the heterogeneity of the microstructure.     

Structural and magnetic properties of ZnO and Zn1−xMnxO nanocrystals

Chemical precipitation of metal-ions from aqueous solution has been successfully used to produce Zn_1?xMn_xO nanocrystals, in the form of nano-powder. X-ray diffraction (XRD) measurements reveal that the as-prepared samples are single-phase materials and their lattice constant changes with the variation of Mn-concentration, which indicates the incorporation of Mn²⁺ into the hosting ZnO. These findings are corroborated by the observation of the well defined six hyperfine lines of Mn²⁺ ion in the electron paramagnetic resonance (EPR) spectra of the samples with a low Mn-concentration, and of a broad EPR line, which manifests the onset of Mn–Mn exchange interaction, in the samples with an elevated value of x.     

Structural,electrochemical and optical properties of 4′-n-alkyl-4-cyanobiphenyl (nCB) – A computational approach

The structures of nCB (n = 6 & 7 where n is the number of carbon atoms in the alkyl chain) have been optimized using the Becke3-Lee-Yang-Parr (B3LYP) hybrid functional with 6–31G+(d) basis set using the crystallographic geometry as input. The electronic structures of the dimer molecules have been computed using the optimized geometries. The spectra of the dimer molecules have been calculated by employing the DFT method. The features of electronic transitions and excited states have been calculated via configuration interaction singles (CIS) with the semiempirical Hamiltonian Zerner intermediate neglect of differential overlap (ZINDO). The photo sensitivity of liquid crystalline alkyl cyanobiphenyl has been presented on ultraviolet (UV) absorption based approach through Density functional theory (DFT) calculations. The structural and electrochemical properties such as HOMO (H), LUMO (L), and energy gap (Eg = E_L – E_H) have been investigated. A comparison of dimers during the different modes of interactions suggests an absorption maxima at longer wavelength for 7CB, indicating the high photo sensitivity. Further, the 6CB dimers exhibit a lower band gap; hence its conductivity is high in comparison with the 7CB dimers.