Arabinda Nayak

Derivative spectra of polycrystalline Zn3P2 thin films

Abstract Polycrystalline Zn3P2 thin films prepared by e-beam evaporation technique on glass substrates kept at 300°C, have exhibited low resistivity value, nearly stoichiometry and good adherence which are very promising features for device applications. Optical studies namely, optical absorption and derivative spectra, have been made which helped to understand not only the nature of the optical band gap (1.55 eV) but also the details of the other transitions related to the band structure (tetragonal) of this compound semiconductor. The results indicated the direct optical transitions at energies greater than 1.70 eV and the energy values for spin-orbit and crystal field splitting are found to be 0.16 and 0.02 eV respectively.

Preparation and characterization of Zn3P2-Cd3P2 solid solutions

Solid solutions of Zn3P2-Cd3P2 systems of the type (ZnxCd1−x)3P2 have been prepared by direct reaction of the constituent elements (Zn, Cd, P) for values of x equal to 0.0, 0.2, 0.4, 0.5, 0.6, 0.8 and 1.0. X-ray diffraction data indicate that all the systems crystallize in tetragonal (α) phase only, exhibiting preferred orientation along the (220) and (224) directions. The lattice parameters, a and c, and the interplanar spacing, d, vary linearly with x, obeying Vegards law. The systems show minimum conductivity at room temperature for composition corresponding to x values in the range 0.4–0.6. Electrical conductivity for all systems is measured in the temperature range 100–450 K. In view of the estimated (low) values of the activation energy the conduction process in the different temperature regions has been attributed to the presence of shallow trapping levels in the systems.

Preparation and characterization of Cd3P2 thin films

Cd3P2 thin films have been deposited on glass substrates by electron beam evaporation technique. The amorphous as‐deposited films, after annealing in vacuum in the temperature range 180–200 °C, developed the crystallinity. Such polycrystalline films exhibited preferential orientation along the (224 400) direction. The films indicated n‐type conductivity with resistivity value around 1.6 Ω cm at room temperature. The composition, ionicity, and Madelung constant, calculated from x‐ray photoelectron spectroscopy data, are Cd3P1.7, 0.08, and 1.68, respectively. From the optical (R and T) and modulation (derivative) spectra, the different parameters, namely, refractive index (n∼3.83), band gap (Eg=0.61 eV), band splitting parameters (crystal field ΔCF=0.03 eV, spin‐orbit Δ0=0.08 eV) have been obtained. Photoluminescence emission related to band‐to‐band transition is observed around 0.51 eV.

Electrical properties of electron-beam-evaporated Zn3P2Cd3P2 alloy films

Abstract Electrical conductivity, Hall mobility and spectral photoconductivity response measurements conducted on polycrystalline (ZnxCd1−x)3P2 thin films, for various compositions (x = 0.0, 0.2, 0.4, 0.5, 0.6, 0.8 and 1.0), are reported in this paper. Zn3P2-rich films are found to be p-type, while Cd3P2-rich films are n-type. Compensation effects are observed for the films with composition x = 0.5 and 0.6, which exhibited comparatively very low conductivity. The photoconduction spectrum indicated broad maxima at the wavelengths corresponding to the optical band gap. The results are discussed taking into account the role of the native defects of these systems, namely, the P interstitials, P vacancies and their associated complexes.

Microstructure and dielectric functions of Ge nanocrystals embedded between amorphous Al2O3 films: study of confinement and disorder

Ge nanocrystals (6–9 nm) embedded between amorphous Al2O3 films were produced in a cluster beam deposition system. The microstructural evaluation and compressive stress experienced by the Ge nanocrystals due to the presence of an oxide layer, nanoparticle size distribution and their changes due to thermal annealing were studied by X-ray diffraction, HRTEM and Raman spectroscopy. Spectroscopic ellipsometry was used to measure the dielectric functions of the deposited films. A multilayer model based on the effective medium approximation was used to analyze the variation of percentage of defects and the extent of disorder with particle size of the nanocrystals. The correlation between the microstructural characteristics and optical properties was established by evaluating standard sum rules. Germanium nanocrystals show visible photo luminescence at room temperature around 3.0 and 2.8 eV. However, a peak shift towards lower energies with increasing particle size due to thermal annealing was not detected. The experimentally observed luminescence is presumably originated due to the presence of oxide-related defect centers at the interface between the germanium nanocrystals and the embedded oxide layers.

Bonding and optical properties of diamond-like hydrocarbon films deposited by plasma decomposition of acetylene—the role of water vapour addition

Abstract Diamond-like hydrocarbon (DLHC)/amorphous hydrogenated carbon (a-C:H) films were deposited on quartz and silicon substrates by DC glow discharge decomposition of C2H2 diluted with H2 and a small amount of H2O (0.8 to 1.4 vol.%) vapour. The presence of H2O in the discharge markedly enhanced the growth rate and diamond-like properties of these films. The atomic oxygen and OH radical produced in the plasma owing to dissociation of H2O vapour, played crucial roles in growing and stabilizing diamond-like carbon films.

Photoluminescence spectroscopic investigation on the quality of diamond films grown in oxy-acetylene combustion flame

Abstract Polycrystalline diamond films were deposited on (100) silicon substrates at 700 °C by oxy-acetylene combustion flame operated under near neutral (C 2 H 2 /O 2 ∼1.0) condition in the open atmosphere. The thickness profile, nucleation density, surface morphology, and microstructure of the diamond films were analysed using surfometry, optical microscopy, X-ray diffraction and scanning and transmission electron microscopy. Photoluminescence (PL) spectroscopy (380 to 680 nm spectral range) was used for studying the distributions of crystalline defects and impurities at different growth regions along the radius of the circular (5–6 mm diameter) films. The film deposited from acetylene feather showed structural and crystalline inhomegeneities. The PL spectra originated from the centre of the growth region indicated a broad bright-blue luminescence band (around 2.81 eV). This bright-blue luminescence band appeared owing to the recombination of trapped electrons at the donor states with holes in the valence band. Substitutional nitrogen incorporated during film growth from open atmosphere possibly created deep donor states. The spectra recorded from the edge of the growth region of the same film exhibited some structural features (other than 2.81 eV) at 2.16, 2.34, (2.63 and 2.75), (3.10 and 3.19) eV. The prominent appearance of such PL emission bands from the optical centres involving isolated nitrogen–carbon vacancy, carbon interstitials and nitrogen–carbon interstitial related defects respectively, indicated that defect/impurity incorporations at the edge of the deposit were higher than the centre of the deposit. The radial gradient of substrate temperature and the non-uniform distribution of flame species/impurities were mainly responsible for such structural inhomogeneities. However, the PL spectra of diamond films deposited by the periodic exposure into feather and outer zone of the flame with time period ( t D : t E =12 s: 3 s) ratio, exhibited the bright-blue luminescence band (around 2.81 eV) and a broad pink luminescence band centred around 2.16 eV. The structure around 2.16 eV was believed to be originated from the nano crystalline diamonds precipitated in the carbon matrix. The PL emission feature was found identical through out the growth region for this film. It is concluded that the thermal recycling to the growing films minimized the structural inhomogeneities.

Band-gap tuning and optical response of two-dimensional SixC1−x : A first-principles real-space study of disordered two-dimensional materials

We present a real-space formulation for calculating the electronic structure and optical conductivity of such random alloys based on the Kubo-Greenwood formalism interfaced with the augmented space recursion (ASR) [A. Mookerjee, J. Phys. C: Solid State Phys. {\bf 6}, 1340 (1973)] formulated with the Tight-binding Linear Muffin-tin Orbitals (TB-LMTO) basis with van Leeuwen-Baerends corrected exchange (vLB) [Singh et al, Phys. Rev B {\bf 93}, 085204, (2016)]. This approach has been used to quantitatively analyze the effect of chemical disorder on the configuration averaged electronic properties and optical response of 2D honeycomb siliphene Si

Disorder induced lifetime effects in binary disordered systems: A first principles formalism and an application to disordered graphene

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Microstructure, dielectric response and electrical properties of polypyrrole modified (poly N-vinyl carbazole–Fe3O4) nanocomposites

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