A. Dussan

Transport mechanism in lightly doped hydrogenated microcrystalline silicon thin films

Boron-doped microcrystalline silicon films have been deposited in a plasma-enhanced chemical vapor deposition system using silane diluted in hydrogen, and diborane (B2H6) as a dopant gas. The temperature dependence of the dark conductivity has been measured from 120to420K in all samples. In the high-temperature range above room temperature, the carrier transport is found to be thermally activated, with a single activation energy that changes with the B2H6 compensation degree. In the low-temperature range (300–120K), variable range hopping (VRH) was established as a predominant electronic transport mechanism for all samples, with the exception of the sample with a diborane concentration of 12.5ppm. A model for Mott’s VRH, referred to as the “diffusional model,” which yields a relation between the conductivity and the localized density of gap states, is presented. Using classical equations from the percolation theory and the diffusional model, the density of states near the Fermi level, as well as the hoppi...

Synthesis of self-organized TiO2 nanotube arrays: Microstructural, stereoscopic, and topographic studies

In this work, titanium dioxide nanotubes were prepared by using titanium foils via electrochemical anodization in ethylene glycol solutions containing different amounts of water and fluoride in the ranges of 1%–3% and 0.15%–0.5%, respectively, to determine their effects on morphology, optical, and crystalline structure properties. Annealing processes were performed on all samples in the range between 273 and 723 K. Morphology and structure properties of the samples were studied by scanning electron microscopy, X-ray diffraction (XRD), and transmission electron microscopy. Titanium dioxide (TiO2) nanotubes, through anodization method, are strongly influenced by conditions, like fluoride concentration and applied voltages. Tube lengths between 2 and 7 μm were obtained, exhibiting different diameters and wall thicknesses. When alternating voltage was applied, the outer surface of the nanotubes exhibited evenly spaced ring-shaped regions, while smooth tubes were observed when constant voltage was applied. Ref...

Microcrystalline silicon thin films: A review of physical properties

In this work we present a study of the optical, electrical, electronic and structural properties of Boron doped hydrogenated microcrystalline silicon thin films (@mc-Si:H). The films were deposited in an RF plasma reactor using as reactive gas a mixture of silane and diborane, both highly diluted in hydrogen. The Boron concentration in the reactive gas was modified from 0 to 100ppm. The addition of Boron to the silicon films not only moves the Fermi energy level to the center of the gap, but also induces changes in all the physical properties. The Boron effect on structural and morphological properties was studied by X-ray diffraction and atomic force microscopy (AFM); the rugosity and grain size increased with the Boron concentration. The absorption coefficient measured by the constant photocurrent method (CPM) at low photon energies also showed an increase, which can be explained and correlated with an increase in the density of state (DOS) in the gap, due to Borons bonding. At high temperatures (T>300K) the controlling transport mechanism is thermally activated; the curves conductivity log versus the inverse of temperature gives straight lines. The activation energy, measured from the valence band, decreases with Boron concentration, as expected, passing through a maximum, corresponding this point to the position of Fermi energy of an intrinsic film. At low temperatures (T<300K) the predominant transport mechanism was variable range hopping (VRH). The behavior of the charge hopping under different electrical fields was followed. Results showed that conductivity remained constant in a VRH regime only for a narrow range of electrical field.

Electrical and optical properties of thin films with a SnS2 ? Bi2S3 alloy grown by sulphurization

In this work, thin films of SnS2 with increased Bi content were grown by sulphurization of a thin film of Sn:Bi alloy, at temperatures around 300°C. The effect of the Bi concentration on the optical, electrical and structural properties was determined through measurements of spectral transmittance, conductivity and x-ray diffraction XRD respectively. It was found that the optical constants (refractive index n, absorption coefficient α and energy gap Eg) and the electrical conductivity are significantly affected by the Bi concentration. In particular, a variation of the energy gap between 1.44 and 1.63 eV and a change of the conductivity greater than three orders of magnitude were observed when the content of Bi in the Sn:Bi alloy varied between 0 and 100 %. The analysis of the XRD measurements allowed us to find that the SnS: Bi films grow with a mixture of the SnS2 and Bi2S3 phases, independently of the Bi content.

Electronic structure and magnetism of Mn-doped GaSb for spintronic applications: A DFT study

We have carried out first-principles spin polarized calculations to obtain comprehensive information regarding the structural, magnetic, and electronic properties of the Mn-doped GaSb compound with dopant concentrations: x = 0.062, 0.083, 0.125, 0.25, and 0.50. The plane-wave pseudopotential method was used in order to calculate total energies and electronic structures. It was found that the MnGa substitution is the most stable configuration with a formation energy of ∼1.60 eV/Mn-atom. The calculated density of states shows that the half-metallic ferromagnetism is energetically stable for all dopant concentrations with a total magnetization of about 4.0 μB/Mn-atom. The results indicate that the magnetic ground state originates from the strong hybridization between Mn-d and Sb-p states, which agree with previous studies on Mn-doped wide gap semiconductors. This study gives new clues to the fabrication of diluted magnetic semiconductors.

Synthesis and characterization of porous silicon as hydroxyapatite host matrix of biomedical applications

In this work, porous-silicon samples were prepared by electrochemical etching on p-type (B-doped) Silicon (Si) wafers. Hydrofluoric acid (HF)-ethanol (C2H5OH) [HF:Et] and Hydrofluoric acid (HF)-dimethylformamide (DMF-C3H7NO) [HF:DMF] solution concentrations were varied between [1:2]—[1:3] and [1:7]—[1:9], respectively. Effects of synthesis parameters, like current density, solution concentrations, reaction time, on morphological properties were studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM) measurements. Pore sizes varying from 20 nm to micrometers were obtained for long reaction times and [HF:Et] [1:2] concentrations; while pore sizes in the same order were observed for [HF:DMF] [1:7], but for shorter reaction time. Greater surface uniformity and pore distribution was obtained for a current density of around 8 mA/cm2 using solutions with DMF. A correlation between reflectance measurements and pore size is presented. The porous-silicon samples were used as substrate for hydroxyapatite growth by sol-gel method. X-ray diffraction (XRD) and SEM were used to characterize the layers grown. It was found that the layer topography obtained on PS samples was characterized by the evidence of Hydroxyapatite in the inter-pore regions and over the surface.

Nanocrystalline Cu2ZnSnSe4 thin films for solar cells application: Microdiffraction and structural characterization

This work presents a study of the structural characterization of Cu2ZnSnSe4 (CZTSe) thin films by X-ray diffraction (XRD) and microdiffraction measurements. Samples were deposited varying both mass (MX) and substrate temperature (TS) at which the Cu and ZnSe composites were evaporated. CZTSe samples were deposited by co-evaporation method in three stages. From XRD measurements, it was possible to establish, with increased Ts, the presence of binary phases associated with the quaternary composite during the materials growth process. A stannite-type structure in Cu2ZnSnSe4 thin films and sizes of the crystallites varying between 30 and 40 nm were obtained. X-ray microdiffraction was used to investigate interface orientations and strain distributions when deposition parameters were varied. It was found that around the main peak, 2ϴ = 27.1°, the Cu1.8Se and ZnSe binary phases predominate, which are formed during the subsequent material selenization stage. A Raman spectroscopy study revealed Raman shifts asso...

Microstructural and Morphological Properties of Nanocrystalline Cu2ZnSnSe4 Thin Films: Identification New Phase on Structure

This paper presents a study of the structural and morphological properties of thin films of compound Cu2ZnSnSe4. Mass (MX) and temperature of the substrate (TS(Cu)) of compound copper (Cu), were varied. All samples were deposited by co-evaporation method in three stages. From measurements of X-ray diffraction it was possible to establish with TS increasing the presence of associated binary phases quaternary compound during the growth process of the material. It was found that the main peak around, 2θ= 27.1°, predominate binary phases Cu1.8Se and ZnSe. Measurements of X-ray diffraction were performed to pure binary compounds, showing a peak corresponding to the main peak found around the compound. Raman shifts showed associated binary compounds with the observed by XRD. In this work, we report for the first time the binary phase identification Cu1.8Se and ZnSe as part of the structure of the stannite CZTSe. Since the Scherrer equation was found that the crystallite sizes ranged between 30 and 40 nm. A correlation between structure and topography superficial is presented.

On the validity of defect density determinations by the recombination-regime modulated photoconductivity technique

In this work we study the modulated photocurrent arising from experiments performed in the recombination regime on amorphous and microcrystalline semiconductor samples. The influence of the illumination intensity on the results obtained from the recombination-regime modulated photocurrent (RRMPC) technique is studied both from measurements and computer simulations. A wide range of density of states (DOS) distributions is used to simulate different material qualities. Applying a computer code that takes into account all thermal and optical transitions involving gap states, modulated photoconductivity experimental data are simulated. The DOS is then reconstructed from the simulated data following the RRMPC method. Simulations performed under different light intensities lead to the empiric definition of an indicator parameter, which allows us to evaluate the validity of the hypotheses of the RRMPC method for a particular experiment. Measurements for microcrystalline and amorphous hydrogenated silicon samples are presented as examples.

Optical and structural properties of GaSb-doped Mn based diluted magnetic semiconductor thin films grown via DC magnetron sputtering

In this work, diluted magnetic semiconductor GaSb:Mn thin films were grown via DC magnetron Co-sputtering on glass substrates. The effect of synthesis parameters on the optical and structural properties was determined through spectral transmittance measurements and X-ray diffraction (XRD), respectively. Substrate temperature was changed from 373 to 523 K and layer thicknesses were obtained between 200 and 330 nm. All samples were subjected to annealing process to 623 K in situ. It was found that the optical constants (refractive index (n), absorption coefficient (), extinction coefficient (), and energy gap (E1)) are significantly affected by the temperature and target power used during the synthesis process. In particular, a variation of the energy gap between 0.58 and 0.98 eV was obtained when the target power of GaSb varied between 80 and 140 W. GaSb, Mn2Sb and SiO2 phases were observed for lowest values of target power, where SiO2 phase is a contribution of the substrate. The dielectric function of the compound with a dependence on synthesis parameters such as, the substrate temperature and deposition time was obtained, both real () and imaginary part (). Analysis of the XRD measurements allowed to find that the (GaSb)Mn films grow with a mixture of the GaSb and Mn2Sb phases, and an amorphous halo associated with the glass substrate. A correlation between synthesis parameters and optical properties is presented. Diluted magnetic semiconductors, like GaSb:Mn, are considered among promising materials for the development of new spin-electronic devices, high speed quantum-mechanical in computational information, other more; in this case, studies on magnetic properties in digital alloys of GaSb/Mn can be realized around of the optimization of Curie temperature. Copyright