Structural,electrical and optical properties of Ge implanted GaSe single crystals grown by Bridgman technique

Structural, optical and electrical properties of Ge implanted GaSe single crystal have been studied by means of X‐Ray Diffraction (XRD), temperature dependent conductivity and photoconductivity (PC) measurements for different annealing temperatures. It was observed that upon implanting GaSe with Ge and applying annealing process, the resistivity is reduced from 2.1 × 10⁹ to 6.5 × 10⁵ Ω‐cm. From the temperature dependent conductivities, the activation energies have been found to be 4, 34, and 314 meV for as‐grown, 36 and 472 meV for as‐implanted and 39 and 647 meV for implanted and annealed GaSe single crystals at 500°C. Calculated activation energies from the conductivity measurements indicated that the transport mechanisms are dominated by thermal excitation at different temperature intervals in the implanted and unimplanted samples. By measuring photoconductivity (PC) measurement as a function of temperature and illumination intensity, the relation between photocurrent (I_PC) and illumination intensity (Φ) was studied and it was observed that the relation obeys the power law, I_PC αΦⁿ with n between 1 and 2, which is indication of behaving as a supralinear character and existing continuous distribution of localized states in the band gap. As a result of transmission measurements, it was observed that there is almost no considerable change in optical band gap of samples with increasing annealing temperatures for as‐grown GaSe; however, a slight shift of optical band gap toward higher energies for Ge‐implanted sample was observed with increasing annealing temperatures. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of NaCl filler on ferroelectric phase and polaron configurations of PVDF films

Polyvinyldene fluoride (PVDF) films filled with NaCl of mass fraction range 1 ≤ W ≤ 6 % were prepared by casting technique. Their crystalline structure, thermal, optical properties and Electron spin resonance (ESR) were examined. X‐ray diffraction (XRD) and differential thermal analysis (DTA) measurements indicated a maximum ferroelectric β‐phase increment at 4%. DTA was used to identify the phase transition temperatures, the order of reaction and the activation energy of melting. The UV‐Visible optical absorption implied a minimum value of the estimated optical energy gap at W = 4%. ESR spectra contained a Lorentizian signal exhibiting a minimum value of the symmetry factor at W = 4%. The energy levels of the optical gap boundaries were though to contribute to ESR transitions. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigations on Sn-doped Ni oxide thin films and their use as optical sensor devices

Tin-doped NiO thin films (5.0, 7.4, and 9.3 at.% Sn) have been prepared by thermal oxidation of vacuum deposited films of pure Ni and Sn elements on glass and silicon substrates. The prepared films were comprehensively characterised by X-ray fluorescence, X-ray diffraction, UV–VIS-NIR absorption spectroscopy, and electrical (ac and dc) measurements. Experimental data indicate that Sn⁴⁺ doping slightly stress the host NiO crystalline structure and change the optical and electrical properties. The electrical and optical behaviours of the Sn-doped NiO films show that they are wide-band semiconductors with band gap 3.69–3.76 eV and have insulating properties. The ac and dc electrical measurements show that it is possible to use Sn-doped NiO as an optical-sensitive oxide in metal-oxide-silicon configurations.     

Growth and characterization of semiconducting cadmium selenide thin films

Semiconducting thin films of cadmium selenide have been grown by conventional thermal evaporation technique. The effect of various growth parameters like rate of deposition and deposition temperature has been studied in detail. Films deposited at room temperature are cadmium rich with segregated selenium globules. A deposition temperature of 453 K has been found to yield stoichiometric, homogeneous films. The films have been analysed for optical band gap and thermal activation energies. Films of low electrical resistivity have been obtained for possible applications.     

Effect of heat treatment on the optical absorption of tin diselenide thin films

Optical absorption measurements were made on tin diselenide thin films grown on glass substrates by thermal evaporation of stoichiometric bulk compound. In the as-deposited films, optical absorption was dominated by an indirect transition at lower photon energies with a band gap of 1.4 eV. The direct allowed transition with a band gap of 1.62 eV was predominant at higher energies. Annealing the film at elevated temperatures resulted in the decrease of both the direct and indirect transition band gaps.     

Preparation,single crystal growth and characterization of bis(tetrabutylammonium)bis(4,5‐dithiolato‐1,3‐dithiole‐2‐thione)copper

The preparation and single crystal growth of bis(tetrabutylammonium)bis(4,5‐dithiolato‐1,3‐dithiole‐2‐thione)copper, (I), are described. The energy gap Eg of (I) is about 2.38 eV. The nonlinear optical susceptibility χ⁽³⁾ is about 1.3×10^‐3 esu at 1064 nm. The characterization of (I) has been performed by electronic absorption, infrared and X‐ray powder diffraction spectroscopy. The thermal behavior of (I) has been investigated by means of thermogravimetric analysis (TGA) and differential thermal analysis (DTA) measurements in air. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural,compositional and optical properties of gallium selenide thin films doped with cadmium

Polycrystalline cadmium doped gallium selenide thin films were obtained by the thermal co‐evaporation of GaSe crystals and Cd grains onto glass substrates. The structural, compositional and optical properties of these films have been investigated by means of X‐ray diffraction, energy dispersive X‐ray analysis and UV‐visible spectroscopy techniques, respectively. Particularly, the elemental analysis, the crystalline nature, the energy band gap, the refractive index, the dispersion energy and static dielectric constant have been identified. The absorption coefficient spectral analysis in the sharp absorption region revealed a direct forbidden energy band gap of 1.22 eV. The cadmium doping has caused a significant decrease in the values of the energy band gap and in all the dispersive optical parameters, as well. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Indium phosphide nanofibers prepared by electrospinning method: Synthesis and characterization

Nine sets of (3 × 3) InP nanofiber samples have been successfully prepared at three different voltages (20, 25, 30 kV) and at three separate heights (5, 7, 10 cm) by electrospinning with a constant precursor flow rate of 0.3 mLh⁻¹. The crystalline structure, thermal, morphologies and nanostructure, electrical, and optical properties of the samples are characterized by X‐ray powder diffractometer (XRD) and thermal gravity‐differential scanning calorimeter (TG‐DSC), scanning electron microscopy (SEM), by Four‐Point Probe Technique (FPPT,) and ultraviolet/visible spectrometry (UV/VIS), respectively. From these measurements, we have found the formation of stoichiometric nanostructured InP with zinc‐blende structure and having lattice parameter of a = 5.874 Å, weight loss of 64.59% and crystallization temperature of 500°C, average fiber diameter of 65.82 nm, the activation energies, E_a, of the samples, and band gap energy, E_g, of the nanofibers developed at constant applied voltage 30 kV. The band gap energies determined at different distances 5, 7, and 10 cm are found to be as 1.29, 1.37, and 1.30 eV, respectively.     

Optical properties of thermally evaporated CdS thin films

CdS thin films of varying thicknesses were deposited on cleaned glass substrates at room temperature by thermal evaporation technique in a vacuum of about 2 x 10^‐5 torr. UV‐VIS spectra of the films were studied using the optical transmittance measurements which were taken in the spectral region from 300 nm to 1100 nm. The absorbance and reflectance spectra of the films in the UV‐VIS region were also studied. Optical constants such as optical band gap, extinction coefficient, refractive index, optical conductivity and complex dielectric constant were evaluated from these spectra. All the films were found to exhibit high transmittance (∼ 60 ‐ 93 %), low absorbance and low reflectance in the visible/near infrared region from ∼ 500 nm to 1100 nm. The optical band gap energy was found to be in the range 2.28 – 2.53 eV. All the films annealed at 300°C for 4 hours in vacuum (∼ 10^‐2 torr) showed a decrease in the optical transmittance with its absorption edge shifted towards the longer wavelength, leading to the result that the optical band gap decreases on annealing the films. Also, on annealing crystallinity of the films improves, resulting in decrease in the optical transmittance. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Time-resolved photoluminescence study of excited states in nanostructured melanin

Abstract

Time resolved emission spectra (TRES), 3D maps of photoluminescence excitation-emission (PLE maps), 3D maps of TRES, and kinetics of photoluminescence (PL) spectra of natural and synthetic melanin, as well as melanin composites with surfactant, sodium dodecyl benzenesulfonate, have been studied. The kinetics of PL and TRES for the melanin and the composites were investigated at low temperatures that allowed to discuss melanin’s intermolecular complexation, nano-aggregation and formation of charge transfer states. Particularly, the radiation of Frenkel excitons from fragments of different degrees of conjugation that are the parts of molecular nanodisks of melanin is observed. Furthermore, charge transfer excitons (CT-excitons) can be formed and observed at both room and helium temperatures. Some bands in the PL spectra of melanin, depending on temperature, behave like excimer radiation. The studies are aiming to understand the electronic properties of various melanins, particularly natural ones.     

Modification of amorphous hydrogenated silicon by co-sputtered aluminum

The electronic and optical properties of a-Si_1?xH_x have been modified by the incorporation of aluminum. Samples were prepared by rf sputtering in a hydrogenated atmosphere from a composite silicon-aluminum target. This paper reports on several modified material parameters including the optical band gap, electrical conductivity, and thermal activation energy. Aluminum concentrations up to 10.6% in the target have been investigated. It is observed that the optical band gap remains constant at 1.83 eV for Al concentrations up to 2.7%. For higher concentrations there is a marked decrease in optical gap. The conductivity initially decreases with small Al concentration and the activation energy increases, characteristic of compensation of the inherently n-type material. For higher Al concentrations the conductivity increases by seven orders of magnitude and the activation energy decreases to a minimum of about 0.2 eV. The increase in conductivity can be explained by both the movement of the Fermi level and the shrinking band gap. Microprobe analyses have also been performed to determine the amount of Al actually incorporated into the films. Finally, implications of these results are discussed and compared to previously reported results on gas phase doping and ion implantation.     

Synthetic Diamond — Basic Research and Applications

Non-equilibrium growth of synthetic diamond layers by chemical vapour deposition (CVD) techniques on heterosubstrates has largely been improved over the past decade. On silicon substrates highly textured and oriented diamond films can be grown with optical transparencies and thermal conductivities suitable for broad-band optical windows and heat spreaders. Boron pulse-doping of homoepitaxial diamond layers leads to high p-conductivity at room temperature allowing the fabrication of Schottky diodes and field effect transistors operating at temperatures up to 1000 K. Other devices such as sensors and detectors are being successfully fabricated. At the same time many basic questions remain to be solved including efficient n-type doping.     

Photobleaching in a g-Ge0.27Sn0.03Se0.7 thin film

Photobleaching was observed by optical transmission measurements performed using constant 655 nm (sub-band gap) illumination of a g-Ge_0.27Sn_0.03Se_0.7 thin (<2 μm) film sample deposited by thermal evaporation of bulk starting materials on silica. The threshold power density for destroying the film was 2.7 kW cm⁻² for a film thickness of 1.75 μm. The effective thermal conductivity was calculated and the temperature increase in the illuminated spots was estimated. A little below the destruction threshold photobleaching may be attributed to band gap increase with thermal annealing. But photobleaching is also observed at low power densities inducing spot temperatures well below the glass transition. No noticeable crystallization of the bleached film region was observed either optically or by X-ray diffraction.     

Optical characterization of As2Se3-Ag4SSe-SnTe amorphous thin films

Amorphous thin films from the system As₂Se₃-Ag₄SSe-SnTe were prepared by thermal vacuum evaporation from the corresponding bulk glassy samples. The film structure and surface morphology were investigated by scanning electron microscopy and atomic force microscopy; the results revealed uniform, smooth and homogeneous coatings. The amorphous chalcogenide films are transparent in a wide spectral range as shown by transmission and reflection measurements in the VIS and NIR regions. The optical band gap was determined and its compositional dependence is discussed in terms of structural considerations and the formation of charged defect centers.     

Structural,optical and electrical characterization of HgxCd1‐xTe polycrystalline films fabricated by two‐source evaporation technique

Two‐source thermal evaporation technique was used to prepare Hg_xCd_1‐xTe thin films onto scratch free transparent glass substrates. The structural investigations revealed that thin films were polycrystalline in nature. Transmittance measurements in the wavelength range (500‐2700 nm) were used to calculate optical constants. The analysis of the optical absorption data showed that the optical band gap was of indirect type. In the composition range 0.05 < x < 0.25 the films exhibited an optical band gap between 1.29 and 0.98 eV. In the same composition range the films were p‐type and exhibited a resistivity, which varied between 10² and 10^‐1 Ω‐cm. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of the charge state on the photoluminescence spectra of melanin

Abstract

Steady-state and time-resolved photoluminescence (PL) spectra of natural and synthetic melanin, as well as melanin composites with surfactant, sodium dodecylbenzenesulfonate, and polymethine dye, astraphloxin, have been studied. The melanin molecules having differently charged peripheral groups with terminal O atoms and OH groups mixed with the negatively charged surfactant or positively charged astraphloxin dye feature new PL spectral bands. The kinetics of PL and time-resolved emission spectra for the melanin and the composites were also investigated and discussed in terms of intermolecular complexation, nano-aggregation and formation of charge-transfer states. The studies are aiming to understand the electronic properties of various melanins, particularly natural ones.     

On the thermal and optical properties of ZnSe and doped ZnSe crystals grown by PVT

Single crystals of ZnSe, phosphorus-doped ZnSe and phosphorus with Gallium-doped ZnSe have been grown by physical vapour transport technique. The thermal properties like thermal diffusivity, effusivity and conductivity are studied using photoacoustic spectroscopy and differential scanning calorimetry, for the use of substrate. Their optical properties are also studied using Raman, photoluminescence and photocurrent for the study of the site symmetry of the phosphorus. Our measurements reveal the possibility of phosphorus going to deep level thus reducing to C_3v level.     

Synthesis and optical properties of poly[4-methacryloxy-(4′-carboxy)-azobenzene]

Abstract

In the present work, the homopolymer built from the free radical polymerization of methacrylic monomer incorporating an azobenzene side-group has been synthesized and structurally characterized. The optical properties such as refractive index, extinction coefficient, absorption coefficient and optical energy band gap of poly[4-methacryloxy-(4′-carboxy)-azobenzene] thin film prepared by high vacuum sublimation method were determined using spectroscopic ellipsometry combined with transmittance measurements.

We found that in the spectrum of extinction coefficient there are absorption bands, which are assigned as the n-π* and π-π* electronic transitions of the azo compound.     

Growth and characterization of 4‐Aminopyridinium‐4‐nitro phenolate single crystals

Organic optical material 4‐Aminopyridinium‐4‐nitro phenolate (4AP4NP) has been synthesized, and single crystals of size 20 x 14 x 6 mm³ have been grown from acetone solvent at room temperature by solvent evaporation technique. The grown crystals have been characterized by X‐ray diffraction to determine the cell parameters, and by FT‐IR technique to confirm the formation of the expected compound. The crystal belongs to monoclinic crystal system with space group P2₁/a.The structural perfection of the grown crystals has been analyzed by high‐resolution X‐ray diffraction (HRXRD) rocking curve measurements. The thermal stability of the compound has been determined by TG‐DTA curves. The transmittance of 4AP4NP has been used to determine the refractive index n; the extinction coefficient K and both the real ε_r and imaginary ε_i components of the dielectric constant as functions of photon energy. The optical band gap of 4AP4NP is 2.4 eV. The dielectric and mechanical behavior of the specimen was also studied. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Light and heat induced interdiffusion in Sb/As2S3 nano-multilayered film

The light and heat induced changes in the optical band gap of Sb/As₂S₃ nanomultilayered chalcogenide film has been studied. Even though the changes in optical bandgap are attributed to the light and heat induced interdiffusion, the diffusional intermixing between the layers is rather different with light and heat. The observed difference in the light and heat induced interdiffusion is due to unequal diffusion coefficients of light and heat predicted by thermal spike model.