Growth of a Si nanocrystal in an oxygen atmosphere. Computer simulation

Growth of a Si nanocrystal in vacuum and in atomic-oxygen atmosphere was simulated on the basis of molecular dynamics. SiO_x nanoparticles (x = 0–0.42) containing up to 302 atoms were obtained. The shape of the nanocrystals was primarily determined by the amount of oxygen contained in them. The growth of nanocrystals in an oxygen atmosphere was accompanied by the formation of oxygen clusters inside a nanoparticle, which gave rise to considerable internal stresses. In most cases, structural relaxation increases the volume of SiO_x nanocrystals and decreases internal pressure. The angular and topological statistical characteristics of Voronoi polyhedra are indicative of a structural irregularity in oxygen clusters formed inside Si nanoparticles.     

Characterizations of pulsed laser deposited SiC thin films

Thin films of silicon carbide (SiC) were prepared using pulsed laser deposition (PLD) on Si(1 0 0) substrates at a temperature of 370 °C. Various structural characterizations showed the development of short-range SiC precipitates in the films. These films were annealed isochronally at temperatures of 800 °C, 1000 °C and 1200 °C for 2 h under an inert environment. Thermally induced crystalline ordering of SiC into β-SiC phase was investigated by X-ray diffraction (XRD), Raman spectroscopy and Fourier transforms infrared (FTIR) spectroscopic measurements. In addition to the crystallization of SiC films, high temperature annealing resulted in the dissolution of carbon clusters found in the as-grown films.     

Green synthesis of metal sulfide nanocrystals through a general composite-surfactants-aided-solvothermal process

In this paper, we developed a generalized and greener composite-surfactants-assisted-solvothermal process (CSSP) to produce colloidal nanoparticles of metal sulfides. X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectra (XPS) revealed that single-molecular-layer type of MoS₂ nanoparticles with diameter 6–10 nm were successfully synthesized. The molecular structure model of the capped MoS₂ nanoparticles was suggested through further examination by infrared spectra. Hexagonal CdS nanocrystals with spherical, triangle, and hollow sphere shapes were controllably synthesized by varying the experimental conditions. A possible in-situ reduction–sulfidation mechanism was proposed for the formation of Ag₂S nanocrystals, where the metal ions were reduced to metallic nanoparticles before the generation of sulfides. The obtained nanocrystals through this CSSP approach could provide the building blocks for the bottom-up approach to nanoscale fabrication in nanoscience and nanotechnology.     

Influence of oxidization temperature on Zn(O,S) films deposited by electron beam evaporation

Zn(O,S) films were fabricated by oxidizing ZnS thin films deposited by electron beam evaporation method onto glass substrates at temperatures of 350–500℃ for 2 h in an atmosphere of oxygen. The XRD and EDX confirmed that the Zn(O,S) films were obtained successfully. The influence of the oxidization temperature on the optical and electrical properties of the Zn(O,S) thin films was investigated. The experimental results show that the Zn(O,S) thin film oxidized at the temperature of 400℃ exhibits better properties than others, with the transmittance of 86% in the visible region, the band gap energy of 3.36 eV and the resistivity of 3.22 × 10³ Ω·cm, which makes it a potential buffer layer of solar cell.     

1540 nm fiber laser excited upconversion luminescence in erbium-doped lead-fluoride nanocrystals

Upconversion luminescence in erbium-doped PbGeO₃-PbF₂-CdF₂-based vitroceramic under 1540 nm infrared excitation is investigated. Luminescence signals around 410, 525, 550, 660 and 850 nm were generated and attributed to the ²H_9/2, ²H_11/2, ⁴S_3/2 and ⁴F_9/2 transitions to the ⁴I_15/2 ground-state, and ⁴S_3/2-⁴I_13/2, respectively. The erbium ions excited-state emitting levels were populated through a combination of stepwise ground-state absorption, phonon-assisted excited-state absorption and cross-relaxation processes. The results also disclosed that all emission signals obtained with vitroceramic samples presented intensities three times higher when compared to the precursor glass samples. In addition, the red emission signal at 660 nm for 1540 nm pumping exhibited an expressively high intensity when compared to the green signal.     

Broad UHF ferromagnetic resonance of iron rich-aluminum pulsed laser deposited thin films

The magnetic susceptibility of Fe–Al off-normal pulsed laser deposited thin films was measured at ultra high frequencies, UHF. Different Fe_1?x–Al_x films from pure Fe to x = 0.2 Al were prepared. The films were ≈40 nm thick and non-crystalline peaks were detected by the X-ray diffractometry studies. The magnetization of the films remained between 2.0 and 1.8 T for composition less than or equal to 20% Al. A magnetic anisotropy, from H_k ≈ 18 Oe for pure Fe to H_k ≈ 130 Oe for 20% Al was measured. These samples exhibited a well-defined ferromagnetic resonance at frequencies between ≈2.0 GHz and 3.8 GHz depending on composition. The broad resonance peaks had a width, at half maximum, w_h, in the interval from 2.5 GHz to 4.0 GHz depending on Al content. After fitting the magnetic hysteresis loops using a simple distribution of anisotropy values, we used the Landau–Lifshitz–Gilbert equation to fit the UHF magnetic susceptibility. From this last fit we obtained a high damping coefficient value (≈4 times higher than that corresponding to Co or CoFe films), explaining this broad ferromagnetic resonance of these Fe_1?x–Al_x films.     

Crystalline and electrical properties of pulsed laser deposited BST on platinized silicon substrates

In order to produce thin film capacitors with elevated capacitance and breakdown strength, pulsed laser deposition of the ferroelectric material Ba_0.6Sr_0.4TiO₃ (BST) has been made on (1 1 1)Pt/Ti/SiO₂/Si substrates. The films are in situ crystallized at 700 °C, polycrystalline in nature, and exhibit a single perovskite phase. This paper reports on both the effect of the oxygen pressure during heating and deposition, and the influence of different modes of introduction during the deposition stage, on the crystalline and electrical properties of the BST films. Orientation of films depends on the deposition oxygen pressure, with a (1 1 1) preferential orientation obtained when depositing under vacuum. XRD characterization reveals that the out-of-plane parameter of BST films increases when depositions are made under lower oxygen pressure. This phenomenon is related to a higher concentration of vacancies and defects in the films grown under low-pressure environment. However, a local introduction of oxygen on the substrate improves the annihilation of these defects. The crystalline orientation of the films is also highly dependent on the residual oxygen pressure during heating. A high (1 1 1) preferentially oriented BST film is obtained when heating and depositing under 0.1 mbar oxygen pressure. The heating atmosphere is thought to influence the out-diffusion of titanium on the surface of the Pt layer, thus modifying the nucleation and growth of BST films. Aluminum electrodes have been deposited on top of the BST films by dc sputtering to measure electrical capacitances. The calculated dielectric constant of in situ crystallized films deposited under 0.1 mbar oxygen pressure exceeds 500 at 100 kHz under zero dc bias. This high dielectric constant value obtained without post-deposition treatment appears to be of great technological interest.     

Tunneling in sub-5 nm La2O3 films deposited by E-beam evaporation

We investigate the leakage current in ultrathin (sub-5 nm) La₂O₃ dielectric films deposited on n-Si (1 0 0) substrates by electron-beam evaporation and annealed in situ in ultra-high vacuum conditions. We show that simple tunneling models both for the direct and Fowler-Nordheim conduction regimes can accurately reproduce the measured current-voltage characteristics over a wide voltage range. In the latter regime, it is shown the importance of considering the series resistance effect to account for the shape of the characteristics. We propose a method to obtain the series resistance’s value based on the linearization of the Fowler-Nordheim plot. Some experimental features in combination with an exploratory analysis of the fitting parameters seem to indicate that the current flow through the structure is mostly localized, which is attributed to the existence of thickness non-uniformities in the oxide layer. In addition, we show that the application of electrical stress creates traps or defects within the insulator that leads to a progressive increase of the leakage current and to a change of the dominant conduction mechanism at the lowest biases.     

Diffusion of vanadium in GaAs

The diffusion of Vanadium has been studied in V-doped GaAs layers (GaAs:V) grown by Metal-Organic Chemical Vapour Deposition (MOCVD) using secondary ion mass spectroscopy (SIMS). The vanadium (V) concentration profiles of sandwiched structures made of alternatively undoped and V doped GaAs layers have shown a concentration independent diffusion coefficient (D_V) for varying V doping levels from 10¹⁸ to 10¹⁹ cm⁻³. Measurements of D_V at 550, 615 and 680 °C indicate that the temperature dependence of D_V can be represented by the Arrhenius equation:  cm² s⁻¹. It is suggested that V diffuses via interstitial sites.     

Effect of oxygen content on barrier properties of silicon oxide thin film deposited by dual ion-beam sputtering

A silicon oxide thin film barrier was prepared with various oxygen contents and its chemical composition, surface morphology and optical and barrier properties were related to the deposition conditions used. Our study showed that under Ar and O₂ assisted process conditions, a stoichiometric silicon oxide thin film formed at a critical oxygen content during deposition of 40-50%. The thin films deposited at the critical condition showed the lowest surface roughness giving similar or higher optical transmittance than that of the bare polycarbonate (PC) substrate. The boiling and tensile strength test performed on the thin film deposited with assisted ions before the deposition process showed improvement in the adhesion between the oxide layer and the polymer substrate. In addition, interface modification to improve for improving the barrier layer properties of the silicon oxide thin film was achieved through the introduction of dual ion beam sputtering without pre-treatment.     

Characterization of nanocluster formation in Cu-implanted silica: Influence of the annealing atmosphere and the ion fluence

High-purity silica plates were implanted with 2 MeV Cu⁺ ions at various ion fluences: 0.7 × 10¹⁶, 3 × 10¹⁶ and 6 × 10¹⁶ ions/cm². After implantation, thermal treatments were performed at 400 °C and 900 °C in either an oxidizing (air) or a reducing (50% H₂ + 50% N₂) atmosphere for 1 h. All the samples were studied by electron paramagnetic resonance, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy (HRTEM), Rutherford backscattering spectrometry and optical absorption. The advantages of the reducing atmosphere (RA) over the oxidizing atmosphere (OA) are clearly observed. When annealed in a RA, the surface plasmon resonance is more intense and a narrower size distribution of the Cu nanoparticles is obtained. The existence of CuO nanoparticles was confirmed by HRTEM, and while both annealing atmospheres favor the formation of CuO nanoparticles, this process is strengthened when the sample is annealed in an OA.     

Magnetic and magnetoelastic properties of cobalt–iron amorphous–nanocrystalline pulsed laser deposited thin films

We prepared pulsed laser deposited planar and cylindrical amorphous–nanocrystalline Co–Fe thin films using the corresponding target with composition Co_1−xFe_x, x = 0, 0.02, …, 1.0. Their room temperature spontaneous magnetization, M_s (film), was always a fraction of the M_s of the corresponding crystalline alloy, M_s (film) = γ M_s (crystal): γ ≈ 0.8 for pure Co, γ ≈ 0.88 for the Co₃₅Fe₆₅ film and γ ≈ 0.94 for pure Fe. Their isotropic magnetostriction coefficient, λ_s, was also determined. From pure Co to 30 at.% Fe λ_s values were similar to those corresponding to the crystalline alloys: from pure Co to 4 at.% Fe was negative and of the order of 10⁻⁶; λ_s increased to 10⁻⁵ up to 25 at.% Fe and achieved 10⁻⁴ from 30 at.% Fe to 90 at.% Fe; λ_s decreased to 10⁻⁵ for pure Fe. A chemical short-range order between the Co atoms surrounded by the Fe ones, increasing the magnetic moment of Fe atoms, was used to explain the observed behavior.     

An electron microscopy study of the crystal growth of schwertmannite needles through oriented aggregation of goethite nanocrystals

The formation of the iron oxyhydroxide schwertmannite has been monitored by time-resolved transmission electron microscopy (TEM) studies. Schwertmannite aggregates are found to form initially as spherical agglomerates of ferrihydrite crystallites, which then begin to grow characteristic needles on their surfaces. High-resolution images of the needles show that they are initially comprised of aligned goethite nanocrystals, which subsequently coarsen to form crystallographically coherent needles of goethite. Thus, needle formation on schwertmannite aggregates can be considered as the first stage in the phase transformation from schwertmannite to goethite. The results of this study suggest that schwertmannite is not a distinct mineral phase, but may be a mixture of ferrihydrite and poorly crystalline goethite with a distinctive morphology directed by the presence of surface-adsorbed sulfate anions.     

Influence of oxygen partial pressure on the properties of undoped InOx films deposited at room temperature by rf-PERTE

Transparent and conductive/semiconductive undoped indium oxide (InO_x) thin films were deposited at room temperature. The deposition technique used is the radio frequency (rf) plasma enhanced reactive thermal evaporation (rf-PERTE) of indium (In) in the presence of oxygen. The influence of oxygen partial pressure on the properties of these films is presented. The oxygen partial pressure varied between 3 × 10^?2 and 1.3 × 10^?1 Pa. Undoped InO_x films, 100 nm thick, deposited at the oxygen partial pressure of 6 × 10^?2 Pa show a conductive behaviour, exhibit an average visible transmittance of 81%, a band gap around 2.7 eV and an electrical conductivity of about 1100 (Ω cm)^?1. For oxygen pressures greater than 6 × 10^?2 Pa, semiconductive films are obtained, maintaining the visible transmittance. Films deposited at lower pressures are conductive but dark. From XPS data, films deposited at an oxygen partial pressure of 6 × 10^?2 Pa show the highest amount of oxygen in the film surface and the lowest ratio between oxygen in the oxide crystalline and amorphous phases.     

Synthesis via an organic molten salt solvent route and characterization of PbS nanocrystals

In this paper, four petals flowers‐like and quasi sphere‐like PbS nanostructures were successfully synthesized by an environment friendly organic molten salt solvent (OMSS) route at 200 °C, with different sulfur sources, e.g. thiourea and sodium thiosulfate, respectively. The as‐synthesized products were characterized by X‐ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), UV–vis absorption spectrum and photoluminescence (PL) spectrum, respectively. It was shown that four petals flowers‐like and quasi sphere‐like PbS nanocrystals were formed. It was also demonstrated that the morphologies of PbS nanocrystals were significantly influenced by different sulfur sources. The ultraviolet‐visible absorption peaks of PbS nanocrystals exhibited a large blue‐shift and the luminescence spectra had strong and broad emission bands centered at 488 nm and 492 nm. The possible formation mechanisms of the PbS nanostructures were discussed. The organic molten salt solvent (OMSS) method is preferable for synthesizing high‐quality PbS nanocrystals. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of the substrate temperature on the structural,optical, and electrical properties of tin selenide thin films deposited by thermal evaporation method

Thin films of tin selenide (SnSe) were deposited on sodalime glass substrates, which were held at different temperatures in the range of 350‐550 K, from the pulverized compound material using thermal evaporation method. The effect of substrate temperature (T_s) on the structural, morphological, optical, and electrical properties of the films were investigated using x‐ray diffraction analysis (XRD), scanning electron microscopy (SEM), transmission measurements, and Hall‐effect characterization techniques. The temperature dependence of the resistance of the films was also studied in the temperature range of 80‐330 K. The XRD spectra and the SEM image analyses suggest that the polycrystalline thin films having uniform distribution of grains along the (111) diffraction plane was obtained at all T_s. With the increase of T_s the intensity of the diffraction peaks increased and well‐resolved peaks at 550 K, substrate temperature, were obtained. The analysis of the data of the optical transmission spectra suggests that the films had energy band gap in the range of 1.38‐1.18 eV. Hall‐effect measurements revealed the resistivity of films in the range 112‐20 Ω cm for films deposited at different T_s. The activation energy for films deposited at different T_s was in the range of 0.14 eV‐0.28 eV as derived from the analysis of the data of low‐temperature resistivity measurements. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Characterization of GaInN/GaN layers for green emitting laser diodes

An enhancement of radiative recombination in GaInN/GaN heterostructures is being pursued by a reduction of defects associated with threading dislocations and a structural control of piezoelectric polarization in the active light-emitting regions. First, in conventional heteroepitaxy on sapphire substrate along the polar c-axis of GaN, green and deep green emitting light-emitting diode (LED) wafers are being developed. By means of photoluminescence at variable low temperature and excitation density, internal quantum efficiencies of 0.18 for LEDs emitting at 530 nm and 0.08 for those emitting at 555 nm are determined. Those values hold for the high current density of 50 A/cm² of high-power LED lamps. In bare epi dies, we obtain efficacies of 16 lm/W. At 780 A/cm² we obtain 22 lm when measured through the substrate only. The 555 nm LED epi material under pulsed photoexcitation shows stimulated emission up to a wavelength of 485 nm. This strong blue shift of the emission wavelength can be avoided in homoepitaxial multiple quantum well (MQW) and LED structures grown along the non-polar a- and m-axes of low-dislocation-density bulk GaN. Here, wavelength-stable emission is obtained at 500 and 488 nm, respectively, independent on excitation power density opening perspectives for visible laser diodes.     

Influence of deposition parameters on composition and refractive index of femtosecond and nanosecond pulsed laser deposited gallium lanthanum oxysulphide

The influence of the deposition parameters on femtosecond and nanosecond pulsed laser deposited gallium lanthanum oxysulphide (GLSO) glass films has been investigated. A comparison between films deposited by femtosecond and nanosecond pulsed laser deposition (PLD) shows that the compositional range of each ablation regime varies significantly; in particular, femtosecond PLD shows a unique potential for selective fabrication of films with a high lanthanum content well outside the conventional glass-melting region. We demonstrate how manipulation of the PLD growth parameters can influence the stoichiometric transfer of the PLD process, leading to films with compositions that differ significantly from the GLSO target material. We also reveal how the refractive index of as-deposited films is dependent upon the composition and briefly discuss the thermal properties of bulk GLSO material of various compositions which indicate the potential for films grown by PLD to be used in optical data-storage device applications.