Absorption and luminescence in amorphous silica synthesized by low-pressure plasmachemical technology

A comparison study of as-deposited by the hydrogen-free SPCVD process silicon dioxide with and without fluorine as well as the one in the form of fused materials is performed to define whether glass forming processing affects its optical properties. Raman scattering, optical absorption in the vacuum ultraviolet region as well as luminescence at different temperatures and various excitation conditions are measured. A difference in Urbach rule parameters and intrinsic defect concentrations for different samples is revealed. No significant impact on the structure responsible for Raman scattering is observed.     

Features of Er3+ luminescence in fluorine-doped amorphous silicon dioxide fabricated by low-temperature plasma CVD

Spectra and photoluminescence kinetics of Er³⁺ ions incorporated in an amorphous host of fluorine-doped silicon dioxide synthesized by surface plasma chemical vapor deposition (SPCVD) are investigated at temperatures of 27–300 K. Luminescence is excited with an Ar⁺ laser at a wavelength of 514.5 nm and with a diode laser at a wavelength of 975 nm. Narrow and well-defined components of Stark manifolds with a small contribution from inhomogeneous broadening intrinsic to Er³⁺ ions in crystalline, but not amorphous, hosts are revealed and identified in photoluminescence spectra. The structure of the Stark manifolds is well-resolved at low temperatures. The presence of the well-resolved Stark structure in the spectra is indicative of stable anion complexes formating the Er³⁺ local neighborhood presumably associated with fluorine incorporation. This neighborhood is formed during the low temperature plasma chemical synthesis and is destroyed upon glass fusion.     

Structural and luminescence properties of amorphous SiO2 nanoparticles

We report an experimental study on the photoluminescence band peaked at 2.7 eV (blue band) induced by thermal treatments in nanometric amorphous SiO₂. In particular the emission dependence on the nanometric particles size as a function of their mean diameter from 7 nm up to 40 nm is investigated. We found that the emission amplitude increases on decreasing the particle diameter, showing a strong correlation between the blue band and the nanometric nature of the particles. By Raman spectroscopy measurements it is evidenced that the SiO₂ nanoparticles matrix is significantly affected by the reduction of size. Basing on the shell-like model, these findings are interpreted assuming that the defects responsible for the photoluminescence are localized on a surface shell of the particles and not simply on their surface. In addition it is found that the generation efficiency of these defects depends on the structural properties of the SiO₂ matrix in the surface shell.     

Fictive temperature measurement of amorphous SiO2 films by IR method

The structure and properties of amorphous materials, in general, change with their thermal history. This is usually explained using the concept of fictive temperature, i.e., the temperature at which the super-cooled liquid state turned into a glassy state. In earlier studies, a simple IR method was used to determine the fictive temperature of silica glasses, both bulk and fiber. In the present study the applicability of the same technique for thin amorphous silica films on silicon was examined. It was found that the IR absorption as well as reflection peak wavenumber of the silica structural band can be used to determine the fictive temperature of amorphous silica films on silicon with an unknown thermal history. Specifically, IR absorbance spectra of an amorphous silica film of thickness greater than 0.5 μm grown on silicon can be taken before and after etching a thin surface layer of 20–30 nm and the peak wavenumber of the difference signal can be compared with the pre-determined calibration curve to convert the peak wavenumber to the fictive temperature. For a film thicker than ∼2 μm, IR reflection peak wavenumber can be converted directly to the fictive temperature of the film by using the calibration curve.     

Intense luminescence of amorphous Eu2O3 prepared by aqueous sol–gel method

Amorphous Eu₂O₃ was prepared by an aqueous sol–gel method. Emission due to the ⁵D₀ → ⁷F_J (J = 0, 1, 2) transitions of Eu³⁺ ions were observed. The dominant transition was the ⁵D₀ → ⁷F₂ red emission of Eu³⁺. The properties of the as-prepared samples were different with changes in the annealing temperature. To investigate the luminescence properties of the amorphous Eu₂O₃, the temperature-dependent photoluminescence (PL) spectra of samples annealed at 600 °C were measured in the temperature range 77–300 K. PL peak positions were unchanged with the change of temperature.     

Anomalous amorphous SiO2 films

Anomalous SiO₂ films have been prepared by sputtering Si in a mixture of Ar-10% O₂ at 77 K. The same sputtering conditions at room temperature yield normal SiO₂ which means that the anomaly is produced by the low temperature deposition. The anomaly reveals itself in several physical properties. The density of the anomalous SiO₂ is 1.72 as compared with 2.20 for bulk and the dielectric constant is about 50% larger than bulk and with a much stronger temperature dependence. The infrared (ir) spectrum of the anomalous SiO₂ is only slightly different from bulk SiO₂ but esr experiments reveal about 3 × 10¹⁸ spins cm which do not exist in bulk SiO₂. These anomalous films are extremely stable: upon heating only a small amount of oxygen (1 part in 10⁵) evolves at 440°C but the density and IR spectrum remain unchanged up to 1300°C. Annealing at 1500°C completely removes the ESR signal and returns the ir spectrum and the density to that of cristobalite. An electron diffraction and transmission electron microscopy study reveals that the anomalous SiO₂ films consist of essentially bulk like SiO₂ clusters about 250 Å in diameter separated by a low density network. The low density network undoubtedly contains unbound O atoms and the SiSi bonds which give rise to the esr signal. The structural model can account for all the anomalous properties.     

溶胶-凝胶法制备MgxZn1-xO及其特性

采用溶胶-凝胶法制备了不同组分(x=0.1～0.3)的MgxZn1-xO前驱体,并对它进行不同温度的热处理(550℃～1000℃).X射线衍射(XRD)结果表明,Mg0.1Zn0.9O具有和ZnO一样的衍射谱,为六方纤锌矿结构,而且随着热处理温度的升高,ZnO衍射峰的强度逐渐增强,半高宽不断减小;Mg元素掺杂浓度增大后,出现了MgO的峰位.扫描电子显微镜(SEM)显示Mg0.1Zn0.9O晶粒粒径分布较均匀;热处理温度升高,晶粒的尺寸不断变大.用室温荧光光谱(PL)分析了经过550～1000℃热处理获得的Mg0.1Zn0.9O粉末,结果发现除了550℃下处理的样品,其它都有紫外发射峰(350nm左右),而且随着热处理温度的升高紫外峰有明显的蓝移现象.     

Electron beam poling in amorphous Ge-doped H:SiO2 films

Amorphous Ge-doped H:SiO₂ films on silica, deposited by matrix-distributed electron cyclotron resonance – plasma enhanced chemical vapor deposition, were irradiated with an electron beam while varying the dose. Using the Maker fringe method, second-harmonic generation was measured in the irradiated regions of the films. With a current of 5 nA, and an acceleration voltage of 25 kV for 25 s, a Ge-doped H:SiO₂ film (3.8 at.% Ge) showed a maximum second-order nonlinearity of d₃₃ = 0.0005 pm/V. In contrast, a H:SiO₂ film with a smaller Ge content (1.0 at.% Ge), showed a large SHG: d₃₃ = 0.06 pm/V when irradiated for 15 s. The second-harmonic generation in the films is caused by a frozen-in electric field induced by charge implantation from the electron beam. The strength of the electric field is determined by two conditions: the trapping centers (numbers, depth) and the remaining conductivity under large electric field.     

RHEED investigations of unusual crystalline states in amorphous SiO2 films

Microcrystalline regions of silicon dioxide with structures unobserved in the macro-state are studied by Reflection High Energy Electron Diffraction (RHEED). Their formation can be understood by considering the existence of several possible relatively stable crystalline states of SiO₂, with free energies higher than those of the known silicon oxides. SiO₂ with structures and lattices identical to the structures and the lattices of the CaF₂-, of the rutile-type GeO₂, and of α-PbO₂ have been identified. Considerations for the state of silicon in some of these structures are also included.     

Seebeck coefficient in amorphous chalcogenide films

The Seebeck coefficient determined for a number of amorphous chalcogenide films is linear in 1/T. Slopes were typically 0.1 to 0.2 eV less than the conductivity activation energies. A parameter A in the expression for thermopower appears to be a measure of the disorder, being large for highly disordered materials and small for annealed stoichiometric materials.     

Conduction mechanism in amorphous Si films

dc conductivity as a function of temperature has been measured for as-evaporated and annealed films of amorphous Si, grown by the vacuum evaporation technique. The experimental data suggest that conduction in the higher temperature range (～175–300 K) is by the thermally activated holes in the localized states near the valence band edge while conduction in the lower temperature range (～77–175 K) is found to be thermally assisted tunnelling in the localized states near the Fermi level. The activation energy for both the processes is found to increase with an increase in the annealing temperature. The average hopping distance, calculated for conduction near the Fermi level, is also found to increase with an increase in the annealing temperature.     

Structure and optical properties of amorphous GeSx films prepared by PLD

Amorphous GeS_x (x = 2, 4, 6) films were prepared by the pulsed laser deposition (PLD) technique. The optical band gaps (E_g^opt) and refractive indices of the films were obtained from the optical absorption spectra and transmission spectra, respectively. The short-wave absorption edges of the films were described using the ‘non-direct transition’ model proposed by Tauc. The dispersion of the refractive index was analyzed in terms of the single-oscillator Wemple–Di Domenico model. The structural units of the films were characterized using Raman spectroscopy. In addition to the basic structural units of edge-sharing and corner-sharing [GeS₄] tetrahedra, there are S–S homopolar bonds in S-rich GeS₄ and GeS₆ films while Ge–Ge bonds exist in stoichiometric GeS₂ film. The results show that the index of refraction decreases while E_g^opt increases with the sulphur content in the GeS_x films. The changes of E_g^opt were discussed in relation to the structure of GeS_x films, which were confirmed by the Raman spectra analysis.     

Improvement of GaInNAsSb films fabricated by atomic hydrogen-assisted molecular beam epitaxy

The effect of incorporation of antimony in GaInNAs films grown by atomic hydrogen-assisted molecular beam epitaxy (MBE) has been investigated. We show that the rate of incorporation of N and In forming GaInNAs do not depend on the Sb beam flux. However, the incorporation of Sb is strongly dependent on the Sb/As₂ flux ratio. Introducing a small amount of Sb (<∼1%) significantly improves the photoluminescence (PL) emission efficiency of GaInNAs, but Sb concentration of >1% rapidly degrades the PL intensity, though a large redshift can still be achieved. Therefore, there is an optimum amount of Sb for the growth of low-strained GaInNAs films to improve the overall optical quality.     

Optical absorption in amorphous InN thin films

Amorphous indium nitride (a-InN) thin films were deposited onto different substrates at temperatures <325 K using RF magnetron sputtering at a rate 0.3–0.4 Å/s. X-ray diffraction patterns reveal that the films grown on the substrates are amorphous. The optical absorption edge, ‘bandgap’ energy, E_g, of a-InN has been determined by spectroscopic ellipsometry over the energy range 0.88–4.1 eV. The absorption coefficient was obtained by the analysis of the measured ellipsometric spectra with the Tauc–Lorentz model. The E_g was determined using the modified Tauc and Cody extrapolations. The corresponding Tauc and Cody optical bandgaps were found to be 1.75 and 1.72 eV, respectively. These values are in excellent agreement with the values of the bandgap energy obtained as fitting parameters in the Tauc–Lorentz model: 1.72 ± 0.006 eV as well as by using spectrophotometry (1.74 eV) and photoluminescence (1.6 eV). The spectral dependence of the polarized absorptivities was also investigated. We found that there was a higher absorptivity for wavelengths <725 nm. This wavelength, ∼725 nm, therefore indicates that the absorption edge for a-InN is about 1.70 eV. Thus, the average value of the measured optical absorption of a-InN film is approximately 1.68 ± 0.071 eV.     

Dark conductivity in amorphous undoped silicon films

The temperature dependence of the dark conductivity was investigated in amorphous undoped silicon films deposited by glow-discharge in a SiCl₄H₂ mixture. Different transport processes were indentified according to the investigated temperature range. The dependence of the dark conductivity was also examined as a function of some deposition parameters. The experimental results are discussed in terms of the two-phase structure of the film.     

Holographic recording in amorphous chalcogenide thin films

Holographic recording by He–Ne laser (line 632.8 nm) light in amorphous As_0.55Se_0.45 thin films for different film thickness and grating period was studied. A strong dependence of the diffraction efficiency of the gratings on the readout light wavelength (650 nm, 805 nm and 1150 nm) was observed. A decrease in diffraction efficiency for longer wavelengths is explained by a decrease in the photoinduced changes of refractive index. It is shown that high efficiency gratings can be recorded in As_0.55Se_0.45 films with a thickness of ∼1 μm.     

Crystallization and decomposition of amorphous silicon-aluminium films

The crystallization and decomposition of vacuum-deposited amorphous silicon-aluminium films have been examined by means of transmission electron microscopy. Depending on the aluminium concentration, the transformation of the metastable amorphous phase into the stable phases of aluminium and silicon proceeds by different reactions such as pre-crystallization of aluminium, polymorphous transformation into supersaturated crystalline solid solutions or eutectic decomposition. The temperature dependence of the eutectic crystallization was measured. The results are discussed in terms of the thermodynamics of amorphous-to-crystalline transformation.