Effects of high-temperature annealing on electron spin resonance in SiOx films prepared by R. F. sputtering system

Electron spin resonance (ESR) spectra are investigated in order to analyze paramagnetic defects in amorphous SiO_x films with 0.8 ≤ x ≤ 1.87 prepared by a co-sputtering of Si-wafer chips and a SiO₂ disk target. Effects of the thermal annealing at 900 °C and 1100 °C on the ESR spectra are also investigated. Four types of silicon dangling bond centers with forms of •Si ≡ Si_3 − nO_n (n = 0, 1, 2 or 3) are assumed in order to simulate the ESR spectra. The random bonding model appears to describe the network structure of the films with x ~ 2, that is, near the stochiometric composition of SiO₂. It is suggested that the structural fluctuation around silicon dangling bonds is larger in the sputtered SiO_x films used in the present work in comparison with those prepared by plasma-enhanced chemical vapor deposition.     

Miscibility gap calculation for Ga1−xInxNyAs1−y including strain effects

A method including strain effects is introduced for calculating the miscibility gap of the Ga_xIn_1−xN_yAs_1−y material system. The Gibbs free energy is computed using the delta lattice parameter model and the conventional solution model. The contribution caused by the strain energy due to the mismatch between substrate and epitaxial layer is added. The spinodal points can be calculated from this expression. The critical temperature above which the solid is metastable has been found to be increased due to strain effects. It is pointed out that the result of the calculation depends on the choice of substrate. The miscibility gaps for Ga_xIn_1−xAs, GaN_yAs_1−y and Ga_xIn_1−xN_yAs_1−y are evaluated by this method for a more realistic model of crystal growth.     

Effects of N2 addition on enhanced scratch resistance of flexible polycarbonate substrates by low temperature plasma-polymerized organo-silicon oxynitride

An investigation was conducted on enhanced scratch resistance of polycarbonate (PC) substrates by low temperature plasma-polymerized organo-silicon oxynitride (SiO_xC_yN_z) with various N₂ flow rates. It was found that the low temperature plasma-polymerized SiO_xC_yN_z with tetramethylsilane (TMS)-O₂-N₂ plasmas in room temperature (23 °C) can be used for improving the scratch resistance of PC substrates. Scratch test demonstrates this improvement. The scratch resistance of PC substrates was greatly enhanced from the overwhelming presence of scratching (90%) on un-treated PC substrates to a complete lack of scratching (0%) on TMS-O₂-N₂ plasma-polymerized PC substrates with steel wool for 200 cycles at 300 g loading. The results of this study indicate the performance of scratch resistance on PC substrates was highly dependent on the surface characteristics of PC substrates. The hardness of PC substrates was determined by the pencil test. The surface morphology of PC substrate was monitored by atomic force microscopy (AFM) and field emitted scanning electron microscopy (FESEM). The atomic compositions and chemical bondings of TMS-O₂-N₂ plasma-polymerized SiO_xC_yN_z were analyzed by X-ray photoelectron spectroscopy (XPS).     

Influence of amorphous buffer layers on the crystallinity of sputter-deposited undoped ZnO films

Undoped ZnO films were deposited by radio frequency (RF) magnetron sputtering on amorphous buffer layers such as SiO_x, SiO_xN_y, and SiN_x prepared by plasma enhanced chemical vapor deposition (PECVD) for dielectric layer in thin film transistor (TFT) application. ZnO was also deposited directly on glass and quartz substrate for comparison. It was found that continuous films were formed in the thickness up to 10 nm on all buffer layers. The crystallinity of ZnO films was improved in the order on quartz>SiO_x >SiO_xN_y>glass>SiN_x according to the investigated intensities of (0 0 2) XRD peaks. The crystallite sizes of ZnO were in the order of SiO_x～glass >SiN_x. Stable XRD parameters of ZnO thin films were obtained to the thickness from 40 to 100 nm grown on SiO_x insulator for TFT application. Investigation of the ZnO thin films by atomic force microscope (AFM) revealed that grain size and roughness obtained on SiN_x were larger than those on SiO_x and glass. Hence, both nucleation and crystallinity of sputtered ZnO thin films remarkably depended on amorphous buffer layers.     

An analysis of the radial distribution function of SIOx

A theory is presented for the short-range order (sro) in the SiO_x alloy system for 0?x?2. The parameters of the sro are taken to be fixed by the properties of amorphous Si and SiO₂, except that the probability of an Si atom having (i?1) oxygen neighbors is left as a free parameter, C_i(x), for i=1 through 5. The areas of the first three peaks in the radial distribution function (RDF) of SiO_x are calculated as a function of x and C_i(x). It is shown that the RDF can be used to differentiate between the two models proposed for the sro in SiO_x namely the SiSiO₂ mixture model and the random bonding model. In the case of SiO powder, a mixture model at the atomic level is found to be consistent with the RDF. It is suggested that accurate RDF's of SiO_x films, which are not currently available, would be useful in determining the sro of the alloy system. The present theory may be extended to analyze the GeO_x and SnO_x alloy systems.     

The network structure of amorphous silicon-carbon alloy

The network structure of amorphous silicon-carbon alloy (a-Si_1−xC_x) has been studied over a wide range of x. The a-Si_1−xC_x thin films were prepared by sputtering silicon and carbon target with argon in radio-frequency magnetron sputtering equipment. The films were characterized by X-ray photoelectron spectroscopy, optical absorption, infrared absorption, and mechanical measurements. The results showed that the network structure could be classified neither as the random covalent network nor as the chemically ordered covalent network. The structure as a whole was close to the random covalent network, but the Si-Si combination at x>0.5 showed a feature of the chemically ordered covalent network. The film at 0.6<x<0.8 was hard and showed a high energy gap, due to the sp³ configuration in Si-C combinations.     

Electron beam probing of silica surface layers on alumina

The electron beam induced self-consistent charge injection and transport in a layered insulator SiO₂–Al₂O₃ is described by means of an electron–hole flight–drift model FDM and an iterative computer simulation. Thermal and field-enhanced detrapping are included by the Poole–Frenkel effect. The surface layer with a modified electric surface conductivity is included which describes the surface leakage currents. Furthermore, it will lead to particular charge incorporation at the interface between the surface layer and the bulk substrate. As a main result the time-dependent spatial distributions of currents j(x,t), charges ρ(x,t), field F(x,t), and potential V(x,t) are obtained. The spatial charge distribution with depth shows a quadro-polar plus–minus–plus–minus structure in nanometer dimension.     

Characterization of the structure of calcium alumino-silicate and calcium fluoro-alumino-silicate glasses by magic angle spinning nuclear magnetic resonance (MAS-NMR)

Calcium aluminosilicate and calcium fluoro-aluminosilicate glasses have been characterized by ²⁹Si, ²⁷Al and ¹⁹F MAS-NMR. The two calcium aluminosilicate glasses examined were based on the composition 2SiO₂ · Al₂O₃ · 2CaO (ART1) and the mineral anorthite 2SiO₂ · Al₂O₃ · CaO (ART2). The observed chemical shifts for ²⁹Si and ²⁷Al agreed with previous studies. The fluorine containing glasses were based on 2SiO₂ · Al₂O₃ · (2−X)CaO · XCaF₂. The ²⁹Si chemical shift moved in a negative direction with increase fluorine content indicating a progressive reduction in the average number of non-bridging oxygens, NBO, attached to a silicon. The ²⁷Al spectra indicated the presence of four coordinate aluminium in the glasses with X=0.0-0.75, but aluminium was present in Al(IV), Al(V) and Al(VI) coordination states in the highest fluorine content glass with X=1.0. The ¹⁹F spectra indicated the presence of F-Ca(n) in low fluorine content glasses and both F-Ca(n) and Al-F-Ca(n) in high fluorine content glasses. We speculate here that the Al-F-Ca(n) species are oxyfluorides [AlO_xF_y]ⁿ⁻, where x=1-6, y=1-6 and n is the charge on the total complex when aluminium is in Al(IV), Al(V) and Al(VI) coordinate states. The reduction in the average number of NBO per silicon with increasing fluorine content is explained by fluorine converting Ca²⁺ to F-Ca(n).     

Reduction of bulk and interface defects by network self-organizations in gate dielectrics for silicon thin film and field effect transistors (TFTs and FETs,respectively)

Studies of binary chalcogenide alloys have established that the onset of network rigidity is generally delayed by a network self-organization resulting in an intermediate phase with significant deviations from mean-field chemical bonding. In Ge_xSe_1−x, the onset of local chemical bonding rigidity occurs for a mean-field coordination, r_c = 2.4 at x = 0.2, but percolation of stress resulting in network rigidity is delayed until r_c = 2.52. This paper demonstrates that low levels of electrically active defects in gate dielectrics for (i) thin film transistors (TFTs) in liquid crystal displays (LCDs), and (ii) aggressively-scaled metal- oxide-semiconductor field effect transistors (MOSFETs) are derived from similar network self-organizations that occur for a narrow range of dielectric compositions. The dielectrics of this article are non-crystalline (nc-) Si_xN_yH_z alloys in which a chemical self-organization occurs during deposition at 300 °C, and nc-(SiO₂)_x(Si₃N₄)_y(ZrO₂)_z alloys in which it occurs during post-deposition annealing at ∼900 °C. For each of these alloys, the values of x, y and z, are approximately 0.3, 0.4 and 0.3.     

A simple method to synthesize α-Si3N4, β-SiC and SiO2 nanowires by carbothermal route

α-Si₃N₄ nanowires, β-SiC nanowires and SiO₂ amorphous nanowires are synthesized via the direct current arc discharge method with a mixture of silicon, activated carbon and silicon dioxide as the precursor. The α-Si₃N₄ nanowires, β-SiC nanowires and SiO₂ amorphous nanowires are about 50–200 nm in stem diameter and 10–100 μm in length. α-Si₃N₄ nanowires and β-SiC nanowires consist of a solid single-crystalline core along the [0 0 1] and [1 1 1] directions, respectively, wrapped within an amorphous SiO_x layer. The direct current arc plasma-assisted self-catalytic vapor–solid and/or vapor–liquid–solid (VLS) growth processes are proposed as the growth mechanism of the nanowires.     

Structural and optical properties of Zn(Mg,Cd)O alloy films grown by remote-plasma-enhanced MOCVD

We report the structural and optical properties of wurtzite-structure Zn(Mg,Cd)O ternary alloys. Wurtzite (0 0 0 1) Zn_1−xCd_xO and Mg_yZn_1−yO films were grown on (11–20) sapphire substrates using remote-plasma-enhanced metalorganic chemical vapor deposition. The large bowing parameters of Zn_1−xCd_xO and Mg_yZn_1−yO ternary alloys are 3.0 and 3.5, respectively, which reflects the large difference of each binary’s electronegativity. We have analyzed the broadening of photoluminescence (PL) in Zn(Mg,Cd)O alloys on alloy content by taking into account the statistical alloy fluctuation and the localization of the exciton, and have clarified that the localization of the exciton strongly affects to PL full-width at half-maximum (FWHM) in Zn(Mg,Cd)O alloys. The alloy broadenings in steady-state PL of Zn(Mg,Cd)O alloys are in good agreement with the calculated tendency by the theoretical model based on the statistical alloy fluctuation, while PL FWHM of Zn_1−xCd_xO is three times larger than the calculated results. Moreover, as another way to confirm alloy broadening, we also have done time-resolved PL measurements and derived the localized depth of the exciton in ZnO-based system, indicating a good agreement with the tendency of PL FWHM broadening.     

Formation of glass in the GeSiS system

The formation of glass in the GeSiS system was investigated. After synthesis of material with the general formula Ge_1?xSi_xS_y, where x was chosen to be 0.05, 0.1, 0.2, 0.3 and y was in the range 1.28–3.6, cylindrical samples were prepared and used for the characterization of glass by means of DTA. It was found that the substitution of germanium with silicon does not lead to any expressive change of the glass transition temperature, crystallization and the onset of melting.     

Efficient ITO–Si solar cells and power modules fabricated with a low temperature technology: Results and perspectives

ITO–SiO_x–nSi semiconductor–insulator–semiconductor (SIS) structures have been produced with a simple spraying technique. It is shown that the structures obtained in such a way may be considered as an induced p–n diode, in which the polycrystalline tin–doped indium oxide (ITO) layer spray deposited on the preliminary treated silicon surface leads to an inversion p-layer at the interface. Solar cells with an active area of 1–4 cm² have been fabricated based on ITO–SiO_x–nSi structures and studied. Under AM0 illumination conditions, the efficiency is nearly 11%, whereas it exceeds 12% for AM1.5 illumination conditions. The theoretical analysis provided in this work shows a good agreement with experimental results and allows for predicting the efficiency of the cells depending on the silicon electro-physical properties.     

Photoluminescence of Si or Ge nanocrystallites embedded in silicon oxide

This paper presents the results of photoluminescence, its temperature dependence and Raman scattering investigations on magnetron co-sputtered silicon oxide films with (or without) embedded Si (or Ge) nanocrystallites. It is shown the oxide related defect origin of the visible PL centers peaked at 1.7, 2.06 and 2.30 eV. The infrared PL band centered at 1.44–1.58 eV in Si–SiO_x, system has been analyzed within a quantum confinement PL model. Comparative PL investigation of Ge–SiO_x system has confirmed that high energy visible PL bands (1.60–1.70 and 2.30 eV) are connected with oxide related defects in SiO_x. The PL band in the spectral range of 0.75–0.85 eV in Ge–SiO_x system is attributed to exciton recombination inside of Ge NCs.     

Optical and magneto-optical properties of {Co0.45Fe0.45Zr0.1/a-Si}n multilayers

The optical and magneto-optical (MO) properties of {CoFeZr(x)–aSi(y)}_n and {CoFeZr(x)–SiO₂(y)}_n composite films with multilayer structure were investigated. It was shown that the form and magnitude of the MO spectra strongly depend of the magnetic and non-magnetic layer thickness and component of non-magnetic material. The MO spectra were analyzed in the framework of the effective-medium approximations and macroscopic Fresnel magneto-optics to ultrathin layers.     

Raman study of the structure of alkali germanosilicate glasses II. Lithium,sodium and potassium digermanosilicate glasses

Polarized and depolarized Raman spectra of lithium, sodium and potassium digermanosilicate glasses of the composition 33 A₂O 67[(1 ? y)SiO₂, y GeO₂] and powder Raman spectra of crystalline Li₂O · 2 GeO₂ and K₂O · 2 GeO₂ are given. From the Raman spectra it is concluded that the structure of the glasses mainly consists of networks of SiO₄ and GeO₄ tetrahedra with one non-bridging oxygen atom, probably mixed in a random way. The structure is comparable to that of crystalline disilicates and digermanates.     

On the oxidation mechanism of microcrystalline silicon thin films studied by Fourier transform infrared spectroscopy

Insight into the oxidation mechanism of microcrystalline silicon thin films has been obtained by means of Fourier transform infrared spectroscopy. The films were deposited by using the expanding thermal plasma and their oxidation upon air exposure was followed in time. Transmission spectra were recorded directly after deposition and at regular intervals up to 8 months after deposition. The interpretation of the spectra is focused on the Si-H_x stretching (2000-2100 cm⁻¹), Si-O-Si (1000-1200 cm⁻¹), and O_xSi-H_y modes (2130-2250 cm⁻¹). A short time scale (< 3 months) oxidation of the crystalline grain boundaries is observed, while at longer time scales, the oxidation of the amorphous tissue and the formation of O-H groups on the grain boundary surfaces play a role. The implications of this study on the quality of microcrystalline silicon exhibiting no post-deposition oxidation are discussed: it is not sufficient to merely passivate the surface of the crystalline grains and fill the gap between the grains with amorphous silicon. Instead, the quality of the amorphous silicon tissue should also be taken into account, since this oxidation can affect the passivating properties of the amorphous tissue on the surface of the crystalline silicon grains.     

Thermal conductivity of mixed alkali silicate glasses at low temperature

Thermal conductivity of glass is one of the fundamental properties of it. However, that has not been studied enough. That of only less than 20 compositions has been measured below the room temperature. In this study, we measured the thermal conductivity of xNa₂O · (100 ? x)SiO₂ and (33 ? y)Na₂O · yCs₂O · 67SiO₂ glasses by a transient heating method in the temperature range from about 150 K to room temperature. The conductivity of xNa₂O · (100 ? x)SiO₂ is found to decrease with the increase in alkali content. The dominant factor of this behavior is the decrease in phonon mean free path, which is due to the increase of non-bridge oxygen. Thermal conductivity of (33 ? y)Cs₂O · yNa₂O · 67SiO₂ is decreased with the increase in Cs₂O/(Na₂O + Cs₂O) ratio. The dominant factor of this behavior is the decrease of sound velocity. However, composition dependence of the phonon mean free path also affects the thermal conductivity. Phonon mean free path of 33Cs₂O · 67SiO₂ glass is longer than that of 33Na₂O · 67SiO₂ glass, and should be related to the change in distribution of structural unit in glass. In addition, phonon mean free path of mixed alkali glasses are shorter than that of single alkali glasses.     

Structure–property correlations in lead borate and borosilicate glasses doped with aluminum oxide

Glass samples from four systems: xPbO–(100?x)B₂O₃ (x = 30, 40, 50 and 60 mol%), 50PbO–yAl₂O₃–(50?y)B₂O₃ (y = 2, 4, 6, 8 mol%), 50PbO–ySiO₂–(50?y)B₂O₃ (y = 5, 10, 20, 30 mol%) and 50PbO–5SiO₂–yAl₂O₃–(45?y)B₂O₃ (y = 2, 4, 6, 8 mol%) were prepared by a melt-quench technique. Characterization of these systems was carried out using density measurements, UV–visible spectroscopy, differential scanning calorimetry (DSC), and ¹¹B and ²⁷Al magic-angle spinning (MAS) solid-state nuclear magnetic resonance (NMR). Our studies reveal an increase in glass density with increasing lead(II) oxide concentration in pure lead borates and also with addition of silica into 50PbO–50B₂O₃ glass. ¹¹B MAS NMR measurements determine that the fraction of tetrahedral borons (N₄) reaches a maximum for the glass containing 50 mol% of PbO in the PbO–B₂O₃ glass series and that N₄ is sharply reduced upon adding small amounts of Al₂O₃ into lead borate and lead borosilicate systems. ²⁷Al MAS NMR experiments performed on glasses doped with aluminum oxide show that the Al³⁺ are tetra-, penta- and hexa-coordinated with oxygen, even without any excess concentration of Al³⁺ over charge-balancing Pb²⁺ cations. ^[5]Al and ^[6]Al concentrations are found to have unusually high values of up to 30%. The results of UV–visible absorption spectroscopy, DSC and density measurements support the conclusions drawn from the NMR studies, providing a consistent picture of structure–property relations in these glass systems.     

Solid-state NMR and XANES studies of lithium and silver silicate gels synthesized by the sol-gel route

The objective of this study is to understand the effect of low temperature sol-gel synthesis on the microstructural properties of lithium [xLi₂O-(1−x)SiO₂; x=0.1-0.8 in steps of 0.1] and silver [xAg₂O-(1−x)SiO₂; x=0.1-0.8 in steps of 0.1] silicate xerogels via solid state nuclear magnetic resonance (NMR) and X-ray absorption near edge structure (XANES) techniques. The Li silicate xerogels were analyzed with solid-state ⁷Li and ²⁹Si NMR and the Ag silicate xerogels were studied with Ag XANES. At high Li loading, ⁷Li NMR shows quadrupolar satellite transitions attributed to LiNO₃, a phase also found with X-ray diffraction (XRD). At low Li loading, both NMR and XRD results show an amorphous xerogel. The silicate network is monitored with ²⁹Si NMR and shows evidence of Li incorporation. For the Ag silicate xerogels, Ag-L-III XANES spectral studies show a local environment similar to AgNO₃ for low Ag loading levels, and an increased Ag oxidation for higher Ag loading levels. Si K edge spectra show only an amorphous phase, with no evidence of a crystalline quartz phase. The electrical conductivity of the lithium silicates was estimated from impedance data and the highest conductivity is exhibited by the 0.3Li₂O-0.7SiO₂ composition xerogel. The conductivity dependence on loading level strongly suggests that the observed conductivity is due to Li⁺ mobility. However, further experimental studies are needed to rule out the possibility that the conductivity is, at least in part, due to H⁺ mobility. Variation in conductivity is explained qualitatively using existing theoretical models.