Laser-induced fluorescence detection of reactive intermediates in diffusion flames and in glow-discharge deposition reactors

We have applied the technique of laser-induced fluorescence (LIF) to detect and spatially resolve chemical intermediates present in the diffusion-flame reaction of silane with fluorine as well as in the glow-discharge decomposition of silicon tetrafluoride. The results demonstrate the power of LIF in the investigation of radical species that may prove dominant in the deposition process.     

Fundamental mechanisms in silane plasma decompositions and amorphous silicon deposition

The mechanisms resulting in amorphous silicon deposition from silane glow discharges are classified in primary processes, (electron impact on silane), and secondary processes such as ion-molecule and radical-molecule reactions. The low pressure limit of a multipole DC discharge is shown to allow the measurement of various cross-sections associated with the primary processes, in particular spontaneous emission. The results from a plasma probing with ion mass spectrometry, laser induced fluorescence and other techniques are described in relevance to the problem of the thin film growth.     

Defects and stresses in MBE-grown GaN and Al0.3Ga0.7N layers doped by silicon using silane

The electric and structural characteristics of silicon-doped GaN and Al_0.3Ga_0.7N layers grown by molecular beam epitaxy (MBE) using silane have been analyzed by the Hall effect, Raman spectroscopy, and high-resolution X-ray diffractometry. It is established that the electron concentration linearly increases up to n = 4 × 10²⁰ cm^?3 with an increase in the silane flow rate for GaN:Si, whereas the corresponding dependence for Al_0.3Ga_0.7N:Si is sublinear and the maximum electron concentration is found to be n = 4 × 10¹⁹ cm^?3. X-ray measurements of sample macrobending indicate a decrease in biaxial compressive stress with an increase in the electron concentration in both GaN:Si and Al_0.3Ga_0.7N:Si layers. The parameters of the dislocation structure, estimated from the measured broadenings of X-ray reflections, are analyzed.     

Quantitation of sulfate solubility in borosilicate glasses using Raman spectroscopy

Raman spectroscopy is used here as an innovative technique to investigate sulfate content in borosilicate glasses. Using Raman spectroscopy after having heated the material, the evolution of sulfate amounts can be followed as a function of temperature, time and chemical composition of the starting matrix. The accuracy of this technique was verified using electron probe micro analysis (EPMA), on two systems of glasses (SiO₂–B₂O₃–Na₂O (SBNa) and SiO₂–B₂O₃–BaO (SBBa)) in order to compare the effect of alkaline or alkaline-earth elements on sulfur speciation and incorporation. To quantitate sulfate content with Raman spectroscopy, the integrated intensity of the sulfate band at 990 cm^?1 was scaled to the sum of the integrated bands between 850 and 1250 cm^?1, bands that are assigned to Qⁿ silica units. Calibration curves were then determined for different samples. The determination of sulfate contents with Raman spectroscopy analysis is possible with an accuracy of approximately 0.1 wt% depending on the composition of the glass. It mainly allows us to follow sulfate removal during the elaboration process and to establish kinetic curves of sulfate release as a function of the viscosity of the borosilicate glass.     

Investigation of defects in low-temperature-grown GaAs using optical transient spectroscopy

Optical transient current spectroscopy (OTCS) has been used to investigate defects in the low-temperature-grown GaAs after postgrowth rapid thermal annealing (RTA). Two samples A and B were grown at 220°C and 360°C on (0 0 1) GaAs substrates, respectively. After growth, samples were subjected to 30 s RTA in the range of 500–800°C. Before annealing, X-ray diffraction measurements show that the concentrations of the excess arsenic for samples A and B are 2.5×10¹⁹ and 1×10¹⁹ cm⁻³, respectively. It is found that there are strong negative decay signals in the optical transient current (OTC) for the annealed sample A. Due to the influence of OTC strong negative decay signals, it is impossible to identify deep levels clearly from OTCS. For a comparison, three deep levels can be identified for sample B before annealing. They are two shallower deep levels and the so-called As_Ga antisite defect. At the annealing temperature of 600°C, there are still three deep levels. However, their structures are different from those in the as-grown sample. OTC strong negative decay signals are also observed for the annealed sample B. It is argued that OTC negative decay signals are related to arsenic clusters.     

Determination of sulfur environments in borosilicate waste glasses using X-ray absorption near-edge spectroscopy

Sulfur can be the waste-loading limiting constituent for vitrification of sulfur-bearing radioactive wastes due to low solubility in silicate melts. Methods to improve sulfur loading would benefit from improved understanding of the structural aspects of sulfur incorporation in borosilicate and other glasses. To this end, sulfur XANES spectra were collected for eight crystalline standards and twenty-four glasses, including borosilicate, phosphate, and borate compositions. Spectra for the standards show a systematic energy shift of the sulfur K-edge from 2469 to 2482 eV, as sulfur valence increases from 2− (in sulfides) to 6+ (in sulfates). Most crucible glasses investigated have simple edges near 2482 eV that indicate sulfur in the form of sulfate only. Other glasses, some synthesized under reducing conditions, have complicated edges, indicating sulfate, sulfite, and more reduced species that may include S, S-S doublets, or short polysulfide chains. Sulfide species (S²⁻) were not dominant in any of the samples over the range of redox conditions investigated. These results indicate that sulfur incorporation is considerably more complex than would be suggested by the conventional interpretation of the redox-dependence of sulfur solubility, which considers only sulfate and sulfide species. Raman data indicate that several of the glasses investigated are not homogeneous with regard to all sulfur species.     

Quantitative measurement of Q species in silicate and borosilicate glasses using Raman spectroscopy

Raman spectroscopy has been used to measure the fraction of tetrahedral silicate units connected at three corners into the network (Q³) in binary lithium silicate glasses and also in the more complex borosilicate glasses used for waste immobilization. Agreement within experimental error was obtained with ²⁹Si MAS NMR measurements of the same samples. Raman provides an alternative method of structural determination for silicon-containing glasses with a high content of paramagnetic species where NMR loses resolution. Analysis was performed on borosilicate glasses containing up to 11.98 mol% Fe₂O₃ and the Q³ values obtained by Raman spectroscopy agree within error with the published ²⁹Si NMR results from borosilicate glasses containing the equivalent quantity of Al₂O₃.     

Characterization of plasticity-induced structural changes in a Zr-based bulk metallic glass using positron annihilation spectroscopy

Changes in the free volume distribution in Zr_58.5Cu_15.6Ni_12.8Al_10.3Nb_2.8 bulk metallic glass with inhomogeneous plastic deformation and annealing were examined using positron annihilation spectroscopy. Results indicate that the size distribution of open volume sites is at least bimodal prior to deformation. The size and concentration of the larger sites, identified as flow defects, changes with processing. The size of the flow defects initially increases with deformation. More extensive deformation shifts the distribution, with a third group of much larger sites forming at the expense of flow defects. This suggests that a critical strain is required for the growth of nanovoids observed elsewhere by HRTEM. Although these observations suggest the presence of three open volume size groups, further analysis indicates that all groups have a similar distribution of chemical species around them as evidenced by the same line shape parameter. This may be due to the disordered structure of the glass and the positron affinity to particular atoms surrounding the open volume regions.     

Etching of silicon trenches in CF4 plasma using photoresist/aluminium masks

The reactive ion etch process (RIE) of silicon in a CF₄/O₂ gas mixture is described. The dependence of the etch rate and the surface quality on used mask material (photoresist, aluminium) is studied. A silicon etch rate of about 1.5 times higher than with resist mask is reached using aluminium as mask material. The free etched surfaces are very rough due to micromasking effects during the etch process. Using a phtoresist mask the surfaces remain clean. The silicon etch depth depends on the thickness of the mask.     

Detection of boron and carbon in flux grown single crystals of hexagonal ferrites using electron energy loss spectroscopy

A method of detecting boron and carbon in the lattice of the flux grown single crystals of hexagonal ferrites using Electron Energy Loss Spectroscopy is given. Our results have led us to conclude that these elements are present as a solid solution and not as inclusions.     

Short-range atomic order in the surface layer of sputter-deposited Al0.6W0.4 thin film investigated using directional elastic peak electron spectroscopy

A mechanism of creating maxima in directional elastic peak electron spectroscopy (DEPES) polar profiles for textured polycrystalline samples and for amorphous samples with a short-range atomic order is proposed and discussed. DEPES was used in investigating the atomic order in Al and W polycrystalline samples. The maxima found in DEPES polar profiles correspond with the texture expected for these samples on the basis of literature data. A method of processing DEPES data of amorphous metallic alloy with a short-range atomic order is proposed and successfully used for an Al_0.6W_0.4 alloy. The results obtained indicate that nanocrystals with an average linear size in the range of 1 nm and with a particular orientation with respect to the sample surface are present in the surface layer of the sample investigated. A concentration of nanocrystals with such orientation equal to 1.9 × 10¹⁹/cm³ was found when using of the contrast obtained for the maximum in the DEPES polar profile, corresponding to an incidence angle of zero. The contrast of DEPES profiles for the Al_0.6W_0.4 sample increases remarkably during annealing at 373 K. An increase in the amount of preferentially-oriented nanocrystals is suggested as the probable reason for this increase.     

Glass formation in and structural investigation of Li2S + GeS2 + GeO2 composition using Raman and IR spectroscopy

Li₂S + GeS₂ + GeO₂ ternary glasses have been prepared and a wide glass-forming range was obtained. The glass transition temperatures increase with the GeO₂ concentration in the glasses. The vibrational modes of both bridging (Ge–S–Ge) and non-bridging (Ge–S⁻) sulfurs are observed in Raman and IR spectra of binary Li₂S + GeS₂ glasses. Additions of GeO₂ to this binary glass increase the bridging oxygen band (Ge–O–Ge) at the expense of decreasing the bridging sulfur band (Ge–S–Ge), whereas the bands associated with the non-bridging sulfurs (Ge–S⁻) remain constant in intensity up to high GeO₂ concentrations. At higher concentrations of GeO₂ (⩾60%), the non-bridging oxygen band, which is not observed at low and intermediate GeO₂ concentrations, appears and grows stronger. From these observations, it is suggested that the added lithium ions favor the non-bridging sulfur sites over the oxygen sites to form non-bridging sulfurs, whereas the added oxygen prefers the higher field strength Ge⁴⁺ cation to form bridging Ge–O–Ge bonds. The structural groups in the Li₂S + GeS₂ + GeO₂ glasses that are consistent with results of Raman and IR spectra are described and are used to develop a structural model of these glasses.