These last few years a great effort has been made to understand the mechanisms of powder formation in silane discharges. It is now well established that powders are negatively charged and thus confined in the plasma. Therefore, one does not expect powders to contribute to the deposition, unless the plasma is switched off. We present here experimental evidence for the deposition of nanoparticles, even in the case of a continuous discharge. Experimental conditions for nanoparticle formation while avoiding powder formation have been determined from light-scattering and transmission electron microscopy measurements. Nanoparticle deposition has been studied by in situ ellipsometry in silane–argon discharges. From a comparison of the growth kinetics and the optical properties of films obtained under continuous and modulated discharges we conclude that nanoparticle deposition can take place even when the discharge is on. The implications of these discoveries on the properties of hydrogenated amorphous silicon are discussed in the framework of previous studies where we showed that low defect density and well relaxed films are deposited under conditions where nanoparticles and powders are formed in the discharge. The improved properties in these films are attributed to the incorporation of nanoparticles.

Using an electron beam melting (EBM) printing machine (Arcam A2X, Sweden), a matrix of 225 samples (15 rows and 15 columns) of Ti-6Al-4V was produced. The density of the specimens across the tray in the as-built condition was approximately 99.9% of the theoretical density of the alloy, ρT. Tensile strength, tensile elongation, and fatigue life were studied for the as-built samples. Location dependency of the mechanical properties along the build area was observed. Hot isostatic pressing (HIP) slightly increased the density to 99.99% of ρT but drastically improved the fatigue endurance and tensile elongation, probably due to the reduction in the size and the distribution of flaws. The microstructure of the as-built samples contained various defects (e.g., lack of fusion, porosity) that were not observed in the HIP-ed samples. HIP also reduced some of the location related variation in the mechanical properties values, observed in the as-printed condition.

Spectroscopic ellipsometry and high resolution transmission electron microscopy have been used to characterize microcrystalline silicon films. We obtain an excellent agreement between the multilayer model used in the analysis of the optical data and the microscopy measurements. Moreover, thanks to the high resolution achieved in the microscopy measurements and to the improved optical models, two new features of the layer-by-layer deposition of microcrystalline silicon have been detected: ͑i͒ the microcrystalline films present large crystals extending from the a-Si:H substrate to the film surface, despite the sequential process in the layer-by-layer deposition; and ͑ii͒ a porous layer exists between the amorphous silicon substrate and the microcrystalline silicon film. © 1996 American Institute of Physics. ͓S0003-6951͑96͒02730-1͔In situ ellipsometry is a powerful nondestructive technique providing detailed information on the growth mechanisms and optical properties of thin films.1 In order to get information on the composition of a film, the real ͗⑀ 1 ͘ and imaginary ͗⑀ 2 ͘ parts of the pseudodielectric function of the system film plus substrate, deduced from the ellipsometric angles ⌬ and ⌿, are compared to those of an optical model based on Bruggeman's effective-medium theory. 2 The differences between the measured and calculated data are minimized by a linear regression analysis. In recent years we have applied this technique to study the layer-by-layer deposition of microcrystalline silicon films.3 In this method, microcrystalline silicon films are obtained by alternating the deposition of hydrogenated amorphous silicon ͑a-Si:H͒ during a time T Si with its exposure to a hydrogen plasma during a time T H . The major conclusions deduced from these studies are: ͑i͒ the nucleation of a crystalline phase within the a-Si:H network takes place once the initially dense a-Si:H film has been converted into porous a-Si:H by the hydrogen plasma exposure, 4 ͑ii͒ the crystallization of the a-Si:H film deposited during the time T Si is related to the diffusion of hydrogen, leading to nanovoid and broken bond formation processes, 5 and ͑iii͒ there is a substrate dependence of the long term evolution of the properties of the already deposited films. 6 Because most of our previous results were based on in situ ellipsometry and because of the increasing complexity of optical models used to fit the experimental data, we have performed an independent validation of the optical models by high resolution transmission electron microscopy ͑HRTEM͒ measurements. An excellent agreement between the results of the optical models and the HRTEM measurements has been achieved, in agreement with previous reports.7 Moreover, the HRTEM measurements have allowed us to improve the optical models and reveal new features of the layer-by-layer deposition of c-Si.Microcrystalline silicon films were codeposited on different substrates by the layer-by-layer technique. Figure 1 shows the experimental and calculated real and imaginary parts of the pseudo...