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Electrical conductivity of porous silicon fabricated from heavily doped p-type silicon is very sensitive to NO2, even at concentrations below 100 ppb. However, sensitivity depends strongly on the porous microstructure. The structural difference between sensitive and insensitive samples is independently confirmed by microscopy images and by light scattering behavior. A way to change the structure is by modifying the composition of the electrochemical solution. We have found that best results are achieved using ethanoic solutions with HF concentration levels between 12% and 13%

We have investigated the photoluminescence spectra of silicon-based nanoscale SiO2 islands obtained by anodization of silicon-based aluminum membranes in a 0.3m sulfuric acid solution under a constant voltage of 25 V. Two ultraviolet emission bands were observed at 290 and 370 nm. After annealing the samples in 900 ℃ in O2, the 290 nm band vanishes, but the 370 nm band still exists. We suggest that the 290 nm band originates from optical transition in the E centers in the SiO2 islands according to its annealing behavior. The 370 nm band is considered to be from Al-related luminescence centers, [AlO4]0, because a decrease of intensity of the 370 nm band is in agreement with that of amount of the Al ion impurities located in the SiO2 islands. This work shows a clear understanding of the light-emitting mechanism of silicon-based SiO2 island array. The obtained result can be expected to have important applications in modem optoelectronics.

Intrinsic tensile stress, which can lead to problems in deposited thin films such as cracking, peeling, and delamination, often develops during the early stages of thin film growth. Many attempts have been made to estimate the tensile stress during crystallite coalescence, both experimentally and analytically. Most recently, using a combination of Hertzian contact mechanics and elasticity theory, Freund and Chason applied the Johnson-Kendall-Roberts (JKR) theory to account for adhesion between crystallites under specific conditions. Other existing contact mechanics models that naturally account for adhesion include the improved Derjaguin-Muller-Toporotov and Maugis-Dugdale theories. The objective of this study is to provide useful analytical and numerical techniques based on these contact mechanics theories for a wide range of conditions that accurately approximate the intrinsic tensile stress that develops during crystallite coalescence. As an analytical method, the Maugis-Dugdale model is proposed as a more general alternative to the JKR model. Parameters such as the contact radius and "net" adhesive force are computed as a function of the relative separation between two adjacent crystallites in a thin gold film. Another useful parameter known as the "jump-to-contact" separation is also calculated by the Maugis-Dugdale and JKR models. For comparison to the analytical models, a finite element method is used to simulate the crystallite coalescence problem. The numerical technique is based on a nonlinear surface interaction element developed to approximate van der Waals adhesion, and allows for full-field analysis of stress and displacement in crystallites. Two different boundary conditions are used, for which corresponding contact radius and tensile stress are computed and compared to the analytical results. As a further study, the length scale effect is also investigated by varying the radius of individual crystallites from 20 nm to 300 nm. It is concluded that in order to estimate the average tensile stress accurately using analytical models, the radius of individual crystallites must be large compared to the contact radius. For small length scales, the finite element approach is more appropriate.

The silver island films were vacuum deposited at two different evaporation rates on unmodified indium tin oxide substrates and the substrates modified with molecules containing thiol, pyridyl, and fluorocarbon groups. The films were characterized by UV-visible spectroscopy and scanning electron microscopy. Large variations in the structure and thickness of the island films that were observed between substrates were rationalized in terms of different chemical affinity of the surfaces to the silver metal.

The erosion rate of an aluminum target bombarded with Ar+ and O2+ ions was simulated using TRIDYN. An abrupt change of the erosion rate is noticed at a critical mole fraction of O2+ ions. This target behavior can also be described by a simple analytical model showing that the abrupt change finds its origin in an avalanche induced by reaction ion implantation. Indeed, by the reduction of the average sputter yield by compound formation, more reactive ions become implanted into the target as the number of implanted ions depends inversely on the average sputter yield. As such, an avalanche situation can develop which finally results in a completely oxidized target. It is also shown that the critical mole fraction depends linearly on the sputter yield of the target material and that target poisoning induced by ion implantation can result in a hysteresis behavior for the target condition.

Between 0 and 20%-25%, VD increases with O2. It is explained by a reactive species implantation in the target as proposed by Depla and De Gryse (ref 15). The curves reach a maximum and the VD values decrease dramatically to reach a minimum at 40%. The sharp decrease is attributed to the compound layer formation on the target, which can be correlated to the RD decrease (Fig. 2). Indeed, it is known that the secondary electron emission is higher for an oxide than for the corresponding metal (refs 5 & 6). Consequently, the discharge impedance and the target voltage decrease. Finally, at high %O2 (>40%), the discharge voltage increases due to the change of the gas mixture from a mainly argon atmosphere to a 100% oxygen atmosphere. Indeed, the oxygen molecules can get electrons to produce O2-species in the discharge; consequently the discharge impedance decreases and the target voltage increases.