Hydrogen-related mechanical stress in amorphous silicon and plasma-deposited silicon nitride

Hydrogen concentrations in amorphous silicon (a-Si) prepared by electron beam evaporation or plasma deposition as well as in plasma-deposited silicon nitride were measured as a function of annealing conditions using the photon-proton scattering method. Comparison with IR evaluations of SiH bonds shows the existence of “quasi-free” (IR inactive) hydrogen in a-Si. The amount of this component can be altered by annealing. The dependence of the mechanical stress on the annealing conditions was determined interferometrically, and the intrinsic contribution was separated in the case of plasma-deposited silicon nitride. As the hydrogen content decreases monotonically with increasing annealing temperature, the initial compressive stress of plasma-deposited a-Si or silicon nitride films changes to a tensile stress. For these films a linear correlation between the amount of mechanical stress and the number of SiH or NH bonds is shown. Evaporated a-Si films show a totally different stress behaviour from that of plasma-deposited a-Si. No correlation of stress with the hydrogen concentration was found for the evaporated a-Si films.     

Heterogeneous effect in carbonized porous silicon

The spectra of photoluminescence from carbonized porous silicon were measured upon the anodic polarization of samples in an oxidative electrolyte. A significant increase in the intensity of a blue-green emission band was observed and attributed to a change in parameters of the silicon-silicon carbide heterostructure. In addition, the carbonized porous silicon exhibits effective electroluminescence in an oxidizing electrolyte.     

Electrodeposition of amorphous silicon in non-oxygenated organic solvent

We report on the synthesis of amorphous silicon using electrodeposition in two non-oxygenated organic solvents, acetonitril and dichloromethane, under controlled atmosphere. In both solvents, tetraethylammonium chloride has been used as a supporting electrolyte and silicon tetrachloride as a silicon precursor. The porosity and the morphology of the electrodeposited silicon layers have been studied as a function of depositing parameters (solvent, voltage, etc.). We will show the formation from dense to highly porous Si deposits as a function of solvent and experimental parameters. To increase the stability of the deposits before exposure to air, a heat treatment under hydrogen has been performed. The chemical and structural characterizations of the deposit by scanning electron microscopy, energy dispersive X-ray spectroscopy and Raman spectroscopy show the formation of pure amorphous silicon.     

Hydrogen catalytic oxidation reaction on Pd-doped porous silicon

The efficiency of Pd-doped porous silicon (PS) as a catalytic material for hydrogen sensing is studied. Pd is deposited by an electroless process on the internal surface of porous silicon. The catalytic behavior of Pd-doped PS samples is estimated and the parameters that influence the kinetics of the chemical reaction are evaluated. The catalytic activity is examined through the kinetics of the chemical reaction, which occurs in low hydrogen content mixtures with air (up to 1% v/v in air), far below the mixture flammability limit. It was found that the catalytic activity of Pd-doped porous silicon at 160°C is significantly higher than that of a planar surface covered with Pd. The dependence of the catalytic activity on processing conditions was also evaluated. These results open important new possibilities for applications in gas sensors     

Properties of Sol–Gel Derived Thin Organoalkylenesiloxane Films

We have prepared film-forming solutions for the growth of dense and porous thin organoalkylenesiloxane (OAS) films based on copolymers of methyltrimethoxysilane and 1,2-bis(trimethoxysilyl)ethane (BTMSE) by a sol–gel process. The chemical composition and microstructure of the OAS films have been studied by IR spectroscopy and spectral ellipsometry in relation to the mole fraction of BTMSE and the water: methoxy groups ratio in solution. The results demonstrate that partial substitution of ethylene bridges for silicon–oxygen bonds in OAS leads to distortion of the regular ladder-like structure characteristic of polymethylsilsesquioxane films and the presence of residual silanol groups, which causes an increase in the dielectric permittivity k of the matrix material. The relative porosity in porous OAS films produced via evaporationinduced self-assembly has been shown to be determined by not only the amount of surfactant added but also the presence of a sufficient amount of silanol groups, participating in the attachment of surfactant molecules, in the matrix copolymer solution. In this connection, an important factor determining the structure of the OAS matrix and its pore structure is control over the amount of water involved in the cohydrolysis process. It has been shown that the samples with a relative porosity of 38% prepared from a film-forming solution containing 47 mol % BTMSE (m = 0.7) and 30 wt % surfactant have k ≈ 2.3 and are potentially attractive materials for use as insulators in integrated circuits.     

Chemical stability of hydrogen-containing boron nitride films obtained by plasma enhanced chemical vapour deposition

The purpose of this paper is the investigation of the dehydrogenation kinetics of boron nitride films during thermal annealing. BN_x:H films on silicon substrates were prepared by remote plasma enhanced chemical vapour deposition at 473 K using a mixture of borazine and helium. IR spectroscopy and ellipsometry were used to characterize the film properties and composition. The films contain a certain amount of hydrogen in B---H and N---H bonds. The breakage kinetics of these bonds is different. The breakage of N---H bonds determines the hydrogen annealing kinetics at 973–1073 K. The low-temperature annealing (673–873 K) of B---H bonds is sensitive to the generation of hydrogen from N---H bonds. Heat treatment leads to ordering of the films.     

Microcrystalline silicon growth on a-Si:H: effects of hydrogen

Spectroscopic ellipsometry and secondary ion mass spectrometry have been performed on structures consisting of a microcrystalline silicon film deposited on different a-Si:H substrates. The substitution of hydrogen by deuterium in the microcrystalline growth allowed us to quantify the long-range effects of hydrogen and to distinguish between the different layers. The thicknesses deduced from ellipsometry measurements were in excellent agreement with the secondary ion mass spectrometry profiles. Moreover, we clearly show that the a-Si:H film on which the microcrystalline silicon is deposited can be modified because of the diffusion of hydrogen necessary to the formation of the microcrystalline silicon. However, this modification strongly depends on the deposition conditions and abrupt interfaces can be achieved in some cases.     

The hydrogen content of austenite after cathodic charging

We have made direct measurements of deuterium concentration profiles and have shown that the concentration of hydrogen (deuterium) in the near-surface layers of 310 austenitic steel following room-temperature cathodic charging in poisoned electrolytes is 0.5 to 0.8 hydrogen atoms per metal atom. There is also considerable microstructural damage, including the formation of an unstable martensite. With no arsenic poison in the electrolyte the hydrogen ratio is about 0.15, and there is low associated damage structure. These hydrogen (deuterium) concentrations are many orders of magnitude greater than those required to embrittle ferritic steels, implying a different embrittlement mechanism in the austenite. The observations of a hydrogen-induced martensite phase in this supposedly phase-stable alloy indicates that hydrogen embrittlement mechanisms involving martensite are no longer excluded.     

Composition and reactivity of porous silicon nanopowders

We have studied the chemical reactivity of silicon powders with distilled water. Nanopowders were prepared through electrochemical etching of silicon wafers, followed by grinding of the porous layer. The composition of the chemical bonds in the powders was determined by IR spectroscopy. The particle size of the powders was evaluated using transmission electron microscopy and nitrogen thermodesorption measurements. According to potentiometry data, the powders differ in their reactivity with water, which can be understood in terms of the particle composition and size.     

Formation of nanosize poly(p-phenylene vinylene) in porous silicon substrate

We report the results of optical investigations in porous silicon (PS)/poly(p-phenylene vinylene) (PPV) systems obtained by filling the pores of silicon wafers with polymer.By scanning electron microscopy (SEM), IR, and Raman spectroscopy, we observed that the porous silicon layer was thoroughly filled by the polymer with no significant change in the structure of the materials. This suggests that there is no interaction between the components. On the other hand, the photoluminescence (PL) spectra of the devices investigated at different temperatures (from 11 to 290 K) showed that both materials are active at low temperatures. Porous silicon has a band located at 398 nm while PPV has two bands at 528 and 570 nm. As the temperature increases, the PL intensity of porous silicon decreases and that PPV is blue shifted. A new band emerging at 473 nm may indicate an energy transfer from the porous silicon to PPV, involving short segments of the polymer. The band of PPV located at 515 nm becomes more dominant and indicates that the nanosize polymer films are formed in the pores of the silicon layer, in agreement with the results obtained by SEM, IR, and Raman analyses.     

Chemical bonds and microstructure in nearly stoichiometric PECVD aSixNyHz

Silicon nitride thin films were prepared by low frequency plasma enhanced chemical vapour deposition in silane, nitrogen and helium mixtures. With different silane flow rates, various thin films with nearly stoichiometric composition but different chemical bonds were prepared with hydrogen content lower than 12%. The films were annealed up to 1100 °C and the hydrogen content was measured by elastic recoil detection analysis and compared to the infrared vibration of hydrogenated modes of silicon and nitrogen atoms. Two kinds of behaviour were observed in the chemical bonding with the annealing temperature: either NH and SiH bonds show a decrease, as is generally reported in the literature, or a strong increase of the SiH one is observed. There is also a dramatic discrepancy between the hydrogen content measured by nuclear technique and by infrared spectroscopy if constant oscillator strengths of the stretching modes of hydrogenated sites are used. As in amorphous silicon, these results confirm the difficulty of deducing the concentration of hydrogen bonded to the silicon atoms in silicon nitride. We suggest that these effects are closely correlated to the local microstructure of the films and specific arrangements around SiH dipoles.     

Stabilization of luminous properties of porous silicon by vacuum annealing at high temperatures

The degradation properties of porous silicon caused by vacuum annealing at high-temperatures have been measured. Vacuum annealing within the temperature range of 150–300°C provided the complete removal of water molecules from the pore surface and partial destruction of hydrogen complexes. Our studies showed that vacuum annealing at 150°C considerably stabilizes the luminous properties of porous silicon: the subsequent UV-irradiation of silicon specimens changed the photoluminescence intensity only by 8–10% of its initial value.     

In situ detection of F2 laser-induced oxidation on hydrogen-terminated Si(111) and Si(110) surfaces by Fourier transform infrared transmission spectroscopy

Although oxidation of silicon has been intensively investigated experimentally and theoretically, the composition and structure of the interface region between the bulk oxide and silicon are not very well known. In this work, we report on the vacuum UV laser-induced oxidation of well-defined atomically flat Si(111)-(1×1):H and Si(110)-(1×1):H surfaces, prepared by wet-chemical processing, using an F₂ laser (157 nm). The silicon samples were irradiated under ultra high vacuum conditions in water and oxygen atmospheres. The photo-induced partial oxidation of the first monolayer, controlled by the oxidant pressure, was monitored by in situ Fourier transform infrared transmission spectroscopy with different photon detectors. The chemical reactions occurring at the surface were monitored by the shift and decreasing strength of the stretching vibration of Si---H surface bonds. The IR absorption spectra recorded during surface processing yield detailed information on the rearrangement of chemical bonds with a sensitivity reaching monolayer resolution.     

Porous silicon and aluminum co-gettering experiment in p-type multicrystalline silicon substrate

The lifetimes of non-equilibrium minority carriers, which bound with the diffusion length, are considered as two important parameters of the low-quality multicrystalline silicon (mc-Si) substrate. Its value defines the quality of the initial substrate. It is also subjected to change as a result of many high-temperature operations during the device fabrication. Therefore, it is necessary to incorporate certain processing steps that either improve or preserve the electronic quality of the mc-Si substrate. In this study, a novel porous silicon and aluminum co-gettering experiment has been applied as a beneficial approach to improve the electronic quality of the low-resistivity mc-Si substrates. Porous silicon layers were prepared by anodization of the n⁺ silicon region by a simple electrochemical etching process using an aqueous HF-based electrolyte, which leads to the creation of porous silicon microcavities. Besides making porous silicon and aluminum co-gettered samples, both phosphorous and aluminum alloy-gettered samples and reference samples were made. The gettering-induced lifetime enhancement in the test samples was monitored by measuring the lifetime/diffusion length of the test samples using two independent methods such as photoconductivity decay (PCD) measurement and the photocurrent generation method (PCM), respectively. The result in both the measurements has shown a reasonably good agreement with each other. Therefore, it is inferred that the applied co-gettering experiment has a synergetic effect to improve the lifetime of the mc-Si substrate.     

Porous silicon and aluminum co-gettering experiment in p-type multicrystalline silicon substrate

The lifetimes of non-equilibrium minority carriers, which bound with the diffusion length, are considered as two important parameters of the low-quality multicrystalline silicon (mc-Si) substrate. Its value defines the quality of the initial substrate. It is also subjected to change as a result of many high-temperature operations during the device fabrication. Therefore, it is necessary to incorporate certain processing steps that either improve or preserve the electronic quality of the mc-Si substrate. In this study, a novel porous silicon and aluminum co-gettering experiment has been applied as a beneficial approach to improve the electronic quality of the low-resistivity mc-Si substrates. Porous silicon layers were prepared by anodization of the n⁺ silicon region by a simple electrochemical etching process using an aqueous HF-based electrolyte, which leads to the creation of porous silicon microcavities. Besides making porous silicon and aluminum co-gettered samples, both phosphorous and aluminum alloy-gettered samples and reference samples were made. The gettering-induced lifetime enhancement in the test samples was monitored by measuring the lifetime/diffusion length of the test samples using two independent methods such as photoconductivity decay (PCD) measurement and the photocurrent generation method (PCM), respectively. The result in both the measurements has shown a reasonably good agreement with each other. Therefore, it is inferred that the applied co-gettering experiment has a synergetic effect to improve the lifetime of the mc-Si substrate.     

Porous silicon nanoparticles containing neurotropic drugs

Porous silicon samples with a pore diameter under 100 nm have been prepared by a two-sided anodic electrochemical etching of single-crystal silicon. We have studied vinpocetine and afobazol sorption on porous silicon. The drugs have been shown to have no effect on the structure of the porous silicon. Comparison of the degree of afobazol and vinpocetine release from the drug delivery systems produced in this study and from microcapsules demonstrates that porous silicon nanoparticles can be used as a means of prolonged drug delivery, suggesting that further pharmacological research is warranted.     

The synthesis and photoluminescence properties of selenium-treated porous silicon nanowire arrays

Here we prepared vertical and single crystalline porous silicon nanowire (SiNW) arrays using the silver-assisted electroless etching method. The selenization was carried out by annealing the samples in vacuum with selenium atmosphere. The selenization treatment at 700?°C is useful for investigating the photoluminescence (PL) properties of porous SiNWs, with an enhancement of 30 times observed. The observed PL peaks blue-shift to 650 nm and the decomposition of the spectrum reveals that three PL bands with different origins are obtained. It is proved that selenization treatment could remove the Si-H bonds on the surface and form Si-Se bonds, which could increase the absorbance of the SiNWs and also enhance the stability of the PL intensity. These Se-treated porous SiNWs may be useful as nanoscale optoelectronic devices.     

The dependence of hydrofluoric acid concentration during anodization on photoluminescence of porous silicon

The mixture of hydrofluoric (HF) acid and ethanol is used as an electrolyte during anodization of silicon. We investigated the effect of the ratio of HF acid to ethanol on photoluminescence. It is concluded that porous silicon anodized with the electrolyte containing 35 or 40% HF acid provides strong photoluminescence. The fact implies the existence of a chemical reaction including ethanol during anodization other than electrochemical reaction.     

The dependence of hydrofluoric acid concentration during anodization on photoluminescence of porous silicon

The mixture of hydrofluoric (HF) acid and ethanol is used as an electrolyte during anodization of silicon. We investigated the effect of the ratio of HF acid to ethanol on photoluminescence. It is concluded that porous silicon anodized with the electrolyte containing 35 or 40% HF acid provides strong photoluminescence. The fact implies the existence of a chemical reaction including ethanol during anodization other than electrochemical reaction.     

Ground state structures and properties of small hydrogenated silicon clusters

We present results for ground state structures and properties of small hydrogenated silicon clusters using the Car-Parrinello molecular dynamics with simulated annealing. We discuss the nature of bonding of hydrogen in these clusters. We find that hydrogen can form a bridge like Si-H-Si bond connecting two silicon atoms. We find that in the case of a compact and closed silicon cluster hydrogen bonds to the silicon cluster from outside. To understand the structural evolutions and properties of silicon cluster due to hydrogenation, we have studied the cohesive energy and first excited electronic level gap of clusters as a function of hydrogenation. We find that first excited electronic level gap of Sin and SinH fluctuates as function of size and this may provide a first principle basis for the short-range potential fluctuations in hydrogenated amorphous silicon. The stability of hydrogenated silicon clusters is also discussed.