V.-P. Lehto

Thermal oxidation of free-standing porous silicon films

We have studied the thermal oxidation of free-standing porous silicon films from room temperature to 730 °C with a differential scanning calorimeter and a thermogravimeter. We have observed three different thermal oxidation processes for the porous silicon. The change of enthalpy (ΔH) and activation energy (Ea) for the first reaction has been calculated. The oxidation of a fresh sample has been compared with those of aged samples, which were stored in dry relative humidity (RH 0%), humid (RH 100%) and normal (RH 25%–35%) laboratory air atmospheres. We also used Fourier transform infrared spectroscopy to clarify the bonds for each process.

Studies of Thermally‐Carbonized Porous Silicon Surfaces

Thermally-carbonized porous silicon films have been prepared by exploiting the dissociation of acetylene. Thermoanalytical methods have been used to study the oxidation behavior of these films in different oxidizing ambients. The results have been compared to other stabilization methods. Due to enhanced adsorption and only slightly reduced specific surface area, the carbonization of porous silicon was found to be an attractive treatment for sensing applications. The possible post-treatments are also discussed.

The room temperature oxidation of porous silicon

The room temperature oxidation of porous silicon was studied using isothermal methods. The oxidation was found to depend on the type of the porous silicon. The microcalorimetric signals from the oxidation of the p+- and n-type porous silicon in dry air were different. In humid air the signals from the oxidation could not be distinguished from the strong signal due to adsorption of water vapour, but when the samples were placed in water similar differences were observed. The reason for differences in reactions is discussed. The oxidation in different liquids was also studied. The signal from reactions in methanol and ethanol were found to be 100 times higher than in water. In FTIR studies the reaction gas produced by reactions between alcohols and the porous silicon, silane (SiH4) was found in the gas. Traces of SiOCH3 and SiOC2H5 groups were also found in FTIR spectra indicating SiOCxHy passivation of the surface.

Color control of white photoluminescence from carbon-incorporated silicon oxide

Color control of the white photoluminescence (PL) from carbon-incorporated silicon oxide is demonstrated. The carbon-incorporated silicon oxide was fabricated by carbonization of porous silicon in acetylene flow (at 650 and 850 °C) followed by wet oxidation (at 650 and 800 °C). It was shown that PL color can be controlled in the range of blue-white and yellow-white by selecting the porosity of starting porous silicon as well as the carbonization and oxidation temperatures. Low-temperature oxidation resulted in bluish light emission in lower porosity series, while high-temperature oxidation promoted yellow-white light emission. The maximal integral intensity of PL was observed after oxidation at 800 °C. It was shown that white PL from carbon-incorporated silicon oxide has blue and yellow-white PL bands originating from different light-emitting centers. The origin of blue PL is attributed to defects in silicon dioxide. Some trap levels at the interface of the carbon clusters and silicon oxide are suggested ...

Effective passivation of porous silicon optical devices by thermal carbonization

Nanostructured porous silicon (PS) optical filters have been proposed for their use in biological and chemical sensing applications. PS, however, presents a reactive surface that must be adequately passivated in order to achieve the required chemical stability mandatory for sensing applications. In the present work, thermal carbonization (TC) by acetylene decomposition is shown to provide effective passivation of a PS internal surface. Thermally carbonized PS optical filters are stable even in strong oxidizing environments such as absolute ethanol. Moreover, it is shown that the TC process, as opposed to more commonly used oxidation treatments, has only minor effects on the optical properties of PS. Thus, the optical performance of PS interference filters is preserved after the carbonization process. In addition, the hydrophilicity of the PS device surface can be adjusted by setting the appropriate TC process temperature. These results show that it is possible to produce low-cost, reusable, chemically sta...

A role of illumination during etching to porous silicon oxidation

The oxidation behavior of porous silicon (PS) has been found to be related to illumination during etching. The autocatalytic oxidation behavior at room temperature arises from the unrelaxed surface induced by the preparation under illumination and can be removed using thermal treatment in a nitrogen atmosphere. The effect is absent in the case of degenerate PS and smaller in p type than in n-type PS. The correlation between the oxidation behavior and the microstructural dimensions is also discussed.

Photo-oxidation studies of porous silicon using a microcalorimetric method

We have used an isothermal microcalorimetric method to study photoinduced effects in porous silicon (PS). In the photo-oxidation investigations using constant wavelength, sharp threshold behavior with threshold energies 3.9 eV for n-PS and 4.7 eV for p+-PS were observed. The two discussed origins for the different threshold energies are based either on the energy gap transitions and enhanced electron transfer from the conduction band to the electron-affinity level of oxygen molecules, or the Si–O bond energy. Also nonlinear dependence on the irradiation intensity was found. Surprisingly, high exothermic signals were observed in measurements made under an inert perfusion. It is proposed that this is associated with relaxation of PS structure, which seems to be more efficiently induced by illumination than thermal treatment.

Investigations of Activation Energy of Porous Silicon Oxidation Using Calorimetric Methods

The oxidation of porous silicon has been studied using differential scanning calorimeter. The oxidation was found to consist of two parts with different activation energies. This indicates the existence of two different reaction mechanism. The results from the hydrogen desorption measurements have been used to study the different oxidation behaviour of the n- and p+-type porous silicon. The results show that the dihydride structure dominates on the surface of the n-type porous silicon, contrary to p+-type porous silicon, where the monohydride is the major structure. Explanations of these features are discussed. Using the activation energy, the surface termination effects are investigated. The best improvement in the activation energy was observed in the sample, whose surface was partially stabilized by ammonium groups.

Characterization of Lactose Powder Surfaces by Isothermal Microcalorimetry

Several lactose samples containing various amounts of amorphicity were studied with an isothermal microcalorimetric technique, which allow to detect the heat and the quantity of water sorption simultaneously. As interaction with vapor is characteristic of different surfaces, the samples were easy to be discriminated from each other by studying sorption behavior. With the crystalline lactose samples, the amount of sorbed water was too minor to be detected reliably with the technique, but differences were found when the energy values (J g−1) were compared. In the future work, the measurement set-up will be improved so that sorption rates less than 0.1 nmol s−1 can be measured repeatably and reliably.

A Microcalorimetric Study on the Role of Moisture in Photolysis of Nifedipine Powder

A well-known photolabile substance, nifedipine, was used as a sample material to test self-constructed irradiation cells and demonstrate their usefulness in photostability studies. The devices were made as accessories for a commercial isothermal microcalorimeter. Several powder samples containing various amounts of moisture were irradiated with monochromatic light as a scan measurement from 700 to 280 nm, and the heat flow evolved in the photodegradation of nifedipine was determined. According to the results, light does not affect the nifedipine molecule directly, but the photodegradation is a result of the combined effects of moisture and light.