The paper reports on the synthesis and structural characterization of amorphous silicon oxide nanowires (SiONWs). The nanowires were prepared using the solid-liquid-solid (SLS) technique and display an average mean diameter in the range of 20-150 nm and 15-20 μm in length. Energy dispersive spectrometry (EDS) analysis revealed that the nanowires consist of Si and O elements in a ratio of approximately 1:2. The result was corroborated by the presence of a single peak at around 103 eV in the X-ray photoelectron (XPS) spectrum. As-prepared SiONWs display blue photoluminescence (PL) centered ∼ 400 nm. The blue luminescence is inhomogeneous with an increase in the PL peak intensity by a factor of 1.8 at certain areas of the surface. Next, we have investigated the effect of high-pressure water vapor annealing (HWA) on the PL and found that while the PL became homogeneous, a decrease of its intensity by a factor 2 was observed. The origin of the blue emission could be attributed to defect centers of oxygen deficiency in the wires.

The effects of high-pressure water vapor annealing (HWA) on the refractive index of PS have been studied. HWA was conducted at 260 °C and 1.3 MPa, for 3 h. The refractive index (real part n and imaginary part k) was estimated by fitting reflectivity spectra. HWA considerably modifies the layers refractive index. It enhances the optical transparency of PS, particularly at short wavelengths down to below 400 nm. Both n and k are significantly reduced by HWA between 400 nm and 850 nm. These results are attributed to the high level of oxidation of PS after HWA. The high transparency of the treated layers enables Si-based photonics in the full visible range and also in the near UV range. Distributed Bragg reflectors (DBRs) have been fabricated. The central wavelengths appear blue-shifted compared to untreated samples due to the reduced refractive index. Almost no changes could be found in the reflectivity properties after one year storage in air for HWA-treated DBRs. Therefore, HWA is very useful for getting stable practical photonics devices in the visible and near UV range.

It is shown that the periodic stacked structures of nanocrystalline porous silicon (nc-PS) layers with controlled densities and elastic properties act as an acoustic band crystal (ABC) device. Supposing that the periodic nc-PS layers are formed by conventional modulated anodization technique to fabricate the multi-layered distributed Brag reflection mirror, the acoustic wave propagation modes are investigated theoretically for various structural parameters. According to the calculation results, a significant acoustic band gaps are generated in the ultrasonic regions due to a big contrast in the elastic constant produced between low-porosity and compact nc-PS layers. The propagation of acoustic wave can be completely suppressed in the characteristic band determined from designed parameters. The present result suggests further possibility of the nc-PS layer as a key component of ABC devices.

It is shown that the acoustic pulse output of nanocrystalline porous silicon (nc-PS) ultrasonic emitter can be enhanced by appropriate control of both the driving mode and the nc-PS structure. The device is composed of a patterned heater electrode, an nc-PS layer, and a single-crystalline silicon (c-Si) substrate. As the sound pressure is proportional to input power in principle, the nc-PS device can generate a high acoustic power under the mode of temporal burst electrical input. The most important limiting factor for maximum electrical input power is a mechanical stress at the electrode/nc-PS interface induced by a rapid interfacial temperature raise rather than a simple electro-migration inside the thin metal film. In fact the maximum sound pressure is enhanced with increasing the hardness and Young's modulus of the nc-PS layer. The present result provides useful information for further progress of the nc-PS acoustic device.

Mechanism of electron transport through planerized nanocrystalline-Si (nc-Si) cold cathode surface emitting devices was investigated. The energy distribution of electrons emitted from nc-Si emitter was obviously not Maxwellian, which was usually obtained at conventional cold cathode devices, but was similar to that from the nanocrystalline porous silicon diode emitter. These results strongly suggest that electrons are emitted quasiballistically from our devices and indicate that the planarized nc-Si layer play an important role in this high efficiency cold cathode emitter.

A high-pressure H2O vapor annealing technique has been applied to nanocrystalline porous silicon (PS). The effects on the photoluminescence (PL) are reported here for PS samples of different initial porosities, in different conditions of the annealing pressure. A drastic enhancement of the PL intensity has been achieved, while both the emission band and the peak wavelength remain almost unchanged. The best results have been obtained with an initial PS porosity of about 68 %. The resulting layers consist of Si nanocrystals embedded into an SiO2 tissue. The high external quantum efficiency obtained in treated PS (23%) may be attributed mainly to a very high surface passivation by high quality oxide and an enhancement in the localization of photoexcited carriers.

Efficient electroluminescence (EL) is obtained at low operating voltages (<3 V) from n+-type silicon- electrochemically oxidized thin nanocrystalline porous silicon (PS)-amorphous carbon-Indium tin oxide (ITO) junctions. The effects of a few nanometer thick amorphous carbon film between PS and ITO on the EL characteristics have been investigated. The carbon film enhances the stability. The EL efficiency is improved due to a reduction of current density and an increase in EL intensity. In addition, the reproducibility from device to device is very much improved by the carbon film. The enhancement in stability should originate from the capping of PS by the carbon film and the high chemical stability of carbon and Si-C bonds, which should prevent PS oxidation. The carbon film acts as an efficient buffer layer between PS and ITO, resulting in enhanced mechanical, electrical and chemical stability of the top contact and providing high reproducibility. The thin carbon film has only positive effects on all the EL characteristics. This is a very important step towards application.

It is demonstrated that a porous polycrystalline silicon (PPS) film is useful as a ballistic electron emitter for excitation source of a flat panel display. A 1.5 µ m polysilicon layer is deposited on a silicon substrate by Low Pressure Chemical Vapour Deposition (LPCVD) technique and subsequently anodised in an ethanoic HF solution and oxidised in a Rapid Thermal Oxidation (RTO) furnace. A thin Au film is deposited onto the RTO-treated PPS layer and used as a top electrode. The electron emission current Ie and the diode current Ips are measured as a function of the bias voltage Vps. Electron emission of which onset voltage is about 8 V rapidly increases with increasing Vps. The Ie value reaches about 2 mA/cm2 for Vps= 20 V at which the emission efficiency defined as Ie/Ips is about 1 %. The emission mechanism has also been investigated in terms of the correlation between the emitted electron energy and the structure of PPS layer. The observed energy distribution curve of output electrons suggests that the PPS layer acts as a ballistic transport medium and the emission occurs based on multiple tunnelling through silicon nanocrystallites. The PPS layer is also formed on the polysilicon layer deposited on a glass substrate by Plasma Enhanced Chemical Vapour Deposition (PCVD) technique. In this case, the film is treated by an electrochemical oxidation (ECO) in an H2SO4 solution. Similar emission characteristics are observed, although the emission current is lower than that formed on silicon substrate. We also demonstrate the 2.6 inches diagonal 53×40 pixels multicolour flat panel display. We name it ballistic electron surface-emitting display device (BSD). BSD shows the possible application to the future flat panel display.

The characteristics of ballistic electron transport in porous silicon (PS) are investigated in terms of the electron emission from PS diodes and the energy distribution of emitted electrons. The energy distributions show a behavior of ballistic electron emission that is quite different with the Maxwellian distribution. This is clearly observed at low temperatures below 150 K where the electrical conduction in PS is dominated by the tunneling mode. At 100 K, the peak position of distribution curve becomes more close to the energy corresponding to the energy gain expected from ballistic transport without any scattering losses. The simulated energy distribution suggests that the electrons having higher energies in a non-equilibrium state travel ballistically in PS via field-induced tunneling process. These results support that electrons can travel ballistically in nanocrystalline layer under a high electric field. The observed ballistic transport indicates the further technological potential of silicon nanocrystallites.

It is demonstrated that porous polysilicon (PPS) diode fabricated on the silicon substrate operates as efficient and stable surface-emitting cold cathode. A 1.5 pm of non-doped polysilicon layer is formed on n-type (100) silicon wafer and anodised in a solution of HF(50%): ethanol = 1:1 at a current density of 10 mA/cm2 for 30 seconds under illumination by a 500W tungsten lamp from a distance of 20 cm. Subsequently, PPS layer is oxidized in a rapid thermal oxidation(RTO) furnace for one hour at a temperature of 700°C. A semi-transparent thin Au film (about 10 nm thick) is deposited onto the PPS layer as a positive electrode and an ohmic contact is formed at the back of the silicon wafer as a negative electrode. When a positive bias is applied to the Au electrode in vacuum, the diode uniformly emits electrons. No electron emission is observed in the negatively biased region. Emission current is about 10−4 A/cm2 at 20V bias, and no fluctuation of emission current is observed as a function of time. Emission current is not affected by a surrounding pressure up to around 10 Pa. It is envisaged that mechanism of this emission is attributed to hot electron tunnelling.

We have studied the optical and the electrical properties of deuterium-terminated porous silicon. The photoluminescence spectrum was different from that of usual hydrogen-terminated porous Si despite both porous Si showing the same structure and the same absorption spectrum. In addition to these basic results, it is found that the replacement of hydrogen with deuterium reduces the degradation not only of photoluminescence but also of electroluminescence. The luminescence degradation mechanism will be discussed from the point of a photoinduced oxidation model.

We have studied the optical properties of deuterium-terminated porous silicon. The photoluminescence spectrum was different from that of usual hydrogen-terminated porous Si despite porous Si showing both the same structure and the same absorption spectrum. These results indicate that the surface vibration of terminated atoms couples to the quantum confined states.

The optoelectronic properties of porous Si (PS) have been investigated in relation to the visible luminescence characteristics. First, the electronic structure has been characterized by spectroscopic ellipsometry and synchrotron orbital radiation (SOR) reflection spectra analyses. A retention of the original crystallinity, accompanied by a band gap widening, is confirmed by the optical reflection spectra and the optical constants behavior. Second, it is shown that there exists an intrinsic correlation between the visible photoluminescence characteristics of PS and its interfacial and electrical properties. These experimental results suggest that in the visible luminescence mechanism of PS, both band modification in Si crystallites and passivation at their surface play a complementary role to each other. The former ensures sufficient electronic excitation and the latter contribute to suppression of nonradiative relaxation.

The optoelectronic properties of porous Si (PS) are presented in terms of electroluminescence (EL), photoluminescence (PL), photoconduction (PC), and optical absorption. Observations of injection-type EL, efficient PL, band-gap widening, and photosensitivities In the visible region are consistent with the quantum size effect model in PS.