Nanocrystalline Porous Silicon Research Articles

Acoustic Emission Characteristics of Nanocrystalline Porous Silicon Device Driven as an Ultrasonic Speaker

It is shown that the dc-superimposed driving mode is more useful for the efficient operation of a novel thermally induced ultrasonic emitter based on nanocrystalline porous silicon (nc-PS) than the conventional simple ac-voltage driving mode. The nc-PS device is composed of a patterned heater electrode, an nc-PS layer and a single crystalline silicon (c-Si) substrate. The almost complete thermally insulating property of nc-PS as a quantum-sized system makes it possible to apply the nc-PS device as an ultrasonic generator by efficient thermo acoustic conversion without any mechanical vibrations. In the dc-superimposed driving mode, the output frequency is the same as the input frequency and a stationary temperature rise is kept constant independent of input peak-to-peak voltage. In addition, power efficiency is significantly increases compared with that in the ac-voltage driving mode without affecting on the temperature rise. The present results suggest the further possibility of the nc-PS device being used as a functional speaker.

Highly Enhanced Efficiency and Stability of Photo- and Electro-luminescence of Nano-crystalline Porous Silicon by High-Pressure Water Vapor Annealing

The effects of high-pressure water vapor annealing (HWA) on the photoluminescence (PL) of quantum-sized silicon materials have been studied for as-anodized or electrochemically oxidized heavily doped n-type porous Si (PS) and electrochemically oxidized p-type PS. PL efficiency and stability are markedly enhanced in all HWA-treated samples. PL peak wavelength remains almost unchanged except for n+-type PS in which a large redshift in the PL emission band was observed due to a structural nonuniformity in the depth direction. The high PL efficiency and stability of HWA-treated PS are attributed mainly to a suppression of nonradiative defect density by complete passivation of silicon nanocrystal surfaces. On the basis of these results, HWA is employed to improve the electroluminescence (EL) characteristics. The device fabricated using an HWA-treated anodized pn+ junction shows improved EL efficiency and stability.

Photosensitized Generation of Singlet Oxygen

This work gives an overview of what is currently known about the mechanisms of the photosensitized production of singlet oxygen. Quenching of pi pi* excited triplet states by O2 proceeds via internal conversion of excited encounter complexes and exciplexes of sensitizer and O2. Both deactivation channels lead with different efficiencies to singlet oxygen generation. The balance between the deactivation channels depends on the triplet-state energy and oxidation potential of the sensitizer, and on the solvent polarity. A model has been developed that reproduces rate constants and efficiencies of the competing processes quantitatively. Sensitization by excited singlet states is much more complex and hence only qualitative rules could be elaborated, despite serious efforts of many groups. However, the most important deactivation paths of fluorescence quenching by O2 are again directed by excess energies and charge-transfer interactions similar to triplet-state quenching by O2. Finally, two recent developments in photosensitization of singlet oxygen are reviewed: Two-photon sensitizers with particular application potential for photodynamic therapy and fluorescence imaging of biological samples and singlet oxygen sensitization by nanocrystalline porous silicon, a material with very different photophysics compared to molecular sensitizers.

Pronounced Photonic Effects of High-Pressure Water Vapor Annealing on Nanocrystalline Porous Silicon

ABSTRACTThe 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 partnand imaginary partk) 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.

Mechanism of a remarkable enhancement in the light emission from nanocrystalline porous silicon annealed in high-pressure water vapor

To clarify the effect of surface passivation on the optical properties of nanocrystalline porous silicon (PS), the photoluminescence (PL) characteristics of PS have been investigated by employing a high-pressure water vapor annealing (HWA). PS samples with various porosities were prepared on (100)-oriented p-type (4Ωcm) single-crystalline silicon wafers by electrochemical anodization. Some samples were then electrochemically oxidized. The HWA treatment was then applied to the prepared PS samples at 0.5–3MPa and 200–300°C for 2–3h. The PL intensities, spectra, and dynamics after HWA were measured in relation to surface analyses by Fourier-transform-infrared (FTIR) spectroscopy. It is shown that the HWA treatment leads to a drastic enhancement in both the PL efficiency and stability. Under the optimum condition, the PS sample exhibits an extremely high external quantum efficiency of 23% at room temperature. According to the FTIR spectra analyses, silicon nanocrystallites in HWA-treated PS are covered with a high-quality SiO2 tissue. The PL decays are found to be longer than those of as-prepared PS, and become closer to a single-exponential behavior near the PL peak wavelength. The observed high efficiency and stability of PL emission from HWA-treated PS is attributed to (i) suppression of nonradiative surface defect density, (ii) uniform passivation by unstrained thin oxides, and (iii) strong localization of excitons in silicon nanocrystals. This low-temperature treatment is very useful for obtaining highly efficient and stable luminescent PS and devices.

High surface area silicon materials: fundamentals and new technology

Crystalline silicon forms the basis of just about all computing technologies on the planet, in the form of microelectronics. An enormous amount of research infrastructure and knowledge has been developed over the past half-century to construct complex functional microelectronic structures in silicon. As a result, it is highly probable that silicon will remain central to computing and related technologies as a platform for integration of, for instance, molecular electronics, sensing elements and micro- and nanoelectromechanical systems. Porous nanocrystalline silicon is a fascinating variant of the same single crystal silicon wafers used to make computer chips. Its synthesis, a straightforward electrochemical, chemical or photochemical etch, is compatible with existing silicon-based fabrication techniques. Porous silicon literally adds an entirely new dimension to the realm of silicon-based technologies as it has a complex, three-dimensional architecture made up of silicon nanoparticles, nanowires, and channel structures. The intrinsic material is photoluminescent at room temperature in the visible region due to quantum confinement effects, and thus provides an optical element to electronic applications. Our group has been developing new organic surface reactions on porous and nanocrystalline silicon to tailor it for a myriad of applications, including molecular electronics and sensing. Integration of organic and biological molecules with porous silicon is critical to harness the properties of this material. The construction and use of complex, hierarchical molecular synthetic strategies on porous silicon will be described.

Highly enhanced photoluminescence of as-anodized and electrochemically oxidized nanocrystalline p-type porous silicon treated by high-pressure water vapor annealing

The effects of a treatment based on high-pressure water vapor annealing (HWA) on nanocrystalline porous silicon (PS) and electrochemically oxidized PS have been investigated in terms of the photoluminescence (PL) efficiency and stability, electron-spin-resonance and infrared spectroscopy. The treatment produces highly efficient and stable luminescent nanocrystalline Si layers emitting red-orange light typical of PS. The PL external quantum efficiency reaches 23% at room temperature. Electron-spin-resonance and infrared absorption analyses show that the HWA treatment provides Si nanocrystals surrounded by a thin layer of oxide with an extremely low non-radiative defect density at the Si/SiO 2 interface. This causes a drastic enhancement in the PL efficiency associated with a high reduction of non-radiative recombinations and a strong localization of excitons in Si nanocrystals. As a practical approach, the HWA technique is very useful for fabrication of efficient and stable optoelectronic nc-Si devices.

The Composition of Fluoride Solutions

Aqueous fluoride solutions underpin much of silicon processing because they are used to strip oxide layers and for cleaning. Depending on their composition and reaction conditions they can be used to create either flat, hydrogen-terminated surfaces or nanocrystalline porous silicon. Nonetheless, confusion reigns regarding the identity and quantity of different solution species as a function of the formal concentration of HF and other additives. The literature is critically surveyed, data and results are reanalyzed, and analytical expressions are derived that allow for the calculation of solution species activities. Above , fluoride solutions are highly nonideal. It is shown that the main constituents of acidic fluoride solutions are , , HF, , and in the range of and even as high as . In these concentration ranges the derived analytical expressions exhibit excellent and approximate agreement with experimental data, respectively. The nature of the HF species that exists in solution, whether it exists as a contact ion pair or as a solvated molecule, remains in doubt.

Highly efficient and stable luminescence of nanocrystalline porous silicon treated by high-pressure water vapor annealing

The effects of a treatment based on high-pressure water vapor annealing (HWA) on nanocrystalline porous silicon have been investigated in terms of the photoluminescence (PL) efficiency and stability. For originally nonluminescent samples with a relatively low porosity, the treatment produces highly efficient and stable luminescent nanocrystalline-Si (nc-Si) layers without affecting the emission wavelength. Under appropriate conditions of pressure (2.6 MPa) and temperature (260 °C), the PL external quantum efficiency reaches 23% at room temperature. Electron-spin-resonance and infrared absorption analyses show that the HWA treatment promotes surface oxidation of nc-Si under a minimized mechanical stress and consequently generates sufficiently passivated nc-Si∕SiO2 interfaces with an extremely low nonradiative defect density. This causes a drastic enhancement in the PL efficiency associated with a strong localization of excitons in nc-Si. As a practical approach, the HWA technique is very useful for fabrication of efficient and stable optoelectronic nc-Si devices.

Three-Dimensional Image Sensing in Air by Thermally Induced Ultrasonic Emitter Based on Nanocrystalline Porous Silicon

Ultrasonic three-dimensional (3-D) image sensing in air is successfully demonstrated by combining a novel thermally induced ultrasonic emission from nanocrystalline porous silicon (nc-PS) device with a condenser microphone array. The nc-PS ultrasonic emitter is composed of a patterned heater electrode, an nc-PS layer, and a single-crystalline silicon (c-Si) substrate. In this device, an ideal probe signal can be generated with little reverberation under impulse operation, since the acoustic output shows a flat frequency response in a wide range due to a quick thermo-acoustic conversion at the device surface without any mechanical vibrations. The 3-D object is detected with a spatial resolution considerably higher than that in the case of conventional techniques based on piezoelectric transducers. It is also shown that the device is available for the detection of several objects with different acoustic reflectance coefficients.

Phased array operation of nanocrystalline porous silicon ultrasonic emitters

To confirm the applicability of thermally induced ultrasound emission from nanocrystalline porous silicon (nc-PS) devices as directional sound emitter, the radiation pattern of one-dimensionally arrayed nc-PS device have been investigated. The nc-PS emitter is fabricated on a p-type Si substrate by conventional electrochemical anodization with subsequent formation of the surface electrode. It is shown that the emission pattern of arrayed devices can be controlled due to flat nature of the frequency response of the individual nc-PS devices. Furthermore we report a technique for wave control by phase-shift operation. This PS array enables the development of a directional sound emitter and intense sound source. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Stable electroluminescence from passivated nano‐crystalline porous silicon using undecylenic acid

Stabilization of electroluminescence from nanocrystalline porous silicon diodes has been achieved by replacing silicon-hydrogen bonds terminating the surface of nanocrystalline silicon with more stable silicon-carbon (Si-C) bonds. Hydrosilylation of the surface of partially and anodically oxidized porous silicon samples was thermally induced at about 90 °C using various different organic molecules. Devices whose surface have been modified with stable covalent bonds shows no degradation in the EL efficiency and EL output intensity under DC operation for several hours. The enhanced stability can be attributed to the high chemical resistance of Si-C bonds against current-induced surface oxidation associated with the generation of nonradiative defects. Although devices treated with 1-decene exhibit reduced EL efficiency and brightness compared to untreatred devices, other molecules, such as ethyl-undecylenate and particularly undecylenic acid provide stable and more efficient visible electroluminescence at room temperature. Undecylenic acid provides EL brightness as high as that of an untreated device. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Fabrication of Nano-Crystalline Porous Silicon on Si Substrates by a Plasma Enhanced Hydrogenation Technique

AbstractA novel plasma hydrogenation method for the fabrication of nano-crystalline structures of silicon as well as the photoluminescence and structural properties of these porous structures is presented. We have observed that the hydrogenation process followed by an annealing treatment results in the formation of nano-crystalline silicon structures where increased temperatures during hydrogenation reduces the grain size. Furthermore, by increasing the time of the hydrogenation process, the density of the silicon grains is increased.Photoluminescence (PL) spectroscopy demonstrated the presence of a direct gap in the visible light range where materials with a smaller grain size emitted light at lower wavelengths, and a higher density of grains resulted in higher amplitudes in the PL spectrum. TEM and SEM characterization of these samples and the structure-emission relationship are also presented.

Ballistic transport mode detected by picosecond time-of-flight measurements for nanocrystalline porous silicon layer

The electron transport mechanism in nanocrystalline porous silicon (nc-PS) with a controlled structure has been studied for a self-supporting sample by time-of-flight (TOF) measurements at room and low temperatures using a picosecond-width UV laser pulse. In contrast to both single-crystalline silicon (c-Si) and hydrogenated amorphous silicon (a-Si:H), the TOF signals of nc-PS show characteristic behavior that involves a ballistic component. The drift velocity vd determined from observed TOF signals shows no signs of saturation with increasing field strength F. At F∼3×104V∕cm, the vd value in nc-PS at room temperature reaches 2.2×108cm∕s. The corresponding electron mean free path is 1.6μm. These values are considerably larger than those in c-Si. The ballistic transport mode becomes clear at low temperatures. The results support the model that electrons can travel ballistically with little scattering losses in a nanocrystalline silicon dot chain interconnected via thin silicon dioxide films.

High Performance Electroluminescence from Nanocrystalline Silicon with Carbon Buffer

Efficient electroluminescence (EL) is obtained at low voltages (3 V) from n+-type silicon/electrochemically oxidized thin nanocrystalline porous silicon (PS)/amorphous carbon/indium tin oxide (ITO) junctions. The carbon film acts as an efficient mechanical and electrical buffer layer between PS and ITO. As a result, the efficiency, stability and reproducibility are markedly enhanced. High efficiency and high brightness have been made possible at low voltages. A diode exhibiting top performance in several parameters has been obtained. A brightness of 3 Cd/m2 has been achieved at 3 V for an external power efficiency of 0.35%. For the first time, voltage-tunable one-peak EL spectra have been obtained. This phenomenon originates from the EL excitation mechanism and the diode high efficiency at low voltages. The EL stability is also enhanced due to the capping function of the carbon film, and the high chemical stability of carbon and Si-C bonds.

Hydrogen surface coverage of as-prepared nanocrystalline porous silicon

Porous silicon (PoSi) surface coverage by hydrogen has been investigated by thermal desorption spectroscopy (TDS). The desorption rate of H 2 and SiH x ( x=1–4) molecules was monitored with a quadrupole mass spectrometer during linear heating of the samples in vacuum. The almost simultaneous desorption of several species containing hydrogen gives rise to complicated TD spectra. The full spectra can be understood as due to H-atom desorption from different bonding states at the PoSi surface, namely a SiH(a) surface monohydride, a SiH 2(a) dihydride and a SiH 3(a) trihydride. In order to obtain clean spectra for a kinetic analysis, as prepared samples were annealed at selected temperatures to isolate the contributions from the different bonding states. A detailed knowledge of the kinetic parameters for the desorption process has been obtained from these experiments. In this paper, the order of the reaction, activation energy and pre-exponential factor for desorption from each H bonding state will be presented.

Enhancing the Sound Pressure of Thermally Induced Ultrasonic Emitter Based on Nanocrystalline Porous Silicon

ABSTRACTIt 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.

Improved Optoelectronic Characteristics of Nanocrystalline Porous Silicon by High-Pressure Water Vapor Annealing

ABSTRACTA 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 SiO2tissue. 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.

Possible Operation of Periodically Layered Nanocrystalline Porous Silicon as an Acoustic Band Crystal Device

ABSTRACTIt 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.

Quasiballistic Electron Emission from Planarized Nanocrystalline-Si Cold Cathode

ABSTRACTMechanism 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.