nc-PS Layer Research Articles

Effect of Bilayer Structure on the Long-Term Stability of Nanocrystalline Porous Silicon Ultrasonic Emitter

Due to a strong quantum confinement effect, both the thermal conductivity α and heat capacity per unit volume C of nanocrystalline porous silicon (nc-PS) are extremely lowered in comparison to those of single-crystalline silicon (c-Si). This high contrast between the thermal properties of nc-PS and c-Si makes it possible to produce an efficient ultrasound emitter device based on interfacial thermoacoustic transfer with no mechanical surface vibrations. The most important parameter of this device is the thermal effusivity (αC)1/2 of the nc-PS layer, since the theoretical acoustic output is in inversely proportional to (αC)1/2. To stabilize the acoustic output by suppressing possible change in (αC)1/2 during a long-term operation, a bilayer structure is introduced into the nc-PS layer in this work. Due to the increased stability against oxidation, the device operates for a long time without any significant degradation. The observed long-term stability ensures the applicability of this emitter in functional acoustic devices such as sensors, speakers, and actuators.

Effects of Thermal Effusivity in Nanocrystalline Porous Silicon on Long-Term Operation of Thermally Induced Ultrasonic Emission

Due to a strong quantum confinement effect, both the thermal conductivity α and heat capacity per unit volume C of nanocrystalline porous silicon (nc-PS) are extremely lower than those of single-crystalline silicon (c-Si). This high contrast between the thermal properties of nc-PS and c-Si makes it possible to produce an efficient ultrasound emitter device for various sensors, speakers, and actuators. The most important parameter in this device is assumed to be the thermal effusivity (αC)1/2 of nc-PS, since the theoretical acoustic output is inversely proportional to the (αC)1/2 of the nc-PS layer. To confirm this assumption, the output stability during a long-term operation has been studied in relation to a change in (αC)1/2. The obtained experimental results indicate that the acoustic output deteriorates with a gradual increase in (αC)1/2 due to the oxidation of nc-PS and that an appropriate nanostructure control with a passivated surface against oxidation is the key issue for obtaining a practical operation time.

Tunable Output Directivity of Thermally Induced Ultrasound Generator Based on Nanocrystalline Porous Silicon

To confirm the usefulness of thermally induced ultrasonic emission from nanocrystalline porous silicon (nc-PS) as a probing source for object sensing in air, its characteristics have been investigated in terms of the input pulse width dependence of both the sound pressure amplitude and the ultrasound emission directivity. The device is composed of a silicon wafer substrate, an nc-PS layer, and a thin metal film surface heater electrode. When the device is driven under a pulse input with various widths, the output sound pressure amplitude remains almost constant, whereas the emission directivity changes from the isotropic mode to the directional mode: the measured directivity angles at pulse widths of 50 and 10 µs are 180 and 71.6°, respectively. These results can be explained consistently with theoretical analyses. The observed controllability of the emission angle is attractive for the development of a three-dimensional ultrasonic image sensor with an area-selective function by a simple pulse-width modulation.

Characteristics of Thermally Induced Ultrasonic Emission from Nanocrystalline Porous Silicon Device under Impulse Operation

To clarify the mechanism of thermoacoustic effects in a nanocrystalline porous silicon (nc-PS) device, ultrasonic emission characteristics have been investigated in relation to dynamic behavior. The nc-PS ultrasonic emitter is composed of a surface heater electrode, an nc-PS layer, and a single-crystalline silicon (c-Si) substrate. Due to a completely flat frequency response, this device emits an ideal impulse acoustic output with no reverberations for pulse driving. The relationship between acoustic output and transient surface temperature change can be well interpreted quantitatively by taking the heat capacity of the heater electrode into account. The experimental fact that the transient surface electrode temperature change is induced uniformly over the whole range of the emission area ensures the directivity of the acoustic pulse output.

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.

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.

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.

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.

A solid-state light-emitting device based on ballistic electron excitation using an inorganic material as a fluorescent film

It has been demonstrated that a solid-state light-emitting device based on ballistic electron excitation generated in nanocrystalline porous silicon (nc-PS) layer can be fabricated using an inorganic material as a fluorescent film. This device is composed of a semitransparent thin Au film, an inorganic fluorescent film (in this case, ZnS: Mn), a nc-PS layer, n-type Si substrate and an ohmic back contact. When a positive bias voltage of higher than 20 V is applied to the Au contact, electrons injected into the nc-PS layer are accelerated towards the outer surface, and then hit a fluorescent film as quasi-ballistic or ballistic electrons. The spectra of emitted light correspond to the original emission band of the deposited fluorescent material. These results show that it is possible to use a variety of materials (such as organic materials and inorganic ones) as fluorescent films for the novel solid-state luminescent devices. This is applicable to multicolor surface-emitting light source and flat panel display.

Characteristics of Light Emission by Ballistic Electron Excitation in Nanocrystalline Silicon Device Formed on a p-Type Substrate

It is demonstrated that planar light emission by ballistic electron excitation is observed in a nanocrystalline porous silicon (nc-PS) device fabricated on a p-type silicon substrate and occurs in a similar way to the case of n-type substrate. The device is composed of a semitransparent thin Au film, an organic fluorescent film, the nc-PS layer, a p-type Si substrate and an ohmic back contact. When a positive bias voltage higher than 20 V is applied to the Au top contact, the diode uniformly emits green light. The light intensity sharply increases with increasing bias voltage. The measured luminescence band coincides well with the original photoluminescence spectrum of the deposited fluorescent material. A hysteresis is observed in the current-voltage and the corresponding luminescence-voltage characteristics. These optoelectronic properties suggest that the light emission is based on excitation of a fluorescent film by ballistic electrons generated in the nc-PS layer. The observed hysteresis is due to an optical feedback effect by which photoexcited electrons in the nc-PS layer participate in the ballistic transport and subsequent impact on the fluorescent film. The present result is useful for applications to functional planar light-emissive devices.

Generation of ballistic electrons in nanocrystalline porous silicon layers and its application to a solid-state planar luminescent device

A principle of planar-type visible light emission is presented using ballistic electrons as excitation source. The device is composed of a semitransparent top electrode, a thin film of fluorescent material, a nanocrystalline porous silicon (nc-PS) layer, an n-type silicon wafer, and an ohmic back contact. When a positive dc voltage is applied to the top electrode with respect to the substrate, electrons injected into the nc-PS layer are accelerated via multiple-tunneling through interconnected silicon nanocrystallites, and reach the outer surface as energetic hot or quasiballistic electrons. They directly excite the fluorescent film, and then induce uniform visible luminescence. This solid-state light-emitting device, regarded as a “vacuum-less cathode-ray tube,” has many technological advantages over the conventional luminescent devices. It may lead to big innovations in the development of large-area thin flat-panel display and other electronic devices.