Hideki Koyama

Efficient visible photoluminescence from porous silicon

It is shown that UV-excited porous Si (PS) exhibits an efficient visible photoluminescence (PL) at room temperature. The PS layers were formed by anodization of p-type and n-type single-crystal Si wafers in aqueous HF solutions. The peak wavelength of PL spectra depends on the anodization parameters including the resistivity and the conduction type of Si substrates. The PL spectra can be tuned to a higher energy side by either adjustment of the anodizing conditions or chemical etching after anodization. These remarkable results can be interpreted as a result of quantum size effects in PS.

Cold Electron Emission from Electroluminescent Porous Silicon Diodes

It is demonstrated that electroluminescent porous silicon (PS) diodes operate as surface-emitting cold cathodes. The PS diode is composed of a semitransparent thin Au film, PS layer, n-type Si substrate and ohmic contact. When a sufficient positive bias voltage is applied to the Au contact in vacuum, the diode uniformly emits electrons as well as visible light. This cold emission is explained by the model that electrons are injected from the substrate, drifted by the field within the PS layer, and ejected as hot electrons through the thin Au film. This mode gives important insight for understanding the electroluminescence mechanism of PS, and shows the potential of PS devices not only for optoelectronic applications, but also for vacuum microelectronic ones.

Visible photoluminescence of Porous Si and its related optical properties

Visible photoluminescence (PL) characteristics of porous silicon (PS) layers have been studied in relation to photoconduction and optical absorption spectra. The PS layers are formed on p- and n-type Si wafers by electrochemical and photoelectrochemical anodization in HF solutions. Some intrinsic correlations are observed between the peak wavelength of PL spectra and the anodization parameters. Other macroscopic observations show that both the photoconductivities and the fundamental absorption edges of PS layers are present in the visible region. This evidence suggests that the optoelectronic properties of PS are strongly modulated by the quantum size effect in microstructures of the PS.

Energy transfer in porous-silicon/laser-dye composite evidenced by polarization memory of photoluminescence

The optical properties of nanocomposites are studied for rhodamine B RhB dye molecules embedded in nanoscale porous silicon PS and porous silica P-silica. The samples are prepared by simple immersion of porous matrix substrates in RhB solutions. Fourier transform infrared spectroscopy suggests that RhB molecules fully penetrate into the porous matrix. Efficient photoluminescence has been observed in all experimental nanocomposites. The measurement of the polarization memory PM effect provides a clear evidence for the presence of an excitation energy transfer from PS to RhB molecules, in contrast to the PM behavior in RhB /P-silica composite.

Radiative Transition with Visible Light in Electrochemical Anodized Polycrystalline Silicon

Radiative transition with visible light was observed by means of a porous layer made from polycrystalline silicon (poly-Si). Photoluminescence (PL) spectra were characterized as a function of initial poly-Si materials. For poly-Si formed by the casting method, a PL spectrum with the peak energy of 2.00 eV and the fullwidth at half maximum (FWHM) of 0.42 eV were obtained. For poly-Si formed by thermal CVD, the peak energy and the FWHM were 1.84 eV and 0.57 eV, respectively. On the basis of these findings, the structure of a highly developed display system, which contains TFT and luminous regions in the same poly-Si layer, was also proposed.

Optical cavity based on porous silicon superlattice technology

It is demonstrated that luminescent porous silicon (PS) is applicable to a vertical optical cavity which operates at wavelengths in the visible region. The PS device consists of a high-reflectivity thin Ag film, a light-emitting PS layer, and a quarter-wavelength multilayer reflector fabricated by precisely controlled PS superlattice (PSSL) technology. The reflectivity of a prepared PSSL mirror is higher than 90% at designed wavelengths. When excited by a UV laser, the device acts as a Fabry-Perot resonator. Based on experimental studies of emission spectra as a function of major design parameters (the number of PSSL pairs and the cavity length), the linewidth is significantly decreased to 6 meV in full width at half-maximum. The change in the linewidth with the cavity length is consistent with the behavior expected from a simplified theoretical analysis. These results confirm that PS is applicable for silicon-based photonic devices.

Visible Electro- and Photoluminescence from Porous Silicon and its Related Optoelectronic Properties

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.

Photoelectrochemical Effects of Surface Modification of n‐Type Si with Porous Layer

Effects of semiconductor surface modification on the photoelectrochemical properties have been studied experimentally and in detail for a porous Si layer (PSL) formed on n‐type Si substrates. The behavior of this electrode is characterized by the fact that the photoanodic current in aqueous electrolytes shows a considerable increase at the initial stage of operation. From the results of the interfacial impedance measurements, a model is proposed that may account for this behavior. According to the model, the initial increase in the photocurrent is due to the penetration of electrolyte into the PSL accompanied by photoanodic oxidation. Characteristics of the PSL oxidized prior to operation and those of the PSL operated in a nonaqueous electrolyte are consistent with this model.

Improved cold electron emission characteristics of electroluminescent porous silicon diodes

The property of electroluminescent porous silicon (PS) diodes as surface-emitting cold cathodes were investigated. The experimental PS diodes consist of thin Au films, PS, n+-type Si substrates, and ohmic back contacts. When a positive bias voltage VPS is applied to the Au electrode with respect to the substrate, electrons are uniformly emitted through the Au contact as well as photons. The cold electron emission characteristics are presented here in terms of the PS layer thickness dependence, effects of rapid thermal oxidation (RTO), and electroluminescence (EL) characteristics. It was demonstrated that both the decrease in the PS layer thickness (dPS) and the introduction of RTO treatment are useful for a significant improvement in the emission characteristics, and that the emission current and efficiency for a RTO-treated diode with dPS=3 μm reach 450 μA/cm2 and 0.2%, respectively, at VPS=27 V. It is also shown that in every case, the Fowler–Nordheim scheme holds in the bias voltage dependence of the e...

Photoelectrochemical Behavior of n-Type Porous-Si Electrodes

A prolonged photoanodic operation of n-type Si in aqueous electrolytes was observed with porous-structure Si electrodes. The n-type porous-Si layers (n-PSL) were formed on n-Si substrates (1-2 Ωcm in resistivity) by anodic treatments in an HF solution. The total output charge Q obtainable from a n-PSL photoelectrochemical cell increased in proportion to the n-PSL thickness d. At d = 50 µm, Q reached 12.5 C/cm2; this was four orders of magnitude greater than was obtained from naked n-Si photoanodes. The H2 evolution rate from the Pt counterelectrode was about 60 µmol/2 h at d = 30 µm. The experimental results of the spectral response suggest an increase in the surface state density of the n-PSL surface with increasing d.