Minoru Araki

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.

CONTROLLED ELECTROLUMINESCENCE SPECTRA OF POROUS SILICON DIODES WITH A VERTICAL OPTICAL CAVITY

It is demonstrated that a porous silicon (PS) optical cavity is available for the electroluminescence (EL) PS diode configuration. The PS diode is composed of a thin Au film, a light‐emitting PS layer, a multilayered PS mirror, a p‐type Si substrate, and an ohmic contact. When a sufficient bias voltage is applied to the diode, a uniform visible light emission is observed through the top contact. The EL intensity is fairly proportional to the driving current over a wide range of operation. The bandwidth of the EL spectrum is significantly reduced in comparison to that of the conventional PS diodes, owing to a resonance effect induced between the Au film and the multilayered PS mirror. These results suggest that the PS technology is a promising process for applications in monolithic integrated optoelectronics.

FABRICATION AND FUNDAMENTAL PROPERTIES OF AN EDGE-EMITTING DEVICE WITH STEP-INDEX POROUS SILICON WAVEGUIDE

It is demonstrated that an edge‐emitting device with a waveguide structure based on luminescent porous silicon (PS) operates at wavelengths in the visible region. The PS device is composed of an upper‐side cladding Al thin film, an active PS layer, and a lower‐side cladding PS layer with a smaller refractive index. When excited by an Ar ion laser with a wavelength of 458 nm, the device operates as an optical waveguide and consequently visible light can be observed from the cleaved facets of the device. The emitted light is significantly polarized along the direction of the transverse‐electric (TE) mode due to the metal cladding layer. These results suggest that PS is useful as a component for silicon‐based photonic integration.

Photoelectronic properties of porous silicon

It is shown that the introduction of a porous structure into single‐crystal Si produces substantial changes in the photoelectronic properties. The porous Si (PS) layer is formed by anodization of p‐type Si wafers in a HF solution. The photoconduction cells used in this study consists of a semitransparent thin Au film, PS, Si substrate, and Al ohmic contact. The photoconductive behavior of PS is characterized by an extremely high dark resistivity, a definite photosensitivity for visible light, and an intrinsic bias voltage dependence of the spectral response. These properties, which are interpreted to be the result of a band‐gap widening in PS, provide further support of the assumption that the visible luminescence of PS is explained by the band scheme.

Precisely tuned emission from porous silicon vertical optical cavity in the visible region

The operating peak energy of a porous silicon (PS) cavity can be completely controlled over a wide range of 1.5–2.2 eV, using a PS‐based Fabry–Perot resonator composed of a light‐emitting active PS layer and two high‐reflectivity mirrors. When the PS devices are excited by a uv laser, quite narrow spectra (10–40 meV in full width at half‐maximum) are observed without any significant signs of side mode. The central photon energy is precisely and continuously tuned simply by changing the anodization parameters. The key issues of the controlled device operation are adjustment of the optical thickness of the active PS layer to an appropriate value and fabrication of the quarter‐wavelength multilayered PS mirror with a high reflectivity. The spectral qualities of the emitted light are also discussed by theoretical analyses on the basis of a simplified model. These results suggest that the PS devices operate as sharp band‐pass optical filter and the PS materials are available for novel silicon‐based microphotonics.

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.

Buried Optical Waveguides of Porous Silicon

It is demonstrated that the refractive-index-controllable nature of luminescent porous silicon (PS) is directly applicable to the development of a three-dimensionally buried optical waveguide. The PS waveguide is fabricated on a p-type silicon wafer by monolithic processes such as photolithography, ion implantation, anodization, and thermal oxidation. An induced high contrast of refractive indices leads to efficient confinement and propagation of visible light. When the active core layer is partially excited by a He-Cd (325 nm) laser, blue emission is observed from a cleaved facet. The PS waveguide is potentially useful as a component of silicon-based photonic integration.

Optoelectronic Effects in Porous Silicon Related to the Visible Luminescence Mechanism

Some optoelectronic effects in porous Si (PS) have been investigated in relation to the visible luminescence mechanism. As regards photoluminescence (PL), particular emphasis is placed on the relationship between photoconduction (PC) and PL excitation (PLE) spectra, the interaction of external electric field and PL emission, and polarization properties of PL Main subjects of electroluminescence (EL) studies reported here are the dynamic behavior of EL operation and the formation of a large-area contact by a conducting polymer (polypyrrole: PP). The observed experimental results (almost complete coincidence of PC spectra with PLE ones, linear polarization memory of PL definite correlation between the polarization degree and the PL efficiency, and comparable response time of electrical PL quenching and EL to the PL decay time) are consistent with our hypothesis that the major process of PL takes place within Si nanocrystallites. The electrical characterization of light-emitting PS diodes with PP contacts ensures the usefulness of the contact formation by electropolymerization as a technique for uniform and efficient carrier injection into PS.