The many interesting and unique physical properties of nanocrystalline-Si/amorphous-SiO2 superlattices stem from their vertical periodicity and nearly defect-free, atomically flat, and chemically abrupt nanocrystalline-Si/SiO2 interfaces. By combining a less than 5% variation in the initial as-grown amorphous-Si layer thickness with control over the Si nanocrystal shape and crystallographic orientation produced via an appropriate annealing process, systems of nearly identical Si nanocrystals having remarkably different shapes (spheres, ovoids, bricks, etc.) have been produced. Such details governing the fabrication of nanocrystalline-Si/amorphous-SiO2 superlattices have dramatic effects on their structural and optical-Raman scattering and photoluminescence-properties. The reliable fabrication of Si-based nanostructures with control over the nanocrystal size, shape, and crystallographic orientation is an important first step in their applications in Si photonics.
A model thin-film system based on SiO2 coating with artificially introduced gold nanoparticles was investigated for the mechanism of 351-nm, pulsed-laser-radiation interaction with well-characterized nanoabsorbers. Damage morphology, represented by craters, provides strong evidence of the important role of the melting and vaporization processes. Measured crater volumes and numerical estimates based on them suggest that crater formation cannot proceed through laser-energy absorption confined within the particle. It instead starts in the particle and then, due to energy transfer, spreads out to the surrounding matrix during the laser pulse.
Nanocrystalline (nc)-Si/amorphous (a)-SiO2 superlattices (SLs) have been studied by transmission electron microscopy, Auger elemental microanalysis (AEM), Raman spectroscopy and optical reflection spectroscopy. Recrystallized Si/SiO2 SL is extremely stable under high temperature annealing (up to 1100 degree(s)C) and aggressive wet thermal oxidation: AEM and Raman spectroscopy of folded acoustic phonons show no changes in periodicity in the growth direction and the abruptness of the nc-Si/a-SiO2 interfaces. Furthermore, Raman spectroscopy in the optical phonon range indicates that the annealing decreases the defect density in the Si nanocrystals, possibly due to Si-Si bond rearrangement accompanied by surface reconstruction and surface defect passivation by oxygen.
Although porous silicon has received the most attention over the last 5 years, other structures containing Si nanocrystallites have recently been shown to emit strong luminescence at room temperature. This presentation reviews the state-of-the-art in the preparation and properties of silicon quantum dot structures, and their use in electroluminescent devices. The materials science and the structural, chemical, electrical, and optical properties of both porous silicon and recrystallized silicon superlattices are discussed. The fabrication of light-emitting devices consisting of these two types of nanoscale silicon materials is then described. Finally, the present and projected stability, efficiency, and speed of these LEDS is reviewed and their integration with silicon microelectronic driver circuits demonstrated.
Since the 1990 discovery that porous silicon emits bright photoluminescence in the red part of the spectrum, light-emitting devices (LEDs) made of light-emitting porous silicon (LEPSi) have been demonstrated, which could be used for optical displays, sensors or optical interconnects. In this paper, we discuss our work on the optical properties of LEPSi and progress towards commercial devices. LEPSi photoluminesces not only in the red- orange, but also throughout the entire visible spectrum, from the blue to the deep red, and in the infrared, well past 1.5 micrometers . The intense blue and infrared emissions are possible only after treatments such as high temperature oxidation or low temperature vacuum annealing. These new bands have quite different properties form the usual red-orange band and their possible origins are discussed. Different LED structures are then presented and compared and the prospects for commercial devices are examined.
We report the results of an extensive optical characterization of the properties light-emitting porous silicon (LEPSi), using optical techniques such as Raman spectroscopy, FTIR, cw photoluminescence (PL) and time-resolved PL spectroscopy. Additional insight is obtained from several nonoptical techniques, such as optical and electron microscopy, atomic force microscopy, and various surface physics tools. We examine how to control the surface passivation of LEPSi and what the consequence for light emission are. Samples with widely different surface chemistry have been prepared by controlling the electrochemical processes during anodization or by selected post-anodization treatments such as low- and high- temperature oxidation. In particular, we discuss the relationship between the presence of Si-H, Si-O-H, and Si-O bonds, and the relative strengths of the red PL line have a microsecond(s) ec decay time and the blue PL having a Nsec decay time. These results are compared to the predictions of the leading models that have been proposed to explain the efficient room-temperature luminescence of porous silicon.