Microstructure of visibly luminescent porous silicon

Abstract

The microstructure of luminescent porous silicon, formed by electrochemical etching of silicon wafers has been characterized by cross-sectional high-resolution transmission electron microscopy. Results of this study reveal the structure to consist of Si crystallites. The crystallites are ∼3.5 nm in size and are randomly distributed throughout the porous Si region.

Luminescence from Si-based structures

“…These luminescence characteristics require confinement in 3 to 5 nm microstructures. Transmission electron microscopy (TEM) [23,24], Raman spectroscopy [25], and X-ray diffraction measurements [26] have confirmed the presence of Sicrystallites with these dimensions. This model also appears to explain the wavelength dependence of the luminescence lifetime after pulse excitation [27].…”

“…There have been reports of TEM and Raman spectroscopy that reveal amorphous regions in the porous silicon layers [24,33,341. It has been suggested that the luminescence originates in these amorphous regions [34,35] since it has previously been shown that thin, amorphous polysilane films grown by homogeneous chemical vapor deposition exhibit efficient (>1%), room temperature photoluminescence [9] and the surface of porous silicon is known to be terminated with hydrogen in the form of SiH and SiH2 [36].…”

Electrochemical Investigation of the Electroluminescent Properties of Porous Silicon

“…Although other possible explanations have been proposed in the literature [13,14], the confinement origin seems now to be well supported by theoretical calculations of the energy levels in small size silicon clusters [15,16,17,18,19] and by several different experimental approaches attesting for the presence of quantum-sized crystallites in porous silicon layers [20,21,22,23]. Following the confinement model, the observation of a simultaneous emission of light during anodic oxidation requires that radiative recombination of carriers takes place within the quantumsized crystallites in the porous layer.…”

Comparison of Porous Silicon Etched Gently and Under Illumination

“…The observed upshift of the fundamental optical absorption energy gap [2], the blue shift in the photoluminescence spectra with increased porosity [3,4] and a correlation of Raman and photoluminescence spectra [5], all indicate the role of quantum confinement. The presence of nanoscale crystallites in porous silicon has been established by high resolution electron microscopy [6,7,8]. Predominantly, the limiting size of the nanocrystals is approximately three nm as predicted by a self limiting process in the anodic electrochemical etching [9].…”