To improve the stability of porous Si (PS) prepared by electrochemical etching, we make use of rapid-thermal oxidation (RTO). During RTO processing, the hydride coverage of the internal surfaces of the pores is replaced by a high-quality oxide while retaining nm-sized Si grains. With increasing process temperature Tox the luminescence is first quenched. It is recovered with comparable intensity for Tox≥700 °C.
Porous silicon (p type) has been exposed to several chemical vapors at various partial pressures. The quenching of the photoluminescence by the adsorbates has been quantified and correlation to the electrical conductivity of the porous silicon sample has been studied. Some gases, e.g., water and benzene, have a small effect on the photoluminescence and on the conductivity, while others, e.g., methanol, reduce the photoluminescence by a factor of 2 and increase the conductivity by four orders of magnitude. This is accompanied with a qualitative change in the current-voltage characteristics. These changes have been found to be reversible and the temporal behavior of the system has been investigated.
Recently a quantum size effect was proposed to be responsible for the blue shift of optical absorption edge and photoluminescence peak wavelength as well as for the porous silicon (PS) formation itself. In the debate about the mechanism of light emission from PS a correlation between particle size and luminescence peak position would be a key test of the confinement approach. In this letter X-ray diffraction results of as-etched PS samples will be presented that indicate a decrease of particle size and an increase of stress in conjunction with a blueshift of photoluminescence wavelength and absorption edge.
We observe a quadratic rise of the absorption coefBcient with excitation energy in photoluminescence excitation spectra of porous silicon. Extrapolation to a, = 0 yields an average band gap of microporous silicon about 0.2 eV above the luminescence line. Good agreement is obtained with an estimate of the band gap from the position of the second luminescence line of porous silicon in the infrared spectral region. Further analysis of the line shape using difFerent luminescence detection energies shows that, in addition to the size distribution of crystallites, there exists a second contribution to the linewidth.During the past years there has been an increased interest in the luminescence properties of porous Si (PS). This material has efBcient luminescence in the visible regime even at room temperature. Although many of studies have been devoted to various aspects of PS, it is not yet clear what is the origin of the light emission; this question is still under intense debate.There are numerous models that have been suggested in the literature. Generally they can be classified into four major categories: Radiative recombination is assumed to occur via quantum confined excitons, localized electronic states on the surface of the crystallites, '4 defects in the oxidic coverage of the crystallites, and even within certain Si-based chemical compounds, like siloxene.Canham was the first to ascribe the visible luminescence to quantum confinement of the excited electronhole pair inside the small Si structures, resulting in a luminescence energy well above the bulk Si band gap.It was further argued that the nonradiative recombination is much reduced due to the good surface passivation and the fact that the carriers are confined insid. e the crystallites and cannot diffuse far away to reach a nonradiative center. Calcott et al. 2 have shown that the luminescence at low temperature has a phonon structure; they argued that this proves that both the absorption of the light and the reemission occurs within the Si crystallites. However, there is a variety of experiments that cannot be simply explained by this model. For example, it was shown that the chemical environment affects the luminescence properties.This result suggests that the enlarged surface area of the PS plays a role in the light emission. The existence of localized states on the surface of the Si network has been shown using various measurements. 3 Furthermore, it was demonstrated that the in&ared emission observed &om PS is related to a radiative recombination process, which involves a dangling bond. state at the surface of a crystallite~o '~T he reason for the diKculty in determining the radia-tive processes in PS is related to the highly nonhomogeneous structure of the material. Crystallites having different sizes and shapes, will result in a broad distribution of confinement energies. Therefore, it is not easy to identify where the absorption as well as the related emission takes place. Several attempts were done to translate the PL spectral shape into a size di...
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