Developments in Luminescent Porous Si

Abstract

The recent observation of room temperature photoluminescence (PL) from porous Si layers (PSLs) has received considerable attention. Bulk crystalline Si does not luminesce efficiently at room temperature due to the indirect nature of the energy bandgap minimum. The PL from PSLs has been attributed to quantum confinement effects in nanometer‐sized crystalline features found in PSLs, typically of high porosity. In this paper, we review some of the major results and discuss the controversies of subsequent research. The basic methods of fabricating luminescent PSLs are described, including conventional anodic etching and chemical etching. The latter technique requires no applied bias, but is modeled as an electrochemical process. Other processing issues are also addressed, particularly with respect to postanodization control of the PL spectrum. Microscopic and spectroscopic studies of the material and optical characteristics of PSLs fabricated by the various techniques are presented and discussed. The basic models for the luminescence mechanism are described, including quantum‐sized crystalline Si, surface passivation, normalSi‐Hx , alloys, and molecular electronics. These models are discussed in terms of the supporting and contradicting evidence. Electroluminescence (EL) studies are discussed, including EL from anodic oxidation of PSLs as well as from light‐emitting diodes. The results are promising for optoelectronic applications. However, fundamental questions about the underlying chemistry, physics, and microstructure remain unanswered. More research will be required before any definitive statements can be conclusively made regarding the luminescence mechanism.

Evolution of nanoporous silicon phase composition and electron energy structure under natural ageing

“…These are the models of quantum confinement [1,2], of surface passivation [3], photoluminescence due to the presence of Si-SiO 2 boundaries [4,5] and some others. However, no one of them can explain all of the observed experimental facts.…”

Stabilisation of surface and photoluminescent properties of porous silicon

“…Similar to the case of bulk silicon, the nonradiative recombination in the PS takes place mainly through defect centres on the surface. Here the silicon atoms with broken bonds can play a part of such defects (see [7]). A decrease observed in the PL intensity is related to changing nature of passivation of the surface.…”

“…In the process of aging, the PL spectra are being transformed: the radiation intensity is reducing and the spectra themselves are shifting towards the short-wavelength range. These processes are associated with adsorption of hydrogen or Si-H x complexes (x = 1, 2, 3) which determine concentration of nonradiative recombination centres (broken bonds of silicon) [6,7], or with oxidation of silicon pore walls. The most important results on stabilisation of the luminescent properties of PS have been obtained using inactivation of the broken bonds of silicon, fast oxidation, and modification of electrolyte which forms the PS [8][9][10].…”

Initial stages of porous silicon formation: An in‐situ investigation of the electronic properties and surface morphology

“…[34,39,40]. However, up to now spectroscopic data have not shown any kind of silicon oxide or silicon fluoride species during or after pore formation.…”