DEVELOPMENTS IN LUMINESCENT POROUS SI

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 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 of the luminescence mechanism are described, including quantum-sized crystalline Si, surface passivation, Si-H(x) allows, 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.