A large amount of work world-wide has been directed towards obtaining an understanding of the fundamental characteristics of porous Si. Much progress has been made following the demonstration in 1990 that highly porous material could emit very efficient visible photoluminescence at room temperature. Since that time, all features of the structural, optical and electronic properties of the material have been subjected to in-depth scrutiny. It is the purpose of the present review to survey the work which has been carried out and to detail the level of understanding which has been attained. The key importance of crystalline Si nanostructures in determining the behaviour of porous Si is highlighted. The fabrication of solid-state electroluminescent devices is a prominent goal of many studies and the impressive progress in this area is described.

The structural and luminescence properties of porous silicon

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Porous silicon: a quantum sponge structure for silicon based optoelectronics

This review is concerned with quantum confinement effects in low-dimensional semiconductor systems. The emphasis is on the optical properties, including luminescence, of nanometre-sized microcrysta...

Low-dimensional systems: quantum size effects and electronic properties of semiconductor microcrystallites (zero-dimensional systems) and some quasi-two-dimensional systems

Silicon is at the heart of the microelectronics revolution. Its dominance over other semiconductors is intimately tied to its superior materials and processing properties and to the tremendous base of technology that has developed around it. Another semiconductor is not likely to displace silicon as the material of choice in electronic applications. Silicon, however, is an extremely inefficient light emitter, and for this reason has not enjoyed the same level of dominance in optical applications.

Porous Silicon: From Luminescence to LEDs

Optical properties of porous silicon

Measurements of x‐ray absorption in the vicinity of the silicon L edge in porous silicon show a blueshift and a broadening of the conduction band edge, consistent with a distribution of quantum confinement energies. The absorption spectrum for porous silicon can be fit by a broadened and energy‐shifted version of the crystalline silicon absorption spectrum. The average quantum shift and broadening used in the fit to the absorption spectrum are in reasonable agreement with the corresponding parameters derived from the photoluminescence spectrum.

Evidence for quantum confinement in porous silicon from soft x‐ray absorption

Using synchrotron radiation we measured the x-ray absorption (XAS) near the C 1s edge for powdered graphite, boron substituted graphite, and a disordered graphitic carbon. The chemical potential of Li intercalated into the same carbons has also been measured as a function of x in Li x C 6 . When Li is intercalated, the 2s electron is transferred to the C host, filling the same levels as probed by XAS. In XAS, final-state energies contain a contribution from the electron-core-hole interaction which mimics the situation in Li intercalation where Li + interacts with conduction electrons

Correlation between x-ray absorption and chemical potential measurements in lithium intercalated carbons

Thin films of ${\mathrm{SrF}}_{2}$ and ${\mathrm{BaF}}_{2}$ grown on clean GaAs(100) surfaces have been studied by high-resolution photoemisison spectroscopy. Similar to the previously studied ${\mathrm{CaF}}_{2}$/GaAs(100) interface, there is evidence of cation-substrate bonding after annealing at temperatures near 600 \ifmmode^\circ\else\textdegree\fi{}C. For ${\mathrm{BaF}}_{2}$, Ba was found to react with As at the interface with an associated loss of Ga and F. For ${\mathrm{SrF}}_{2}$, the Sr 3d core-level shift also suggests cation-substrate bonding. The valence-band maxima for GaAs and the respective alkaline-earth fluorides (${\mathrm{CaF}}_{2}$, ${\mathrm{SrF}}_{2}$, ${\mathrm{BaF}}_{2}$) were measured directly by linear extrapolation of the high-kinetic-energy edge of the respective valence bands. The resulting valence-band offsets were found to follow the same trend for coverages up to 2--4 monolayers.

Photoemission study of the formation of SrF2/GaAs(100) and BaF2/GaAs(100) interfaces.

Bragg reflection model for x-ray absorption near edge structure in crystalline solids

The interface between ZnSe and GaAs (100) has been studied as a function of annealing temperature by high‐resolution photoemission spectroscopy using synchrotron radiation. From an analysis of chemical shifts and relative intensities of the atomic core levels, we find an interfacial Ga2Se3 layer forms with loss of Zn and As at the interface. The valence band offset between ZnSe and GaAs is found to be 1.25±0.07 eV, from photoemission measurements of the top of the valence band.

Y. Gao

Papers

Evidence for quantum confinement in porous silicon from soft x‐ray absorption

Correlation between x-ray absorption and chemical potential measurements in lithium intercalated carbons

Photoemission study of the formation of SrF2/GaAs(100) and BaF2/GaAs(100) interfaces.

Bragg reflection model for x-ray absorption near edge structure in crystalline solids

Photoemission study of the ZnSe/GaAs (100) interface: Composition and band offset