Photonic crystals are materials patterned with a periodicity in dielectric constant, which can create a range of 'forbidden' frequencies called a photonic bandgap. Photons with energies lying in the bandgap cannot propagate through the medium. This provides the opportunity to shape and mould the flow of light for photonic information technology.

Photonic crystals: putting a new twist on light

The term photonic crystals appears because of the analogy between electron waves in crystals and the light waves in artificial periodic dielectric structures. During the recent years the investigation of one-, two-and three-dimensional periodic structures has attracted a widespread attention of the world optics community because of great potentiality of such structures in advanced applied optical fields. The interest in periodic structures has been stimulated by the fast development of semiconductor technology that now allows the fabrication of artificial structures, whose period is comparable with the wavelength of light in the visible and infrared ranges.

http://ieeexplore.ieee.org/iel5/6354/16969/00781867.pdf

Photonic crystals

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

Organometallic chemistry on silicon and germanium surfaces.

Porous silicon layers grown on nondegenerated p‐type silicon electrodes in hydrofluoric acid electrolytes are translucent for visible light, which is equivalent to an increased band gap compared to bulk silicon. It will be shown that a two‐dimensional quantum confinement (quantum wire) in the very narrow walls between the pores not only explains the change in band‐gap energy but may also be the key to better understanding the dissolution mechanism that leads to porous silicon formation.

Porous silicon formation: A quantum wire effect

Macropore formation in n‐type silicon is a self‐adjusting phenomenon characterized by a specific current density at the pore tip. At this specific current density, the dissolution reaction changes from the charge‐transfer‐limited to the mass‐transfer‐limited regime. The passivation of the pore walls in hydrofluoric acid is caused by a depletion of holes due to the n‐type doping of the substrate. Equations based on these findings are presented and allow us to precalculate the dimensions of the pores. The validity of the model and its mathematical description is verified in experiments. Pores of a depth up to the wafer thickness and aspect ratios of 250 were etched using this method.

The Physics of Macropore Formation in Low‐Doped p‐Type Silicon

Three‐dimensional structures in silicon are increasingly coming into use for the fabrication of mechanical and electrical devices. The fabrication of deep trenches is one of the most important problems in VLSI (very large scale integration) technology. In this study the spontaneous trench formation in n‐type silicon immersed in hydrofluoric acid under anodic bias is demonstrated and the resulting microstructures are characterized. Trenches with arbitrary cross sections and high aspect ratios for microelectronic (e.g., 42 μm depth and 0.6 μm diam) and for power component application (e.g., 60 μm depth and 10 μm diam) have been produced by a standard masking technique. The trench formation is explained by a model which takes into account the conditions of the space charge region the minority carrier current and the crystal orientation. A passivating sidewall layer is not needed in this model. The dimensions and the shape of anodically etched trenches can be varied over a wide range by adjusting the critical parameters.

https://iopscience.iop.org/article/10.1149/1.2086525/pdf

Formation Mechanism and Properties of Electrochemically Etched Trenches in n‐Type Silicon

Electrochemistry of Silicon

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.

Volker Lehmann

Papers

Porous silicon formation: A quantum wire effect

The Physics of Macropore Formation in Low‐Doped p‐Type Silicon

Formation Mechanism and Properties of Electrochemically Etched Trenches in n‐Type Silicon

Electrochemistry of Silicon

Rapid‐thermal‐oxidized porous Si−The superior photoluminescent Si