Indirect evidence is presented that free‐standing Si quantum wires can be fabricated without the use of epitaxial deposition or lithography. The novel approach uses electrochemical and chemical dissolution steps to define networks of isolated wires out of bulk wafers. Mesoporous Si layers of high porosity exhibit visible (red) photoluminescence at room temperature, observable with the naked eye under <1 mW unfocused (<0.1 W cm−2) green or blue laser line excitation. This is attributed to dramatic two‐dimensional quantum size effects which can produce emission far above the band gap of bulk crystalline Si.

Silicon quantum wire array fabrication by electrochemical and chemical dissolution of wafers

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

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

LIGHT-emitting devices based on silicon would find many applications in both VLSI and display technologies, but silicon normally emits only extremely weak infrared photoluminescence because of its relatively small and indirect band gap1. The recent demonstration of very efficient and multicolour (red, orange, yellow and green) visible light emission from highly porous, electrochemically etched silicon2,3 has therefore generated much interest. On the basis of strong but indirect evidence, this phenomenon was initially attributed to quantum size effects within crystalline material2, but this interpretation has subsequently been extensively debated. Here we report results from a transmission electron microscopy study which reveals the structure of the porous layers that emit red light under photoexcitation. Our results constitute direct evidence that highly porous silicon contains quantum-size crystalline structures responsible for the visible emission. We show that arrays of linear quantum wires are present and obtain images of individual quantum wires of width <3 nm.

Visible light emission due to quantum size effects in highly porous crystalline silicon

http://nathan.instras.com/documentDB/paper-69.pdf

Porous silicon: a quantum sponge structure for silicon based optoelectronics

An experimental and theoretical study of the formation and microstructure of porous silicon

A systematic study is presented of the effects of silicon dopant type, resistivity, current density, and hydrofluoric acid concentration on the formation and properties of porous silicon. Cross‐section transmission electron microscopy revealed the presence of two distinct microstructures. The structure formed is determined by the doping level with the transition occurring near degeneracy. A model of the anodisation process is presented which is based on the semiconducting properties of the material and which explains the formation of the two different types of porous structure observed.

Microstructure and formation mechanism of porous silicon

Orientation dependence of high speed silicon crystal growth from the melt

The laser melting of amorphous Si is accurately modelled by computer calculations. It is found that the thermal conductivity of the amorphous phase must be set at approximately 10−2 W/cm K, a value much lower than that of crystalline material to obtain close agreement with experimental measurements. This low value is, however, consistent with the thermal conductivities of other amorphous materials. The results of the computations, when compared with experimental observations, confirm that the melting point of ion implanted amorphous Si is below that of crystalline Si, with a best estimate in the range 1185–1385 K.

Computer simulation of high speed melting of amorphous silicon

Morphological instability occurring during high‐velocity Si crystal growth from an impurity containing melt is examined in detail. The experimental conditions are achieved by annealing an ion‐implanted Si layer with pulsed laser radiation. Computer modeling is employed to understand the heat flow and impurity diffusion behavior that occurs. The stability and size of impurity segregation cells observed to occur under particular conditions are related to the predictions of morphological stability theory.

N. G. Chew

Papers

An experimental and theoretical study of the formation and microstructure of porous silicon

Microstructure and formation mechanism of porous silicon

Orientation dependence of high speed silicon crystal growth from the melt

Computer simulation of high speed melting of amorphous silicon

Growth interface breakdown during laser recrystallization from the melt