Showing papers by "Leigh T. Canham published in 1991"

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

Abstract: 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.

Atmospheric impregnation of porous silicon at room temperature

Abstract: Microporous and mesoporous Si layers contain a very large surface area that affects both their optical and electrical properties. Secondary ion mass spectroscopy (SIMS) analysis is used for the first time to simultaneously monitor all the major impurities on that surface. SIMS data on a microporous layer demonstrate that its chemical composition changes dramatically with time during ambient air exposure. Similar trends are observed for mesoporous layers. Extended storage in air at room temperature converts the hydride surface of freshly anodized layers to that of a contaminated native oxide. Characterization techniques need to take the metastability of the hydride surface into account since the structural, optical, and electrical properties of porous Si can consequently change with time upon exposure to ambient air. Low‐temperature photoluminescence and spectroscopic ellipsometry data on freshly anodized and ‘‘aged’’ microporous and mesoporous layers are chosen to illustrate typical changes in optical pro...

The Structure of Porous Silicon Revealed by Electron Microscopy

Abstract: This detailed electron microscope study of porous silicon compares the different structures of macro-, meso- and microporous material. Mesoporous silicon of high porosity (~80%) exhibits efficient red photoluminescence at room temperature. Transmission electron microscopy provides strong direct evidence that this visible luminescence arises from dramatic carrier confinement in quantum-size, crystalline silicon structures. Images of undulating, interconnected ‘quantum wires’ of widths <3nm are shown.

Sims Analysis of the Contamination of Porous Silicon by Ambient Air

Abstract: Microporous silicon layers contain an enormous surface area (> 500 m 2 cm −3 ) that influences their structural, optical and electrical properties. When freshly etched the pore wall surface can be extremely clean and composed primarily of hydrogen and fluorine. Extended storage in ambient air however will convert this clean hydride surface into that of a contaminated native oxide. Using dynamic SIMS profiling we demonstrate here that slow oxidation at room temperature by ambient air is accompanied by impregnation with atmospheric boron and sulphur but that levels of calcium and sodium for example, remain exceedingly low. We conclude that the pore wall surface is very efficiently protected from particulate - related airborne species but is susceptible to contamination from small molecules present in the atmosphere at trace levels.

Electrical and Optical Properties of Si and Si1−xGex, Alloy Grown by the Ultra-High Vacuum Vapour Phase Epitaxy Method

Abstract: Epitaxial growth and p-type doping in Si and Si1−xGex alloy by the ultra-high vacuum vapour phase epitaxy (UHV-VPE) method using SiH4/GeH4/B2H/H2 mixture is reported. Growth temperatures as low as 610°C were studied for Ge contents between x=0 to 0.25. Secondary Ion Mass Spectroscopy (SIMS) data of boron dopant profiles in si and Si1−xGex structures are presented. Strong luminescence attributed to the strained Si1−xGex alloy is obtained. The bandgap in the alloy layer obtained from our luminescence data is compared with published data. Unambiguous electroluminescence from a Si1−xGex multiple quantum well p-n structure grown by UHV-VPE, supported by the photoluminescence and photoconductivity measurement, is reported for the first time.