Porous Si

Positronium formation was also found in porous Si obtained by anodization of crystalline silicon in HF acid solutions. Itoh, Murakmi and Kinoshita [11] found a long-lived ( 10 ns) component in the positron lifetime spectrum measured by conventional PALS. The authors investigated Ps behavior in porous Si at various temperatures by means of PALS with a monoenergetic pulsed positron beam [12],[13]. 

Several positron annihilation studies on amorphous Si02 (not only bulk but also thin films) revealed that Ps formation occurs with high efficiency in 

The Ps formation in Si02 can be seen in AMOC data more clearly ([20]). From the two-dimensional (2D) AMOC 

At 15 keV, about 90% of positrons are implanted in the Si substrate. In the young age region, annihilation in the Si substrate is dominant. Thus, 

It should be noted that the S parameters of both o-Ps pick-off and free-positron annihilation are lower than that of the Si substrate, because positrons predominantly annihilate with electrons of oxygen in the Si02 network. Only p-Ps self-annihilation has a higher S value than that of Si. The S parameter observed in conventional Doppler- broadening-of-annihilation radiation is the average of p-Ps, o-Ps, and free-positron annihilation. Therefore, if the Ps fraction decreases due to the presence of defects, impurities, etc., the intensity of the narrow momentum component due to p-Ps self-annihilation decreases, and as a result the averaged S parameter decreases. 

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Mason M D, Credo G M, Weston K D and Buratto S K 1998 Luminescence of individual porous Si chromophores Phys. Rev. Lett. 80 5405-8... 

Fig. 4 Electron micrograph of porous Si sacrificial substrates for ferroelectric nanotubes, from KTH (Stockholm) x9000...

In a Si zero-dimensional system the strong quantum confinement can increase the optical infrared gap of bulk Si and consequently shift the optical transition energies towards the visible range [65,66]. This is the reason for which silicon nanocrystals (Si-NCs) with a passivated surface are used as the natural trial model for theoretical simulations on Si based light emitting materials, such as porous Si or Si nanocrystals dispersed in a matrix. In this section we present a comprehensive analysis of the structural, electronic and optical properties of Si-NCs as a function of size, symmetry and surface passivation. We will also point out the main changes induced... 

Many quite different approaches to alleviating the miserable light emission in bulk Si ( 10 4 quantum efficiency at 300 K) have been proposed and are actively being explored.9"14 Some, such as Si i xGex quantum well or Si/Ge superlattice structures, rely on band structure engineering, while others rely on quantum confinement effects in low dimensional structures, as typified by quantum dots or porous Si (rc-Si15). Still another approach is impurity-mediated luminescence from, for example, isoelectronic substitution or by the addition of rare earth ions. An overview of results obtained with some of these methods is given below. However, in order to understand more fully the reasons why such different approaches are necessary, it is appropriate to review first what creates the optical emission problem in crystalline Si (c-Si). 

Anti-atrazine antibodies were coupled to a Si microchip comprising 42 coated porous flow channels. The porous Si layer was achieved by anodizing the channel in 40% HF/96% ethanol (1 1). Detection of atrazine was achieved in a competitive format. The chemiluminescence signal decreased when a known amount of horseradish peroxidase-labeled atrazine was added. This decrease was used to determine the amount of unlabeled atrazine in the sample [1030],... 

Materials obtained by pyrolysis of pitch-polysilane blends have been extensively studied as carbon materials containing Si [157-161], For some of these materials, ca. 600mAh/g of Crev for Li insertion, as well as small irreversible capacities and small hysteresis effects, were reported. It has been shown that the materials contain nanodispersion of Si-O-C and Si-O-S-C instead of nanodispersed Si particles [162-165], Furthermore, the oxygen and sulfur contents are proved to be correlated to the irreversible capacity. There is a report about the fabrication of porous Si negative electrodes with 1-D channels, where the usefulness of the fabricated negative electrodes for rechargeable microbatteries is also suggested [166],... 

L. T. Canham, Appl. Phys. Lett. 57 1046 (1990). First paper on photoluminescence from porous Si. 

Light emission from porous Si was discovered by Canham (1990). The phenomena are complex and depend on the Si nanocrystal size (Uosaki, 1997). 

Instead of pressurizing a-Si, Deb et al. [263] obtained a densified a-Si via pressurizing porous Si. They prepared films of porous Si having crystallite of —5 nm (on average). In situ measurements of X-ray diffraction patterns and... 

Figure 14. Raman spectra of porous Si in a compression-decompression cycle [263]. In the compression process, the characteristic spectrum of nanocrystalline Si disappears above M3 GPa and a broad amorphous feature emerges. In the decompression process, the characteristic spectrum of the LDA form grows below 9 GPa, which indicates an HDA-to-LDA transition.

Since positron annihilation spectroscopy is highly sensitive to atomic defects in solid materials, positron annihilation experiments have been carried out extensively on silicon (Si) and silicon dioxide (Si02), both of which are extremely important for the microelectronic device industry. While several reviews are available [1], those reviews are mainly focused on positron (not positronium) annihilation behavior because positronium (Ps) formation dose not occur in bulk crystalline Si. Recent positron annihilation experimental studies revealed that Ps formation occurs in some Si-based thin films, such as porous Si and hydrogenated amorphous Si furthermore, Ps formation is dominant in high-purity amorphous Si02 thin films. In this chapter, Ps annihilation characteristics in Si and Si02 thin films will be discussed from the experimental point of view. 

Figure 9.3(a) shows positron lifetime spectra of a porous Si thin film at the sample temperatures of 25°C (initial), 300°C, and 500°C, and Figure 9.3(b) shows positron lifetime spectra measured at 500°C and at 200°C after 500°C annealing. Strong temperature dependence was observed in the long-lived component. 

Figure 1 (a) Bright field TEM image in plane view of a porous Si layer with 70 % porosity prepared from p type ( 3.10 n.cm) [100] Si substrate. Pores (in white) are separated by Si walls (in black), (b) Film thickness derived from N2 adsorption isotherm at 77 K for a porous Si layer ( ) extracted from the pore size distribution of cylindrical pores having the same section area as real pores, (o) from the geometrical surface, (a) are film thickness for MCM 41 (5.5 nm). Solid line shows a t-curve obtained by the semi-empirical law FHH and currently proposed to describe adsorption on a non porous substrate. 

TRANSPORT PROPERTIES OF Nb THIN FILMS DEPOSITED ON POROUS Si SUBSTRATES... 

N. Koshida and K. Echizenya, Characterization studies of p-type porous Si and its photoelectro-chemical activation, J. Electrochem. Soc. 138, 837, 1991. 

A. Nakajima, T. Itakura, S. Watanabe, and N. Nakayama, Photoluminescence of porous Si, oxidized then deoxidized chemically, Appl. Phys. Lett. 61(1), 46, 1992. 

V. Petrova-Koch, T. Muschik, A. Kux, B. K. Meyer, and F. Koch, Rapid-thermal-oxidized porous Si— The superior photoluminescent Si, Appl. Phys. Lett. 61(8), 943, 1992. 

N. Koshida, M. Nagasu, and Y. Kiuchi, Impedance spectra of T type porous Si-electrolyte interface, J. Electrochem. Soc. 133, 2283, 1986. 

N. Koshida, H. Koyama, and Y. Kiuchi, Photoelectrochemical behavior of n-type porous-Si electrodes, Jpn. J. Appl. Phys. 25, 1069, 1986. 

T. Nakagawa, H. Koyama, and N. Koshida, Control of structure and optical anisotropy in porous Si by magnetic-field assisted anodization, Appl. Phys. Lett. 69(21), 3206, 1996. 

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