P.G. Han

Sensitivity and stability of porous polycrystalline silicon gas sensor

Abstract Porous silicon (PS) used as a sensing material for micro-sensors has attracted a lot of attention in recent years. Owing to the large surface area (∼200 m 2 cm −3 ) and high chemical activity, porous silicon would be a good choice for gas sensor applications. In this paper, a resistivity sensor using porous polycrystalline silicon (PPS) thin films was fabricated by anodization and standard lithographic technique. Analytical measurements on the sample reveal that these sensing porous poly-Si films consist of many pores with a diameter of ∼100 nm. A high sensitivity to ambient pressure and ethanol vapor was observed. The sensitivity and stability of PPS gas sensor have been characterized and analyzed as a function of gas concentration and storing duration. In particular, we will discuss the mechanism of the interaction between the surface state of PPS and ambient gases in detail. Suggestions to further improve the sensor stability will also be proposed.

Study of luminescent porous polycrystalline silicon thin films

Luminescent porous poly‐Si thin films can be obtained by electrochemicaletching of phosphorus‐doped poly‐Si filmsdeposited by low‐pressure chemical‐vapor deposition. As‐deposited poly‐Si film has no photoluminescence but all porous poly‐Si films, large area or micron‐size patterns, show comparable orange‐red photoluminescences to those obtained from crystal Si. High‐resolution atomic force microscopy and scanning electron microscopy analyses show that all porous poly‐Si films have smooth surfaces and uniform thicknesses, and are composed of Si grains (∼150 nm) with nanopores (∼20 nm) formed around the surfaces. The pores increase with anodization time, and grow preferentially along the poly‐Si grain boundaries and the Si 〈100〉 crystal directions. The evolution of the microstructure is analogous to that of the etching of a coral ball layer due to sea water.

Porous polycrystalline silicon conductivity sensor

Porous polycrystalline silicon (PPS) based conductivity sensors were fabricated and characterized in this work. The PPS sensors show excellent sensitivity for detecting ambient pressures, gas species as well as temperature. Results show that the current increases significantly as the pressure decreases and an over two orders of magnitude change has been detected when the pressure was reduced to 10−2 atm. We also find that both ethanol and acetone vapors can significantly change the conduction in the sensors. In acetone vapor, the device even displays diode-like characteristics. Electrical conduction mechanisms of the sensors in vacuum and organic vapors are proposed. The fabrication process can be easily integrated with very large scale integrated technology.

Investigation of the surface silica layer on porous poly-Si thin films☆

Abstract The surface properties of porous poly-Si (PPS) have profound effects on the characteristics of optoelectronic devices and PPS-based gas and bio-chemical sensors. In this work, the effects of plasma dry etching on the surface structures and optical characteristics of PPS film are investigated. Experimental results show that a thin crystalline silica layer is formed on the anodically etched rough surface of the PPS films. When this layer is removed, the photoluminescence intensity decreases sharply, whereas the micro-Raman resonance peak (near 517 cm−1) does not shift but its intensity increases significantly. The mechanisms for these observations are discussed in detail.

A novel approach for fabricating light-emitting porous polysilicon films

Abstract By etching back the as-oxidized polysilicon using reactive ion, a uniform porous polysilicon structure with significant enhancement of photoluminescence (PL) intensity was formed. We further found that the PL peak is centered at around 680 nm and is independent on the porosities or sizes of Si micropores. These results indicate the light emission in the samples should not be a consequence of the quantum confinement. Instead, the 680-nm peak should be due to the non-bridged oxide hole centers (SiO·) at the oxidized grain boundaries of the polysilicon.

Physical structure of light-emitting porous polycrystalline silicon thin films

Abstract This paper reports the surface electronic structure of light-emitting porous polycrystalline silicon (PPS) using X-ray photoelectron spectroscopy (XPS). We find that the PPS films with strong photoluminescence (PL) effect can only be observed in thin film with trace amount of silicon nanoclusters and the luminescence can be enhanced remarkably with proper passivation of the PPS surface. Incomplete oxidation of silicon (Si 3+ or Si 2+ ) does not lead to visible PL. We further estimate that the average size of silicon nanoclusters is in the range of 20–30 A in the sample having PL emission.

Effect Of Nickel In Large Grain Poly-Si Film Formed By Nickel Induced Lateral Crystallization and New Grain Enhancement Method

Large grain poly-silicon film (poly-Si) with high material quality and uniformity can have numerous novel applications such as providing a low cost alternative to form silicon-on-insulator (SOI) substrates and a breakthrough technology to ultra-dense 3-dimensional multi-layer SOI like devices and circuits. Nickel Induced Lateral Crystallization (NILC) of amorphous Si (a-Si) has been studied intensively, yet the grains are still small (∼ 1 μm). Recently, we have reported a novel method by combining NILC and a new annealing (at above 900 °C) to form poly-Si film with very large grains ranging from 10 μm to 100 μm. The film has good quality and the TFTs formed are highly comparable to SOI TFTs. This work further reports the effect of Ni to the new large-grain poly-Si film.

Thermal stability of cobalt and nickel silicides in amorphous and crystalline silicon

Metal silicides have been widely used in microelectronic industries, especially as contact material to reduce the series resistance of source, drain and gate regions in MOSFETs. Among all silicides, cobalt-disilicide (CoSi/sub 2/) and nickel-monosilicide (NiSi) have been demonstrated to be two of the most promising silicide materials for future ULSI, thin film transistor (TFT) and novel devices. They have the advantages of having the lowest resistivities (/spl sim/14 /spl mu/ohm-cm), good thermal stability (up to 700-900/spl deg/C), low formation temperature (/spl sim/400-600 /spl deg/C) and little or no resistivity degradation on narrow lines/gates. Moreover, for CoSi/sub 2/, it can have low film stress (lattice mismatch with silicon (Si) is only 1.2%), less lateral gate-source/drain silicide overgrowth, good resistance to HF and plasma etching, and do not react with oxide below 900/spl deg/C. For NiSi, it has the advantages of less Si consumption, no reaction with N/sub 2/ and a simple single step annealing. This paper aims to provide a first study to explore and compare the thermal stability and process windows of NiSi and CoSi/sub 2/ in amorphous Si (a-Si) and single-crystalline Si (c-Si) substrates after 30 minutes long time annealing.

Formation mechanism of light-emitting porous silicon prepared by reactive ions etching

Porous silicon (PS) offers many potential advantages for the realization of optoelectronic applications. However, the electrochemical anodizing method used to fabricate porous silicon has resulted in many undesirable sub-effects, such as impurities due to reactive residents, nonstabilized surface, nonuniform pore structure, and high film stress, leading to contamination of conventional VLSI processes. In this paper, we report a new dry etching method to produce the light-emitting PS. The formation mechanism of this film is proposed. Using the samples prepared with this new method, results demonstrate that the luminescence in this film does not have strong correlation with size of the nano-structures and we therefore suggest that the luminescence in this film is not due to the quantum confinement effects.

Photoluminescent porous polycrystalline silicon

Recently, visible photoluminescence (PL) has been observed in porous Si layers formed by anodized or stain-etched polycrystalline Si. The new luminescent porous poly-Si (PPS) films can be formed on glass substrates or integrated into parts of Si ICc, hence opening up numerous novel opto-electronic device designs and applications. In this paper present new results on the effect of etching conditions and XRD peaks on PL emission from PPS films. Possible explanations and models, including preliminary studies on PPS formation kinetics, are also proposed and discussed.