Porous and Nanoporous Semiconductors and Emerging Applications
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Porous and Nanoporous Semiconductors and Emerging Applications Helmut Föll, Jürgen Carstensen, and Stefan Frey Chair for General Materials Science, Faculty of Engineering, Christian-Albrechts-University of Kiel, D-24143 Kiel, Germany ABSTRACT Pores in single crystalline semiconductors can be produced in a wide range of geometries and morphologies, including the “nano” regime. Porous semiconductors may have properties completely different from the bulk, and metamaterials with e.g. optical properties not encountered in natural materials are emerging. Possible applications of porous semiconductors include various novel sensors, but also more “exotic” uses as, e.g. high explosives or electrodes for micro fuel cells. The paper briefly reviews pore formation (including more applied aspects of large area etching), properties of porous semiconductors and emerging applications. INTRODUCTION The dominant role of silicon in microelectronics together with the discovery of light-emitting nanoporous Si in 1991 [1, 2] has focused electrochemical investigations on this semiconductor even before 1991, and a large range of possible applications has emerged (for a recent review see [3]). While porous silicon was already discovered in 1957 by Uhlir [4], (but not recognized for what it was), it took until 2000 to find conditions for pore production in Ge [5, 6]. Investigations of porous III-V semiconductors (mainly GaP, GaAs, InP) were also done more recently; see [7]. Pores in GaN [8], SiC [9], and ZnSe [10] have also been reported in the meantime, and the exploration of the available parameter space is an ongoing activity in many laboratories (cf. [11]). POROSIFICATION OF SEMICONDUCTORS It is helpful to first define a few terms to classify the tremendous variety of pores (cf. Fig. 1). Pore geometry refers to (average) diameters of pores and distances between pores, i.e. to the pore dimensions, while pore morphology addresses the pore shape (e.g. cylindrical, branched, facetted, fractal, …).
c) b) a) Fig. 1: Variety of pores in semiconductors: (a) Mesopores in Si, (b) Macropores in Si with lithographic prestructuring, (c) “Curro”-“Crysto” transition in InP.
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According to IUPAC standards, micro-, meso- or macropores refer to pores with typical dimensions of < 2 nm, 2 nm – 50 nm, or > 50 nm, respectively. The term “nanopores” thus is open to interpretation; here we use it somewhat loosely for pore dimensions well below 1 µm. Generally, porosification can be obtained by anodizing the semiconductor in a suitable electrolyte, under suitable conditions. The necessary chemical reaction at the semiconductorelectrolyte interface is generally a mixture of direct dissolution, oxide formation, and oxide dissolution, with details sensitive to many parameters, e.g. electrolyte chemistry, applied potential or current density, temperature, flow conditions of the electrolyte, doping type and level of the semiconductor, illumination state of the semiconductor (front or backside) and surface conditions (polished, rough, masked). The o
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