New progress on p-type macroporous silicon electrodissolution
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New progress on p-type macroporous silicon electrodissolution Paolo Bettotti, Zeno Gaburro, Luca Dal Negro, Lorenzo Pavesi INFM and Department of Physics, University of Trento Povo (TN), ITALY. ABSTRACT We discuss fabrication of macroporous structures, both random and periodical, on p-type silicon samples by electrochemical etching using aqueous and organic electrolytes. We have obtained different lattice structures starting from an unique lithographic mask. Organic compounds used in this work were Dimethylformamide (DMF) and Dimethylsulfoxide (DMSO). INTRODUCTION Photonic crystals, i.e., structured material whose dielectric constant exhibits a translational symmetry, and thus energy band structure, have attracted interest both for theory and applications. Techniques allowing fabrication of photonic crystals in Si have the intriguing perspective of a potential integration of photonic devices within electronic CMOS technology. Two-dimensional photonic crystals in Si can be fabricated by electrochemical anodization of substrates with periodically pre-patterned surface, in HF based solutions. The procedure is known as macroporous Si formation. This process can produce – in a single pass – equal cylindrical pores, with constant diameter and very large aspect ratios, i.e. with pore depths up to a few hundred times larger than the pore diameter. The key characteristic of this procedure is the possibility of obtaining a strongly anisotropic anodization of the Si substrate. Such large necessary anisotropy can be achieved due to the basic characteristics of the macroporous Si formation. In one of the most accepted (although not complete) model [1], the anodic dissolution of Si is assumed to occur because of the presence of positive carriers (holes) at the Si-electrolyte interface. Si dissolution occurs preferentially at the pore tips [2,3]. However, if the hole supply is large, significant hole currents can be injected to the pore wall regions, causing pore widening during the anodization. In n-type Si, the growth of pores of very large aspect ratios can be achieved by limiting the hole supply from the substrate [4]. In p-type Si, however, hole density is set by the doping and cannot be easily controlled in the wall regions. Thus, due to lateral dissolution, only limited aspect ratios are achievable. Photonic crystals in p-type Si, on the other hand, are desirable for a number of reasons. First, the experimental setup does not require a light source to the backside of the wafer to generate holes. Second, the obtained macroporous Si could be easily further used as substrate for nanoporous silicon, whose formation also requires hole injection and is best achieved in p-type Si. Nanoporous layers could be then impregnated by active media, such as Er ions, thus leading to optically active photonic crystals. Third, p-type substrates are largely preferred for CMOS processes, suggesting easier integration potentiality with conventional electronics. It has been shown that macropores can be formed in p-type Si by anodization in HFcont
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