The Formation of Porous Silicon Layers Formed in a Non-Aqueous Electrolyte
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INTRODUCTION Previously, it has been shown that the photoelectrochemical etching of silicon in HF-acetonitrile (MeCN) solutions results in a process free of the complications caused by water.[ 1] A mechanism for the (100) silicon dissolution in HF containing electrolytes has been proposed, and is shown in Fig. 1.[1] The initial surface is hydrogen terminated,[2,3] and the reaction begins with the generation of holes, either thermally generated or photogenerated. The holes drift under applied bias to the solution interface where they can react, Fig. 1A. A radical and a proton are formed as a result of the hole initiated reaction. The radical can be easily oxidized thereby injecting an electron into the conduction band resulting in current doubling. The positively charged silicon is complexed with a fluoride ion, Fig. IC. Due to the highly electronegative nature of fluoride,[1,4] the adjacent silicon-hydrogen bond is destabilized thereby increasing its reactivity. The destabilized silicon-hydrogen bond is then oxidized forming a radical and a proton. The second radical is easily oxidized, Fig. ID, leading to a difluoride terminated surface. The fluoride surface destabilizes the silicon backbonds allowing HF to add across them, Fig. IF. The high quantum efficiency (>3 for (100) silicon) shows that the intermediates are capable of electron injection. The A H h+ Si \H . F" 'e\S
/S
Si
C
B S1 H+""FS1/ S Si \H .
-
\Si./ S/. Si\H F-e"P\ F
\si/
F
\s/"H F\ SS,F 2-HF \i \F \ F/ \ F
D
E
"si//
S1 \F
"si/ F
e- \"/ \Si,/ Si F"
Si
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Fig. 1. Mechanism for the electrochemical dissolution of (100) silicon in fluoride containing electrolytes. 339 Mat. Res. Soc. Symp. Proc. Vol. 358 01995 Materials Research Society
high current density, absence of molecular hydrogen, and formation of relativelylarge, nonbranched pores in MeCN indicate that water (and hydroxide) complicate and slow the reaction sequence.[1] The electrochemical etching of p-type (100) silicon in HF-MeCN was shown to form an unique pore structure that exhibits red-orange photoluminescence (PL) and electroluminescence (EL). The pore morphology for (100) silicon etched in a non-aqueous electrolyte began with faceting along (I ll) planes followed by the formation of deep, non-branching pores at the base of the original (11) facets. The pores were 1-2 gm in diameter, and could be made very deep (>150 jim).[ 1] Several reaction mechanisms have been presented to explain the formation of porous silicon in aqueous solutions. [5-7] However, the aqueous pore formation mechanisms fail to adequately account for the role of water and the contributions of crystal orientation. The purpose of this paper is to examine the electrochemical dissolution of silicon in HFMeCN electrolyte with respect to the effects of crystal orientation on the mechanism of pore formation. The previously proposed mechanism for (100) silicon dissolution is extended to include (111) silicon.
RESULTS The electrochemical behavior of (100) and (111) n-Si in 2M HF, 0.25M TBAP, MeCN is similar
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