| Abstract
| - A systematic study of the etching behavior of one-dimensional (1-D) Si nanowires (SiNWs) in various HFand NH4F etching solutions is reported. The concentration and pH dependences of the etching time (whichis inverse to the “stability”) of the SiNWs in these solutions were investigated. A V-shaped bimodal etchingcurve was observed for HF solutions with concentrations of 0.5−40%. Specifically, SiNWs exhibit highstability in both low (0.5%) and high (40%) concentrations of HF solution, with the lowest stability (i.e.,fastest etching rate) occurring at 2% (1 M) HF solution. With NH4F, the time needed to totally etch away theSiNWs sample decreases with increasing concentration (from 1−40%). The opposite is true when the pH ofthe NH4F solution was maintained at 14. These surprising results were rationalized in terms of “passivation”of the SiNW surfaces by HF or related molecules via hydrogen bonding for Si−H-terminated surfaces in HFsolutions (with low pH values) and by NH4+ ions via ionic bonding for Si−O--terminated surfaces in NH4Fsolutions (with high pH values), respectively. Furthermore, it was found that SiNWs are stable only in relativelynarrow pH ranges in these solutions. When SiNWs are etched with HF, the stability range is pH = 1−2where the surface moieties are Si−Hx species (x = 1−3). When SiNWs are etched with NH4F, the stabilityrange is pH = 12−14 where the surface moieties are mainly Si−(O-)x species (x = 1−3). These rationaleswere confirmed by attenuated total reflection Fourier transform infrared spectroscopy measurements, whichshowed that, while etching SiNWs with HF gave rise to Si−Hx surface species, no Si−Hx species were observedwhen SiNWs were etched with NH4F. The latter finding is at odds with the corresponding results reportedfor the two-dimensional (2-D) Si wafers where etching with either HF or NH4F produces Si−Hx species onthe surface. This difference suggests either that the etching mechanisms for NH4F versus HF are different forSiNWs or, more likely, that the Si−Hx surface species produced in NH4F solutions are so unstable that theyare hydrolyzed readily at pH > 4. The similarities and differences of the etching behaviors and the resultingsurface speciations between the 1-D SiNWs and the 2-D Si wafers suggest that the nanoscale structures aswell as the low dimensionality of SiNWs may have contributed to the rapid hydrolysis of the surface Si−Hxspecies in NH4F solutions, especially at high pH values.
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