Three-dimensional etching profiles and surface speciations (via attenuated total reflection-fourier transform infrared spectroscopy) of silicon nanowires in NH4F-buffered HF solutions: a double passivation model.

A systematic study of the etching behavior, in terms of three-dimensional profiles, of one-dimensional (1-D) silicon nanowires (SiNWs) in NH(4)F-buffered hydrofluoric acid (BHF) solutions of varying concentrations and pH values and the surface speciations of the resulting etched SiNW surfaces, as characterized by attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy, is reported. It was found that SiNWs are stable only in relatively narrow pH ranges of the BHF solutions. The results are rationalized in terms of a "double passivation" model. When SiNWs are etched in BHF solutions with pH values of 1-3, the surfaces are passivated with hydrogen (inner layer) giving rise to surface moieties such as Si-H(x) species (x = 1-3); at high HF concentrations, the H-terminated Si surfaces are covered with a hydrogen bonding network of HF and related molecules (oligomers, etc.), providing an outer-layer passivation. When SiNWs are etched in BHF solutions with pH values of 11-14 (by adding a strong base such as NaOH), the surfaces are oxygen-terminated with surface moieties such as Si-(O(-))(x)() species (x = 1-3); at high NH(4)F concentrations, the negatively charged Si surfaces are stabilized by NH(4)(+) ions via ionic bonding, again providing outer-layer passivation. In BHF solutions with pH values of 3-11, the surface speciation, consisting of Si-(OH)(x)(O(-))(y) (x + y = 1-3) species, is unstable and etched away rapidly. The surface speciations of SiNWs etched in various BHF solutions were explored via ATR-FTIR spectroscopy. It was found that, while etching SiNWs with HF-rich BHF solutions with pH < 4 gave rise to Si-H(x)() surface species, no surface Si-H(x) species were observed with SiNWs etched in BHF solutions with pH >/= 4 (HF/NH(4)F </= 1:1). In sharp contrast, etching of two-dimensional (2-D) Si wafers with either HF, NH(4)F, or BHF etchants produces Si-H(x) species on the surface. Finally, while HF is a much more efficient etchant than NH(4)F for 2-D Si surfaces, NH(4)F is found to be as efficient an etchant as HF for SiNWs (this work). We believe that these differences can be attributed to the nanometer size (corresponding to the roughness scale in 2-D wafers) of SiNWs on one hand and the lack of "passivation" by the hydrogen bonding network of HF and related molecules at pH >/= 4 on the other. These two factors, among others, contribute to the rapid hydrolysis of the surface Si-H(x)() species (and the etching of the SiNWs), particularly in BHF solutions with low HF/NH(4)F ratios and high pH values (pH >/= 4).