In situ scanning tunneling microscopy (STM) was employed to examine the electrochemical etching process of an n-Si(111) electrode in dilute NH4F solutions under potential control. Time-dependent STM images have revealed prominent effects of microscopic structures of Si on the rate of its dissolution. Multiple hydrogen-terminated Si atoms at the kink and step sites were eroded more rapidly than the monohydride Si step. This presumably resulted from the difference in reactivity of these hydrogen-terminated Si species. It is demonstrated that the density of kinks plays a main role in controlling the etching rate of Si. In the absence of kinks, not only the monohydride but also the dihydride steps were found to be stable. The etching rate of the monohydride step is substantially increased from a negligible value to 15 nm/min by the introduction of kink sites. The average etching rate for a dihydride step was 32 nm/min. Overall, the difference in the reactivity guides the dissolution of Si in a layer-by-layer fashion.
The electrochemical etching of (110) n-Si surfaces in aqueous NH 4 F has been studied for the first time by in situ atomic resolution scanning tunneling microscopy. It was found that the degree of interfacial order was profoundly dependent upon applied potential. At potentials near or positive of the open-circuit potential, pit corrosion occurred that resulted in disordered surfaces. At potentials negative of the open-circuit potential, highly anisotropic layer-by-layer dissolution took place that rendered a rough surface atomically flat. The scanning tunneling microscopy images clearly showed an interfacial structure characterized by zigzag chains along the <110> direction. The distance between every other Si atom on the zigzag chain was measured to be 3.8 A, while the chain periodicity was determined to be 5.4 A. These values serve to establish that controlled negative-potential etching resulted in the formation of an ideal H-terminated (110) Si:H -(1 x 1) structure. Time-dependent scanning tunneling microscopy was employed to gain insight into the role of dihydride Si atoms in the anisotropic etching process. High-resolution images allowed the evaluation of absolute etching rates along the <110> direction through an exact numerical count of the Si atoms dissolved per unit time.
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