The etching of Si(100) surfaces in ultrapure water was studied with a combination of infrared spectroscopy (FTIR) and scanning tunneling microscopy (STM). While the FTIR results show that the initially rough H/Si(100) surface becomes highly homogeneous during etching, a phenomenon generally associated with surface smoothing, STM images reveal that the homogeneity is associated with the formation of well-defined etch hillocks. After many hours of etching, the resulting H-terminated surface is composed of stripes of atomically flat Si(100) terminated by etch hillocks bounded by {111}- and {110}-oriented microfacets. Polarization analysis of the Si-H stretching modes provides strong evidence for uniform dihydride-termination of the flat regions, with the narrow (approximately 25 A) width of these stripes allowing for relaxation of steric strain between neighboring dihydrides. The unusual hill-and-valley etch morphology is attributed to the effects of steric strain on the reactivity of sites on the etched surface.

Surface infrared spectroscopy is used to examine the initial phases of oxidation of hydrogen-terminated Si(100) in ultrapure water containing dissolved oxygen. Analysis of both Si−O and Si−H vibrational modes suggests that oxide growth occurs in patches and reveals that much of the surface remains unoxidized after 5 h of immersion in O2(aq). Isotopic labeling experiments are used to demonstrate that the same type of surface etching that takes place in O2-free water occurs in the presence of O2(aq). Evidence for surface homogenization during the earliest stages of this etching process suggests that etching might play a critical role in smooth oxide growth, a conclusion supported by the near absence of detectable surface oxidation when etching is virtually eliminated by lowering the solution pH.