CHEMICAL ETCHING OF VICINAL SI(111) - DEPENDENCE OF THE SURFACE-STRUCTURE AND THE HYDROGEN TERMINATION ON THE PH OF THE ETCHING SOLUTIONS

Infrared spectroscopy is used to study the etching process of stepped Si(111)9-degrees surfaces as a function of the pH of the etching HF solutions. This process results in complete H termination of the silicon surface, including terraces, steps, and defects; the surface structure can therefore be well studied using infrared (IR) spectroscopy. Polarized IR absorption spectra of the Si-H stretching vibrations(i.e., in the region 2060-2150 cm-1) vary dramatically as the pH of the etching solutions increases from 2.0 to 7.8. In general, higher pH solutions yield sharper bands and more easily assigned spectra, making it possible to identify the step and terrace species and thus to infer the surface structure and step morphology (i.e., to investigate the etching process). The data are explained by a model involving different etching rates for each individual surface species: The highest rate of removal is for isolated adatom defects located on (111) planes and the lowest is for the ideally H-terminated (111) planes themselves. For proper conditions of pH and etching time, atomically straight steps are formed due to faster removal of kinks than etching of the straight step themselves. The influence of steric hindrance is invoked to account for such preferential etching.