Study of the Structure and Chemical Nature of Porous Si and Siloxene by STM, AFM, XPS, and LIMA

Abstract

In situ scanning tunneling microscopy (STM) and ex situ atomic force microscopy (AFM) were used to examine the surface morphology of anodized p-St(i,00) electrodes in F -containing solutions. In addition to the formation of a mainly pitted and rough surface, in situ STM observation of anisetropic etching of St(100) in dilute (1%) HF showed the formation of well-defined features, such as peninsulas, a 27 ran wide V-groove, and many protruding 5 nm wide micropyramids.High-resolution in situ STM resolved atomic features at the V-groove limiting (111) facets. Although this slightly etched Si sample contained no quantum pillars, it luminesced orange under UV irradiation, in the same way as a porous Si layer prepared by anodization in a more concentrated HF (1:1 HF:EtOH) solution. A loosely bound surface porous Si layer as thick as 100 nm was revealed by AFM and a 2 ~m 2 square depression could be fabricated in this layer by exerting stronger compressive force. The chemical nature of the surface film prepared by anodic etching in 1:1 HF:EtOH was further probed by x-ray photoemission spectroscopy (XPS), transmission Fourier transform infrared spectroscopy (FTIR), and laser ionization microanalysis (LIMA) techniques. These results support the explanation that the photoluminescence from porous Si can be caused by a chemically modified (St/H/O) layer on the surface (e.g., a siloxene-iype material).Chemical and electrochemical etching of silicon has been applied to the fabrication of submicrometer features on the surface and the production of porous Si layers. ~ Because of the high porosity of porous St, it can be oxidized readily in an O2-rich environment to give a 1 ~m thick dielectric SiO2 layer for insulating electronic elements in integrated circuits. 2 Porous Si has als0 been suggested as a potential optical material for light emitting deyices. 3 Preferential (anisotropic) etching of Si along the (110) and (100) directions is used in microfabrication. 4 A typical chemical etching solution for Si contains an oxidant to react with the surface Si atoms to form higher oxidation states, such as Si(II) and St(IV), that react with F-species to produce soluble complexes. The oxidizing strength of the oxidant plays a crucial role in determining the shape of the etched surface features. Too strong an oxidant, like nitric acid (HNO3), leads to isotropic (orientation independent) etching. 5 Chemical etching of Si with an aqueous KOH solution is strongly anisotropic; this solution has been used to produce high aspect ratio surface features? Generally speaking, the etch rate of Si follows the order of (100) > (110) > (111). 4 The mechanism and causes of this anisotropic etching are of interest and have been studied in attempts to improve Si micromachining capabilities. 6' 7 One tentative model 4 involves correlating the dangling bond density on each of the low index Si surfaces with their reactivities, but the result failed to account for the marked difference in etch rate of 600:300:1 in 44 weight percent (w/o) KOH for the (100 ):...

Macroporous p-Type Silicon Fabry−Perot Layers. Fabrication, Characterization, and Applications in Biosensing

“…As the current densities are increased larger pore sizes can be obtained (Figure 1). The root mean square of the surface roughness is in the range 0.3-0.5 nm for porous silicon layers etched at current densities <200 mA/cm 2 , which is in good agreement with a recent study by Yau et al 15 The surface roughness was only determined for samples etched at current densities less than 200 mA/cm 2 because the macropores generated at higher current densities prevent an accurate measurement of this parameter with SFM. 16 The pore radius is approximately exponentially dependent on the current density (Figure 2).…”

“…As the current densities are increased larger pore sizes can be obtained (Figure ). The root mean square of the surface roughness is in the range 0.3−0.5 nm for porous silicon layers etched at current densities <200 mA/cm 2 , which is in good agreement with a recent study by Yau et al The surface roughness was only determined for samples etched at current densities less than 200 mA/cm 2 because the macropores generated at higher current densities prevent an accurate measurement of this parameter with SFM 1 SFM images of freshly etched porous p-silicon layers etched at different current densities: (A) 1.5 × 1.5 μm 2 image etched at 150 mA/cm 2 ; (B) 5 × 5 μm 2 image etched at 295 mA/cm 2 ; (C) 5 × 5 μm 2 image etched at 370 mA/cm 2 ; (D) 5 × 5 μm 2 image etched at 440 mA/cm 2 ; (E) 5 × 5 μm 2 image etched at 515 mA/cm 2 ; (F) 5 × 5 μm 2 image etched at 600 mA/cm 2 .…”

Silicon Growth at the Two-Dimensional Limit on Ag(111)

“…Survey spectra (shown in Figure S5, Supporting Information) of the sample surface show only peaks attributed to Ag, Si, O, and adventitious C. The Si 2p peak (Figure 3a) is shown to vary significantly with sample depth, consistent with significant changes in the local chemical environment of the Si atoms. At the surface (i.e., sputter time of 0 s), this peak can be fit by components corresponding to Si 0þ , Si 4þ , and an intermediate peak attributable to intermediate oxidation states 37 or SiÀAg interaction, 38 as well as a component for the nearby Ag 4s peak. These results are similar to those obtained in situ on lower temperature growth silicene phases.…”

Electrochemical etching of Si(001) in NH4F solutions: Initial stage and {111} microfacet formation

“…3,4 Allongue et al recently reported a layer-by-layer etching process of n-Si͑111͒ in NaOH solutions at cathodic potentials, showing long range ordered ͑111͒ terraces terminated by monohydride. 7,8 From the practical point of view related to the development of the large scale integrated ͑LSI͒ technology, it is more desirable to have an atomic level understanding of the chemical etching process of Si͑001͒ rather than Si͑111͒ in buffered-HF solutions. On the other hand, chemical etching processes of the Si͑001͒ surface are more complicated than those of Si͑111͒, because the Si͑001͒ surface is expected to be terminated by dihydride in the uppermost layer.…”

“…The potentials reported here are referred to a saturated calomel electrode ͑SCE͒. 8 Apparently, step bunching occurs on the extensively etched Si͑001͒ surface, producing the appearance of stairs with step heights greater than the height of a monolayer. The STM was a Nanoscope III ͑Santa Barbara, CA͒ with a modified electrochemical cell employing two Pt wires as the reference and counter electrodes.…”