Porous silicon host matrix for deposition by atomic layer epitaxy
Introduction
Macroscopically uniform thin layers of tin oxide can be deposited by atomic layer epitaxy (ALE) onto extremely structured surfaces of porous silicon (PS) [1]. However, there is no evidence of the layers being continuous on a microscopic scale. Most of the analytical techniques such as Rutherford backscattering spectrometry (RBS) or secondary ion mass spectrometry (SIMS) provide average concentrations or depth distributions. They are therefore not suited for microanalysis of nanostructures owing to their limited characteristic spatial resolution. Direct observation of nanostructures is the domain of high resolution transmission electron microscopy (HRTEM) and scanning probe microscopic techniques, such as scanning tunnelling microscopy and atomic force microscopy (AFM).
Porous silicon after anodization is known to provide nanometre scale structures in two ways: (i) the size of the pores and (ii) the thickness of the wall separating the voids. Any material brought into the pores of a typical diameter of 3–12 nm, without destroying the original skeleton structure, should be considered amorphous or nanocrystalline. In a nanocrystal all the attributes of the low dimensional structure, such as band gap widening and quantum confinement, should be present. This not only enables the study of these effects in a defined environment but also gives rise to exciting new applications. The luminescent properties of the porous silicon host matrix might be rendered more stable through pore filling and might be wavelength tunable by the addition of such nanosize crystallites of foreign material. This is the motivation of the present study.
Section snippets
Experimental details
(100) oriented p-type silicon wafers of 0.001 Ω cm resistivity were anodically etched in HF:H2O:EtOH 1:1:2 electrolyte applying a current density of 50 mA cm−2. The resulting columnar type porous layers were of 70% porosity as determined by gravimetry. The average diameter of the pores was 14 nm according to BET measurements. The thickness of the PS layer was always 2 μm. The PS structures were oxidized slightly at 300 °C in N2:O2 15:1 ambient for 30 min in order to stabilize the surface.
The ALE
Results and discussion
As the ALE process is governed by the chemisorption of the precursors, the growth rate is independent of precursor pulse times when the condition for surface saturation is achieved. Although the process theoretically is described as layer-by-layer growth [3], recent studies of ALE deposited metal oxides 4, 5have shown that island or Stranski–Krastanov type processes are possible on amorphous or polycrystalline substrates, leading to the formation of polycrystalline thin films. Generally, these
Conclusion
ALE deposited SnO2 layer growth is affected by nucleation on both flat and porous surfaces. This forms the base for the creation of uniformly distributed nanocrystallites of SnO2 in a PS host matrix. Regardless of the deposited material, possible nucleation in ALE processes must be considered when coating porous silicon surfaces.
ALE is an efficient tool in the preparation of nanocrystalline tin oxide structures in PS. Optical, electrical and gas sensing properties of PS based structures can be
Acknowledgements
This work was supported by a bilateral programme of cooperation between the National Committee for Technological Development of Hungary (OMFB) and the Technology Development Centre (TEKES), Finland. Part of the costs was covered by the COPERNICUS PORSIS project (CP940963) of the European Communities.
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