Porous Silicon from Hydrogenated Amorphous Silicon: Comparison with Crystalline Porous Silicon
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ring seal (12 mm inner diameter) against a circular aperture in the bottom of the cell. The geometry of the cell and counter electrode insured a good current homogeneity over the a-Si:H was Anodization surface. performed at constant current density in ethanoic HF solution, and the electrode potential (relative to the reference electrode) was monitored as a function of time during the anodization. The preparation space was explored by varying four parameters: a-Si:H doping (from 0.1% to 3% B), HF concentration (from 5% to
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50 mA/cm ) and anodization time (up to final peeling-off of Fig.]: Electricalresistivity of our a-Si:Hstarting the a-Si:H layer). The thickness of the obtained material. porous layers was determined using VIS/IR reflectance spectrophotometry [12], before and after anodization. An accurate measurement (±0.01 lIm) of the porous layer thickness was deduced by difference. Results: a study of the anodizationprocess Figure 2 shows a typical variation of potential versus time during constant-current anodization of a-Si:H. The large value of the initial potential is clearly associated with the ohmic drop across the a-Si:H layer: for a resistivity psi=106 f2cm, a thickness of only d=l gim and a typical current density J=10 mA/cm 2, the ohmic drop pJd reaches 1 V, which is several orders of magnitude larger than that for the case of a typical c-Si wafer. As anodization proceeds, potential evolves in a complex manner. The potential/time curve can be conveniently described as . the succession of four regions, labelled3 A-D in Fig.2.
In region A, the potential decreases
linearly with increasing time. This decrease is clearly related to formation of a porous layer of thickness A. Since the resistivity of the electrolyte is much smaller than that of a-Si:H, the ohmic drop is now PsiJ(d-A) instead of PsiJd. The slope dV/dt is indeed found to closely match -psiJ(dAldt), with A determined as stated above. In region B, the potential decreases dramatically to a value of -0.4 V, comparable to that found for c-Si. However, there is no significant variation of A during this stage. Next, the potential remains constant for some time (rgonential remaincess co ntfor s Ve time (region D), then increases to -b V (region D), which is the value observed when stainless steel is anodized in the
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Fig.2: Typical potentialtime curve during Fg2 yia oeta/iecredrn anodization of a-Si:H. Electrolyte: 25% ethanoic HF. Current density: 10 mA/cm 2 . a-Si:H layer: 21gun, 0.2% boron. 404
same electrolyte. The whole a-Si:H layer is seen to peel-off around the transition between region C and region D. Keeping in mind that the resistivity of the electrolyte is much smaller than that of a-Si:H, these observations can be accounted for as follows. The fast decrease of the potential in region B corresponds to a progressive short-cir
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