Depth Gradients in Porous Silicon: How to Measure Them and How to Avoid Them
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electrolyte compositions of (HF (50%) : C2H 5OH) (1:1), (1:0.9) and (1:1.1). Before the etching process the samples were cleaned in an ultrasonic bath using propanol and thereafter they were rinsed in deionised water. In order to analyse changes in the porosity of a probe layer (200mA/cm 2, 8s ; 1.1 gm), filling layers were etched below and above the test layers with a current density of 1O0mA/cm 2 whereby the etching times of these layers are varied from 12s (ý ljtm) up to 240s (•.20jim). The resulting layer system was analysed by a Perkin-Elmer X2 reflectance spectrometer (9000cm-1-50000cm'). The spectra were fitted using Looyenga [9] effective medium theory as described in [8]. For the on-line measurements of the thickness oscillations we have used a laser diode operating at 785nm (power
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