Nanotopography development on silicon electrodes by analysis of Brewster angle real time spectroscopy

The nanostructure formation of n type silicon photoelectrodes in fluoride containing solutions was investigated by in situ Brewster angle reflectometry BAR and atomic force microscopy AFM . The corrugation process near the first current maximum of the divalent tetravalent dissolution regime preserves the 111 surface lattice symmetry as proven by auto correlation analysis of corresponding AFM images the structure alignment is related to the terrace orientation of the original H terminated Si samples. The observed cathodic shift of the open circuit potential OCP for increased light intensities 240 mV for intensities between 1 and 40 mW cm2 allowed further shaping of the nanostructures it was possible to invoke alternately divalent and tetravalent paths of the silicon dissolution process at a constant potential. Oxidation in the tetravalent dissolution regime was observed to affect statistically rough areas faster than the nanostructures. As a result, an increase of the aspect ratio of 40 60 could be achieve

Phase relations between photocurrent and in situ reflectance during photoelectrochemical dissolution of silicon

The phase relation between local maxima of photocurrents and those of in situ Brewster angle reflectance BAR is investigated upon dissolution of silicon photoelectrodes in diluted NH4F. In the region of photocurrent oscillations, charge flow and optical response can be related by a linear equation which explains the observed positive phase shift of the reflectance. In the transition regime, between porous silicon formation and electropolishing, commencing surface oxidation results in a negative phase shift of the reflectance. It is shown that this observation points to a change in the dissolution mechanism of the surface near region. A step function, convoluted with the charge flow, is applied to model the optical behavior. This approach is compared to optical multi layer analysis in which simultaneously increasing sub surface porosity and surface roughness are considere

Real time monitoring of SiO2 Si 111 interlayer etching by Brewster angle reflectometry

A transitory etching regime after SiO2 dissolution and before bulk Si 111 etching in neutral NH4F solutions was monitored by in situ Brewster angle reflectometry BAR . An observed intermediate increase of the BAR reflectance signal is attributed to a fast dissolution of a stressed strained interlayer beneath the SiO2 Si 111 interface. Similar effects were observed on thin thermal oxides 18.2nm , grown on float zone silicon, as well as on ultra thin native oxides 1.2nm on Czochralsky silicon. Native oxide covered samples showed an increased surface roughness in the course of interlayer dissolution while the surface is progressively covered with compounds of fluorinated silicon. The etch rate, determined by atomic force microscopy AFM and compared to the etch rate of bulk silicon, is increased by a factor of four. In the limit of extended etching, the known low etch rates for silicon in 40 NH4F are observed. Structural and chemical properties of the interfacial layer were analyzed by synchrotron radiation photoelectron spectroscopy SRPES which confirmed the presence of Si3 4 valence states throughout the interlayer and by near open circuit potential N OCP dark current measurements. As a result, oxide etch rates in NH4F in the pH range 7 8 as well as the silicon interlayer depth can be assessed by in situ BAR