Characterization of thin porous silicon films formed on n(+)/p silicon junctions by spectroscopic ellipsometry

Optical and structural properties of thin porous silicon (PS) layers electr ochemically formed in the outermost part of 0.3 mu m thin ni emitters of co mmercial p/n(+) silicon shallow junctions were studied by spectroscopic ell ipsometry (SE). The SE data were evaluated with a multiparameter fitting pr ocedure based on the Bruggeman effective medium approximation by using a mi xture of fine-grain polycrystalline Si and voids. The fit results show that the thicknesses of PS films grown at a constant current density of 50 mA/c m(2) in 30% hydrofluoric acid (HF) with etching times of 1.5 and 3.5 s are 54 and 105 nm, respectively. When the formation time is less than 2.5 s, th e porosity is a constant 60% throughout the entire PS layer. A porosity gra dient occurs across the layer for formation times greater than 2.5 s, resul ting in a layer of up to 85% porosity near the surface after 3.5 a. This in creasing porosity is due to chemical dissolution by the HF electrolyte, occ urring as a simultaneous reaction to the electrochemical PS formation. All the PS layers grow at a rate of 32 +/- 3 am s(-1), which is independent of the PS formation time and is not affected by the phosphorus doping,gradient in the emitter. The calculated number of elementary charges, z, needed to dissolve one Si atom from the bulk is 3.3. Comparison of the SE fit with to tal reflectance measurements confirms the validity of the SE modeling. PS l ayer thicknesses were measured independently of the SE experiments by combi ning secondary ion mass spectroscopy measurements and surface profiling. (C ) 2000 The Electrochemical Society. S0013-4651(99)01-029-0. All rights rese rved.