  Porous silicon (PSi) layers has been prepared in this work via
photoelectrochemical (PEC) etching process of an n type silicon wafers of two
resistivities (3.5 ohm.cm and 0.02 ohm.cm) in hydrofluoric (HF) acid of 24.5
precent concentration at different etching times (5 to 25 min). The irradiation
has been achieved using laser beam of 2W power and 810 nm wavelength. We have
studied the morphological and structural properties of PSi layers using the
techniques of Xray Diffraction (XRD) and Scanning Electron Microscopy (SEM) and
Gravimetric method. The Xray Diffraction data shows that the structure aspect
of PSi layers remains crystalline as well as the decreasing of diffraction
angle (thetaB) of Xray from PSi layers (29 to 26 degree) and increasing of the
lattice parameter values of PSi structures with increasing of etching times
from 5 to 25 min., and the resistivity of silicon substrates from 0.02 to 3.5
ohm.cm. The nanocrystallite size is decreasing from (20.72 to 5.13 nm) with
increasing of etching times, and the resistivity of silicon substrates. The SEM
images shows that the values of pore width and PSi layer thickness increases
from (0.5 to 6.25 micrometer) and (6.7 to 47 micrometer) respectively with
increasing of etching times and silicon substrates resistivities, while the
values of the thickness of walls between pores has been varied from (1.25 to
0.03 micrometer) with increasing of etching times and silicon substrates
resistivities. The pore shape of pores has been varied from Cylindrical to
Rectangular and to Starful with varied of etching conditions. Further the
measured specific surface area of PSi layers has been increased from (7.43 to
235.35 m2/cm3) with increasing of etching times and silicon substrates
resistivities.
