2. Experimental
Ultrafast laser texturing is achieved by exposing float zone (FZ)
Si (1 0 0) wafers of thickness 280 mm and resistivity of 1 O cm to a
train of femtosecond (130 fs) pulses from a Spectra Physics
Ti–sapphire laser system (1 mJ pulse energy; frequency 1 kHz)
having a wavelength of 800 nm. The experiments are carried out
in 800 mbar pressure of SF6 gas. Large area texturing is obtained
by scanning the sample under the laser beam using a computer
controlled precision x–y stage. Samples with different texture
heights are prepared by varying the average number of laser
pulses impinging on the silicon surface. The average numbers
of laser pulses are controlled by varying the velocity at which
the samples are scanned under the laser beam. Further details
describing the processing of the samples is described in Ref [35].
One of the ultrafast laser textured silicon samples also undergoes
a thermal annealing step in the presence of oxygen at 1100 1C for
7 h. Next, chemically textured (CT) silicon surfaces are obtained
by anisotropic etching in a mixture of sodium hydroxide,
isopropyl alcohol and de-ionized water [42]. After chemical
texturing, 70 nm of silicon nitride is deposited using a plasma
enhanced chemical vapor deposition (PECVD) system to serve as
an antireflection coating. Lastly, density graded porous silicon
surfaces (PS) are obtained by a technique described in reference
[8] investigated for photovoltaic applications. This is very similar
to the nanoporous surfaces reported by NREL [43]. The morphology
of the textured surfaces is investigated using a Zeiss SUPRA 40
scanning electron microscope.
Total integrating scattering (sum of specular reflection,