This research was undertaken to provide an understanding of the nature, origin and desorption mechanism of species found on the porous silicon (PS) surface and the changes that occur when PS is stored under varying conditions.
The PS used in this work was produced from p-, high-resistivity FZ c-Si substrates.Three types of commonly used HF-based electrolytes were chosen for anodisation,
under the same process conditions. With the resulting samples, temperature programmed desorption (TPD) coupled with mass spectrometry were used to identify species liberated at different temperatures. FTIR was also used to investigate the nature of surface species on PS and hence to infer how these give rise to the observed volatile products. After various modifications, the TPD system with the custom-made heating unit and the appropriate methodology were developed to suit the present work.
Freshly anodised PS in the vacuum chamber at room temperatures gave somewhat enhanced peaks due to air components (0 ⁺, N₂⁺ and/or CO⁺, O₂⁺ and CO₂⁺) and, most significantly, an increase in F-containing species (e.g. F ⁺), derived from the electrolyte.
On heating, the main desorbed species were found be hydrogen, silane, Si-Fx species, and Hx-Fx species. TPD spectra for hydrogen showed two peak maxima with a "hump".
This implies two types of hydrogen environments; these were assigned as Si-H (lower temperature peak) and Si-H₂ (higher temperature peak) species on the PS surface. The
temperature difference between the two peaks was similar (~100K) in all three cases. This shows that hydrogen desorption occurs similarly from PS prepared using the three
different electrolytes. It also suggests that hydrogen adsorption during PS formation occurs analogously in the three electrolytes.
Silane was observed to desorb at 575K. It is proposed that this comes from -SiH3 groups on the PS surface, possibly after reaction with sorbed water. A mechanism is suggested.
In contrast, desorption of Si-Fx species was found to be sensitive to the nature of the electrolyte. The lower temperature peaks from the TPD experiments are assigned to
H₂SiF₆ , SiF ₄ and perhaps H₂SiF₂ (by-products from anodisation) sorbed on PS. They are held relatively weakly by electrostatic and/or van der Waals forces. The higher
temperature peak assigned to SiF₃ + may be explained in terms of migration of F atoms followed by Si-SiF₃ bond breakage. The various Hx-Fy products derive from species
present in the HF electrolytes.
To investigate changes in PS under typical storage conditions, samples were kept in (i) a blue wafer box, (ii) a screw-top white box and (iii) a similar box in a vacuum desiccator. The PS was then analysed by FTIR after various time intervals. After one month, only PS stored under condition (iii) was unchanged. The other samples showed evidence of oxidation, attributed to hydrolysis, fonnation of silanol (SiO-H) species, and back-bond oxidation of Si-Hx groups. Further ageing revealed inclusion of C-H species on PS.
This work is a contribution to understanding of PS behaviour, and is relevant to its applications in electronic devices and sensors
