  Cryptocrystal layers of ammonium silicon fluoride (NH4)2SiF6 were grown on
silicon wafers by dry etching using the vapors of HF:HNO3 solution at room
temperature. As-grown layers are composed of white granular crystalline film
with thicknesses of up to 8 micrometer which were synthesized with growth rates
of around 1 micron/hour. The crystallinity was analysed by X-ray diffraction
which indicates an isometric hexoctahedral system(4/m -32/m) with Fm3m space
grouping of (NH4)2SiF6 cryptohalite crystals. These results have been confirmed
by the presence of the vibrational absorption bands of (NH4)2SiF6 species by
FTIR transmission. Strong absorption bands were observed in the infrared at
480cm-1, 725cm-1, 1433cm-1 and 3327cm-1 and assigned to N-H and Si-F related
vibrational modes of (NH4)2SiF6. Annealing above 150oC leads to the formation
of individual crystals with sizes up to 20 micrometer on the surface, thus
indicating the posibility of forming solid compound layers with fine grain
sizes on silicon.
