ADSORPTION AND DECOMPOSITION OF WATER ON SI(100) - A TPD AND SSIMS STUDY

The adsorption and decomposition of water (D2O) on Si(100) has been studied using temperature-programmed desorption (TPD) and static secondary ion mass spectroscopy (SSIMS). At 100 K, D2O adsorbs both molecularly and dissociatively, forming SiOD and SiD; dissociative adsorption dominates at low exposures. Molecularly adsorbed D2O, when present in small amounts, dissociates between 100 and 200 K to form additional SiOD and SiD. At high coverages (monolayer and multilayer), molecular desorption at 170 K dominates. When the dissociative adsorption channel is nearly saturated, a new saturable D2O TPD peak appears (225 K). Although thermal decomposition of SiOD, to form Si-O-Si and SiD, starts as low as 300 K, most SiOD decomposes between 500 and 750 K with an activation energy of 16.5 +/- 1.5 kcal/mol and a pre-exponential factor of 10(4.2 +/- 0.5) s-1. Most SiD decomposes to yield gaseous D2 between 750 and 850 K. A small fraction (10%), probably hydrogen-bonded to nonbridge-bonded oxygen defects (.O-Si), is, however, more stable and decomposes, releasing D2, between 850 and 1000 K. Surface O atoms desorb as silicon oxide between 850 and 1000 K. In contrast to earlier reports, our results imply that an H-free silicon oxide layer cannot be prepared under UHV conditions through wet-oxidation of Si(100).