We studied the thermal stability of hafnium oxides and silicates with femtosecond pump/probe photoelectron spectroscopy, employed to monitor photovoltage shifts in-the underlying Si substrate induced by the absorption of femtosecond pulses of 800 nm light. Annealing of hafnium oxides and silicates, deposited on thin SiON interlayer oxides grown on lightly doped Si (100) substrates, reveals an abrupt onset of charging at elevated temperatures. Core level photoemission and transmission electron microscopy were used to correlate the observed charge injection at elevated temperatures with structural and chemical changes in the SiON and HfO2 dielectric layers. (C) 2005 American Vacuum Society.