SiO2 films were grown in dry oxygen on Si (100) substrates at various tempe
ratures, then annealed for various times and temperatures. The density of t
he oxides was found to depend on the growth temperature, and on the anneal
time and temperature. The amount of positive charge trapped in these oxides
during vacuum ultraviolet hole injection was found to be inversely related
to the oxide density, regardless of how the oxide was prepared. Comparison
of the density of trapped holes with the density of E' centers detected by
electron spin resonance indicates that E' centers cannot account for most
of the trapped positive charge. A model to explain these observations is pr
oposed.