Impurity centers in silicon films on sapphire

The MOS Hall technique has been used to measure the temperature dependence of the average electron and hole mobilities at various depths into the film. Electron and hole concentrations were also determined as a function of temperature and distance from the film surface. It has been found that a second ionization level, in addition to the intentionally introduced arsenic impurity, exists in the n‐type films at approximately 0.1 eV below the conduction band. This level has been associated with the ${\mathrm{SiO}}_4^{+}$ complex known to be present in oxygen‐rich silicon. The density of this added impurity increases with depth into the film and has, therefore, been associated with the mobility gradient. The effect of this complex on the temperature dependence of the mobility is examined and the appearance of a temperature‐independent neutral impurity scattering mechanism is evident. Neutral impurities in p‐type films also are suggested to explain the hole‐mobility‐vs‐temperature data and they may be related to the ${\mathrm{B‐SiO}}_4^{+}$ molecule known to exist in oxygen‐rich boron‐doped silicon. The effect of stress on the mobility has been suggested as a possible cause for the reduction in electron and hole mobilities at elevated temperatures where phonon scattering appears dominant.