Electron-induced extended-fine-structure measurements of thin-film growth and reaction

The application of two electron-beam-induced extended-fine-structure (EFS) techniques [surface extended energy-loss fine structure (SEELFS) and extended appearance-potential fine structure (EAPFS)] to the study of thin films has been demonstrated by measurements on three well-characterized compositional phases of titanium deposited on Si(111). The EFS above the Ti L₂,3 edge have been measured for an unannealed (20 °C) pure Ti overlayer, a 250 °C-annealed layer (a Si-rich Ti overlayer), and a 400 °C overlayer (a silicide phase). Data were analyzed by using two routines from extended x-ray-absorption fine structure (EXAFS): the standard optical transform (including Δl=+1 phase shifts) and the ratio method which is independent of phase shifts. The Ti L₂,3edge EFS satisfies the dipole pseudo-selection-rule of EAPFS, validating the use of Δl=+1 phase shifts in the EAPFS analysis. The agreement between the measured spectra obtained with SEELFS and EAPFS is very good and is additional confirmation of the use of dipole phase shifts in SEELFS analysis.

The nearest-neighbor atomic spacings for both the 20 °C and 250 °C unreacted overlayers were determined by the standard analysis to be 2.93±0.02 Å, in good agreement with the predicted value of 2.915 Å for the two unresolved near-neighbor spacings at 2.89 and 2.94 Å of bulk Ti. Application of the ratio method to this data confirms these results and also shows that the 250 °C-annealed film, measured at room temperature, exhibits a higher degree of structural disorder than the 20 °C film. The absence of additional peaks in the radial distribution function obtained from the EFS and the good straight line fits of the ratio method suggest that the silicon diffuses via grain boundaries. Measurements of the 400 °C data showed a local structure similar to TiSi. The nearest-neighbor pair in this film was determined to be a Ti—Si bond with a spacing of 2.39±0.04 Å, also in good agreement with the predicted value of 2.37 Å, again for two unresolved near-neighbor atomic separations at 2.30 and 2.44 Å of bulk TiSi.