ISOTOPE EFFECTS FOR MEGA-ELECTRON-VOLT BORON IONS IN AMORPHOUS-SILICON

The depth profiles of B-10 and B-11 implanted into amorphous silicon h ave been analyzed by secondary ion mass spectrometry. Implantation ene rgies between 0.4 and 5.0 MeV were used, and each sample was sequentia lly implanted with both B-10 and B-11 without changing the acceleratio n voltage but only the field in the mass analyzing magnet. A shift bet ween the two profiles is clearly resolved and has been carefully studi ed as a function of ion energy. A maximum shift of 3.5% in mean projec ted range (R(p)) is revealed at 0.6-0.8 MeV [R(p)(B-11) > R(p)(B-10)], and for higher energies the ratio R(p)(B-11)/R(p)(B-10) decreases slo wly to a value of approximately 1.006 at 5.0 MeV. This reverse shift ( heavier isotope penetrates deeper) is attributed to a larger electroni c stopping cross section (S(e)) for B-10 than for B-11 at a given ener gy E where S(e) is similar to E(p) and p > 0. The experimental data fo r R(p)(B-11)/R(p)(B-10) and R(p)(B-11) are compared with calculations, and it is demonstrated that the variation of R(p)(B-11)/R(p)(B-10) wi th ion energy hinges strongly on the S(e) vs E dependence. A close vel ocity proportional dependence (p=0.50+/-0.03) is found to be valid up to approximately 300 keV, and then p decreases gradually with a maximu m in S(e) (p=0) at approximately 2.0 to 2.5 MeV. A semiempirical expre ssion is presented for S(e) and shown to yield excellent agreement wit h both the relative isotope shift and the absolute range values; the d eviations are less than 0.2% and 3.0%, respectively.