Dose Dependence of Ion Beam Mixing of Au on Amorphous and Single Crystalline Si and Ge
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DOSE DEPENDENCE OF ION BEAM MIXING OF Au ON AMORPHOUS AND SINGLE CRYSTALLINE Si AND Ge
D. B. POKER, 0. W. HOLLAND AND B. R. APPLETON Solid State Division, Oak Ridge National Laboratory,
Oak Ridge,
TN 37831
ABSTRACT The rate of ion induced mixing of 700 A Au layers vapor deposited on amorphous and single crystalline Si substrates held at room temperature was measured as a function of dose using 300 key Si, 350 keV Ar, and 525 keV Kr ion beams. Mixing profiles were measured at various fluences by Rutherford backscattering techniques and were found to be consistent with mixed layers whose thicknesses increased with ion dose. Mixing compositions, which were stoichiometric over the entire mixed region at Au-28.5 at. % Si, were found to be independent of ion species or implant fluence. For all ion species the dose dependence of mixing was closer to linear than the square root power law reported previously [13. In addition, the mixing rate for Au on single crystalline substrates was significantly higher than Au on substrates amorphised by Si (self-ion) implantation at liquid nitrogen temperature. No difference was found between the mixing rates when the amorphous substrates were prepared by room temperature implantation. Preliminary results indicate similar behavior for the Au/Ge couple.
INTRODUCTION The Au/Si system is considered in mixing studies as a representative metal-semiconductor system that exhibits rapid mixing, forming an amorphous alloy with constant stoichiometry over the mixed region [1,2]. It has been suggested that chemical driving forces are instrumental to the mixing process [2,3]. During ion irradiation non-equilibrium defect production and enhanced defect mobility can occur, providing an environment for the chemical potentials to equilibrate. The influence of such forces could be tested by examining the effect of mixing on single crystal and amorphous Si substrates. In this study we have measured the mixing rates of Au on these substrates to determine whether the free energy difference influences the mixing process.
EXPERIMENTAL PROCEDURE Samples were cut from wafers of Si. Half of each sample was 30 2 amorphised with 150 keV Si to a dose of 2 x 1016 cmusing either of two procedures. Initially the amorphisation was performed with the samples mounted on a liquid nitrogen cooled copper block in vacuums of 10-7-10-8 Torr. It was suspected, however, that condensation of hydrocarbons from the vacuum were being decomposed on the sample during implantation [4,5]. In an effort to reduce the formation of carbon on the sample surface later amorphisations were performed at room temperature. These samples were then 2 exposed to 0.254 nm UV light at an intensity of 0.15 W cmfor 30 minutes to decompose any carbon which might have formed. After either method of amorphisation the samples were organically cleaned, etched in HF, and rinsed with deionized water before mounting into an oil-free vacuum system for thin-film deposition. Films of Au, -700 R thick, were evaporated onto the Mat.
Res. Soc. Symp.
Proc. Vo
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