Germanium Partitioning and Interface Stability During Rapid Solidification of Gesi Alloys
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results are presented for the Ge/Si system. PARTITIONING AND DIFFUSION MEASUREMENTS In general, the partition coefficient is a function of interface velocity v and ranges from its equilibrium value k. during slow solidification to a value of unity at sufficiently fast velocities. For a number of systems [10,11], nonequilibrium partitioning is well described by the Continuous Growth Model (CGM) [12], which gives in the dilute limit
k - s+ v/vD
k+v/vD
(1)
where vD is the diffusive speed. At v = vD, k is in mid-transition between its equilibrium value and unity. In the nondilute CGM with a Henry's law approximation, k becomes a function of v, T, Xh, and thermodynamic data available from the phase diagram. Laser melting and rapid solidification experiments were performed to determine k and D, for Ge using the generally accepted method [10,13]. Films of Ge.,Sil_, on silicon-on-insulator (SOI) and on silicon-on-sapphire (SOS) were fabricated using a variety of techniques (ion implantation, electron beam evaporation, and molecular beam epitaxy) [13]. The SOI substrates consisted of (100) oriented Si on about 1 jm of thermally oxidized silicon while the SOS substrates were (100) Si on (1"02) A12 0 3 . The Si film thicknesses ranged from 0.2 to 0.6 pm. Samples were melted using a pulsed XeC1 excimer laser (308 nm, 30 ns fwhm), with some samples preheated by a continuous wave CO 2 laser to access slower regrowth velocities. The values of k and DI for each sample were determined by coupling numerical simulations of liquid phase diffusion and solute partitioning at the interface with experimental measurements of the final Ge concentration profile using grazing exit angle Rutherford backscattering spectrometry (RBS). The simulations required inputs of the initial concentration profile, the measured melt depth vs time history [14], and values for k and D 1. The output was the final Ge concentration profile, which was matched to the experimental profile by suitable choices of k and D1 . Diffusivity measurements were obtained by first allowing both k and DI to vary. Values for D, from numerous samples were then averaged to obtain a Ge liquid phase diffusivity of 2.5 x 10- cm 2/s with a standard deviation of 0.5 x 10- cm 2/s. These measurements corresponded to D, near the melting point of Si and, to within experimental scatter, were independent of Ge composition over the range studied (< 10% Ge). With D, fixed at 2.5 x 10- cm 2 /s, simulated post-irradiation Ge profiles were once again matched to experimental profiles with only k allowed to vary. Figure 1 shows such simulated and experimental RBS spectra for scattering events from Ge. A sample of 50 nm Geo.,Sio.9 on SOS was laser melted to a depth of 150 nm and rapidly solidified at 1.23 m/s. (Velocities are specified for the interface 50 nm from the surface.) The spreading of the initial Ge profile to greater depths gives a measure of diffusivity while the surface peak is indicative of partitioning. As can be seen, the experimental profile is well matched by simulations corre
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