Thin Films of CoSi 2 Co-Deposited onto Si 1-x Ge x Alloys
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temperature was reduced at 40 'C/min and was held at 550 'C for the deposition of a homoepitaxial 200 A Si buffer layer and a heteroepitaxial 800 A Si0. 80Ge0.20 layer. Sharp 2×1 RHEED patterns were observed along the Si [110] azimuth indicating a well ordered surface. Cobalt metallization was achieved with one of three methods: Co and Si co-deposited at 4500 C in a system described elsewhere [10] (co-deposited samples); Co deposited on a sacrificial Si layer at room temperature ("sequential" samples); Co deposited directly onto the SiGe film at room temperature ("direct" samples). In all cases Co deposition was followed by a thermal anneal for 10 minutes at 400-700' C. Results for the "direct" samples have been reported previously [6,7]. Co-deposited and sequential samples were prepared by depositing -47 A of Co and 153 A of Si to produce a stoichiometric CoSi2 layer -150 A thick. An additional codeposited sample was prepared at 400 'C without annealing. In order to study the bonding at the Co/SiGe interface a sample was also prepared by depositing 2 A of Co onto Si0 .goGe0 .2o at room temperature followed by annealing at 450 'C. Similar structures were employed by Tung et al. as CoSi2 templates for the growth of CoSi2 on Si(100) [13,20]. Structural properties of the co-deposited films were determined with EXAFS and XRD. EXAFS data were collected at room temperature in total electron yield (TEY) mode at the Co Kedge at beamline X-1 1 at the NSLS. The thick (-150 A) CoSi 2 layers were measured ex situ, while the Co template was transferred under UHV to the analysis chamber of our growth system and was measured in situ. Fourier filtering was performed with the MacXAFS package [14] and structural parameters were extracted by non-linear fitting to the standard EXAFS equation [15] with a program written in-house. The edge shift E0, coordination number N, bond-length R, and EXAFS Debye-Waller factor c 2 were used as adjustable parameters in the fits. XRD data were collected in the 0-20 mode with Cu Ka radiation on a Rigaku Geigerflex diffractometer equipped with a (0001) graphite monochromator. Surface and interface composition and morphology of the films were examined with AES, SEM, and AFM. AES data were collected for the Co and Ge LMM lines in pulse-count mode with a Physical Electronics CMA spectrometer. The primary beam voltage was 3 keV. AFM data were collected with a Park Scientific BD2-210 instrument and 400 A cantilevers. SEM images were acquired with a JEOL 6400 field-emission scanning electron microscope. Sheet resistance was measured with a Magne-Tron M-700 four-point probe on rectangular samples. The sample size was between 15x9 and 18x12 mm, and the probe spacing was 1.588 mm. Appropriate geometrical correction factors were applied [16]. RESULTS The Fourier transforms of the EXAFS data for the co-deposited are shown in Fig. 1. Corresponding structural parameters are given in Table 1. Unlike films obtained by annealing Co deposited directly on Sil-xGex [7], co-deposited films exhibit a CoSi2-type crystal s
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