Segregation and Trapping of Gold During Ion-Induced Crystallization of Amorphous Si
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interface velocities of typically -lAk/sec being controlled by ion beam current. The temperature dependence of this ion-induced growth is described by an activation energy of only 0.2-0.3 eV. By the use of ion-induced growth, we can reduce the temperatures to values such that the Au diffusive velocity is comparable to the interface velocity. Parameters important in characterizing the segregation process are the interfacial segregation coefficient k' (the ratio of the solute concentration in the c-Si to that in the a-Si at the interface) and the diffusive escape distance, D/v (the ratio of diffusivity away from the interface to the interface velocity). As D/v approaches the interatomic spacing, the probability increases for solute atoms to be engulfed or trapped by an advancing interface. For ion-beam-induced crystal growth at 2500 C, D/v is -0.1A if the diffusivity retains its usual thermal value. However, as we will discuss, the diffusivity is also enhanced above thermal values by the ion-beam irradiation which results in longer escape distances. EXPERIMENTAL Silicon samples were implanted with Au ions at an energy of 3 MeV to a dose of 1016 ions/cm 2 , resulting in an a-Si surface layer approximately 1.6gtm thick with the Au at a depth of approximately llgm. The samples were then furnace annealed to redistribute the Au uniformly throughout the amorphous layer. Typical annealing conditions were 485'C for 24 hours at 10-7Torr, resulting in a-Si layers 1.Lltm thick with Au concentrations -0.2at.%. Ion-beam-induced crystallization experiments were then carried out in the temperature range of 250-420 0 C using 2.5 MeV Ar ion bombardment. Irradiations were made through 1mm diameter apertures at doses of 2x10 16 - 4x10 17 ions/cm 2 and at low dose rates (1.4x1013 - 1.4x1014 ions/cm 2sec) to avoid beam heating. The a-Si thickness and Au depth profiles were measured using the Rutherford Backscattering and channeling techniques with 2 MeV 4 He ions. The samples were also analyzed using transmission electron microscopy (TEM). RESULTS AND DISCUSSION Figure 1 shows Rutherford backscattering and channeling spectra of a sample2 17 before and after irradiation at 320'C with 2.5 MeV Ar at a dose of 2x10 ions/cm and dose rate of 7x10 13 ions/cm 2 sec. The Au profile shows classic segregation and zone-refining with the moving interface. The Au depth profiles for 250 and 320 0 C irradiations as a function of dose are plotted in Figure 2. The dose rate was constant at 7x10 13 ions/cm 2 sec yielding an interface velocity of -3A/sec. This is the first time that such zone-refined profiles have been seen in the solid phase. They display the classic features of the segregation process. But there are, however, several quite novel features. First, it is remarkable that Au is trapped in c-Si at concentrations at least ten orders of magnitude greater than the equilibrium solubility in c-Si 9). Transmission electron microscopy shows this c-Si to be defect free without evidence of Au precipitation. However Rutherford backscattering and ch
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