Diffusion of Gold and Platinum in Amorphous Silicon

By means of radiotracer experiments the diffusion of Au and Pt in radio-frequency-sputtered amorphous silicon (a-Si) was investigated. Specimens of a-Si with homogeneous doping concentrations of Au or Pt in the range 0$\, - \,$1,7~at.\% were produced by co-sputtering of Si and Au or Pt, respectively. An additional tiny concentration of radioactive $^{195}$Au or $^{188}$Pt, about 10~at.ppm, was implanted at ISOLDE. The resulting Gaussian distribution of the implanted atoms served as a probe for measuring diffusion coefficients at various doping concentrations. It was found that for a given doping concentration the diffusion coefficients show Arrhenius-type temperature dependences, where the diffusion enthalpy and the pre-exponential factor depend on the doping concentration. From these results it was concluded that in a-Si Au and Pt undergo direct, interstitial-like diffusion that is retarded by temporary trapping of the radiotracer atoms at vacancy-type defects with different binding enthalpies. In the case of Au in a-Si, the influence of thermal-annealing-induced structural relaxations on the diffusivity has been investigated. It was found that this behaviour arises from an agglomeration of vacancies which results in trap deepening. To check this idea, further experiments were done in which some specimens with different doping concentrations were exposed to particle irradiation during diffusion annealing. These specimens were implanted with $^{195}$Au at ISOLDE and subsequently diffusion-annealed under 1~MeV-N$^+$ irradiation at the pelletron-type linear accelerator of the Max-Planck-Institut für Metallforschung in Stuttgart, Germany. It was found that high implantation doses in combination with great diffusion lengths give rise to complicated shapes of the diffusion profiles. These were shown to be due to diffusion retardation on the vacancy-rich side and diffusion enhancement combined with localized diffusion retardation on the self-interstitial-rich side of the $^{195}$Au implantation peak.