Interstitial-Mediated Diffusion in Germanium under Proton Irradiation

We report experiments on the impact of 2.5 MeV proton irradiation on self-diffusion and dopant diffusion in germanium (Ge). Self-diffusion under irradiation reveals an unusual depth independent broadening of the Ge isotope multilayer structure. This behavior and the observed enhanced diffusion of $B$ and retarded diffusion of $P$ demonstrates that an interstitial-mediated diffusion process dominates in Ge under irradiation. This fundamental finding opens up unique ways to suppress vacancy-mediated diffusion in Ge and to solve the donor deactivation problem that hinders the fabrication of Ge-based nanoelectronic devices.

Figure 1

Concentration profiles of ${}^{74}\mathrm{Ge}$ measured with TOF-SIMS before (as-grown: thin dashed line) and after annealing (symbols) at the temperature and time indicated. Only every 6th data point is shown for clarity. The ${}^{74}\mathrm{Ge}$ profile from the covered part of the sample (blue squares) represents Ge diffusion under equilibrium condition. The corresponding black solid line shows the expected Ge diffusion profile for self-diffusion under equilibrium conditions at $600°\mathrm{C}$ taking into account published self-diffusion data [34]. The ${}^{74}\mathrm{Ge}$ profile obtained from the proton irradiated area of the sample (red circles) indicates an enhanced Ge diffusion under irradiation with respect to equilibrium conditions. The corresponding black solid line shows the numerical simulation of self-diffusion under irradiation assuming different boundary conditions for vacancies and self-interstitials (see text for details). The corresponding concentration profiles of vacancies and self-interstitials normalized to the respective equilibrium concentration (see right $y$ axis) are displayed by the lower and upper green dashed lines, respectively.

Figure 2

Concentration profiles of $B$ in Ge measured with TOF-SIMS after diffusion annealing at the temperature and time indicated. For clarity only every 6th data point is shown. The $B$ profile from the covered part of the sample (blue squares) represents $B$ diffusion under equilibrium conditions. This profile equals the as-grown profile because $B$ diffusion in thermal equilibrium is very slow [4]. The $B$ profile obtained from the proton irradiated area of the sample (red circles) indicates an enhanced $B$ diffusion under irradiation with respect to equilibrium conditions. The black solid line shows a simulation of $B$ diffusion based on Fick’s second law with a concentration independent effective diffusion coefficient of $7.5\times {10}^{-16}{\mathrm{cm}}^2{\mathrm{s}}^{-1}$ which exceeds the equilibrium diffusion of $B$ by several orders of magnitude.

Figure 3

Concentration profiles of $P$-implanted Ge measured with TOF-SIMS before (thin black dashed line) and after diffusion annealing (symbols) at the temperature and time indicated. Only every 6th data point is shown for clarity. The $P$ profile from the covered part of the sample (blue squares) represents $P$ diffusion under equilibrium conditions. The corresponding black solid line shows the expected $P$ profile for diffusion under equilibrium conditions at $600°\mathrm{C}$ taking into account published data [7]. The $P$ profile obtained from the proton irradiated area of the sample (red circles) indicates a retarded $P$ diffusion under irradiation with respect to equilibrium conditions. The corresponding black solid line is a numerical simulation of the concentration dependent $P$ diffusion that reveals a factor of 20 lower effective diffusion coefficient of $P$ under irradiation compared to equilibrium conditions. Note, the 10 nm thick ${\mathrm{SiO}}_2$ cap layer was not removed for the SIMS analysis and considered in the simulations of $P$ diffusion by an offset in the profile. The high $P$ concentrations in the peak region of the implanted $P$ profiles likely reflect the formation of $P$ clusters.