Ga-implantation in Ge: Electrical activation and clustering

The electrical activation and clustering of Ga implanted in crystalline Ge was investigated in the (0.3-1.2)x1021 Ga/cm3 concentration range. To this aim, Ge samples implanted with 50 keV gallium, and annealed at several temperatures up to 650 deg. C, have been subjected to a detailed structural and electrical characterization. The substrate was maintained at 77 K during implantation to avoid the formation of the honeycomb structure that occurs during implantation at room temperature of heavy ions at high fluence. Secondary ion mass spectrometry analyses indicated a negligible Ga diffusion and dopant loss during the thermal annealing. The carrier concentration in the recrystallized samples measured by Hall effect showed a maximum concentration of active Ga of ∼6.6x1020 Ga/cm3. A remarkable Ga deactivation occurred with increasing the annealing temperature from 450 to 650 deg. C although the sheet resistance did not change considerably in this temperature range. It turned out that the carrier concentration reduction is balanced by the enhancement of the hole mobility that exhibits a steep variation with the concentration of the ionized scattering centers in this range. A simple model is proposed to explain the experimental results taking into account the thermally activated Ga clustering. These studies, besides clarifying the mechanism of Ga deactivation in Ge, can be helpful for the realization of future generation devices based on Ge.