Insights for void formation in ion-implanted Ge

a r t i c l e i n f o The formation of voids in ion-implanted Ge was studied as a function of ion implantation energy and dose. (001) Ge substrates were self-implanted at energies of 20–300 keV to doses of 1.0 × 10 13 –1.0 × 10 17 cm − 2. Transmission electron microscopy revealed clusters of voids just below the surface for implant energies ≤ 120 keV at a dose of 2.0 × 10 15 cm − 2 and complete surface coverage for an implant energy of 130 keV and doses ≥ 1.0 × 10 16 cm − 2. Void clusters did not change in size or density after isothermal annealing at 330 °C for 176 min. The initial void formation is discussed in terms of the vacancy clustering and " microexplosion " theories with a damage map detailing the implant conditions necessary to produce voids. There is renewed interest in Ge as an alternative channel material in complementary metal-oxide-semiconductor devices due to its higher free carrier mobility and dopant activation compared to Si. However, the evolution of damage in ion-implanted Ge as a function of implantation conditions remains poorly understood. It is known that for a critical dose, Ge undergoes a crystalline (c-Ge) to amorphous (α-Ge) phase transition [1], and at significantly higher doses exhibits voiding within the α-Ge layer forming a porous structure with surface cavitation [2–6]. However, the threshold ion implanta-tion conditions for void formation remain basically unknown. Over the past 30 years, there has been much debate as to what mechanism governs the formation of the porous structure in ion-implanted Ge. Currently, there are two main theories of void formation for Ge: vacancy clustering and so-called " microexplosions ". The vacancy clustering theory invokes the inefficient recombination of Ge point defects during ion-implantation [7], where once a critical point defect population is created by ion-implantation, excess vacancies cluster into pores [3,8–13] in order to minimize the dangling bond density. In contrast, the microexplosion theory is based on the creation of voids through pressure waves and thermal spikes caused by the overlap of ion cascades [6,14–16]. In principle, it is possible to determine which theory better models void formation by selecting appropriate implant conditions and observing the resulting microstructure after implantation. If vacancy clustering is the governing mechanism, then varying depth and concentration of the vacancy profile should have an effect on the size …