UC Berkeley

Diffusion is the most fundamental mass transport process in solids characterized by point defect-diffusing atom interactions. In order to predict diffusion processes of impurities in a solid, the diffusion mechanisms, i.e., interactions between point defects and diffusing species, must be well understood. While the diffusion parameters and mechanisms are well known in silicon, very limited knowledge exists for diffusion parameters and mechanisms for Ge and SiGe alloys. As Ge and SiGe alloys are introduced in the new generations of microelectronic devices, the diffusion behavior in these materials must be studied.

The simultaneous diffusion of As, Si, and Ge in a Si0.95Ge0.05 alloy has been studied using a structure with an isotopically enriched SiGe layer. The diffusion in both intrinsic and extrinsic conditions was carried out between 900 to 1180°C. From the numerical fitting of the diffusion profiles, the diffusion mechanisms are determined. The simultaneous As and self-diffusion have been successfully modeled as a combination of the vacancy diffusion mechanism with doubly negatively charged vacancies, the interstitial-assisted mechanism with neutral self-interstitials, and a As2V clustering process. The diffusion mechanisms are the same as those for As in pure Si except for the clustering process, which is common for donors in pure Ge. The effective equilibrium diffusion pre-exponential factor for As in Si0.95Ge0.05 is determined to be 129 cm2/s while the diffusion activation enthalpy is 4.27 eV. The diffusion parameters for As in this alloy composition are very close to those in pure Si. Future experiments to study As diffusion in different alloy compositions are proposed. In this way, a transition from the As-in-Si-like diffusion mechanism to As-in-Ge-like diffusion mechanism can be identified.

Proton irradiation enhanced B diffusion in Ge has also been studied. The proton irradiation introduces excess self-interstitials which are virtually non-existing under equilibrium conditions. A molecular beam epitaxially grown structure with six B-doped layers was used for the radiation enhanced diffusion studies. We found B diffusion is enhanced by many orders of magnitude under this non-equilibrium condition. The effective B diffusion enthalpy under 2.5 MeV proton irradiation with 1.5 µA beam current is found to be 0.48 eV for temperatures from 400 to 500°C. This effective enthalpy is much lower than the 4.65 eV found under equilibrium conditions. From the radiation enhanced B and self-diffusion experiments, we conclude that the interstitial-mediated diffusion mechanism is dominant under the proton irradiation condition. We can also conclude that B diffusion is indeed driven by self-interstitials under equilibrium conditions. We further propose some future experiments to help identify the exact B diffusion mechanism(s) and the charge states of the B-defect pairs in Ge.