Diffusion in Germanium and Silicon-Germanium Alloys Eugene E. Haller, University of California-Berkeley, DMR Background: Diffusion is the most fundamental mass transport process in solids. In order to predict diffusion processes of impurities in a solid, the diffusion mechanisms must be well understood. While the diffusion parameters and mechanisms are well known in Si, very limited knowledge exists for Ge and Si-Ge alloys. As Ge and Si-Ge alloys are introduced in the new generations of microelectronic devices, the diffusion behavior in these materials must be understood in greater detail. Si substrate Above: Concentration of B as a function of depth as measured by secondary ion mass spectrometry for the as-grown structure and samples irradiated at a temperature ranging from 400 to 748°C. Except for the one labeled 3 hr, all irradiated samples were annealed for 1 hour. The 748°C sample was irradiated with 3.2 μA of a 2.5 MeV proton beam whereas others were irradiated with 1.5 μA. Clearly, the diffusion broadenings are observed for all samples annealed under proton irradiation. This indicates orders of magnitude enhanced diffusion relative to the equilibrium condition. Cap layer 100 nm natural Si layer Approach: We have investigated proton- irradiation-enhanced B diffusion in Ge. Proton irradiation is performed in order to introduce excess self-interstitials which are virtually non- existent under equilibrium conditions in Ge. For this study, we used a molecular beam epitaxially grown structure with six thin B-doped layers. We found that B diffusion is enhanced by many orders of magnitude under this non-equilibrium condition. Furthermore, we established that the interstitial-mediated diffusion mechanism is dominant under the proton irradiation condition.

Technological Outputs: Ge has the potential to replace Si in future generations of microelectronic devices because of its higher electron and hole mobilities. As in the case of Si, successful implementation of Ge-based devices demands the precise control of dopant distributions during fabrication and any subsequent thermal processing steps. This precise engineering control requires a thorough understanding of the atomic diffusion mechanisms in Ge. The determination of the diffusion mechanisms through the diffusion work presented here will enable optimal processing of high speed devices down to the nanometer scale. Education: Christopher Y. Liao, who completed his Ph.D. last month( 7/2010), performed the diffusion research in Ge and Si-Ge. The skills Chris developed during his time as a student at Berkeley allowed him to find a challenging position in a solar energy conversion company. International Collaboration: This investigation has been performed in collaboration with the group of Prof. H. Bracht in Münster, Germany, and the group of Prof. A. Nylandsted Larsen at the University of Aarhus, Denmark. Development of Instrumentation: C. Y. Liao, in collaboration with scientists at the Lawrence Berkeley National Laboratory (LBNL), has designed and built a vacuum heater (see picture above) which will be attached to a tandem proton accelerator. This will allow us to design and perform the diffusion studies under proton irradiation at Berkeley. Diffusion in Germanium and Silicon-Germanium Alloys Eugene E. Haller, University of California-Berkeley, DMR