Diffusion of boron and silicon in germanium

Abstract

This study used, for the first time, a combination of ion implantation and Secondary Ion Mass Spectroscopy (SIMS) to dope and characterise diSusion proSles of B and Si in Ge. By comparison of concentration proEles of B implanted in diSerent orientation in crystalline and pre-amorphised Ge targets, it has been established that the channelling phenomenon plays an important role. For diffusion studies, B was introduced in Ge to different doses and en- ergies. Subsequently the specimens were subjected to various thermal budgets using furnace annealing in the temperature range 80D-900 °C, either with a surface protection layer or in evacuated ampoules. Annealed boron profiles showed an immobile concentration peak and a diffusion tail. Diffusion coe & cients were derived by computer simulation using concentration prohles of aa implanted and annealed samples. The measured diffusivity, around two orders of magnitude lower than reported previously using electrical techniques, was consistent with the variation in implantation dose, annealing ambient and protective layer. Using B doped epitaxial Ge layers grown by Molecular Beam Epitaxy, it was further conSrmed that im- plantation induced defects are not responsible for slow B diSusion. An activation energy of 4.6(^:0.3) eV and a pre-exponential factor of 1.2x10^ cm^/s can be assigned for B diEusion in Ge in the temperature range studied. The results cast doubt over the prevailing belief of a vacancy diffusion mechanism for B diffusion in Ge. Additionally, the solid solubility of B in Ge at 875 °C has been estimated to be ^2x10^^ atoms/cm^. For Si diffusion in Ge, studied in the temperature range 750-875 °C using furnace an- nealing, an activation energy of 3.2 (±0.3) eV and a pre-exponential factor of 9.7cm^/s is assigned which agrees well with the values from the literature. The anomalous surface peak observed in the annealed profiles is attributed to implantation induced defects. The simi- larity of the activation energy of Si didusion in Ge and Ge self-diffusion is suggestive of a similar diffusion mechanism for both the processes. It is well known that boron shows a diffusion behaviour which is opposite to all other dopants as well as to Ge diffusion in Si-rich Si-Ge alloys. As an implication of the present study to B diSusion in Si-Ge alloys, it seems highly unlikely that a changeover of diffusion mechanism occurs as a function of Ge content in the alloy spectrum.

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