11 February 2013
Dopant diffusion depends on the point defects in the substrate material. Point defects are created by moving lattice atoms from their lattice positions of the substrate material. Since point defects can be charged, the density of point defects depends on the Fermi level of the substrate, i.e. the electron or hole density. The mechanism of dopant atom diffusion in Germanium (Ge) is forming pairs with point defects that move through the crystal. Some dopants prefer vacancies, and others prefer interstitials, and experiments have revealed higher vacancy than interstitial levels in germanium. Our simulation and experiments have shown that doubly negatively charged vacancies assist Phosphorous (P) and Arsenic (As) diffusion, whereas vacancies of other charge states do not.
Boron (B) is a common dopant for the source and drain doping of p-MOSFETs. Experiments have shown very slow boron diffusion in germanium, which is advantageous for source/drain regions of a p-MOSFET transistor. To scale MOSFETs, extremely shallow source and drain doping profiles are desired. The slow diffusion nature of boron in germanium renders it a good dopant to form ultra-shallow p-n junctions for p-MOSFETs.
Silicon and germanium are group IV elements, and are very resistive. To render them conductive, group III and V elements are used as dopants in Si and Ge. Group V elements (P and As) have five valence electrons whereas group IV elements only have four. When an As atom replaces a Si or Ge atom in the native lattice, one of its valence electrons will be free to move and contribute the electric current conduction.
When phosphorous, arsenic, and boron are implanted in the Si or Ge substrate, the dopant atoms (P, As, B) are not completely ionised. If the dopants are not ionised they can’t provide the semiconductor with free carriers enabling electrical conduction. The activation level describes the maximum concentration of ionised dopant in the substrate. It is a material property depending on both the dopant and the substrate. For source and drain of a MOSFET transistor, the lowest resistance possible is desirable.
In our experiment, high boron background doping provides a lot of holes in germanium. The holes can suppress the electron generation and the doubly negatively charged vacancies. This allows us to analyse the neutral vacancy assisted arsenic diffusion. The knowledge of the nature of dopant diffusion allows for better optimisation of Ge-based devices. Once we understand the diffusion mechanism of n-type dopants in germanium, our work could help the diffusion process and help process integration in Ge CMOS fabrication. Arsenic alone experiences appreciable diffusion in Ge, and our results show that if it is compensated by a boron background doping it will hardly diffuse at all.
A fascinating experiment is to diffuse n-type dopants in Ge nanowires. In a nanowire, only one-dimensional diffusion occurs for any dopant. The diffusion profile is no longer a superposition of point diffusion sources like in the bulk materials. Therefore, reduced lateral diffusion profiles should be observed in a nanowire compared with the bulk material. If this is true, a shallow p-n junction can be achieved naturally in nanowire MOSFETs. Nanowire MOSFETs are considered to be the ultimate MOSFET architecture due to their unsurpassed scaling capability.
As silicon is approaching its scaling limit, new materials are required to boost the performance of CMOS integrated circuits. So far the highest hole mobility is measured in bulk Ge, rendering Ge a very promising candidate material for p-MOSFETs. In the case of an n-MOSFET transistor, the present consensus is to use III-V materials such as GaAs. The challenge for CMOS technology is therefore to integrate both types of transistors on a common substrate (most likely Si). In the next few years, the emerging materials and devices will be tested and qualified at nanoscale dimensions. This will put the semiconductor industry in an informed position to zero in on the optimum material selection for future devices in the post-silicon era.
This article is based on the Letter: Arsenic diffusion in boron doped germanium (new window).
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