Atomic transport in three-dimensional nanostructures of silicon and germanium
Experimental Condensed Matter Physics
Final Report Abstract
In the course of this DFG project the preparation of silicon and germanium nanostructures applying different lithography methods in conjunction with cryogenic deep reactive ion etching was investigated. Whereas nanostructures of silicon were prepared successfully even from isotopically modulated silicon wafers, similar structures of germanium could not be realized due to severe difficulties in germanium dry etching. Thus, only studies on atomic transport in silicon nanostructures could be performed. We studied the self-diffusion of silicon in nanopillars down to 70 nm in diameter by means of atom-probe tomography. Surprisingly, we did not observe any significant difference between self-diffusion in nanopillars compared to bulk samples. This reveals that the properties of native point defects, i.e., of vacancies and self-interstitials are not significantly different in bulk and confined silicon samples with diameters down to 70 nm. Based on this result we decided not to study self-diffusion under different experimental conditions realized by e.g., oxidation/nitridation as it is to be expected that the self-diffusion in nanostructures is not altered compared to bulk samples by these surface treatments. This expectation was later confirmed by Kiga et al. (2020), who studied self-diffusion under oxidation in silicon nanopillars and bulk samples. Instead, we decided to focus on dopant diffusion in silicon nanostructures. An important prerequisite for such studies is to characterize the electronic doping level of the structures on the nanoscale. In this context we utilized electrochemical voltage (ECV) profiling to characterize the carrier concentration profile of an array of nanopillars, where the area between the nanopillars was filled with photoresist. We found that ECV profiling could not provide reproducible results and decided to focus on scanning spreading resistance microscopy (SSRM) as possible technique to determine the distribution of electrically active dopants in silicon nanopillars. We tested this technique on nanopillars with well-known dopant profiles and could identify by means of extensive numerical simulations of the SSRM data, the impact of electronic surface/interface states and preparation-induced defects on the SSRM measurements. COMSOL Multiphysics has been used as finite element solver for SSRM simulations. We recognized that a quantitative characterization of the active dopant distribution in nanostructures requires not only suitable calibration samples and a sophisticated sample preparation but also a combined analysis that involves SSRM measurements and COMSOL simulations. After we successfully demonstrated the applicability of SSRM for quantitative measurements of the active defect concentration in silicon nanostructures, we studied the diffusion of dopants, i.e., boron and phosphorous, in undoped silicon nanopillars prepared on top of heavily doped silicon wafers. Surprisingly, we observed a retarded diffusion of both boron and phosphorous in nanopillars compared to the diffusion of these dopants in bulk silicon. This result is described by an injection of vacancies from the pillar shell into the pillar. This presumably stress-induced vacancy injection is not at variance to our finding that self-diffusion in silicon pillars equals that in bulk samples because self-diffusion in silicon is mediated by both vacancies and self-interstitials to nearly the same extend whereas boron and phosphorous diffusion is mainly mediated by self-interstitials. An injection of vacancies from the pillar shell causes a supersaturation of vacancies in the pillar structure and thereby an undersaturation of self-interstitials compared to thermal equilibrium conditions. Accordingly, boron and phosphorous diffusion is expected to be retarded in the pillar whereas self-diffusion remains unaffected compared to thermal equilibrium conditions because an enhanced contribution of vacancies to self-diffusion is balanced by a retarded contribution of self-interstitials. In summary, in the course of the project we not only gained valuable information about self- and dopant diffusion in Si nanostructures but also have further developed the SSRM technique as a tool to quantify the concentration and distribution of active dopants in silicon nanostructures. This technique in conjunction with numerical simulations of SSRM bears the potential to characterize the carrier distribution also in other semiconductor materials with nanoscopic resolution.
Publications
- Highly ordered silicon nanopillar by nanoparticle lithography, Micro-Nano-Integration, 6. GMM Workshop, 1-5 (2016)
G. Hamdana, M. Bertke, T. Südkamp, H. Bracht, H. S. Wasisto, and E. Peiner
- Large-area fabrication of silicon nanostructures by templated nanoparticle arrays, Proc. SPIE 10248, Nanotechnology VIII, 1024808 (2017)
G. Hamdana, M. Bertke, T. Südkamp, H. Bracht, H. S. Wasisto, and E. Peiner
(See online at https://doi.org/10.1117/12.2264995) - Towards fabrication of 3D isotopically modulated vertical silicon nanowires in selective areas by nanosphere lithography, Microelectron. Eng. 179, 74-82 (2017)
G. Hamdana, T. Südkamp, M. Descoins, D. Mangelinck, L. Caccamo, M. Bertke, H. S. Wasisto, H. Bracht, and E. Peiner
(See online at https://doi.org/10.1016/j.mee.2017.04.030) - Self-diffusion in single crystalline silicon nanowires, J. Appl. Phys. 123, 161515 (2018)
T. Südkamp, G. Hamdana, M. Descoins, D. Mangelinck, H. S. Wasisto, E. Peiner, and H. Bracht
(See online at https://doi.org/10.1063/1.4996987) - Quantitative scanning spreading resistance microscopy on n-type dopant diffusion profiles in germanium and the origin of dopant deactivation, J. Appl. Phys. 125, 085105 (2019)
J. K. Prüßing, G. Hamdana, D. Bougeard, E. Peiner, and H. Bracht
(See online at https://doi.org/10.1063/1.5066617) - Defect distribution in boron doped silicon nanostructures characterized by means of scanning spreading resistance microscopy, J. Appl. Phys. 127, 055703 (2020)
J. K. Prüßing, T. Böckendorf, G. Hamdana, E. Peiner, and H. Bracht
(See online at https://doi.org/10.1063/1.5134558) - Retarded boron and phosphorus diffusion in silicon nanopillars due to stress induced vacancy injection, J. Appl. Phys. 131, 075702 (2022)
J. K. Prüßing, T. Böckendorf, F. Kipke, J. Xu, P. Puranto, J. L. Hansen, D. Bougeard, E. Peiner, and H. Bracht
(See online at https://doi.org/10.1063/5.0078006)
