Rapid Ge Diffusion Along Si/SiO2 Interfaces During High Temperature Oxidation for Quantum-Scale Structures

Abstract

Abstract Body: There is continued interest in the combination of Si and Ge in unique geometries to further improve the performance of transistors [1]. A novel diffusion mechanism of Ge along an oxidizing Si/SiO2 interface was reported, which resulted in the formation of strained Si nanowires [2]. By taking advantage of this diffusion mechanism, there is the potential of forming site-controlled nanowires, quantum dots, and arbitrary nanoshapes. However, there is limited understanding of the diffusion mechanism that results in the formation of these nanostructures. To investigate this mechanism, alternating 15nm thick layers of Si and Si0.7Ge0.3were grown in a superlattice and patterned into fins via electron beam lithography. The final pattern contained multi-layered Si/SiGe fins with widths varying from 70 to 280nm with a 100nm layer of Si substrate exposed at the base. This extended Si surface enabled the observation Ge’s diffusion down the fin’s sidewall during high temperature oxidation. Ge’s lateral diffusion was observed over a range of times at temperatures between 800 and 950°C. Cross-sectional TEM samples were prepared after each anneal and the diffusion length was measured through analysis of HAADF-STEM images, utilizing z-contrast. The diffusion length was measured after four different oxidation times at each temperature and the diffusivity was extracted. An Arrhenius plot of the data indicated the activation energy of Ge diffusion down the oxidizing interface is 2.50 +/- 0.3 eV. This is approximately half of activation enthalpy for Ge’s movement into bulk Si or through the oxide [3], [4]. We propose that the Ge may be diffusing through the suboxide along the Si/SiO2 interface and is subsequently reinserted back into the Si during oxidation. This is further supported by the linear correlation between the thickness of the thermally grown oxide and the distance the Ge diffuses down the Si sidewall. The difference in the activation energy compared to bulk interdiffusion helps to explain why the nanowires and quantum dots are formed within these materials during oxidation. In addition to dry oxidation anneals, anneals in a wet oxidizing ambient (steam) will be discussed. A model for the diffusion using the Florida object-oriented process and device simulator (FLOOXS) was developed. This model was used to assist in the extraction of diffusivities and is being further developed to help predict possible structures using this phenomenon. Preliminary results using the process simulator’s model will be compared to the experimentally observed evolution of the nanostructures. This work is supported by Sandia National Laboratories, which is managed and operated by NTESS under DOE NNSA contract DE-NA0003525. [1] T. David et al., “New strategies for producing defect free SiGe strained nanolayers,” Sci. Rep., vol. 8, no. 1, pp. 1–10, Feb. 2018, doi: 10.1038/s41598-018-21299-9. [2] W. M. Brewer, Y. Xin, C. Hatem, D. Diercks, V. Q. Truong, and K. S. Jones, “Lateral Ge Diffusion During Oxidation of Si/SiGe Fins,” Nano Lett., vol. 17, no. 4, pp. 2159–2164, Apr. 2017, doi: 10.1021/acs.nanolett.6b04407. [3] R. Kube et al., “Composition dependence of Si and Ge diffusion in relaxed Si1−xGex alloys,” J. Appl. Phys., vol. 107, no. 7, p. 073520, Apr. 2010, doi: 10.1063/1.3380853. [4] Y. Dong, Y. Lin, S. Li, S. McCoy, and G. Xia, “A unified interdiffusivity model and model verification for tensile and relaxed SiGe interdiffusion over the full germanium content range,” J. Appl. Phys., vol. 111, no. 4, p. 044909, Feb. 2012, doi: 10.1063/1.3687923.