Origin of the Puzzling Diffusion Behaviors of Cu and Ag in Semiconductors Are Unraveled
Update: 2017-03-02 15:11:40      Author: yangjuan@csrc.ac.cn

Atomic diffusion is one of the critical factors in determining the performance and stability of semiconductor electronic and optoelectronic devices. Group IB elements Cu and Ag are important contact materials in semiconductor devices due to their low resistivity and high resistance to electromigration damage. However, they have very different diffusion behaviors in semiconductors: Cu diffuses much faster than Ag in covalent semiconductors like Si and GaAs, but Ag diffuses faster than Cu in more ionic II-VI semiconductors such as CdS and CdTe despite Ag has larger size than Cu. These puzzling behaviors have not only scientific interest but also technology applications because they hindered some practical application of Cu and Ag in semiconductor devices. For example, because of challenges related to the high diffusivity of Cu in Si, one has to use Ag as contact materials in Si solar cell despite Ag is more expensive. On the other hand, because Ag diffuses faster than Cu in more ionic CdTe, replacing Cu by Ag has failed because the fasr diffusion of Ag introduce instability despite Ag has better doping properties than Cu in CdTe. Despite the importance of Cu and Ag in semiconductor technology, the mechanisms behind their distinct diffusion behaviors in covalent and ionic semiconductors have not been addressed.

Working with Hui-Xiong Deng, Jun-Wei Luo, and Shu-Shen Li of the Institute of Semiconductors, CAS, Prof. Su-Huai Wei at the Beijing Computational Science Research Center have unraveled the underlying mechanisms of the puzzling diffusion behavior of Cu and Ag in semiconductors by combining the first-principles calculations and group theory analysis. They show that the important roles of the Coulomb energy, strain energy, and most importantly crystal symmetry enforced s-d coupling plays important roles in determining the diffusion behaviors of Cu and Ag in the covalent and ionic semiconductors. The s-d coupling is absent in pure covalent semiconductors but increases with the ionicity of the zinc-blende semiconductors, and the coupling strength of Cu, owing to its higher d orbital energy, is much larger than Ag. In conjunction with Coulomb interaction and strain energy, the s-d coupling is able to explain all the diffusion behaviors of Cu and Ag in covalent to ionic semiconductors. The calculated results and explanation, therefore, provide deep understanding on the important diffusion behavior of impurity in semiconductors and enables us to engineering doping in semiconductors through diffusion, e.g., by designing alternative diffusion barriers for Cu and Ag or choosing different diffusers.

For more information, please see the paper: “Origin of the distinct diffusion behaviors of Cu and Ag in covalent and ionic semiconductors”, H.-X. Deng, J.-W. Luo*, S.-S. Li, and S.-H. Wei*, Phys. Rev. Lett. 117, 165901 (2016), (Pub. 11 Oct. 2016). DOI: 10.1103/PhysRevLett.117.165901.

This research was supported by Development Fund of China Academy of Engineering Physics and NSFC.

 

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Fig. 1. The diffusion energy curves of Cu, Cu+, Ag, and Ag+ in the group IV elemental compounds, Si and Ge; group III-V compounds, GaAs and GaSb; and group II-VI compounds, CdS and CdSe. The Ta’ and Tc’ sites for Cu and Ag diffusion are slightly different.

 

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Fig. 2. The diffusion pathways of Cu in (a) Si and (b) CdS semiconductors. The red dashed lines indicate the [111] (or equivalent directions). In Si, the Cu diffusion path is exactly along the [111] (or equivalent directions). However, in CdS, it deviates from the [111] direction (or equivalent directions). We label the new sites near the Ta and Tc as the Ta’ and Tc’, respectively.


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