Intermediate Band (IB) Induced by Nitrogen Chemical Complexes in Silicon

Nitrogen hyperdoping has been proposed to generate intermediate band (IB) in silicon. This enables two photon absorption of low energy photons. The enhanced IR absorption adds photogenerated carriers to the carriers generated through photon absorption via valence band-conduction band transitions. In this work we include the influence of the ubiquitous oxygen in silicon on N hyperdoping. To evaluate that effect Density Functional Theory (DFT) calculations have been performed, where the exchange and correlation functional MGGA,[1] Pseudo-Dojo pseudopotential,[2] a moderately accurate basis set, and 7x7x7 k-points for sampling the Brillouin zone have been used. To reduce the defect perturbation of the Si supercell, the latter was chosen large (3x3x3 Si unit cells, that is 216 Si atoms), which led to extensive computation. This heavy computation was unavoidable in order to have adequate atomic system that matches the concentrations of N and O species in hyperdoped Cz silicon. The symmetry reduced the k-points, for instance to 172 in the case of the substitutional nitrogen (Ns). This makes one complex per 216 Si atoms, that is a N concentration of 2.3E20/cm3, which is within the hyperdoping concentration range. The DFT calculations have shown that high concentration of N and O impurities in silicon bonded to vacancies (V), or located in interstitial sites as well as clusters of such species significantly transform Si energy band structure.<br/>VN complexes in silicon are N atoms in substitutional sites; by occupying tetravalent vacancy sites, they are negatively charged (Ns-). The band structure has shown a higher density of bands due to degeneracy removal by the perturbation caused by Ns-. These results are analyzed in twelve VxNyOz complexes for odd numbers of N atoms (y=1,3,5,7). The negatively charged complexes drastically effect, and may totally control, the band structure of the IB material. The complexes with odd numbers of N atoms in silicon have been discussed as deep levels by Voronkov et al [3].<br/>The DFT calculations have shown also that added oxygen atom does not fundamentally change the energy band structure. This result has been verified for a wide range of N-related complexes detected by high resolution FTIR mapping along the depth of the specimen. We used bevel polished samples to enable the depth profiling of the N-O complexes. The samples are obtained from heat-treated N doped Cz and N doped FZ silicon wafers. The identification of N-O complexes combined with the DFT calculations demonstrated that O does neither hinder the hyperdoping of silicon with N nor the formation of an IB in silicon. The fact that annealed N free Cz Si has not produced IB, and never been reported to do so, is a sign of the oxygen ineptitude to making an intermediate band in silicon. We propose that the reason for the little influence of O on the band structure is the nature of the Si-O-Si bridging bonds, which are well relaxed.<br/>[1] K. Lejaeghere, G. Bihlmayer, T. Björkman, P. Blaha, S. Blügel, V. Blum, Reproducibility in density functional theory calculations of solids, Science 351 (6280), aad3000 (2016) doi: 10.1126/science.aad3000<br/>[2] http://www.pseudo-dojo.org/paperlist.html<br/>[3] G. I. Voronkova, A.V. Batunina, V.V. Voronkov, R. Falster, V.N. Golovina, A.S. Guliaeva, N. B. Tiurina, Electrical properties of nitrogen doped float-zoned silicon annealed in the range of 200 to 900C, Thin Solid Films, 2350 (2010).