Investigation of the origin of deep levels in CdTe doped with Bi

Abstract

Combining optical ͑low temperature photoluminescence͒, electrical ͑thermoelectric effect spectroscopy͒, and structural ͑synchrotron X-ray powder diffraction͒ methods, the defect structure of CdTe doped with Bi was studied in crystals with dopant concentration in the range of 10 17 -10 19 at./ cm 3 . The semi-insulating state observed in crystals with low Bi concentration is assigned to the formation of a shallow donor level and a deep donor recombination center. Studying the evolution of lattice parameter with temperature, we postulate that the deep center is formed by a Te-Te dimer and their formation is explained by a tetrahedral to octahedral distortion, due to the introduction of Bi in the CdTe lattice. We also shows that this model agrees with the electrical, optical, and transport charge properties of the samples.

Obtaining an intermediate band photovoltaic material through the Bi insertion in CdTe

“…The results of this simulation demonstrated that the Te-Te configurational equilibration position is far from a dimer formation in these conditions and instead a simple Bi cd substitution without structural distortion appears. This theoretical result is different from the prediction of Saucedo et al [9], but a tetrahedral to octahedral distortion is in fact what should happen in CdTe:Bi. Du [14] points out that Bi in CdTe located at the Cd position and interacting with oxygen substituting tellurium (Bi cd +0 Te ) adopts a C 3v symmetry that is coherent with a Jahn Teller distortion [15].…”

“…This result implies a high oxygen co-doping. However, these aforementioned experiments [8,9] are made restricting oxygen concentration range to the 10 14 -10 15 cm~3 level [16]. Therefore this cannot be the explanation.…”

“…Previous works have found that Bi spontaneously diffuses throughout the CdTe lattice [8,9] but as Bi is a big atom to be placed in the interstices, the diffusion coefficient of Bi in CdTe cannot be determined. The explanation of this phenomenon by Hage-Ali et al [17] is consistent with a complex defect formation in which V Ct ¡ and Bi, -should be addressed.…”

“…The structure and electronegativity of the atoms in CdTe permit the formation of a neither-ionic nor-covalent bond that yields a direct band-gap not far from the optimum band-gap for the conversion efficiency [7]. Previously, several experimental results demonstrated that CdTe doped with Bi can enhance, in some cases, the photon absorption of the incident solar radiation [8,9].…”

“…Different hypotheses have been proposed to explain this deep level in CdTe:Bi. Saucedo et al [9] states that Bi introduced into CdTe and in a Cd position (Bi C£ ¡) compensates the V cd and forms the semi-insulating CdTe when a tetrahedral to octahedral distortion is induced, and consequently Te atoms are displaced forming two Te-Te dimers around the Bi cd . We calculated, using Density Functional Theory (DFT), the formation energy of a model supercell containing two Te-Te dimers around Bicd [10].…”