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Electronic and Optical Properties of Spinel TCOs : SnZn 2 O 4 , SnCd 2 O 4 , and CdIn 2 O 4
| Content Provider | Semantic Scholar |
|---|---|
| Author | Segev, David |
| Copyright Year | 2005 |
| Abstract | Using the band-structure method, we have studied the electronic and optical properties of the transparent conducting oxides SnZn2O4, SnCd2O4, and CdIn2O4. We analyzed the atomic and orbital characters of the band edge states and explained the general trends observed in the fundamental band gap, the optical band gap, the energy difference between the first and the second conduction bands, and the electron effective mass. General rules for designing more efficient transparent conducting oxides are proposed. 1. Objectives SnZn2O4, SnCd2O4, and CdIn2O4 are ternary compounds that can exist in the spinel structure. They have emerged as promising transparent conducting oxides (TCOs), which are transparent and at the same time conductive, thus are suitable for solar cell applications. However, despite many recent studies of these compounds, many of their physical properties are still unknown. Their band structure and optical properties are not well established, nor are the reasons for their combined transparency and conductivity. 2. Technical Approach Using first-principles band structure and total energy methods as implemented in the VASP code, we studied the structural, electronic, and optical properties of the three compounds in normal and inverse spinel structures [1]. In particularly, we investigated the wavefunction characters of the band edge states and the relationship between the crystal structures and the combined transparency and n-type conductivity in these compounds. 3. Results and Accomplishments 3.1 Crystal Structure In the “normal” spinel oxide AB2O4 with Oh symmetry, 1/8 of the tetrahedral voids in a face-centered-cubic (fcc) close-packed oxygen sublattice are occupied by A atoms and 1/2 of the octahedral voids are occupied by B atoms. There exists also an “inverse” spinel structure, where the tetrahedral sites are occupied by B atoms and octahedral sites are occupied randomly by an equal number of A and B atoms. From the total energy calculations (Table I), we find that SnZn2O4 and SnCd2O4 are more stable in the inverse spinel structure, whereas CdIn2O4 is more stable in the normal spinel structure. The relative stability between the normal and the inverse structures can be explained by the Coulomb interaction, and by the tendency for Zn to form four-fold covalent bond [2]. 3.2 Band Structure A high-performance n-type TCO should simultaneously satisfy two requirements: (i) large optical band gap as well as energy separation between the CBM and the second conduction band (SCB), for transparency; (ii) a low CBM with respect to the vacuum level, and a small effective mass, for high dopability and good conductivity. As a consequence of the second condition, a low VBM, which is a common characteristic of oxides, is also required. For the three compounds in the spinel structure, we find that (a) the G12v valence band maximum (VBM) state consists mostly of O p and cation d states of the octahedral site, with some d character also from the cation at the tetrahedral site. (b) The G1c conduction band minimum (CBM) state consists predominantly of O s and cation s states of the tetrahedral site, with some s character from the cation at the octahedral site. (c) The SCB state has the G2'c representation, and consists mostly of O s and cation s states of the tetrahedral site only. (d) The G15v state, which determines the optical band gap (see below), consists of mostly O p and cation d states of the tetrahedral site. With this analysis, we can now explain the changes in the band gap and the splitting between the first two conduction bands, E12 = ESCB ECBM, as a function of the crystal structures. The results for Eg and E12 are listed in Table I. We find that when SnZn2O4 changes from the normal to the inverse spinel structure, i.e., when Sn and half of the Zn change sites, both the band gap and E12 increases. This is because when Sn moves to the octahedral site, the VBM energy decreases, due to the lower Sn 4d orbital energy compared to that of the Zn 3d orbital, and, hence, the reduced p-d repulsion at the octahedral site. Moreover, since Zn has a much higher 4s orbital energy than the Sn 5s orbital, the |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | https://digital.library.unt.edu/ark:/67531/metadc780240/m2/1/high_res_d/860496.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
