Abstract
1. Introduction
Post-transition-metal oxides, such as In2O3 and SnO2, have been extensively used as transparent conducting oxides (TCOs), a unique class of materials that combine the high electrical conductivity and optical transmission in the visible or near-infrared spectral range, which are needed in various optoelectronic devices, such as solar cells, flat-panel displays, light emitting diodes (LEDs) and transparent transistors[
In contrast to the case of In2O3, the band gap problem of SnO2 is still not fully understood, largely because of the lack of detailed experimental measurements. Analogous to bixbyite In2O3, rutile SnO2 also has the inversion symmetry and the dipole-forbidden transitions between the band edge states. As a result, one may expect different fundamental and optical gaps of SnO2 as well, which, however, has never been reported in experiments. The single-photon and two-photon absorption experiments showed an optical gap of SnO2 at ~3.60 eV, which has been widely taken as its “band gap”[
Despite the absence of direct measurements on its fundamental gap, more insight can be obtained from the recent experiments where SnO2 was employed as the electron transport layer (ETL) in both dye-sensitized and perovskite solar cells to replace the commonly used TiO2[
Figure 1.(Color online) Band gaps and band alignments between rutile SnO2 and TiO2. In (a) the band gaps of SnO2 is taken from Ref. [
To address this issue, in this work, we revisit the band gap problem for the rutile SnO2 by using first-principles calculations. Different-level computational methods and functionals, such as PBE, PBE + G0W0, HSE06 and HSE06 + G0W0 are employed and the results are carefully compared. We find that the HSE06 calculations yield a VB offset of 0.38 eV between SnO2 and TiO2, generally obeying the common-anion rule. The fundamental gap is calculated to be 2.96 eV, which is much smaller than the previously reported value. The conduction band (CB) offset is then found to be 0.45 eV, in good agreement with experiments[
2. Theoretical calculation
Our calculations are carried out using the projector augmented wave (PAW) method[
3. Results and discussion
Fig. 2(a) illustrates the tetragonal rutile structure for both SnO2 and TiO2 with the space group
Figure 2.(Color online) (a) Primitive unit cells and (b) calculated total and partial density of states for rutile SnO2 and TiO2. The green, blue and purple balls represent Sn, Ti and O respectively. (c) Side view of the atomic configuration for SnO2/TiO2 heterojunction (top panel).
The first two terms on the right-hand side are the core-level to VBM energy separations for pure bulk TiO2 and SnO2, respectively, and the third term
The calculated band alignment for SnO2 and TiO2 is shown in Fig. 1(b). We find that the VBM of SnO2 is 0.38 eV lower in energy than that of TiO2, which generally follows the common-anion rule. The slightly lower VBM of SnO2 can be attributed to the more polar Sn4+ ions, compared to Ti4+ ions. The calculated fundamental gaps of SnO2 and TiO2 are 2.96 and 3.15 eV, respectively, with the latter slightly larger than the experimental value of 3.03 eV[
Figure 3.(Color online) (a) Band structure of SnO2 along two high symmetry lines M–Γ–Z. (b) Optical absorption coefficient of SnO2. The valence band maximum is set at zero. In (a), the red and green arrows represent the possible optical transitions at Γ-point and away from Γ-point, respectively.
Table Infomation Is Not EnableIt should be noted that the fundamental gap of SnO2 obtained here is quite different from the values reported by the previous calculations[
One may argue that the hybrid functional and/or GW approximation may have different impacts on the fundamental gaps of SnO2 and TiO2, because of the different component of their CBM states, as shown in Fig. 1(b). To clarify this point, we consider a series of metal oxides, including ZnO, CdO, Ga2O3, In2O3 and GeO2, for which the CBM and VBM components are all similar to that of SnO2. Table 3 gives the calculated fundamental gaps by the HSE06 + G0W0 method, together with the available experimental data. We find that although the HSE06 + G0W0 calculations can result in reasonable fundamental gaps for IIB oxides ZnO and CdO, which have large d orbital component at the VBM, it significantly overestimates the band gaps of IIIA oxides Ga2O3, In2O3 and IVA oxides GeO2, which have similar band edge wavefunction characters as SnO2. We find the differences between the calculated and experimentally measured fundamental gaps
Figure 4.(Color online) Differences between the HSE06 + G0W0 calculated and experimentally measured band gaps
Recently, a hard X-ray photoelectron spectroscopy (HAXPES) measurement showed that the sharp increase of the VBM occurs at ~3.6 eV below the Fermi energy for undoped SnO2[
Figure 5.(Color online) Calculated DOS of SnO2 for the valence bands below VBM level. The right-hand side shows the DOS with enlarged energy scale below the VBM at
4. Conclusion
In conclusion, we have performed a systematic first-principles calculations and symmetry analysis to revisit the long-standing band gap problem for SnO2. Different-level computational methods and functionals, such as PBE, PBE + G0W0, HSE06 and HSE06 + G0W0 are employed. We find that in all these calculations the band gap of SnO2 is similar or slightly smaller than that of TiO2. The HSE06 calculations yield a fundamental gap of 3.0 eV and a much larger optical gap of 3.6 eV for SnO2, which is consistent with recent experimental measurement of the band alignment between SnO2 and TiO2, but smaller than the previously reported fundamental band gap of 3.65 eV for SnO2. The discrepancy between the fundamental and optical gaps is attributed to the inversion symmetry of rutile SnO2 and the resultant dipole-forbidden transitions between the VBM and CBM, similar to what has been found in In2O3. On the other hand, we find that the HSE06 + G0W0 method, as employed in previous calculations, would overestimate the fundamental gaps of both TiO2 and SnO2 as well as a series of other oxides, including Ga2O3, In2O3 and GeO2. More experimental tests of our predictions are called for. Because SnO2 is widely used in optoelectronic devices, our new understanding of the band structure and optical properties of SnO2 should have large impact on the future design of optoelectronic materials.
Acknowledgment
We acknowledge the computational support from the Beijing Computational Science Research Center (CSRC). This work was supported by the Science Challenge Project (No. TZ2016003), the National Key Research and Development Program of China (No. 2016YFB0700700), and the Nature Science Foundation of China (No. 11634003, 51672023, U1930402 ).
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