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Thickness-Induced Metal-Insulator Transition in Sb-doped SnO2 Ultrathin Films: The Role of Quantum Confinement.

Ke C, Zhu W, Zhang Z, Tok ES, Ling B, Pan J - Sci Rep (2015)

Bottom Line: A thickness induced metal-insulator transition (MIT) was firstly observed in Sb-doped SnO2 (SnO2:Sb) epitaxial ultrathin films deposited on sapphire substrates by pulsed laser deposition.With the shrinkage of film thickness, the broadening of the energy band gap as well as the enhancement of the impurity activation energy was studied and attributed to the quantum confinement effect.Based on the scenario of impurity level pinning and band gap broadening in quantum confined nanostructures, we proposed a generalized energy diagram to understand the thickness induced MIT in the SnO2:Sb system.

View Article: PubMed Central - PubMed

Affiliation: Microelectronics Centre, School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798.

ABSTRACT
A thickness induced metal-insulator transition (MIT) was firstly observed in Sb-doped SnO2 (SnO2:Sb) epitaxial ultrathin films deposited on sapphire substrates by pulsed laser deposition. Both electrical and spectroscopic studies provide clear evidence of a critical thickness for the metallic conductivity in SnO2:Sb thin films and the oxidation state transition of the impurity element Sb. With the shrinkage of film thickness, the broadening of the energy band gap as well as the enhancement of the impurity activation energy was studied and attributed to the quantum confinement effect. Based on the scenario of impurity level pinning and band gap broadening in quantum confined nanostructures, we proposed a generalized energy diagram to understand the thickness induced MIT in the SnO2:Sb system.

No MeSH data available.


The energy band gap of different thickness SnO2:Sb thin films as extracted from the XPS valence band spectra.The dash line shows the least-squares fit to a power-law function: , from which a remarkable quantum confinement effect is revealed.
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f5: The energy band gap of different thickness SnO2:Sb thin films as extracted from the XPS valence band spectra.The dash line shows the least-squares fit to a power-law function: , from which a remarkable quantum confinement effect is revealed.

Mentions: Depending on the above analyses of Sb 3d3/2 core-level spectra, Sb(III) is preferred for the ultrathin SnO2:Sb film, and this oxidation state transition gives rise to the MIT. To understand this film thickness related phenomena, the quantum confinement effect should be considered as the film thickness approaches its Bohr radius ()17. Referring to the energy band diagram of the SnO2:Sb/Al2O3 structure, as shown in the inset of Fig. 5, the electrons in the films are confined in the potential well formed by Al2O3 substrate and vacuum. It’s generally accepted that the energy band gap () will be significantly broadened when the electrons are squeezed. To explore the thickness effect on the electronic structure of the SnO2:Sb films, we extracted the values of from the XPS spectra (shown in Fig. 3) by subtracting the conduction band filling () from the VBM (see the part 3 and Fig. S3 in Supplementary Information). In addition to , the band gap renormalization should also be considered to calculate the fundamental , since all the SnO2 films were highly doped with Sb. Bandgap renormalization caused by electron-electron interaction and electron-impurity interaction has been extensively studied in multiple material systems, e.g. CdO, ZnO and SnO213192324252627. Based on Egell et al.’s13 and Walsh et al.’s27 works the total band gap narrowing () in our samples can be estimated as: , where N is the electron concentration. Accordingly, the fundamental can be calculated by equation, . A prominent increase of with the decrease of film thickness was observed, as shown as in Table 1 and Fig. 5. By analytic fitting to the thickness-dependent shown in Fig. 5, the shift scales as , here denotes the thickness of thin film. The magnitude of the scaling power is found to have a similar value with the cases of SnO2 quantum wires () and quantum dots () predicted by first principles calculations, where stands for the radius28. Besides the quantum confinement effect induced band structure change, the strain effect on band structure should also be considered, since it usually exists in heteroepitaxial thin films. Recently Zhang et al. reported that the tensile strain presents in a series of In2O3 epitaxial thin films with thickness ranging from 35 to 420 nm, due to the 1.6% lattice mismatch between the In2O3 film and YSZ substrate. In addition, a 0.08 eV red shift to the band gap of In2O3 films was found to be caused by the thickness modulated tensile strain29. Accordingly, the strain effect in our samples should also be considered for band gap analysis. It has been well established that the residual strain in an epitaxial film is highly dependent on the lattice mismatch between the film and substrate. Specifically to (101)-SnO2 film on -Al2O3 substrate, the lattice mismatch along the SnO2 direction, which is parallel to the direction of Al2O3, is very high, i.e. 11.45%30. With such a big lattice mismatch, the substrate induced strain will be relaxed through a series of quasiperiodic misfit dislocations running along the direction of SnO230. In addition, based on Matthews-Blakeslee theory3132, the critical thickness for (101)-SnO2 epitaxial film to preserve perfect lattice matching with Al2O3 substrate can be estimated to be less than one SnO2 monolayer, i.e. 0.26 nm. This means the lattice relaxation happens in the first one or two layers of SnO2 from the SnO2-Al2O3 interface. The thinnest film in this work is 3.1 nm-thick, which is considerably greater than the critical thickness. Accordingly we believe the strain in the samples in this work has been relaxed. To experimentally study the strain, symmetric 2θ-ω XRD scans across the Al2O3, SnO2 (101) and Al2O3 reflections for SnO2:Sb films with different thicknesses were collected and shown in the Fig. S4. Based on the SnO2 (101) reflection positions and calculated out-of-plane lattice distances in the Table S1, it can be seen that the film thickness variation does not affect the lattice parameters. This confirms the strain in the samples has been relaxed. As a result, the strain induced bandgap shift is insignificant to this study.


Thickness-Induced Metal-Insulator Transition in Sb-doped SnO2 Ultrathin Films: The Role of Quantum Confinement.

Ke C, Zhu W, Zhang Z, Tok ES, Ling B, Pan J - Sci Rep (2015)

The energy band gap of different thickness SnO2:Sb thin films as extracted from the XPS valence band spectra.The dash line shows the least-squares fit to a power-law function: , from which a remarkable quantum confinement effect is revealed.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4663506&req=5

f5: The energy band gap of different thickness SnO2:Sb thin films as extracted from the XPS valence band spectra.The dash line shows the least-squares fit to a power-law function: , from which a remarkable quantum confinement effect is revealed.
Mentions: Depending on the above analyses of Sb 3d3/2 core-level spectra, Sb(III) is preferred for the ultrathin SnO2:Sb film, and this oxidation state transition gives rise to the MIT. To understand this film thickness related phenomena, the quantum confinement effect should be considered as the film thickness approaches its Bohr radius ()17. Referring to the energy band diagram of the SnO2:Sb/Al2O3 structure, as shown in the inset of Fig. 5, the electrons in the films are confined in the potential well formed by Al2O3 substrate and vacuum. It’s generally accepted that the energy band gap () will be significantly broadened when the electrons are squeezed. To explore the thickness effect on the electronic structure of the SnO2:Sb films, we extracted the values of from the XPS spectra (shown in Fig. 3) by subtracting the conduction band filling () from the VBM (see the part 3 and Fig. S3 in Supplementary Information). In addition to , the band gap renormalization should also be considered to calculate the fundamental , since all the SnO2 films were highly doped with Sb. Bandgap renormalization caused by electron-electron interaction and electron-impurity interaction has been extensively studied in multiple material systems, e.g. CdO, ZnO and SnO213192324252627. Based on Egell et al.’s13 and Walsh et al.’s27 works the total band gap narrowing () in our samples can be estimated as: , where N is the electron concentration. Accordingly, the fundamental can be calculated by equation, . A prominent increase of with the decrease of film thickness was observed, as shown as in Table 1 and Fig. 5. By analytic fitting to the thickness-dependent shown in Fig. 5, the shift scales as , here denotes the thickness of thin film. The magnitude of the scaling power is found to have a similar value with the cases of SnO2 quantum wires () and quantum dots () predicted by first principles calculations, where stands for the radius28. Besides the quantum confinement effect induced band structure change, the strain effect on band structure should also be considered, since it usually exists in heteroepitaxial thin films. Recently Zhang et al. reported that the tensile strain presents in a series of In2O3 epitaxial thin films with thickness ranging from 35 to 420 nm, due to the 1.6% lattice mismatch between the In2O3 film and YSZ substrate. In addition, a 0.08 eV red shift to the band gap of In2O3 films was found to be caused by the thickness modulated tensile strain29. Accordingly, the strain effect in our samples should also be considered for band gap analysis. It has been well established that the residual strain in an epitaxial film is highly dependent on the lattice mismatch between the film and substrate. Specifically to (101)-SnO2 film on -Al2O3 substrate, the lattice mismatch along the SnO2 direction, which is parallel to the direction of Al2O3, is very high, i.e. 11.45%30. With such a big lattice mismatch, the substrate induced strain will be relaxed through a series of quasiperiodic misfit dislocations running along the direction of SnO230. In addition, based on Matthews-Blakeslee theory3132, the critical thickness for (101)-SnO2 epitaxial film to preserve perfect lattice matching with Al2O3 substrate can be estimated to be less than one SnO2 monolayer, i.e. 0.26 nm. This means the lattice relaxation happens in the first one or two layers of SnO2 from the SnO2-Al2O3 interface. The thinnest film in this work is 3.1 nm-thick, which is considerably greater than the critical thickness. Accordingly we believe the strain in the samples in this work has been relaxed. To experimentally study the strain, symmetric 2θ-ω XRD scans across the Al2O3, SnO2 (101) and Al2O3 reflections for SnO2:Sb films with different thicknesses were collected and shown in the Fig. S4. Based on the SnO2 (101) reflection positions and calculated out-of-plane lattice distances in the Table S1, it can be seen that the film thickness variation does not affect the lattice parameters. This confirms the strain in the samples has been relaxed. As a result, the strain induced bandgap shift is insignificant to this study.

Bottom Line: A thickness induced metal-insulator transition (MIT) was firstly observed in Sb-doped SnO2 (SnO2:Sb) epitaxial ultrathin films deposited on sapphire substrates by pulsed laser deposition.With the shrinkage of film thickness, the broadening of the energy band gap as well as the enhancement of the impurity activation energy was studied and attributed to the quantum confinement effect.Based on the scenario of impurity level pinning and band gap broadening in quantum confined nanostructures, we proposed a generalized energy diagram to understand the thickness induced MIT in the SnO2:Sb system.

View Article: PubMed Central - PubMed

Affiliation: Microelectronics Centre, School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798.

ABSTRACT
A thickness induced metal-insulator transition (MIT) was firstly observed in Sb-doped SnO2 (SnO2:Sb) epitaxial ultrathin films deposited on sapphire substrates by pulsed laser deposition. Both electrical and spectroscopic studies provide clear evidence of a critical thickness for the metallic conductivity in SnO2:Sb thin films and the oxidation state transition of the impurity element Sb. With the shrinkage of film thickness, the broadening of the energy band gap as well as the enhancement of the impurity activation energy was studied and attributed to the quantum confinement effect. Based on the scenario of impurity level pinning and band gap broadening in quantum confined nanostructures, we proposed a generalized energy diagram to understand the thickness induced MIT in the SnO2:Sb system.

No MeSH data available.