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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.


XPS results of the SnO2:Sb films with different thicknesses in the binding energy range from −2 to 16 eV.For comparison, the valence band spectrum of pure Al2O3 substrate is also presented. The substrate effect on the valence maximum denotation and conduction band electron measurement of the SnO2:Sb can be neglected due to little contribution from Al2O3 below 7 eV.
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f3: XPS results of the SnO2:Sb films with different thicknesses in the binding energy range from −2 to 16 eV.For comparison, the valence band spectrum of pure Al2O3 substrate is also presented. The substrate effect on the valence maximum denotation and conduction band electron measurement of the SnO2:Sb can be neglected due to little contribution from Al2O3 below 7 eV.

Mentions: In order to get a better insight into this thickness induced MIT, XPS was employed to investigate the electronic structure of the SnO2:Sb thin films. Figure 3 presents the photoemission spectra of four samples in the binding energy ranging from −2 to 16 eV. This region encompasses the conduction band, the bulk band gap, and the valence band. By zooming into the zero-binding-energy region, a prominent Fermi-Dirac-like cutoff associated with electrons in the conduction band is observed in samples A to C, which confirms the metallic property of these SnO2:Sb films at room temperature. These results are consistent with the resistivity measurements shown in Fig. 2. The electron occupation in the conduction band vanished as the film thickness scaled down to 3.1 nm in sample D. The absence of a Fermi-edge cutoff clearly indicates its insulator nature. According to these XPS results, the thickness induced MIT in SnO2:Sb thin films is further confirmed. In addition, the electron emission signal from conduction band region was found to gradually reduce as the film thickness decreased (Fig. 3). This occupation decrement implies a thickness induced reduction of free electron concentration. To verify the above behavior, the room-temperature electron concentrations () were measured by Hall effect as summarized in Table 1, in which a clear shrinkage of electron concentration is revealed. In addition, the valence band maximum (VBM) of each sample can be quantified by extrapolating the leading edge of valence band (VB) spectra in Fig. 3 to intersect with the background base line. This procedure has been used to determine the VBM position of conventional semiconductor materials1314. The resolution of extrapolation of our samples is found to be ±0.05 eV. The VBM values are summarized in the Table 1. It is clear that the VBM shows a red shift to lower binding energy side with the decrease of film thickness (i.e. the shrinkage of electron concentration shown in Fig. 3 and Table 1). This VBM shift can be interpreted by the Burstein-Moss (BM) effect originated from the filling of the conduction band15. Considering a free-electron profile of Sn 5 s conduction band for SnO2, the magnitude of BM induced VBM shifts in XPS spectra is directly related to the occupied conduction bandwidth, which can be calculated by 13:


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)

XPS results of the SnO2:Sb films with different thicknesses in the binding energy range from −2 to 16 eV.For comparison, the valence band spectrum of pure Al2O3 substrate is also presented. The substrate effect on the valence maximum denotation and conduction band electron measurement of the SnO2:Sb can be neglected due to little contribution from Al2O3 below 7 eV.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f3: XPS results of the SnO2:Sb films with different thicknesses in the binding energy range from −2 to 16 eV.For comparison, the valence band spectrum of pure Al2O3 substrate is also presented. The substrate effect on the valence maximum denotation and conduction band electron measurement of the SnO2:Sb can be neglected due to little contribution from Al2O3 below 7 eV.
Mentions: In order to get a better insight into this thickness induced MIT, XPS was employed to investigate the electronic structure of the SnO2:Sb thin films. Figure 3 presents the photoemission spectra of four samples in the binding energy ranging from −2 to 16 eV. This region encompasses the conduction band, the bulk band gap, and the valence band. By zooming into the zero-binding-energy region, a prominent Fermi-Dirac-like cutoff associated with electrons in the conduction band is observed in samples A to C, which confirms the metallic property of these SnO2:Sb films at room temperature. These results are consistent with the resistivity measurements shown in Fig. 2. The electron occupation in the conduction band vanished as the film thickness scaled down to 3.1 nm in sample D. The absence of a Fermi-edge cutoff clearly indicates its insulator nature. According to these XPS results, the thickness induced MIT in SnO2:Sb thin films is further confirmed. In addition, the electron emission signal from conduction band region was found to gradually reduce as the film thickness decreased (Fig. 3). This occupation decrement implies a thickness induced reduction of free electron concentration. To verify the above behavior, the room-temperature electron concentrations () were measured by Hall effect as summarized in Table 1, in which a clear shrinkage of electron concentration is revealed. In addition, the valence band maximum (VBM) of each sample can be quantified by extrapolating the leading edge of valence band (VB) spectra in Fig. 3 to intersect with the background base line. This procedure has been used to determine the VBM position of conventional semiconductor materials1314. The resolution of extrapolation of our samples is found to be ±0.05 eV. The VBM values are summarized in the Table 1. It is clear that the VBM shows a red shift to lower binding energy side with the decrease of film thickness (i.e. the shrinkage of electron concentration shown in Fig. 3 and Table 1). This VBM shift can be interpreted by the Burstein-Moss (BM) effect originated from the filling of the conduction band15. Considering a free-electron profile of Sn 5 s conduction band for SnO2, the magnitude of BM induced VBM shifts in XPS spectra is directly related to the occupied conduction bandwidth, which can be calculated by 13:

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.