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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.


Core-level Sb 3d3/2 XPS spectra of SnO2:Sb films (filled squares) with different thicknesses: (A) 188.0 nm, (B) 31.3 nm, (C) 7.9 nm, and (D) 3.1 nm. The spectra were fitted to a Shirley background (green line) together with the Voigt profiles for combined Sb(V) and plasmon satellite (blue dash lines) and Sb(III) (red dash line). The fitting curves (black solid line) are seen to match well with the experimental data points. The denotation exp., fit., and bg. represent experimental data, fitting curve, and back ground, respectively.
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f4: Core-level Sb 3d3/2 XPS spectra of SnO2:Sb films (filled squares) with different thicknesses: (A) 188.0 nm, (B) 31.3 nm, (C) 7.9 nm, and (D) 3.1 nm. The spectra were fitted to a Shirley background (green line) together with the Voigt profiles for combined Sb(V) and plasmon satellite (blue dash lines) and Sb(III) (red dash line). The fitting curves (black solid line) are seen to match well with the experimental data points. The denotation exp., fit., and bg. represent experimental data, fitting curve, and back ground, respectively.

Mentions: As shown in Fig. 4, the fitting of the Sb 3d3/2 core lines into two Voigt components gives a good description of the overall core line shape in which the lower binding energy peak represents the Sb(III). However, the higher binding energy component is complicated, since it may be the combination of main Sb(V) (screened final state) and plasmon satellite (unscreened final state) peaks. Due to the relatively low doping concentration of Sb in the samples and also the very thin film thickness, the signal-to-noise ratio of the Sb 3d3/2 spectra is too low to separate the peaks. Accordingly, a broad high binding energy peak is used to fit the spectra. The plasmon satellite peak in the XPS core lineshapes in degenerately doped semiconductors has been well documented131920. It usually appears as a shoulder at the higher energy side of the core line. In addition, according to Egdell et al.’s works, the plasmon satellite peak is strongly correlated to the electron concentration, i.e. the lower of electron concentration, the weaker of plasmon satellite peak13. Since the sample D is non-metallic, which means the electron concentration is low, its high binding energy peak does not have unscreened component. Accordingly, the observation of a narrower high binding energy peak in sample D can be explained.


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)

Core-level Sb 3d3/2 XPS spectra of SnO2:Sb films (filled squares) with different thicknesses: (A) 188.0 nm, (B) 31.3 nm, (C) 7.9 nm, and (D) 3.1 nm. The spectra were fitted to a Shirley background (green line) together with the Voigt profiles for combined Sb(V) and plasmon satellite (blue dash lines) and Sb(III) (red dash line). The fitting curves (black solid line) are seen to match well with the experimental data points. The denotation exp., fit., and bg. represent experimental data, fitting curve, and back ground, respectively.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f4: Core-level Sb 3d3/2 XPS spectra of SnO2:Sb films (filled squares) with different thicknesses: (A) 188.0 nm, (B) 31.3 nm, (C) 7.9 nm, and (D) 3.1 nm. The spectra were fitted to a Shirley background (green line) together with the Voigt profiles for combined Sb(V) and plasmon satellite (blue dash lines) and Sb(III) (red dash line). The fitting curves (black solid line) are seen to match well with the experimental data points. The denotation exp., fit., and bg. represent experimental data, fitting curve, and back ground, respectively.
Mentions: As shown in Fig. 4, the fitting of the Sb 3d3/2 core lines into two Voigt components gives a good description of the overall core line shape in which the lower binding energy peak represents the Sb(III). However, the higher binding energy component is complicated, since it may be the combination of main Sb(V) (screened final state) and plasmon satellite (unscreened final state) peaks. Due to the relatively low doping concentration of Sb in the samples and also the very thin film thickness, the signal-to-noise ratio of the Sb 3d3/2 spectra is too low to separate the peaks. Accordingly, a broad high binding energy peak is used to fit the spectra. The plasmon satellite peak in the XPS core lineshapes in degenerately doped semiconductors has been well documented131920. It usually appears as a shoulder at the higher energy side of the core line. In addition, according to Egdell et al.’s works, the plasmon satellite peak is strongly correlated to the electron concentration, i.e. the lower of electron concentration, the weaker of plasmon satellite peak13. Since the sample D is non-metallic, which means the electron concentration is low, its high binding energy peak does not have unscreened component. Accordingly, the observation of a narrower high binding energy peak in sample D can be explained.

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.