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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 activation energies of Sb in (a) SnO2:Sb10 films with thickness of 3.1 nm, 7.9 nm and (b) SnO2:Sb1 films with thickness of 3.5 nm, 8.7 nm.
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f7: The activation energies of Sb in (a) SnO2:Sb10 films with thickness of 3.1 nm, 7.9 nm and (b) SnO2:Sb1 films with thickness of 3.5 nm, 8.7 nm.

Mentions: where is the residual electrical resistivity and is Boltzmann constant. For sample C, the is extracted from data below 150 K in which a semiconducting behavior was observed. Fig. 7(a) gives the fitting results showing that for samples C and D are and , respectively. Indeed, a prominent enhancement in the thinner film is observed, which is consistent with Fig. 6. Based on equation (2), with the decrease of doping concentration the impurity level will move downward relatively to the CBM as well as the vacuum level in the bulk region. Referring to Fig. 6, this downward shift of impurity level will lead to a higher critical thickness for the size induced MIT. Experimental implementation for this prediction has been done and demonstrated in Fig. 7(b). Similar to the 10% Sb doped SnO2 (SnO2:Sb10), a series of 1% Sb doped SnO2 (SnO2:Sb1) thin films were fabricated with the same parameters and characterized by Hall measurement. The deposition rate has been recalibrated as 0.58 Å per laser ablation for the new target of SnO2:Sb1. Figure 7(b) shows the temperature dependent resistivity of SnO2:Sb1 thin films with the critical thickness found to be around 8.7 nm, which is larger than that of SnO2:Sb10. The for this 8.7 nm thick SnO2:Sb1 thin film is calculated to be . This value is large that the sample C with thickness of 7.9 nm and 10% Sb, which implies that the 8.7 nm thick SnO2:Sb1 thin film is “more insulating” than the 7.9 nm thick SnO2:Sb10 thin film. For the thinner SnO2:Sb1 film with thickness of 3.5 nm, its insulator nature is confirmed and the EA is . Compared with that of sample D which has 10% Sb and thickness of 3.1 nm, the of Sb in the SnO2:Sb1 thin film increased substantially, which is consistent with the conclusions deduced from Fig. 6. At present, we look back to the previous analysis of the Sb 3d3/2 core-level spectra shown in Fig. 4, the observed oxidation state transition from Sb(V) to Sb(III) can be understood by the enhancement of the activation energy (electron binding energy) and the strong self-trapping of electrons by the Sb sites. Hence, both the experimental results and theoretical analysis indicate a quantum confinement induced MIT in the SnO2:Sb films and the existence of operational size limit for its microelectronic applications.


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 activation energies of Sb in (a) SnO2:Sb10 films with thickness of 3.1 nm, 7.9 nm and (b) SnO2:Sb1 films with thickness of 3.5 nm, 8.7 nm.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f7: The activation energies of Sb in (a) SnO2:Sb10 films with thickness of 3.1 nm, 7.9 nm and (b) SnO2:Sb1 films with thickness of 3.5 nm, 8.7 nm.
Mentions: where is the residual electrical resistivity and is Boltzmann constant. For sample C, the is extracted from data below 150 K in which a semiconducting behavior was observed. Fig. 7(a) gives the fitting results showing that for samples C and D are and , respectively. Indeed, a prominent enhancement in the thinner film is observed, which is consistent with Fig. 6. Based on equation (2), with the decrease of doping concentration the impurity level will move downward relatively to the CBM as well as the vacuum level in the bulk region. Referring to Fig. 6, this downward shift of impurity level will lead to a higher critical thickness for the size induced MIT. Experimental implementation for this prediction has been done and demonstrated in Fig. 7(b). Similar to the 10% Sb doped SnO2 (SnO2:Sb10), a series of 1% Sb doped SnO2 (SnO2:Sb1) thin films were fabricated with the same parameters and characterized by Hall measurement. The deposition rate has been recalibrated as 0.58 Å per laser ablation for the new target of SnO2:Sb1. Figure 7(b) shows the temperature dependent resistivity of SnO2:Sb1 thin films with the critical thickness found to be around 8.7 nm, which is larger than that of SnO2:Sb10. The for this 8.7 nm thick SnO2:Sb1 thin film is calculated to be . This value is large that the sample C with thickness of 7.9 nm and 10% Sb, which implies that the 8.7 nm thick SnO2:Sb1 thin film is “more insulating” than the 7.9 nm thick SnO2:Sb10 thin film. For the thinner SnO2:Sb1 film with thickness of 3.5 nm, its insulator nature is confirmed and the EA is . Compared with that of sample D which has 10% Sb and thickness of 3.1 nm, the of Sb in the SnO2:Sb1 thin film increased substantially, which is consistent with the conclusions deduced from Fig. 6. At present, we look back to the previous analysis of the Sb 3d3/2 core-level spectra shown in Fig. 4, the observed oxidation state transition from Sb(V) to Sb(III) can be understood by the enhancement of the activation energy (electron binding energy) and the strong self-trapping of electrons by the Sb sites. Hence, both the experimental results and theoretical analysis indicate a quantum confinement induced MIT in the SnO2:Sb films and the existence of operational size limit for its microelectronic applications.

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.