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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.


Generalized energy diagram of the SnO2:Sb thin films in the parameter space of film thickness and Sb doping concentration, which is based on the scenario of quantum confinement induced band gap broadening and impurity level pinning.
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f6: Generalized energy diagram of the SnO2:Sb thin films in the parameter space of film thickness and Sb doping concentration, which is based on the scenario of quantum confinement induced band gap broadening and impurity level pinning.

Mentions: More specifically, the quantum confinement induced broadening can be attributed to two distinct effects: the increase of ionization energy and the decrease of electron affinity. This size induced electronic structure variation has been intensively studied in the pure IV, III-V, II-VI and IV-VI semiconductor nanostructures2833343536373839. As such, the influence of quantum confinement effect on the physical properties of impurity doped semiconductor nanostructures is expected. Based on the first principles calculations, as the nanocrystal size decreases, (i) the impurity activation energy is enhanced404142, (ii) effective Bohr radius is squeezed84143, (iii) impurity wavefunction becomes more localized434445, (iv) a valence state transition for the impurity is driven9, and (v) the energetic level of impurity is pinned referring to the vacuum level8. Based on the above analyses, a generalized energy diagram in the parameter space of film thickness and Sb doping concentration, for the impact of quantum confinement on our thickness-varied SnO2:Sb thin films was deduced and shown in Fig. 6. In this diagram, the ionization energy increases and the electron affinity decreases with the reduction of film thickness within the confinement region. At the same time, the impurity level is independent of the film thickness, which means it is pinned relative to the vacuum level. On the other hand, the position of the impurity level relative to the conduction band minimum (CBM) which is defined as activation energy () can be controlled by the doping concentration (). In the bulk region, the value of is given by the following equation4647:


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)

Generalized energy diagram of the SnO2:Sb thin films in the parameter space of film thickness and Sb doping concentration, which is based on the scenario of quantum confinement induced band gap broadening and impurity level pinning.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f6: Generalized energy diagram of the SnO2:Sb thin films in the parameter space of film thickness and Sb doping concentration, which is based on the scenario of quantum confinement induced band gap broadening and impurity level pinning.
Mentions: More specifically, the quantum confinement induced broadening can be attributed to two distinct effects: the increase of ionization energy and the decrease of electron affinity. This size induced electronic structure variation has been intensively studied in the pure IV, III-V, II-VI and IV-VI semiconductor nanostructures2833343536373839. As such, the influence of quantum confinement effect on the physical properties of impurity doped semiconductor nanostructures is expected. Based on the first principles calculations, as the nanocrystal size decreases, (i) the impurity activation energy is enhanced404142, (ii) effective Bohr radius is squeezed84143, (iii) impurity wavefunction becomes more localized434445, (iv) a valence state transition for the impurity is driven9, and (v) the energetic level of impurity is pinned referring to the vacuum level8. Based on the above analyses, a generalized energy diagram in the parameter space of film thickness and Sb doping concentration, for the impact of quantum confinement on our thickness-varied SnO2:Sb thin films was deduced and shown in Fig. 6. In this diagram, the ionization energy increases and the electron affinity decreases with the reduction of film thickness within the confinement region. At the same time, the impurity level is independent of the film thickness, which means it is pinned relative to the vacuum level. On the other hand, the position of the impurity level relative to the conduction band minimum (CBM) which is defined as activation energy () can be controlled by the doping concentration (). In the bulk region, the value of is given by the following equation4647:

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.