Investigation of the structural, optical and electrical properties of thin films of Pb2+-doped SnO2 and study of the TiO2/PbxSn1-xO2 heterostructure

Author(s)

Stevan Brayan Oliveira dos Santos

• Document type: Master's Dissertation
• Press: Bauru. 2021-04-09.
• Institution: Universidade Estadual Paulista (Unesp). Faculdade de Ciências. Bauru
• Defense date: 2021-03-18
• Advisor: Luis Vicente de Andrade Scalvi

Abstract

The research accomplished in this work consisted on the production of SnO2 samples in the form of powders and thin films, doped with Pb2+, and in the combination with TiO2, forming a heterostructure. Thin films were deposited using the sol-gel-dip-coating technique. For the powders, the characterization was limited to morphological and compositional analysis, however, for the films, morphological, structural, optical, electrical and electro-optical characterizations were performed, in addition to electrical characterizations for heterostructures. Results of EDX and SEM confirmed the presence of lead in the material and allowed to estimate the effective doping of SnO2. These results indicated that the morphology of the films is defined by the deposition technique and not by the doping. X-ray diffraction (XRD) measurements confirmed the formation of nanocrystalline SnO2 in the rutile type structure, with no peaks related to other structures being identified, regardless of doping, suggesting that the doping of SnO2 with Pb2+ gives rise to a solid solution. Optical characterizations have shown that for doping up to 4%, the transmittance is around 80-90% and, for doping above this value, the transmittance starts to decrease. The thin films showed two distinct behaviors: for doping up to 1%, the bandgap shows an increase, associated here with the Burnstein-Moss effect, and it decreases for doping above 1%, reaching a minimum value of 2.89 eV for a high doping of 25%. On the other hand, Urbach energy decreases up to 1% doping and increases considerably for doping above this, indicating greater disorder and intrabandgap states with energy close to the valence band. These results can be explained considering that for low doping, Pb2+ is entering mainly as interstitial and acting as a donor, whereas for greater doping, it enters the matrix mainly substituting Sn4+, acting as an acceptor. The IxV measurements showed an increase in resistivity with doping, associated here with the charge compensation generated by the dopant along with the increase on electron scattering. The activation energy for ionization of oxygen vacancies increases with doping, until it is no longer observed for doping higher than 1%. Data of photoconductivity and decay of the photoexcited current showed a higher decay rate of carrier trapping/recombination and a lower capture energy at 300K, however, with decreasing temperature, samples with higher doping showed the effect of persistent photoconductivity (PPC). The electrical characterization of the heterostructure showed a typical diode behavior for samples with higher doping. It is expected that results obtained so far for Pb2+ doped SnO2 concerning optical characterization and photoconductivity, may contribute to the production of new devices where control of the bandgap and/or PPC are desired. (AU)

• FAPESP's process: 18/25241-4 - Investigation of structural, electrical and optical properties of the hybrid structure PbxSn1-xO2/TiO2: interface analysis and influence of substrate temperature
• Grantee: Stevan Brayan Oliveira dos Santos
• Support Opportunities: Scholarships in Brazil - Master