Investigação das propriedades estruturais, ópticas e elétricas de filmes finos de SnO2 dopado com Pb2+ e estudo da heteroestrutura TiO2/PbxSn1-xO2
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Data
2021-03-18
Autores
Santos, Stevan Brayan Oliveira dos
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Universidade Estadual Paulista (Unesp)
Resumo
Este trabalho consistiu na produção de pós e filmes finos de SnO2 dopado com Pb2+, depositados através da técnica sol-gel-dip-coating. Para os pós, a caracterização se limitou a análise morfológica e composicional, porém, para os filmes foram realizadas caracterizações morfológicas, estruturais, ópticas, elétricas e eletro-ópticas. A combinação com TiO2, formando assim uma heteroestrutura, também é analisada, e caracterizada eletricamente. Resultados de EDX e MEV confirmaram a presença de chumbo no material e permitiu estimar a dopagem efetiva do SnO2, enquanto medidas de TG e DTA não mostraram diminuição de massa ou picos endotérmicos/exotérmicos referentes a formação de PbO e/ou PbO2. Os resultados indicam que a morfologia dos filmes é definida pela técnica de deposição e não pela dopagem. Medidas de difração de raios-X (DRX) confirmaram a formação de SnO2 nanocristalino na estrutura do tipo rutilo, não sendo identificados picos referentes a outras estruturas cristalinas, independente da dopagem, sugerindo que a dopagem do SnO2 com Pb2+ dá origem a uma solução sólida. Os filmes finos apresentaram dois comportamentos distintos: o bandgap aumenta para dopagens até 1%, associado aqui ao efeito Burnstein-Moss, e diminui para dopagens acima de 1%, chegando ao valor mínimo de 2,89 eV para uma alta dopagem de 25%. Por outro lado, a energia de Urbach diminui para até 1% de dopagem e aumenta consideravelmente para dopagens acima disto, indicando maior desordem e estados intrabandgap próximos a banda de valência com o aumento na dopagem, efeito relacionado à localização do íon na matriz de SnO2. Medidas de IxV mostraram um aumento da resistividade com a dopagem, associado aqui à compensação de carga gerada pelo dopante concomitante com o espalhamento gerado por mudanças estruturais. A energia de ativação para ionização de vacâncias de oxigênio aumenta com a dopagem, até que não é mais observada para dopagens superiores a 1%. Medidas de fotocondutividade e decaimento da corrente fotoexcitada mostram maior taxa de aprisionamento/recombinação de portadores e uma menor energia de captura em 300K, entretanto, com a diminuição da temperatura, os filmes de maiores dopagens apresentaram o efeito de fotocondutividade persistente (PPC). A caracterização elétrica das heteroestruturas mostraram um comportamento típico de diodo para os de maior dopagem. Espera-se que os resultados ópticos e de fotocondutividade obtidos até o momento, para o SnO2 dopado com Pb2+, possam contribuir para a produção de novos dispositivos como células solares e lasers semicondutores, onde o controle do bandgap é desejado.
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.
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.
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Palavras-chave
SnO2:Pb, TiO2, Heteroestrutura, Caracterização óptica, Caracterização elétrica, Fotocondutividade, Pb-doped SnO2, , Heterostructure, Optical characterization, Electrical characterization, Photoconductivity