Avaliação de propriedades ópticas e eletrônicas da heteroestrutura GaAs/SnO2: Eu na forma de filmes finos
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Data
2019-04-12
Autores
Bueno, Cristina de Freitas
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Universidade Estadual Paulista (Unesp)
Resumo
A proposta deste trabalho é a investigação e compreensão de propriedades ópticas e eletrônicas da heteroestrutura formada pelo semicondutor III-V GaAs e o semicondutor óxido SnO2. A deposição de filmes finos de GaAs é feita por evaporação resistiva, e a de filmes finos de SnO2 dopados com o íon terra-rara Eu3+ pelo processo sol-gel-dip-coating, combinando um material semicondutor com alta mobilidade eletrônica e transição direta (GaAs), com semicondutor de bandgap largo (SnO2) e condutividade naturalmente do tipo-n, onde a emissão de Eu3+ é bastante eficiente. Amostras desses dois materiais foram investigadas preliminarmente de forma separada, como filmes finos, ou pós de SnO2:Eu prensados na forma de pastilhas. Fotoluminescência foi medida em heteroestruturas GaAs/SnO2:2%Eu com tratamentos térmicos em baixa temperatura (200 e 400°C), enquanto filmes de SnO2:2%Eu isolados apenas apresentaram picos de emissão do Eu3+ quando tratados com temperatura elevada (1000°C), porém com baixa intensidade. A hipótese para esse fenômeno foi associada com aglomerados de Eu3+ na superfície das amostras. Medidas de XAFS têm possibilitado o estudo da incorporação do dopante Eu na matriz SnO2, e na compreensão do mecanismo da luminescência encontrada. Análises de XANES mostraram que o átomo de európio permanece no estado de oxidação trivalente após a síntese da solução e tratamentos térmicos feitos, e que a heteroestrutura apresenta menos distorção na rede e estrutura mais ordenada quando comparada com filmes de SnO2:2%Eu depositados diretamente sobre vidro. Medidas de decaimento de condutividade fotoexcitada são fundamentais na compreensão do transporte elétrico da heteroestrutura, gerando parâmetros tais como a captura por defeitos, e apontam para a possibilidade de aprisionamento de portadores na barreira de potencial interfacial. Esse tipo de medida realizada com irradiação de diferentes fontes monocromáticas e em diferentes temperaturas (50-200 K) em GaAs/SnO2:2%Eu (200°C/1 h), mostrou-se mais lento principalmente abaixo de 100 K, devido à captura de elétrons na interface entre os filmes, sendo também influenciado pela existência de aglomerados de Eu3+ na superfície de SnO2. Assim, a combinação da fotoluminescência existente na heteroestrutura com as propriedades elétricas, dadas pelos resultados de decaimento da condutividade, mostra a potencialidade desta heteroestrutura para a criação de dispositivos optoeletrônicos.
The purpose of this work is the investigation and comprehension of optical and electronic properties of the heterostructure, formed by the III-V semiconductor GaAs and the oxide semiconductor SnO2. The deposition of GaAs thin films is accomplished by the resistive evaporation technique, and thin films of SnO2, doped with the rare earth ion Eu3+, by the sol-gel-dip-coating process, combining a semiconductor material with high electronic mobility (GaAs), with a wide bandgap semiconductor (SnO2) which is naturally n-type, where Eu3+ emission is quite efficient. Samples of these two materials were initially investigated separately, as thin films, or SnO2:Eu powders pressed into pellets. Photoluminescence was measured in GaAs/SnO2:2%Eu heterostructures with thermal annealing at low temperature (200 and 400°C), while SnO2:2%Eu films showed Eu3+ emission peaks only when treated with higher temperature (1000°C), but with low intensity. The hypothesis for this phenomenon was associated with Eu3+ agglomerates on the sample surfaces. XAFS measurements have allowed the study of the incorporation of the Eu doping in the SnO2 matrix, and in the understanding of the mechanism of luminescence found. Analysis of XANES showed that the europium atom remains in the trivalent oxidation state after the synthesis of the solution and thermal annealing done, and that the GaAs/SnO2:2%Eu heterostructure presents less distortion in the lattice and more ordered structure when compared to films of SnO2:2%Eu deposited directly on glass. Photoinduced conductivity decay measurements have helped to understand the electrical transport in the heterostructure, generating parameters such as capture by defects, and leads to the possibility of capturing carriers at the interfacial potential barrier. This type of measurement performed with irradiation of different monochromatic light sources and at different temperatures (50- 200 K) in GaAs/SnO2:2%Eu (200°C/1 h), was shown to be slower, mainly below 100 K, due to the electron capture at the interface between the films, being also influenced by the existence of agglomerates of Eu3+ on the SnO2 surface. Thus, the combination of the photoluminescence existing in the heterostructure with the electrical properties, given by the results of the photoinduced conductivity decay, shows the potentiality of this heterostructure for the creation of optoelectronic devices.
The purpose of this work is the investigation and comprehension of optical and electronic properties of the heterostructure, formed by the III-V semiconductor GaAs and the oxide semiconductor SnO2. The deposition of GaAs thin films is accomplished by the resistive evaporation technique, and thin films of SnO2, doped with the rare earth ion Eu3+, by the sol-gel-dip-coating process, combining a semiconductor material with high electronic mobility (GaAs), with a wide bandgap semiconductor (SnO2) which is naturally n-type, where Eu3+ emission is quite efficient. Samples of these two materials were initially investigated separately, as thin films, or SnO2:Eu powders pressed into pellets. Photoluminescence was measured in GaAs/SnO2:2%Eu heterostructures with thermal annealing at low temperature (200 and 400°C), while SnO2:2%Eu films showed Eu3+ emission peaks only when treated with higher temperature (1000°C), but with low intensity. The hypothesis for this phenomenon was associated with Eu3+ agglomerates on the sample surfaces. XAFS measurements have allowed the study of the incorporation of the Eu doping in the SnO2 matrix, and in the understanding of the mechanism of luminescence found. Analysis of XANES showed that the europium atom remains in the trivalent oxidation state after the synthesis of the solution and thermal annealing done, and that the GaAs/SnO2:2%Eu heterostructure presents less distortion in the lattice and more ordered structure when compared to films of SnO2:2%Eu deposited directly on glass. Photoinduced conductivity decay measurements have helped to understand the electrical transport in the heterostructure, generating parameters such as capture by defects, and leads to the possibility of capturing carriers at the interfacial potential barrier. This type of measurement performed with irradiation of different monochromatic light sources and at different temperatures (50- 200 K) in GaAs/SnO2:2%Eu (200°C/1 h), was shown to be slower, mainly below 100 K, due to the electron capture at the interface between the films, being also influenced by the existence of agglomerates of Eu3+ on the SnO2 surface. Thus, the combination of the photoluminescence existing in the heterostructure with the electrical properties, given by the results of the photoinduced conductivity decay, shows the potentiality of this heterostructure for the creation of optoelectronic devices.
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Palavras-chave
Arseneto de gálio, Dióxido de estanho, Heteroestrutura, Európio, Radiação Síncrotron, XANES, EXAFS, Decaimento de condutividade fotoinduzida, Gallium arsenide, Tin dioxide, Heterostructure, Europium, Síncrotron radiation, Photoinduced conductivity decay