Ultrathin films of Al2O3 deposited on Si were submitted to rapid thermal annealing in vacuum or in oxygen atmosphere, in the temperature range from 600 to 800 degrees C. Nuclear reaction profiling with subnanometric depth resolution evidenced mobility of O, Al, and Si species, and angle-resolved x-ray photoelectron spectroscopy revealed the formation of Si-Al-O compounds in near-surface regions, under oxidizing atmosphere at and above 700 degrees C. Under vacuum annealing all species remained essentially immobile. A model is presented based on diffusion-reaction equations capable of explaining the mobilities and reproducing the obtained profiles.
Krug et al. Reply: In a Comment [1] on our recent Letter [2], Copel discusses features in our XPS data that had not been explained, presents new XPS and MEIS data, and questions the validity of the diffusion-reaction model that we proposed.The first point raised by Copel is the origin of the Si 2p photoelectron peak at 98.5 eV binding energy appearing in the analysis of Al 2 O 3 films on Si(001) after annealing in O 2 at 800 ± C for 30 s, which was unclear in our Letter [2]. Thickness inhomogeneities [1] should indeed be the reason for the above-mentioned peak, which is absent from Copel's results as well as from our results for annealing at lower temperatures. Film inhomogeneities could be induced by extrinsic impurities, as suggested by Copel, or avoided by annealing in ultrahigh vacuum prior to oxidation, as performed by Copel [1]. Such annealings [3] densify the films, reduce oxygen diffusivity during subsequent oxidation, and may result in the observed enhanced stability. In this case, it would be Copel's merit to have found optimal preoxidation annealing conditions.Given the presence of film inhomogeneities in the sample oxidized at 800 ± C in our work, the assumptions underlying our diffusion-reaction model are questioned [1]. On the contrary, we understand that by detecting Si migration and fixation in the Al 2 O 3 film in essentially thickness inhomogeneity-free conditions, Copel provided an important confirmation of our model adequacy. In fact, without oxygen diffusion and reaction one cannot explain Si migration, partial replacement of Al by Si, and Al loss (not entirely quantified in [1]). Furthermore, the specific criticism addressed in the last sentence of the Comment [1] concerning oxygen removal from Al 2 O 3 by excess Si can also be justified. An estimate of the coefficients in the model equations from experimental data is required in order to quantify the relative contributions of different diffusion and reaction processes. Since this information is not yet available, we used the simplest assumption of reactions yielding stoichiometric compounds according to thermodynamic stability data, which is not necessarily valid. Unfortunately, due to partial superposition of Al and Si signals, the MEIS technique used by Copel did not add the necessary information in this sense.An additional, important issue [4] not addressed by us or by Copel regards oxygen diffusion from the gas phase to the Al 2 O 3 -SiO 2 ͞Si interface. This constitutes a basic assumption of the model, as oxygen is taken to trigger Si diffusion and subsequent reaction in the Al 2 O 3 network. In order to investigate oxygen transport, we annealed Al 2 O 3 films on Si(001) in 18 O 2 for various times at 700 ± C, a temperature at which we observed Si migration and fixation without any effect attributable to thickness inhomogeneities. Much thicker (36 nm) films were deposited and oxidized as before [2], aiming at privileging information from the surface and bulk of the films, without contribution of solid-solid interfaces. Excitation curve...
RESUMO: Na literatura, encontramos algumas classificações para os enunciados dos problemas, mas existem escassas discussões sobre como elaborar bons problemas para serem utilizados com a metodologia de Resolução de Problemas. Nesse sentido, o objetivo do presente estudo é apresentar características fundamentais de um Problema Eficaz para o Ensino de Ciências. A partir das investigações desenvolvidas ao longo da última década, propomos que um Problema Eficaz deve contextualizar a temática à realidade do aluno, suscitar a reflexão crítica acerca do assunto abordado, motivar o estudante e tornar a proposição passível de ser pesquisada. Nossa intenção com este artigo é oferecer subsídios aos professores na elaboração e utilização de problemas investigativos em suas aulas.
A metodologia da Resolução de Problemas: uma proposta interdisciplinar sobre agrotóxicos na Educação de Jovens e Adultos Resumo Este artigo apresenta a análise de uma experiência de utilização da metodologia de Resolução de Problemas (RP) no Ensino Médio na modalidade da Educação de Jovens e Adultos (EJA). O objetivo desta investigação é analisar as formas de contribuição da sequência didática implementada para o desenvolvimento de conhecimentos conceituais, procedimentais e atitudinais relacionados com os impactos ambientais que os agrotóxicos podem causar. Para a coleta de dados, foram utilizados o Diário de Campo dos pesquisadores, o questionário de avaliação da proposta e a gravação do áudio da aula em que os estudantes vivenciaram a RP. Os resultados apontam que a sequência didática implementada favoreceu as aprendizagens supracitadas quanto à RP e aos conhecimentos científicos trabalhados.
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