Estados eletrônicos e alinhamento de bandas em heterojunções por espectroscopia de elétrons

Orientadores: Fernando Alvarez, Florestano EvangelisteTese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb WataghinResumo: Apresentamos um novo método experimental para determinar o alinhamento de banda em heterojunções e aplicamos este para estudar a heterojunção c-Si/a-SixC1-x:H. Esse método utiliza uma versão moderna de uma espectroscopía muito utilizada: a espectroscopía de yield fotoelétrico excitada com fótons na região do ultra-violeta próximo. No espectro de yield podemos determinar as bordas da banda de valência do filme e do substrato devido ao grande intervalo dinâmico desta técnica. Também apresentamos um novo método para carbonizar a superfície de silício. Aplicando este método fomos capazes de crescer heterojunções c-SiC/c-Si a temperaturas inferiores das que são reportadas na literatura. A estrutura do filme e a morfologia foram estudadas in situ por RHEED e ex-situ por AFM, respectivamente. Encontramos que o crescimento ocorre em ilhas cristalinas orientadas preferencialmente na direção (001). As propriedades eletrônicas do filme foram investigadas in situ por XPS, UPS e LEYS-CFS. A análise das estruturas associadas aos níveis de caroço e a associadas aos plamons mostrou que temos um interface abrupta entre o filme estequiométrico e o substrato. O uso do LEYS-CFS permitiu a determinação da descontinuidade de banda de valência na interface embora o filme de SiC tenha nucleação em ilhas e não recobra completamente o substrato de Si. O valor encontrado para DEv foi 0.75± 0.08 eV. Esse resultado é discutido dentro da teoria existente na literaturaAbstract: We present a new experimenta1 method for determining band lineups at the semiconductor heterojunctions and apply it to the c-Si(100)/a-SixC1-x:H heterostructure. This method uses a modern version of an old spectroscopy technique: the photoeletric yield spectroscopy excited with photons in the near ultra-violet UV range. It is shown that both substrate and overlayer valence- and tops can be identified in the yield spectrum due to the high escape depth and the high dynamica1 range of the technique, thus a1lowing a direct and precise determination of the band lineup. We a1so present a new method to carbonize silicon surface. Applying this method we were able to grow c-SiC/c-Si heterojunctions at lower temperatures than the one reported in the literature. The c-SiC film structure and morphology were investigated in situ by Reflection High-Energy Electron Diffraction (RHEED) and ex-situ by Atomic Force Microscopy (AFM), respectively. It was found that the growth proceeds through the nucleation of cubic and relaxed crysta1line islands preferentia1ly oriented in (001) direction. The average island size increases with carbonization time. The electronic properties were investigated in situ by X-ray and UV photoemission spectroscopies and by photoelectric yield spectroscopy excited with low energy photons (visible and near -UV range). The ana1ysis of high resolution core level and plasma excitation spectra unambiguously shows that only stoichiometric SiC is present at the interface and demonstrates that the interface itself is abrupt. The use of the photoelectric yield technique operated in the constant final state mode a1lowed us to determine the va1ence band discontinuity a1though the SiC overlayer was cluster-like and did not cover uniform1y the Si substrate. The found va1ue is 0.75± 0.08 eV. This result is discussed in the framework of the current lineup theoriesDoutoradoFísicaDoutor em Ciência

Bad news has wings : dread risk mediates social amplification in risk communication

Social diffusion of information amplifies risk through processes of birth, death, and distortion of message content(1). Dread risk—involving uncontrollable, fatal, involuntary, and catastrophic outcomes (e.g., terrorist attacks and nuclear accidents)—may be particularly susceptible to amplification because of the psychological biases inherent in dread risk avoidance. To test this, initially balanced information about high or low dread topics was given to a set of individuals who then communicated this information through diffusion chains, each person passing a message to the next. A subset of these chains were also re-exposed to the original information. We measured prior knowledge, perceived risk before and after transmission and, at each link, number of positive and negative statements. Results showed that the more a message was transmitted the more negative statements it contained. This was highest for the high dread topic. Increased perceived risk and production of negative messages was closely related to the amount of negative information that was received, with domain knowledge mitigating this effect. Re-exposure to the initial information was ineffectual in reducing bias, demonstrating the enhanced danger of socially transmitted information