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The carbon vacancy (VC) has been suggested by different studies to be involved in the Z1/Z2 defect-a carrier lifetime killer in SiC. However, the correlation between the Z1/Z2 deep level with VC is not possible since only the negative carbon vacancy (V−C) at the hexagonal site, V−C(h), with unclear negative-U behaviors was identified by electron paramagnetic resonance (EPR). Using freestanding n-type 4H-SiC epilayers irradiated with low energy (250 keV) electrons at room temperature to introduce mainly VC and defects in the C sublattice, we observed the strong EPR signals of V−C(h) and another S = 1/2 center. Electron paramagnetic resonance experiments show a negative-U behavior of the two centers and their similar symmetry lowering from C3v to C1h at low temperatures. Comparing the 29Si and 13C ligand hyperfine constants observed by EPR and first principles calculations, the new center is identified as V−C(k). The negative-U behavior is further confirmed by large scale density functional theory supercell calculations using different charge correction schemes. The results support the identification of the lifetime limiting Z1/Z2 defect to be related to acceptor states of the carbon vacancy.

Impacts of reduction of deep levels and surface passivation on carrier lifetimes in p-type 4H-SiC epilayers are investigated. The authors reported that the carrier lifetime in n-type epilayers increased by reduction of deep levels through thermal oxidation and thermal annealing. However, the carrier lifetimes in p-type epilayers were not significantly enhanced. In this study, in order to investigate the influence of surface passivation on the carrier lifetimes, the epilayer surface was passivated by different oxidation techniques. While the improvement of the carrier lifetime in n-type epilayers was small, the carrier lifetime in p-type epilayers were remarkably improved by appropriate surface passivation. For instance, the carrier lifetime was improved from 1.4 μs to 2.6 μs by passivation with deposited SiO2 annealed in NO. From these results, it was revealed that surface recombination is a limiting factor of carrier lifetimes in p-type 4H-SiC epilayers.

In this paper, we have investigated reliability of n- and p-type 4H-SiC(0001) metal-oxide-semiconductor (MOS) devices with N2O-grown oxides and deposited oxides annealed in N2O. From the results of time-dependent dielectric breakdown (TDDB) tests, it is revealed that the N 2O-grown oxides have relatively-high reliability (4-30Ccm -2 for n- and p-MOS structures). In addition, the deposited SiO 2 on n- and p-SiC exhibited a high charge-to-breakdown of 70.0 and 54.9Ccm-2, respectively. The n/p-MOS structures with the deposited SiO2 maintained a high charge-tobreakdown of 19.9/15.1Ccm -2 even at 200 °C. The deposited SiO2 annealed in N2O has promise as the gate insulator for n- and p-channel 4HSiC(0001) MOS devices because of its high charge-to-breakdown and good interface properties.

We study abstract macroeconomic systems in which expectations play an important role. Consistent with the recent literature on recursive learning and expectations, we replace the agents in the economy with econometricians. Unlike the recursive learning literature, however, the econometricians in the analysis here are Bayesian learners. We are interested in the extent to which expectational stability remains the key concept in the Bayesian environment. We isolate conditions under which versions of expectational stability conditions govern the stability of these systems just as in the standard case of recursive learning. We conclude that Bayesian learning schemes, while they are more sophisticated, do not alter the essential expectational stability findings in the literature.

Made available in DSpace on 2016-08-29T14:10:19Z (GMT). No. of bitstreams: 1 tese_469_.pdf: 769783 bytes, checksum: f897c912d8e80a0c8885f4c119931e9e (MD5) Previous issue date: 2005-08-09 O presente trabalho objetivou identificar a identidade social de japoneses residentes na Grande Vitória (ES) a partir de falas sobre o próprio grupo e suas relações cotidianas, verificando-se também a existência de indícios de preconceito ou de tratamento discriminatório. Participaram da pesquisa 20 sujeitos, japoneses ou descendentes, de ambos os sexos, jovens e adultos. Os dados foram coletados através de entrevistas que exploraram os seguintes temas: namoro e conjugabilidade, padrões estéticos, preconceitos e estereótipo, identidade social e adaptação. Os resultados indicaram a existência de semelhanças e diferenças no que diz respeito à identidade do japonês capixaba, definida através de comparação com as características identitárias de japoneses de outros Estados. A principal semelhança identificada foi a flexibilidade de identidade social de nipo-brasileiro, que permite que em determinados momentos façam uso da identidade japonesa e, em outros momentos, da identidade brasileira, sempre procurando manter uma identidade social positiva. A principal diferenças que caracterizou os japoneses locais foi a apropriação de características atribuídas ao capixaba: ele é mais tranqüilo, mais fechado e mais normal. Foram encontrados também alguns indícios de discriminação e preconceito contra os japoneses locais. Tanto a receptividade quanto a adaptação ao contexto capixaba foram considerados boas, apesar de ainda indicarem um certo estranhamento por parte dos dois grupos, um em relação ao outro. Alguns fatores, como a imigração tardia para o Estado, o número reduzido de japoneses e uma comunidade japonesa economicamente homogênea, parecem afetar o tipo de relação estabelecido entre Capixabas e Japoneses e, conseqüentemente, a construção das identidades sociais.