Microstructure development of HfO_x thin films

ABSTRACTMorphological and crystallographic characteristics of HfOx with different oxygen concentrations (0 ≤ x ≤ 2.5) and thicknesses (18 nm ≤ t ≤ 310 nm) were analyzed in this work before and after thermal annealing at 700°C in N2 atmosphere for 1h. The morphology of the as-deposited, low oxygen concentration films (t = 100 nm) is formed by a well-defined granular structure with grains around 20 nm in diameter. For higher oxygen concentration, the roughness increases, as a consequence of a very porous surface morphology. At the same time, the crystallographic structure changes from HCP with a {0002} preferred orientation to an amorphous structure as oxygen concentration increases. As the thickness of the HfO2 films increases, we also observed the formation of a high surface porosity, with pore diameter ranging from 80 nm to 120 nm. The changes observed in morphology and crystallinity of the films, as we increase the concentration of contaminants, occurs because of the lowering of the adatoms surface diffusion, which prevents them from reaching lower energy points during the formation of the microstructure of the films. Furthermore, this lowered surface diffusion favors the process of grain renucleation, which leads to roughness increasing, and to an enhancement of the formation of porous surfaces. After annealing, all films exhibit the monoclinic crystallographic phase associated with HfO2. Surface morphology of the films is consistent with a polycrystalline structure with grain diameters varying between 10 nm and 200 nm. As their size increases, the grains become very faceted, a finding consistent with the improvement in film crystallinity. Our results suggest that long-time annealing promotes the diffusion of oxygen from the SiO2-Hf interface to the film, compensating any O2 deficit in the film. Formation of the monoclinic phase is also favored by the improvement in the film stoichiometry promoted by thermal treatment. Also, the critical size of the nuclei, associated with a grain growth in a particular crystallographic orientation, decreases as a consequence of the high temperature during thermal annealing. Therefore, formation of faceted grains and increasing of surface roughness are favored in thicker films.</jats:p

Raman and infrared spectroscopy studies of carbon nitride films prepared on Si (100) substrates by ion beam assisted deposition

Carbon nitride films prepared by ion beam assisted deposition were studied by Raman and infrared spectroscopy, as a function of nitrogen ion energy (200, 400, and 600 eV) and the ion to atom arrival rate ratio R(I/A) ranging from 0.9 to 2.5. The composition ratio N/C in the film determined by elastic recoil detection analysis was found to be proportional to R(I/A); however, the film growth is possible only if R(I/A) is smaller than a critical value of chemical sputtering yield. This value was found to be 0.21. The maximum value of N/C obtained was 0.9 (47 at.% of nitrogen) for the film deposited with ion energy of 400 eV. In order to understand the effect of nitrogen incorporation on the structure of the films, the parameters were determined from the Raman spectra analysis of the films and found to show strong dependences of the nitrogen content in the films; the behaviors of the G peak position and width, and I D/I G ratio are correlated with the structure changes in the films

Recombinant xylanase production by Escherichia coli using a non-induced expression system with different nutrient sources

The application of enzymes for sustainable and low-environmental impact industrial processes requires high-level enzyme production at low-cost. A promising strategy is the use of a high efficiency heterologous protein expression system using E. coli and the pT7BsXA vector encoding the GH11 xylanase from Bacillus subtilis with promoter, replication origin and signal peptide sequences from B. subtilis (Ruller et al. 2006). This expression system produces high amounts of enzyme that are secreted to the culture broth. The present study aimed to maximize the xylanase production by this system through evaluation of culture medium composition. Different culture media previously described in the literature together with compositions derived from agro-industrial residues were evaluated. A culture medium derived from agro-industrial residues using sugarcane molasses as carbon source showed a 9-fold increase in enzyme production (195,000 U/L) in relation to LB medium and the lowest production cost, which was 8.5-fold lower than LB medium using sugarcane molasses as carbon source and brewer's yeast as vitamin source in shaker experiments. In a bioreactor experiment the best production medium promoted an 8.5-fold higher production at a 10.8-fold lower cost as compared to shaker LB cultivation.Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Verdartis Desenvolvimento Biotecnol Ltda, Av Dra Naldir Ribeirao Preto,1805,Jardim Dr Paulo, BR-14056680 Ribeirao Preto, SP, BrazilUniv Sao Paulo, Fac Filosofia Ciencias & Letras Ribeirao Preto, Dept Quim, Ave Bandeirantes 3-900, BR-14040900 Ribeirao Preto, SP, BrazilUniv Estadual Paulista, Fac Ciencias Farmaceut, Dept Bioprocessos & Biotecnol, Rodovia Araraquara Jau,Km 01,Campos Ville, BR-14800903 Araraquara, SP, BrazilFundacao Oswaldo Cruz, FIOCRUZ, Rua Sao Jose S-N, BR-61760000 Eusebio, CE, BrazilUniv Estadual Paulista, Fac Ciencias Farmaceut, Dept Bioprocessos & Biotecnol, Rodovia Araraquara Jau,Km 01,Campos Ville, BR-14800903 Araraquara, SP, BrazilFAPESP: 2008/53426-7FAPESP: 2010/50328-