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- Electrical AC behaviour of MPCVD diamond Schottky diodesPublication . Pereira, L.; Rodrigues, A.; Gomes, Henrique L.; Pereira, E.The present work reports some experimental results on the electrical AC behaviour of metal-undoped diamond Schottky diodes fabricated with a free-standing MPCVD diamond film (5 mum thick). The metals are gold for the ohmic contact and aluminium for the rectifier. The capacitance and loss tangent vs, frequency shows that capacitance presents a relaxation maximum at frequencies near 10 kHz at room temperature. Although the simple model (small equivalent circuit) can justify the values for the relaxation, it cannot justify the departure from the Debye model, also verified in the Cole-Cole plot. Taking into account the existence of traps in the depletion region, a best fit to the experimental results was obtained. The difference between the Fermi level and the band edge of 0.2-0.3 eV is in agreement with the activation energy found from the loss tangent analysis. The capacitance with applied voltage (Mott-Schottky plots) gives a defect density of 10(16) cm(-3) with contact potentials near 0.5 V and the profile of defect density obtained shows a major density (approx. 10(17) cm(-3)) in a layer with a thickness less than 50 nm from the junction, decreasing by one order of magnitude with increasing distance. Finally a structural model is proposed to explain the AC behaviour found. (C) 2001 Elsevier Science B.V. All rights reserved.
- Optimized Gauss and Cholesky algorithms for using the LMMSE decoder in MIMO/BLAST systems with frequency-selective channels - Reduced-complexity equalizationPublication . Silva, J. C.; Souto, N.; Cercas, F.; Rodrigues, A.; Dinis, Rui; Jesus, S. M.The LMMSE (Linear Minimum Mean Square Error) algorithm is one of the best linear receivers for DSCDMA (Direct Sequence-Code Division Multiple Access). However, for the case of MIMO/BLAST (Multiple Input, Multiple Output/Bell Laboratories Layered Space Time), the perceived complexity of the LMMSE receiver is taken as too big, and thus other types of receivers are employed, yielding worse results. In this paper, we investigate the complexity of the solution to the LMMSE and the Zero-Forcing (LMMSE without noise estimation) receiver's equations. It will be shown that the equation can be solved with optimized Gauss or Cholesky algorithms. Some of those solutions are very computationally efficient and thus, allow for the usage of the LMMSE in fully-loaded MIMO systems.
- O Freixo Duarte de Armas: a história e recuperação da árvorePublication . Martins, L. M.; Silva, C. A.; Sousa, H.; Mariano, Alexandra de Brito; Madeira, S.; Sintra, A.P.; Leal, F.; Ferreira-Cardoso, J.; Duarte, J.; Rodrigues, A.; Quintas, M. C.
- Microelectrical characterisation of diamond films: an attempt to understand the structural influence on electrical transport phenomenaPublication . Pereira, L.; Pereira, E.; Gomes, Henrique L.; Rodrigues, A.; Rees, A.; Cremades, A.; Piqueras, J.The properties of microelectrical conduction in microwave plasma assisted chemical vapour deposition (MPCVD) diamond films were investigated using an atomic force microscopy probe, giving a morphological map of the electrical conduction with a spatial resolution better than 500 nm. Also, a cathodoluminescence map with a spatial resolution of about 1 mu m was obtained, giving the possibility of correlating the defects involved in the different carrier transport phenomena. Using micro-Raman analysis several bands could be identified. It is found that the defects responsible for the cathodoluminescence (CL) blue band are responsible for the major part of the electrical conduction in diamond films, while the defects localised in < 111 > surfaces, responsible for the green CL emission, could be involved in a less conductive process. (C) 2000 Elsevier Science S.A. All rights reserved.
- Employing the Block Fourier algorithm for solving the LMMSE receiver equation under variable channel conditionsPublication . Silva, J. C.; Dinis, Rui; Rodrigues, A.; Cercas, F.; Souto, N.; Jesus, S. M.The LMMSE (Linear Minimum Mean Square Error) algorithm are one of the best linear receivers for DS-CDMA (Direct Sequence-Code Division Multiple Access). However, for the case of MIMO/BLAST (Multiple Input, Multiple Output / Bell Laboratories Layered Space Time) with high loading, the perceived complexity of the LMMSE receiver is taken as too big, and thus other types of receivers are employed, yielding worse results. In this paper, we investigate the complexity of the solution to the MIMO LMMSE receiver's equations using Block-Fourier algorithms, for both steady and unsteady channel situations.