No abstract

Electrical properties of high-dose (1.6X 10' at./cm') H+-implanted B-doped silicon have been investigated using transient capacitance spectroscopy, capacitance-voltage, and spreading resistance profiling.The role of hydrogen is twofold: to interact with the defects created by ion implantation, modifying their electrical properties, and to neutralize the shallow-acceptor dopants. The evolution of the defects responsible for the deep levels and the depth of the neutralized region have been investigated after isochronal annealing at various temperatures up to 800'C. Deep-level transient spectroscopy spectra show three hole traps; two of them, H(0.67), H(0.33), have been tentatively identified as vacancy-hydrogen complexes (VH2, VH3) while the attribution of the third, H(0.23), detected in the samples annealed at 400-450'C, is uncertain. As a function of the heat treatment, the total number of defects is strongly reduced at 300'C, it increases for T )300 'C and at the highest temperatures, namely 800'C, the defects disappear. The first decrease is attributed to the formation of neutral VH4 complexes and the disappearance to complete decoration of point defects or their agglomeration. The thickness of the passivated region has a minimum at 300'C, which corresponds to the formation of the stable VH4. At lower temperatures, the hydrogen necessary for the passivation is the unbonded one and presumably comes from the implantation process itself. At higher temperature, H comes from the breaking of the VH4 or H"complexes. These results are in good agreement with our previous studies concerning the role played by hydrogen in affecting the crystal properties of silicon.

The changes that the UV absorption spectrum and the photophysics of uracil undergo under hydrogen substitution or deprotonation, were studied theoretically within the CS-INDO/CI scheme. First of all this method was tested on uracil. It was then used for the calculation of the electronic structure of excited states (Sn, Tn) of a large number of uracil derivatives (1-, 3- and 5-methyluracil; 1,3-, 1,5- and 3,5-dimethyluracil; 5-fluoro- and 5-chlorouracil), including some anions (1- and 3-methyluracil anion). The excited states were obtained in the singly-excited configuration interaction approximation (S-CI) and the correlation effects on (pi pi*) states were studied by including the most important doubly- and triply-excited configurations in the CI. The S-CI wavefunctions were used for the calculation of the most important electronic matrix elements for spin-orbit coupling. The photophysics of these compounds is discussed using Jablonski diagrams.

As a first step in the interpretation of the spectroscopic changes connected with stereoisomerization of a,w-diphenylpolyenes, a " quantitative " justification for the diffuse spectrum of stericallyhindered isomers in the near u.-v. region has been attempted taking trans-and cis-stilbene as test molecules. Potential energy curves have been obtained for two low-frequency vibrational modes, namely the twisting of the essential single bonds and the torsion of the central double-bond, in both the initial ('A) and final ('B) electronic states. The corresponding sets of vibrational energies and wavefunctions have been obtained by expressing the potential functions as power series and by expanding the wavefunctions in a basis of harmonic oscillator eigenstates. Franck-Condon distributions calculated for the the two vibrational modes have shown that the " twisting " rather than the " torsion " mode is responsible for the blurring of the fine structure in the conjugation band of cisstilbene.* the choice of stilbene as the first term of this series is supported by some peculiar similarities between the u.-v. spectra of stilbenes and those of a,w-diphenylpolyenes regarding the longestwavelength band.

No abstract