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The influence of a sputter-deposited titanium adhesion layer on glass and Si (1 1 1) substrates with or without exposure of titanium layer surface to the atmosphere on the growth mode of the tantalum films was studied. It was found that, because of the well match of the cell constants between the deposited Ta and the underlying Ti, the tantalum films continuously deposited on the adhesion layer of titanium grown without breaking vacuum always grew with the orientation following that of the bottom titanium layer and nucleated bcc α phase of tantalum with low degrees of orientation. After exposure of the bottom titanium layer surface to atmosphere, the absorption and diffusion of oxygen and other impurity atoms on the surface would promote a higher lever of oxidation and favor to form more and more titanium suboxides at the Ti surface. The increase of titanium suboxides at the Ti surface change the lattice mismatch between the underlying titanium film and the bcc-Ta film, which disturbed the nucleation of α-Ta and promoted the growth of β-Ta. Consequently, the electrical properties of the Ta/Ti bi-layer films were significantly changed.

DX center-like traps were detected and characterized by DLTS in the n + -Ga 0.7 Al 0.3 As buffer layers of PIN heterostructures. Their thermal capture barriers were measured by an original method that used the intrinsic superlattice layer as a light source to photoionize the captured electrons. The traps are very close to the conduction band edge, and have a large capture barrier. They are located at 0.04 eV and 0.01 eV under the conduction band.

We have studied the behaviour of electron irradiation induced defects in δ-doping GaAs superlattices, using capacitance-voltage and deep level transient spectroscopy. For the planar doping used (10 11 SSi cm −2 ) with a distance of 70 a between each plane, we observed a delocalization of the carriers, in agreement with theoretical calculation. The defects created by the irradiation are identical to the defects created in bulk GaAs. However, we observed in addition two new defects which are tentatively ascribed to a complex formed between the silicon dopant and the arsenic vacancy.

We first show how the characteristical properties of defects (energy level capture cross-sections) in systems such as single heterostructures and superlattices can be deduced from the corresponding properties in a bulk material. Concluding that these characteristics can be obtained once the band structure of the system is known we discuss as illustration the problems encountered with the two principal defects present in GaAs and related alloys namely the EL2 defect and the DX center.

We describe how the electrical characteristics (energy level in the gap, electron capture cross section) a defect has in a bulk material are modified when this defect is placed in the well or the barrier of a superlattice. The relations between these characteristics we deduce are verified in GaAs-GaAlAs superlattices using electron irradiation induced defects as test. The implications of these considerations to the characterization of deep levels in superlattices as well as of the superlattices themselves are discussed.

The DX center has been studied by Deep Level Transient Spectroscopy in series of GaAs-Ga1-xAlxAs (x = 0.3) and GaAs-AlAs short period superlattices. The existence or not of this center can be understood if its energy level is linked to the L miniband. This suggest a possible way to suppress them by using specific superlattices.