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We study, by time-resolved luminescence with subpicosecond resolution, the tunneling of electrons out of a narrow well (NW) coupled to a wider well (WW} by a thin barrier. The relative energy positions of the NW ground state and of the WW first excited level are changed by adjusting the WW width. Near resonance, and for narrow enough barriers, the transfer time of the electrons to the WW ground state is close to 2 ps and is analogous to LO-phonon-assisted intersubband relaxa-0 tion. When the barrier thickness increases above 40 A, this transfer time increases exponentially with the barrier thickness. The barrier thickness above which the variation becomes exponential strongly depends on the energy mismatch of the levels in the isolated wells and is indicative of interface roughness. The dependence of the decay times on barrier thickness for purposely misaligned samples is always exponential. 42 7021 1990 The American Physical Society 7022 B. DEVEAUD et al. 42 wells are selectively doped p type to a level of about 10" cm, the barriers are undoped. In the first series, designed to be "on resonance, " the well widths are kept at 60 and 140 A, the barrier being changed from 75 to 30 A (the well widths of 60 and 140 A correspond approximately to the resonant case when taking into account the p-type doping and thus the transfer of roughly half of the holes from the NW to the WW). In the second series, designed to be "off resonance, " the well widths are kept to 60 and 120 A, the barrier being changed from 80 to 15 A [for this series, the samples are undoped: this increases the decay times by about a factor of 2 (Ref. 26)]. In the third series, the barrier is kept at about 40 A, the NW at 60 A, and the WW width is changed from 120 to 190 A, thus changing the resonance conditions. In the following, the samples will be labeled according to the NW width, barrier width, and WW width in A.

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The study of the physical properties of the superlattices (Ga 1−xAlxAs)n1(GaAs)n2/GaAs(001) requires the knowledge of their structural parameters n1, n2 and x and of their crystalline state. X‐ray diffraction enables the existence of such artificial superstructures, obtained by molecular‐beam epitaxy, to be revealed and their characteristic parameters to be determined. Two experimental techniques are used: a standard powder goniometer and a double‐crystal diffractometer. The diffraction diagrams directly yield the superperiod and the average Al concentration in the superlattice. The value of x is determined by refinement between observed and calculated structure factors. The results are even more accurate when the number of observed satellite peaks for a given periodicity is greater; this number depends at the same time on the Al composition x, the n1/n2 ratio, the periodicity and its dispersion and the shape of the interfaces. This method is illustrated by a few examples with superlattice periods varying from 51 to 397 Å. The consequence on the X‐ray diagrams of various defects (dispersion in n1 and n2, superperiod gradient, Al diffusion) is discussed.

The formation and identification of binary compounds in Au-InP system has been reported. The contact reaction was investigated in the temperature range 320–360 °C by the combined use of 2-MeV He+ ions backscattering spectrometry, x-ray diffraction, TEM, and SEM. The results give evidence that both In and P form compounds with gold; formation of oxides has not been observed. A model of metallurgical structure of Au/InP contacts annealed at temperatures below or equal to 360 °C has been proposed.