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A concept for the fabrication of highly symmetric quantum dots that are coherently embedded in a single crystalline matrix is demonstrated. In this approach, the formation of the quantum dots is induced by a transformation of an epitaxial 2D quantum well into an array of isolated precipitates with dimensions of about 25 nm. The formation process is driven by the immiscibility of the constituent materials resulting from their different lattice structures. The investigated PbTe/CdTe heterosystem combines two different cubic lattices with almost identical lattice constants. Therefore, the precipitated quantum dots are almost strain free and near thermodynamic equilibrium they exhibit the shape of small-rhombo-cubo-octahedrons.The PbTe/CdTe quantum dots, grown on GaAs substrates, display intense room temperature luminescence at wavelength around 3.2 µm, which makes them auspicious for applications in mid-infrared photonic devices.

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We have applied positron annihilation spectroscopy combined with sheet resistance measurements to study the electrical isolation of thin ZnO layers irradiated with 2 MeV O + ions at various fluences. Our results indicate that Zn vacancies, the dominant defects detected by positrons, are produced in the irradiation at a relatively low rate of about 2000 cm −1 when the ion fluence is at most 10 15 cm −2 and that vacancy clusters are created at higher fluences. The Zn vacancies introduced in the irradiation act as dominant compensating centers and cause the electrical isolation, while the results suggest that the vacancy clusters are electrically inactive.

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