In x Ga 1−x As 1−y N y / GaAs single quantum wells emitting at room temperature in the wavelength range λ=(1.3–1.55) μm have been studied by photoluminescence (PL). By increasing temperature, we find that samples containing nitrogen have a luminescence thermal stability and a room temperature emission efficiency higher than that of the corresponding N-free heterostructures. The temperature dependence of the PL line shape shows a progressive carrier detrapping from localized to extended states as T is increased. Finally, the extent of the thermal shift of the free exciton energy for different y indicates that the electron band edge has a localized character which increases with nitrogen content.

We present a complete study both by experiments and by model calculations of quantum dot strain engineering, by which a few optical properties of quantum dot nanostructures can be tailored using the strain of quantum dots as a parameter. This approach can be used to redshift beyond 1.31μm and, possibly, towards 1.55μm the room-temperature light emission of InAs quantum dots embedded in InGaAs confining layers grown on GaAs substrates. We show that by controlling simultaneously the lower confining layer thickness and the confining layers’ composition, the energy gap of the quantum dot material and the band discontinuities in the quantum dot nanostructure can be predetermined and then the light emission can be tuned in the spectral region of interest. The availability of two degrees of freedom allows for the control of two parameters, which are the emission energy and the emission efficiency at room temperature. The InAs∕InGaAs structures were grown by the combined use of molecular beam epitaxy and atomic layer molecular beam epitaxy; their properties were studied by photoluminescence and photoreflectance spectroscopies and by atomic force microscopy; in particular, by means of photoreflectance not only the spectral features related to quantum dots were studied but also those of confining and wetting layers. The proposed approach has been used to redshift the room-temperature light emission wavelength up to 1.44μm. The optical results were analyzed by a simple effective-mass model that also offers a rationale for engineering the properties of structures for efficient long-wavelength operation.

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The energies of bipolaron states in threeand two-dimensional systems are calculated variationally, treating the electron-phonon interaction in the Frohlich approximation, and separating the relative motion from the center-of-mass motion. The bipolaron is bound if the electron-phonon coupling constant a is larger than 6 in three dimensions and 2 in two dimensions, provided the ratio g=e /eo is smaller than a critical value which depends on n. The theory yields the free-polaron energies as given in the Lee, Low, and Pines approximation for arbitrary values of the electronphonon coupling constant o;. For o; larger than -10 and -4 in the threeand two-dimensional cases, respectively, the lowest free-polaron self-energies are obtained in the strong-coupling approach; it is shown that a trial harmonic envelope wave function, localized both in the center-ofmass and relative coordinates, with a coherent phonon state gives a bound localized bipolaron state of lower energy. It is interesting for application to high-T, superconductivity that the bipolaron bounds more easily in two dimensions than in three, and that the mean value of the pair radius is a few angstroms. Furthermore, bipolaron states obtained when the linear total momentum is conserved have intrinsically high mobility, which is also an important condition to make the bipolaron mechanism consistent with high-temperature superconductivity.