We have studied the band-gap reduction of GaAs1−xNx alloys with N contents between 0.1% and 1.5% using the absorption measurements in a series of samples grown by molecular-beam epitaxy. At room temperature, we observed a redshift of the band edge of about 205 meV for 1% of nitrogen. To interpret this effect, we assume that the incorporation of nitrogen in GaAs breaks the symmetry of the system and introduces localized N states that are weakly coupled to the extended states of the semiconductor. We have also studied the temperature dependence of the band gap of GaAsN alloys. The band-gap energy difference between 15 and 300 K decreases from 110 meV for GaAs, to 70 meV for GaAs0.985N0.015. All these experimental results can be explained by the band anticrossing model [W. Shan et al., Phys. Status Solidi B 223, 75 (2001)] with a coupling constant CNM of 2.7 eV, taking into account the contribution of tensile strain to the band-gap reduction.
An extensive study of time-resolved photoluminescence is made in the temperature range 1.8 to 50 K on nine samples of type-I1 GaAs-A1As (001) superlattices. In previous publications, the region is identified where the states derived from X, valleys are lower than those derived from X,, valleys, in agreement with a simple model involving the competition between X-type valley anisotropy and X valleys splitting caused by the small lattice mismatch between AlAs and GaAs. An analysis is given of the luminescence processes and of their evolution with temperature, as a function of AlAs and GdAs layer thicknesses. Excitons localized by interface roughness are found to decay through radiative processes at low temperature. A model of thermal excitation from the tail of localized exciton states is introduced, which helps to identify the pertinent parameters of temperature behaviour in these samples.On presente une etude de la photoluminescence resolue en temps de neuf echantillons de superreseaux GaAs/AlAs (001) de type I1 dans l'intervalle de temperature 1,8 a 50 K. Dans une publication prkcedente, nous avions identifie la region ou les etats derives de la vallee X, sont plus bas que ceux issus de vallees X,,, d'aprks un modele invoquant la competition entre l'effet de I'anisotropie de masse des vallees X et l'eclatement de ces memes vallees d t au petit desaccord de maille entre GaAs et AlAs. On presente une analyse des processus de luminescence et de leur evolution avec la temperature, en fonction des epaisseurs des couches de GaAs et dA1As. On trouve que le declin des excitons localisks par la rugosite d'interface est domine par les processus radiatifs B basse temperature. On propose un modele d'excitation thermique i partir d'une queue d'etats localists, qui met en evidence les parametres pertinents du comportement en temperature de ces echantillons.
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