In this paper we discuss the conduction of heat in solids excited by a periodically time-dependent source, commonly encountered in photothermal experiments. In the analysis, we take into account the relaxation time necessary for the onset of the heat flux which leads to a second time derivative in the heat diffusion equation. It is shown that there exists a critical modulation frequency at which the heat flux changes its diffusive character to a wave propagation process with the velocity of sound. Evidence is provided for the possibility of detecting second sound in solids by means of a photothermal experiment.

Low-temperature photoluminescence spectroscopy was used to study AlxGa1−xAs intrinsic epitaxial layers in the range of aluminum content between 0.45<x<0.9 in the region where the band gap minimum is indirect. Nonintentionally doped samples were grown by liquid phase epitaxy very near the saturation temperature (∼800 °C). The heterostructures consisted of an AlxGa1−xAs layer grown on top of a GaAs Si-doped (n∼2×1018 cm−3) substrate with orientation (100) 2° toward the (110). There is little information on the zero phonon excitonic lines for Al compositions x≳0.6, because the appearance of these lines depends on several factors such as internal strength, crystalline defects, and impurities. In our samples the excitonic lines were observable up to x=0.9. Transitions related to bound-to-free carrier as well as their phonon replicas were observable too. The data obtained from the spectra fitted with multigaussian lines allowed us to estimate the dependencies on the Al composition, x, of the bound exciton peak, the bound exciton binding energy, and the exciton localization energy. The acceptor ionization energy attributed to carbon residual impurities in As sites and its dependence on x was also obtained. Fitting by Gaussian lines the phonon region, the PL spectra fitted better with the TO(X) AlAs-type branch rather than the LO(X) AlAs-type, with a linear dependence with x in the studied range. The phonon energies of the GaAs type seem to behave almost constant and it was difficult to assign them to the LO(X) GaAs type or to TO(X) GaAs type due to the scatter of the data for high values of x. Other phonon replicas from the bound exciton and the electron-to-acceptor carbon transition (e-A°) coincide with the LO(X) AlAs type and TA(X), TA(X)+LA(X) and two LA(X) phonon energies. All the dependencies we present here are valid for aluminum compositions in the range 0.48<x<0.90.

Atomic layer epitaxy (ALE) was used to grow CdTe and CdSe quantum wells (QWs) with 1 to 4 monolayer thickness. With this ultra-thin quantum wells (UTQWs) it is possible to cover the red-blue spectral range. Low temperature photoluminescence spectroscopy reveals that most of these UTQWs present very intense and narrow emission and, depending on growth conditions, no thickness fluctuations are observed over the whole sample area. The heterostructures were grown at different temperatures between 260 and 290 °C. Both CdTe and CdSe UTQWs grown at the higher temperature presented blue-shifted emission compared to those grown at lower temperatures. This effect is attributed to Cd losses by reevaporation and Cd substitution by Zn. This effect of the temperature can be employed for fine tuning of the UTQW emission. The critical thickness of CdSe/ZnSe grown by ALE is estimated to be 5 ML. 1 Introduction After the first blue-green laser diode, produced in 1991 from wide-band-gap II-VI semiconductors [1], many efforts have been addressed to improve their lifetime and spectral range; it is desirable to produce full color emission devices based on II-VI semiconductors. Zn 1-x Cd x Se/ZnSe quantum wells (QWs) with x < 0.25 and thickness of a few nm have been employed as the active region of blue-green LEDs and lasers grown on GaAs substrates. To extend the emission range towards lower energies, ternary alloys with a higher Cd content are required. However, the increase of Cd in the QW increases the lattice mismatch with the ZnSe barrier, the consequence is that strain and alloy disorder produce the degradation of the optical and structural properties of the QWs. In order to overcome these difficulties a broader range of materials have been employed. For example, full color LED emission has been demonstrated with ZnCdSe/ZnCdMgSe pseudomorphic QWs grown on InP [2]; CdZnSSe quaternary alloys have been used to produce laser emission in the 2.48 to 2.21 eV range (bluish green to green-

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