This paper addresses the effect of generation of confined LO phonons by drifting electrons in quantum wells. We have obtained a general formula for the phonon increment as a function of the phonon wave vector? the electron drift velocity, and parameters of the structure, The kinetic parameters of the drifting electrons are estimated by using momentum and energy balance equations for electron scattering by the confined optical phonons. We have performed numerical estimates of the phonon increment, as well as the phonon lifetimes, and found that AlAs/GaAs/AlAs and GaSb/InSb/CaSb quantum well structures with high drift velocities can demonstrate the effect of generation of the coherent confined optical modes. Essentially, the phonon increment has a maximum as a function of the wave vector. This implies a strong selection of the generated phonon modes. We briefly discuss the nonlinear electron mechanism which stabilizes the increase of the phonon populations and provides for the steady-state phonon generation.

This letter addresses the effect of generation of confined LO phonons by drifting electrons in quantum wells. We have derived a general formula for the phonon increment as a function of phonon wave vector, electron drift velocity, and structure parameters. Numerical estimates of the phonon increment and the phonon lifetimes have shown that AlAs/GaAs/AlAs and GaSb/InSb/GaSb quantum well structures can demonstrate the effect of coherent LO phonon generation by the electric current.

Original results are presented on the development of new stable and reproducible silicon diode temperature sensors (DTSs), characterized by high interchangeability, with the temperature response curve controlled by the current. For these sensors, in a broadened range of temperatures 4.2-500 K, the influence of Joule heating and p-n junction noise on the accuracy of temperature measurement has been minimized. The results of theoretical and experimental investigations are presented concerning the contributions of different currents to the sensor characteristics that allow the DTS performance to be optimized. The limiting values of temperature measurement uncertainty for the DTSs have been established. The advanced DTSs are intended for the range of low to middle temperatures and have record technical characteristics for cryogenic applications.

The infrared absorption spectra of polycrystalline LaMnO3 and CaMnO3 measured in the frequency range of the transverse optical phonons are reported, discussed, and compared with calculations of the lattice dynamics. For LaMnO3 the measured spectrum is consistent with the expected orthorhombic structure. For CaMnO3 the spectrum reflects a symmetry lower than that of a cubic perovskite. We find a close correspondence between the spectra of the two materials which suggests a similarity in their structures. [S0163-1829(99)09441-2].

Abstract Far-infrared transmittance of a SrTiO3 thin film deposited on a sapphire substrate by laser ablation was measured in the frequency range 25–200 cm–1 at temperatures from 5K up to 300 K. An interesting difference between the temperature behaviour of the polar soft mode in the thin film and the one in bulk material was observed. Below the structural transition near 105 K the mode stops softening and its damping increases on further cooling. The values of the total low-frequency permittivity in the film are substantially lower than those in bulk SrTiO3. The suggested explanation is based on the fact that the film contains relevant internal stress.

Infrared spectroscopy is a well-known powerful tool for characterizing the semiconductor thin films. First, we review description of thin films in terms of multiple reflection and transfer matrix. We apply this technique to ferroelectric thin films to obtain information which is important as a feedback for their preparation. We measured the transmittance and reflectance spectra of several perovskite materials and demonstrated potential of this technique. These measurements enable much more direct observation of the soft mode temperature dependence than other techniques as specular reflectance of bulk samples.

We calculated the room temperature dielectric response in the far-infrared range of a hypothetical polycrystal of lead titanate (PT), using the effective medium theory. These calculations were done on the basis of the mode assignment proposed in the recent Raman scattering experiments on PT single crystals [Foster et al., Phys. Rev., B48, 1993, 10160]. The calculated function reveals good agreement with the experimentally measured room temperature dielectric response of a polycrystalline PT thin film. We concluded that the effective medium theory can be successfully used for the direct comparison of dielectric responses of ferroelectric thin film and those of bulk materials and for checking out the validity of different mode assignments as well.

Results recently obtained from IR reflectivity and far-IR transmission measurements of various ferroelectric and related crystals, bulk ceramics and thin films are summarized. In almost all cases the complex dielectric function was evaluated and compared with lower-frequency results. Transmission measurements provide a more accurate way than reflectivity for evaluating the dielectric response.

We report on the first quantitative evaluation of the soft-mode behaviour of the relaxor PLZT 9.5/65/35. The transmission spectra of a PLZT thin film deposited on a sapphire substrate were measured in the region at temperatures from 300 up to 523 K, and in the range at 8 - 300 K. We fitted the spectra using the model of classical oscillators, and calculated the parameters of the observed lattice modes. The results were compared with those for bulk ceramics, and satisfactory agreement was achieved. The soft mode is heavily damped, and below room temperature it shows a strong hardening on cooling. Above room temperature, its eigenfrequency levels off, but the corresponding dielectric loss maximum continues to soften weakly down to at 523 K, where the mode becomes overdamped. The lower-frequency data require an additional relaxation below the measured range. Comparison of all of the available data at room temperature points to strong polydispersive losses in the Hz range, and another dispersion region at lower frequencies. We assign them to fluctuations of the polar nanocluster volume, and to reorientations of the cluster polarization, respectively. Their contribution to the low-frequency permittivity is much stronger than that of the soft mode.

Infrared reflectivity and Raman spectra of hot-pressed PLZT 8/65/35, 9.5/65/35 and 2/95/5 ceramics show only weak temperature changes in the first two (ferroelectric) compositions but much richer spectra and pronounced temperature changes in the third (antiferroelectric) composition, including a soft phonon mode.