Segregation of column III atoms during molecular beam epitaxy of III-III′-V semiconductor compounds causes nonabrupt interfaces and a surface composition different from the bulk one. To derive concentration profiles, a thermodynamical equilibrium model has been used for a long time. This model applies well to describe segregation processes at high growth temperatures, but fails in predicting concentration profile variations with substrate temperature. We have thus developed a kinetic model which correctly takes into account the evolution with the growth temperature. We apply this model to the case of indium segregation in the GaxIn1−xAs/GaAs system. The calculated indium concentration profiles are compared to those obtained with the thermodynamical equilibrium model. A kinetic limitation of segregation is shown to appear at low substrate temperatures and sufficiently high growth rates. This limitation is predicted to arise below 400 °C for a growth rate of 1 monolayer/s for In segregation in the GaxIn1−xAs/GaAs system.
We compare forward and backward Raman scattering results on folded acoustical phonons in GaAs-AlAs superlattices with a detailed theoretical analysis of their dispersion properties and light scattering activity. By forward scattering, which involves phonons with a vanishing wave vector, we first get evidence of zone-center gaps, in quantitative agreement with the elastic model predictions. We also check the zone-center selection rules and conclusively prove the assignment of the light scattering on folded acoustical phonons to a modulated photoelastic (Brillouin) process. In backscattering experiments, one creates phonons with a finite wave-vector and the zone-center selection rules are relaxed. We quantitatively describe this phenomenon, and demonstrate that the backscattering intensities directly reflect the coupling between folded branches and the related zoneboundary gap magnitude.An excellent agreement between measured and calculated intensities is obtained. Finally we emphasize the great sensitivity of the gaps and intensities, contrary to the backscattering frequency shifts, to the supercell inner structure. This greatly enhances the interest of Raman scattering as a tool for characterizing periodic structures.
Abstract. In this paper we report on the observation of response of a Bloch oscillator at room temperature to a THz-field of a frequency larger than the Bloch frequency. The oscillator consisted of a semiconductor superlattice structure, with an applied dc voltage giving rise to a dc electron drift current. Submitting the oscillator to a field at a frequency of 3.3 THz caused a sizeable reduction of the current; the TH2-field was generated by use of intense THz-radiation pulses focused on an antenna coupled to the superlattice. We attribute the THz-field induced reduction of the current to a frequency modulation of the Bloch oscillations of electrons at the frequency of the THz-field, leading to reduction of the electron drift velocity and, consequently, of the current.Keywords: Superlattice; Bloch oscillator; 'hahertz radiation.According to Bloch [I], the electric conductivity by electrons moving in a periodic potential of atoms in a crystal superimposed with a potential of a homogeneous dc electric field is a consequence of scattering of the electrons due to defects and thermal vibrations of the atoms. Zener [2] pointed out that ballistic electrons, i.e. electrons moving without being scattered, should perform oscillations, today denoted as Bloch oscillations, because of the Bragg reflection of the electrons when their kinetic energy corresponds to the upper boundary of the energy band due to the periodic potential. Besides Bloch oscillations, i.e. electron motion within a band, electron transport between different bands separated by forbidden bands was recognized as origin of dielectric breakdown at high dc fields [2]. The eigen frequency of the Bloch oscillation, the Bloch frequency, is determined by the dc voltage across a spatial period of the periodic potential. Esaki and Tsu [3] proposed to study Bloch oscillations in semiconductor superlattices with periods that are, on the one hand, large enough for sustaining, without dielectric breakdown, a sufficiently large voltage across a period and, on the other hand, small enough to allow almost ballistic propagation of electrons over many periods and predicted that under the condition of the occurrence of Bloch oscillations a negative differential conductance should appear in the current-voltage characteristic * Permanent address: Institute for Physics of Microstructures, Russian Academy of Sciences, 46 UIjanov Str., 603600 Nizhny Novgorod, Russia.
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