Photoreflectance studies of optical transitions in type II (GaAs)_m(AlAs)_nsuperlattices

Abstract

Photoreflectance (PR) spectra of two type II [001](GaAs)m(AlAs)n superlattices (SLs) have been measured at 77 K. In the conventional picture of the envelope-function approximation, the lowest conduction state originates, in the first sample, from the Xz point of the AlAs Brillouin zone (z being the growth direction) whereas it originates from the Xx,y point in the second sample. Our spectra exhibit Franz–Keldysh oscillation (FKO) features and interband transition lines. The origin of the built-in electric field within the samples is discussed and its strength calculated from FKOs. For interpreting our spectra of interband optical transitions, a least-squares fit of the data to the Aspnes third-derivative functional form has been performed as well as computation of the optical transition energies. From the energy and amplitude of the interband transition lines in PR spectra, we showed that the two SLs are pseudo-direct, i.e. the ground optical transition in any of them is direct in the k space and takes place at the Γ point of the SL Brillouin zone. All the other interband transitions appearing in the SL spectra are also direct in k space.

Self‐Sensing Cementitious Composites with Hierarchical Carbon Fiber‐Carbon Nanotube Composite Fillers for Crack Development Monitoring of a Maglev Girder

“…At the same time, the concentration of pyrolysis carbon atoms landing on the catalyst cracking surface was high and it is easy to cause the local concentration of pyrolysis carbon atoms saturated, thus forming amorphous carbon and causing the catalyst deactivated through encapsulation, as observed that low yield CNTs corresponding with large amounts of amorphous carbons were formed. [ 54 ] With the increase of temperature to 650 °C, more obvious CNT growth was observed all over the CF surface. At 750 °C, both the diameter and the length of CNTs dramatically increased, indicating the catalyst is highly efficient and durable at this critical temperature.…”