Photo- and electro-reflectance spectroscopy of δ-doped GaAs/AlAs multiple quantum well structures

The photoreflectance (PR) and contactless electroreflectance (CER) spectroscopies have been used to study optical transitions, electronic structure and internal electric fields in beryllium δ-doped GaAs/AlAs multiple quantum wells (MQWs) designed for terahertz sensors. QW widths ranged from 3 to 20 nm while doping densities varied from 2 × 10 ¹⁰ to 2.5 × 10 ¹² cm ^-2. From the Franz-Keldysh oscillations in PR and CER spectra the surface electric field strength was established. The optical spectra were found to exhibit excitonic behaviour up to acceptor density of 2.5 × 10 ¹² cm ². The origin of spectral features was identified on a basis of their dependence on optical bias and calculations of electronic structure under electric field. Modulation spectra of lightly doped samples were found to be dominated by symmetryallowed excitonic transitions while in highly doped samples additional features associated with symmetry forbidden transitions, coming into play due to internal electric field, were revealed. Selective detection of terahertz radiation by sensors based on the studied structures is demonstrated.