DETERMINATION OF BUILT-IN ELECTRIC-FIELDS IN DELTA-DOPED GAAS STRUCTURES BY PHASE-SENSITIVE PHOTOREFLECTANCE

Built-in interface electric fields in MBE-grown delta-doped and other layered structures were studied by a modification of photoreflectance spectroscopy, which allowed us to separate contributions originating from various regions of the structures. The modifications explores differences in temporal responses of photoreflectance contributions and the respective differences in the phase shift between photoreflectance signal and modulated pump. Proper selection of mudulation frequency and phase angle of a phase-sensitive detector allowed us to suppress one of the contributions and to extract the other. The proposed technique of phase separation, along with the Fourier analysis of Franz-Keldysh oscillations, enabled us to obtain quantitatively defined energy band diagrams of delta-doped and SIN+ structures. It was found that Fermi level is pinned by the interface states both at the GaAs undoped buffer/semi-insulating substrate interface and at the n+-GaAs buffer/n+-GaAs substrate interface. These interface states are presumable due to defects which arises at the initial stages of the molecular beam epitaxy.