A study of the Franz-Keldysh oscillations in electromodulation reflectance of Si-delta-doped GaAs by a fast Fourier transformation Fourier transformation ͑FT͒ has been used in the theoretical line shape analysis of Franz-Keldysh oscillations ͑FKOs͒ in detail by numerical simulations. FKOs from the surface-intrinsic-n ϩ GaAs structure were obtained in photoreflectance ͑PR͒ measurements with various modulation light intensities and with different strengths of bias light illumination, which were used to change the static electric field in the intrinsic layer of the sample. The FT spectra of the PR spectra, including the real part, imaginary part, and the modulus, were very consistent with the theoretical line shapes. The ratio of the square root of the reduced mass (ͱ L /ͱ H ) and the ratio of transition strength of the electron heavy hole to the electron light hole were obtained from the FT spectra. In addition, the electric field in the intrinsic layer of the sample without and with bias illumination and the modulation field induced by photomodulation were also obtained.

Complex Fourier transformation (CFT) has been employed to analyse contactless electroreflectance (CER) spectra from an undoped-n + GaAs structure with various ac modulations and dc bias voltages. The CFT spectra of CER have been compared with those of photoreflectance (PR). It has been found that the CER non-flat modulation is between the built-in electric field and a larger electric field which increases with the modulation voltage. The result has been explained by the screening of the applied modulation electric field in one of the two half modulation cycles and the trapping of electrons in surface states in the other half modulation cycle. The dc bias does not change the CER spectra, hence their CFT spectra. This is because of the screening of the applied dc bias electric field.

Self-assembled InAs quantum dots (QDs) have been fabricated by depositing 1.6, 1.8, 2.0 and 2.5 monolayer (ML) InAs on surfaces of the undoped-n+ (UN+) type GaAs structure. Room temperature contactless electroreflectance (CER) was employed to study the built-in electric field and the surface Fermi level pinning of these QD-covered UN+ GaAs samples. The CER results show that 1.6 ML InAs QDs on GaAs do not modify the Fermi level, whereas for samples with more than 1.6 ML InAs coverage, the surface Fermi level is moved to the valence band maximum of GaAs by about 70 meV (which is independent of the InAs deposition thickness) compared to bare GaAs. It is concluded that the modification of InAs coverage on the Fermi level on the GaAs surface is due to the QDs, rather than to the wetting layer.

Photoreflectance spectra of doped molecular‐beam‐epitaxy GaAs films grown on semi‐insulating GaAs substrate, modulated by different pump beams, are studied. From the interference effect of two light beams reflected from different interfaces of a sample, the PR signals from the surface of the MBE film and that from the interface of film and substrate can be distinguished in the spectra. The behavior of the electric fields of the space charge region at the surface of GaAs and the interface of GaAs/SI‐GaAs is also investigated.

No abstract