Electro-optical properties of GaAs doping superlattices containing extreme electric fields

We present SIMS, electrical ( I V and C V), photoluminescence and electro-optical (modulated absorption and reflectance) measurements on a number of MOCVD grown n-p-n-p-...-p and n-i-p-i-...-p GaAs doping superlattices with individual layer thicknesses of between 18 and 70 nm, and n- or p-doping of 10¹⁸ cm^-3. A comparison of the SIMS, PL and electrical data shows that C/v measurements are particularly sensitive to disorder in this type of sample, specifically because the separation between dopants is comparable with the thickness of individual layers in many cases. The application of a bias to the outer layers of the sample allows modulation of the built-in fields in the structure, allowing very large fields to be generated over a limited region of the sample, before the eventual breakdown of the diode, particularly when small 400 μm diameter mesa structures are used. The electro-modulated transmission and reflectance spectra have been measured for light incident normal to the layers, for samples containing internal fields of up to 700 kV/cm. For photon energies close to the band gap of GaAs, the modulation properties of the structures are consistent with the Franz-Keldysh mechanism, up to the highest internal fields, with no evidence for the anomalous behaviour reported elsewhere. At lower photon energies, a dispersionless modulation in optical transmission is observed whose origin is related to the linear and quadratic electro-optic effects. In this region, an anomalous dependence on optical polarisation is observed.