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Field-induced changes of the transmittance of quantum-well laser structures with five ternary wells and quaternary barriers and waveguides, grown on InP substrates, have been investigated at low temperatures. The method selectively picks up states in undoped regions that are particularly sensitive to electric fields: the heavy-hole exciton in quantum wells, responding by the quantum confined Stark effect, and the band gap of the thicker waveguides which responds by the Franz–Keldysh effect. Both effects are used to determine the transition energies and the electric field in the respective regions. It is found that in typical laser structures electrons and holes are localized to single wells as their coupling is overcome by the internal field. The internal field corresponds approximately to the ratio of the gap energy of the waveguide material and the thickness of undoped regions. Stark localization persists even if the internal field is reduced by forward bias down to a level where diffusion currents lead to electroluminescence. The narrow linewidth of the response of the localized heavy-hole excitons allows to resolve small splitting due to inhomogeneities of the wells. The evolution of the spectra with bias voltage and the comparison of the fields derived for quantum-well and waveguide regions, yield information on the distribution and on the homogeneity of the field, giving access to space charges.

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