Abstract

We present a new kind of the modulation spectroscopy designed for the contactless investigation of highly conducting layers (such as two-dimensional electron gas) in semiconductor hetcrostructures. Widely used photolumincsccncc and photoreflectance measurements provide the worth information on the electron energy spectrum, the composition and doping of semiconductor layers, but they are incapable of determining the electron conductivity which is one of the most important characteristic. The method we propose is based on radio-frequency electric current effect on the semiconductor heterostructure with simultaneous measurement of the optical reflectance of the probing light. The modulated ac field acts selectively upon the electrons in the highly conducting layers giving rise to the reflectance variation. Being measured in such a way the modulation spectra are sensitive to the electron conductivity. In the experiments we have used modulation doped hetcrostructures GaAs/AlGaAs subjected to the electric field (about 10³V/cm) with a frequency of 25 MHz. At room temperature die ac field produces only small variation in the reflectance, but cooling to the temperature 77K results in the reflectance increase by a factor of 10³. The commonly used photoreflectance response is much less dependent of die temperature. The ac current induced reflection (CIR) spectrum is shown in the figure together with the photoreflectance (PR) spectrum.

© 1996 IEEE