ELECTRIC-FIELDS SEPARATION BY PHASE SELECTION IN MODULATION SPECTROSCOPY OF PHOTOREFLECTANCE

The built-in electric fields in a MBE grown delta-doped GaAs homojunct ion have been investigated by the techniques of photoreflectance and p hase suppression. Two Franz-Keldysh oscillation features originating f rom two different fields in the structure superimpose with each other in the photoreflectance spectrum. By properly selecting the reference phase of the lock-in amplifier, one of the features can be suppressed, thus enabling us to determine the electric fields from two different regions. We have demonstrated that only two PR spectra, in-phase and o ut-phase components, are needed to find the phase angle which suppress es one of the features. The electric field in the top layer is 3.5 +/- 0.2 x 10(5) V cm(-1), which is in good agreement with theoretical cal culation. The electric field in the buffer layer is 1.2 +/- 0.1 x 10(4 ) V cm(-1), which suggests the existence of interface states at the bu ffer/substrate interface. (C) 1997 Elsevier Science Ltd.