ON THE MECHANISMS OF PHOTOREFLECTANCE IN MULTIPLE QUANTUM WELLS

Mechanisms of photoreflectance of MQW structures are studied. The photomodulation of the built-in surface electric field is analyzed. It is argued that in MQW structures unlike to bulk crystals the electric field does not result in a Franz-Keldysh-effect but rather in a Stark effect of subbands and excitons and a corresponding change of wavefunctions and oscillator strengths. The photoreflectance line shape is not of third derivative but of first derivative type in the case of MQW structures. Experimental photoreflectance spectra are reported which confirm these conclusions and allow the derivation of sublevel energies, broadening parameters, and exciton binding energies.