BELOW GAP PHOTOREFLECTANCE OF SEMIINSULATING GAAS

Below-gap structures in photoreflectance spectra of semi-insulating Ga As were observed originating from electroabsorption of light reflected from the back surface. The peak amplitude of these structures was str ongly temperature dependent within the range (78-290) K. The different lineshape at low temperature in comparison to that at room temperatur e suggested different physical mechanisms. This conclusion was proven experimentally using a two phase lock-in technique by which a fast (X) and slow (Y) component of the spectra could be separated. A fit proce dure provides the pure back surface reflection effect for both compone nts. The X mode dominating at low temperatures is attributed to electr omodulation of the field-broadened excitonic absorption tail whereas t he Y mode originates from the Franz-Keldysh electroabsorption.