Below gap photoreflectance of semi‐insulating GaAs

Below‐gap structures in photoreflectance spectra of semi‐insulating GaAs were observed originating from electroabsorption of light reflected from the back surface. The peak amplitude of these structures was strongly temperature dependent within the range (78–290) K. The different lineshape at low temperature in comparison to that at room temperature 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 procedure provides the pure back surface reflection effect for both components. The X mode dominating at low temperatures is attributed to electromodulation of the field‐broadened excitonic absorption tail whereas the Y mode originates from the Franz–Keldysh electroabsorption.