The sizes of coherent band states in semiconductors, derived from the Franz-Keldysh effect

Electroabsorption spectroscopy with small modulation voltages has been used to study coherent states above the gap of In0.53Ga0.47As. At very low fields only excitonic effects are observed which disappear in fields as small as 6 kV cm(-1) to become part of the Franz-Keldysh oscillations, the response of the continuum states. Different sets are observed for the three valence bands which vary with field in perfect accordance with theory. The range of oscillations, which at moderate fields extends over 0.5 eV, increases linearly with field and is directly related to the coherence length of the continuum states. The coherence length in the ternary sample is 160 nm, more than twice as large as in a quaternary sample and larger than the value derived from Stark ladder transitions in superlattices.