THE SIZES OF COHERENT BAND STATES IN SEMICONDUCTORS, DERIVED FROM THEFRANZ-KELDYSH EFFECT

Electroabsorption spectroscopy with small modulation voltages has been used to study coherent states above the gap of In0.53Ga0.47As. At ver y low fields only excitonic effects are observed which disappear in fi elds as small as 6 kV cm(-1) to become part of the Franz-Keldysh oscil lations, the response of the continuum states. Different sets are obse rved for the three valence bands which vary with field in perfect acco rdance with theory. The range of oscillations, which at moderate field s extends over 0.5 eV, increases linearly with field and is directly r elated 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 ladd er transitions in superlattices.