Temperature dependence of eigen-energies observed in optical transmittance of doped and undoped InGaAs/InAlAs MQWs

Energies of interband optical transition in InGaAs/InAlAs multi-quantum well (MQW) structures were studied using one undoped and two n-type modulation-doped specimens grown by MBE. Step-like allowed transitions were observed and identified between 120 K and 330 K. In a 10 nm-thick quantum well (QW), three typical eigen states of the conduction subband (CS) were discriminated in a 0.52 eV range of energy in the QW. Doped and undoped MQW specimens had almost the same transition energies from the valence subband to the CSs. In a highly doped specimen of 1.5×10¹² cm^-2 electrons per QW, the inter-ground-state optical transition was prohibited by the band filling effect. In the low doped specimen of 5×10¹¹ cm^-2 electrons per QW, however, the same transition below the Fermi energy level was observed at 120 K. Moreover, localized states or an impurity band with an activation energy of 25 meV associated with the ground and the second higher CSs, were observed. No localized state was observed in the undoped ones.