Carrier tunneling in III-V and II-VI semiconductor heterostructures.

摘要

This dissertation describes experimental and theoretical studies of carrier tunneling in semiconductor heterostructures and optical properties of neutron irradiated quantum wells. Unambiguous experimental evidence for the dramatic dependence of hole tunneling rates on in-plane momentum in (Ga,In)As/(Al,In)As asymmetric double quantum wells (ADQWs) is presented. Holes generated near the bandedge tunnel on hundred picosecond time scales, whereas holes excited with large excess energy tunnel on subpicosecond time scales. The mechanism responsible for this increase of three orders of magnitude in the hole tunneling rate is nonresonant delocalization of hole wavefunctions by band mixing in the valence band. The carrier density and temperature dependencies of tunneling dynamics are presented. A simple kinetic model developed for electron LO-phonon assisted tunneling shows good qualitative agreement with experimental data.; Exciton tunneling in wide gap, II-VI semiconductors was studied using (Zn,Cd)Se/ZnSe ADQW. The strong Coulomb interaction in II-VI semiconductors makes the tunneling process significantly different from that in III-VI ADQWs. Fast (1 ps) and complete recovery of the narrow well exciton absorption was observed after resonant femtosecond pulse excitation. The observed dynamics contradict the theory of independent electron and hole tunneling. The theory of exciton tunneling was developed. Theoretical analysis shows that tunneling of the exciton as a whole entity with the emission of only one LO-phonon is very slow. Instead, the exciton tunnels via an indirect state in a two-step process whose efficiency is dramatically enhanced by the Coulomb interaction.; The optical properties of neutron irradiated GaAs/Ga,Al)As multiple quantum wells are investigated. Sharp room temperature exciton features and a 21 ps carrier lifetime are demonstrated in neutron irradiated multiple quantum wells. Carrier lifetime reduction is consistent with the presence of EL2 defects that are efficiently generated by fast neutrons. The influence of the gamma rays accompanying neutron irradiation is discussed. Neutron irradiation provides a straightforward way to control the carrier lifetime in semiconductor heterostructures with minor deterioration of their excitonic properties.