Nonlinear electron transport in asymmetric double quantum well structures

Abstract

We study the effect of asymmetric potential profile on the electron mobility µ of a double quantum well (DQW) structure. In a DQW, along with confinement effect, the coupling of subband states between the wells further splits the energy levels thereby acting as an additional degree of freedom to regulate the subband electron structure properties. We introduce asymmetry (1) in doping concentrations in the barriers, (2) in the shape of the wells, i.e., taking one square and another parabolic well and (3) by applying an electric field F to the potential profile. We calculate the low temperature multisubband mobility µ by considering non-phonon scattering mechanisms such as ionized impurity (IMP-), interface roughness (IR-) and alloy disorder (AL-) scatterings. We show that the mobility exhibits nonlinear behavior as a function of doping concentration and external electric field due to difference in the effects of the scattering potentials on µ mediated by intersubband interactions. When the system becomes symmetric the subband states of the wells undergo resonance leading to a sudden dip in µ. In a GaAs/AlGaAs system, µ is mostly decided by µII while in GaAs/InGaAs system both µII and µAL control µ due to larger effect of AL-scattering because of the alloy channel. However, the dip in µ is mostly decided by the IR-scattering. In case of field induced GaAs/AlGaAs asymmetric structure, the dip in µ is also partly affected by the IMP-scattering. We note that the dip in µ is because of sudden change in the distribution of subband wave functions near resonance which affects the scattering rate matrix elements substantially. Our results of nonlinear mobility in asymmetric double quantum well structures can be utilized to study the effect of coupling in double quantum well devices.