Effects of asymmetric MgZnO barriers on polar optical phonon-limited electron mobility in wurtzite ZnO thin films

Abstract

MgZnO barriers are commonly applied to passivate wurtzite ZnO films to enhance electron mobility, while the Mg mole fraction x is usually controlled below 0.4 to avoid phase separation. Few theoretical analyses have focused on electron mobility at large x since the phase separation leads to a complex scattering mechanism. This work investigates the effects of asymmetric MgZnO barriers on electron mobility, which is one source of complexity. Four asymmetric quantum wells simultaneously contribute to the electron mobility in proportions when the wurtzite and rock salt coexist in the mixed-phase MgZnO barriers with large Mg mole fractions. Besides, built-in electric fields also contribute to the asymmetry by tilting the bands. The polar optical phonon-limited electron mobility in asymmetric MgxZn1−xO/ZnO/Mg0.45Zn0.55O quantum wells is simulated between 176 and 333 cm2/V s as x ranges from 0.1 to 1. Our calculations show that confined optical phonons play a leading role in the quantum well with wurtzite barriers. Interface optical phonons are primary in the wells with rock salt barriers since most electrons are pushed close to the interface by the strong built-in electric field. The results indicate that wurtzite barriers are more favorable to achieving stable high mobility above 238 cm2/V s as the Mg mole fraction ranges from 0.14 to 0.33, which is commonly applied in practice.