Interfacial and bulk polaron masses in Zn_1-xMg_xO/ZnO heterostructures examined by terahertz time-domain cyclotron spectroscopy

The cyclotron resonance of polarons in Zn_1-xMg_xO/ZnO heterostructures (with 0.15 < x < 0.22) was studied by terahertz time-domain spectroscopy. Low-temperature magnetoconductivity spectra of the 2D electron gas at the Zn_1-xMg_xO/ZnO interface determined the polaron density, mass, and scattering rate. The cyclotron mass of 2D polarons was found to increase significantly with magnetic field B from 0.24 m_e at B = 2 T to 0.37 m_e at B = 7.5 T. A nonlinear cyclotron frequency with B was also observed for 3D polarons in ZnO. The findings are discussed in the context of polaron mass renormalization driven by the electron-LO-phonon and electron-acoustic phonon interactions.