Ternary mixed crystal effects on optical phonons in wurtzite ZnO quantum wells with asymmetric MgZnO barriers

Abstract

Optical phonons significantly relate to fundamental limits on dynamical processes in layered polar semiconductor devices. MgZnO layers commonly serve as barriers of ZnO quantum wells (QWs) to improve the electric and optical properties by quantum confinement while keeping the Mg fraction below 0.4 to avoid the crystal phase transformation from wurtzite to rocksalt. This work investigates ternary mixed crystal effects on optical phonons in wurtzite ZnO quantum well with asymmetric barriers of MgxZn1-xO and MgyZn1-yO over a full range of Mg mole fractions. Since MgZnO could be wurtzite (WZ) and rocksalt (RS) phases, the QWs can be denoted as WZ/WZ/WZ, WZ/WZ/RS, and RS/WZ/RS, respectively. Both asymmetries in the Mg fractions and crystal structures of the two barriers affect the optical phonons in QWs. RS/RS/RS quantum wells are also discussed for comparison. Since oscillatoring electrostatic potential of optical phonons is only permitted in the anisotropic wurtzite but forbidden in istropic rocksalt, more modes of confined optical phonons are found in a WZ/WZ/WZ QW, followed by WZ/WZ/RS one, and the least in the RS/WZ/RS one, while there is no confined optical phonons in RS/RS/RS quantum wells. Besides, the difference between the Mg fractions in two barriers leads to more modes of interface optical phonons due to the two unequal interfaces. The modes contained in the optical phonons spectra as well as their frequency ranges, dispersion relations, and electrostatic potentials, vary with Mg fraction in barriers. The strain on interfaces induced by the lattice mismatch leads to a slight blue shift of the dispersion relation as Mg fraction increases. Based on the results of this paper, the optical phonons related to electronic and optoelectronic properties in QWs could be further understood and adjusted.