Observation of negative thermo-optical coefficient in cubic MgZnO thin films

We report on the investigation of thermo-optical coefficient (dn∕dT) in cubic MgZnO thin films through temperature-dependent (10–295K) optical transmission measurements. As a result of potential fluctuation, we observe an anomalous thermal behavior of the band gap in the ternary MgZnO alloy, which limits the observation of negative dn∕dT up to 200K. The negative dn∕dT is evidenced by the phase shift from the prominent Fabry-Pérot interference fringes and modeled by taking into account the thermal and electronic background contribution, as well as the Wannier exciton effect. It is found that the large thermal expansion in cubic MgZnO plays a key role in the observed negative dn∕dT.