Room-temperature cavity-polaritons in planar ZnO microcavities fabricated by a top-down process

Angle-dependent energy shifts in the near-band edge emission peak were observed at room temperature in planar ZnO microcavities (MCs) fabricated by a top-down process that simultaneously maintains high radiative performance of a ZnO active layer and high reflectivity of wide-bandwidth distributed Bragg reflectors (DBRs). An approximately 2λ-thick ZnO active layer with a thickness gradient less than 10 nm/mm (i.e., an angle of gradient less than 2 arc sec) across the entire 10 × 5 mm2 device area was formed by thinning a bulk single crystal of ZnO grown by the hydrothermal method, whose typical threading dislocation densities were lower than 102 cm-2. The DBRs consisting of 10 and 12 pairs of SiO2/ZrO2 multilayers with a thickness gradient of 0.2 nm/mm/pair were deposited by using the nearly surface-damage-free reactive helicon-wave-excited-plasma sputtering method as the top and bottom mirrors, respectively. The quality factor of a passive cavity consisting of the same DBR stacks was in the range between 670 and 720 for the areal size of 1 mm in diameter. Angle-resolved photoluminescence spectra of the ZnO MCs measured at different positions with a macroscopic spot size of 80 μm in diameter exhibited distinct emission from the lower branch of cavity polaritons with apparent detunings ranging from -40 meV to 40 meV at room temperature.