Growth of ZnO/GaN distributed Bragg reflectors by plasma-assisted molecular beam epitaxy

Doktorsavhandling, 2016

This thesis describes epitaxial growth of ZnO/GaN distributed Bragg reflectors by hybrid plasma-assisted molecular beam epitaxy on GaN(0001). The unique hybrid approach employed the same growth chamber for continuous growth of both ZnO and GaN without exposing the layers to the ambient conditions. The Bragg reflectors consisted up to 20 periods as verified with cross-sectional transmission electron microscopy. The maximum achieved reflectance was 77 % with a 32 nm wide stopband centered at 500 nm. A profound study of the ZnO and the ZnO/GaN growth processes was carried out including growth along both ZnO(0001) and ZnO(000-1) directions. The impact of growth temperature, Oxygen flow-rate and the Zn-flux on the ZnO growth rate, structural quality and surface and interface morphology, was investigated in detail. The layers were studied with a wide range of materials characterization techniques such as x-ray diffraction, scanning electron microscopy, atomic force microscopy, secondary-ion mass spectroscopy and transmission electron microscopy. Low-temperature growth as well as two-step low/high-temperature deposition was carried out where the latter method improved the Bragg mirror reflectance. Samples grown along the ZnO(0001) direction yielded a better surface morphology as revealed by scanning electron microscopy and atomic force microscopy. It was observed that the growth rate of ZnO decreased when the Oxygen flow rate was increased. This is unexpected with respect to the common knowledge in the molecular beam epitaxy research community. A detailed study of this effect involving optical emission spectroscopy of the O-plasma, revealed that the cause was an overall decrease of the amount of the active O provided by the plasma source. Reciprocal space maps showed that ZnO(000-1)/GaN reflectors are relaxed whereas the ZnO(0001)/GaN DBRs are strained. The ability to n-type dope ZnO and GaN makes the ZnO(0001)/GaN DBRs interesting for various optoelectronic cavity structures such as blue vertical surface emitting lasers and novel cavity-polariton devices. This is the first time ZnO/GaN DBRs have been demonstrated.