Abstract
A symmetric AlN-ZnO/ZnO/AlN-ZnO double heterojunction structure was consecutively deposited onto silicon substrate using cosputtering technology and then annealed at 700 °C under vacuum ambient for 30 min. The crystalline quality of the ZnO film in the heterojunction structure was significantly improved as verified by X-ray diffraction (XRD) and photoluminescence (PL) measurements. Improvement on the crystalline structure was ascribed to the stress in the ZnO active film, which was effectively buffered by the underlayered AlN-ZnO layer. Native oxygen vacancies in the ZnO film also were effectively suppressed due to a little diffusion of the Al atoms from the cosputtered AlN-ZnO layer, and led to an increase in the carrier concentration. Such ZnO film deposited onto the homogeneous AlN-ZnO buffer layer emitted an intense near-band-edge emission, and the deep level emission was absent. The ultraviolet emission was further enhanced by covering an AlN-ZnO barrier laye, which was a consequence of the improvement on the carrier confinement. Accordingly, single ultraviolet emission with a quality ZnO crystalline structure, which is very promising for application in short-wavelength optoelectronic devices, was realized from the ZnO film sandwiched by the homogeneity of the cosputtered AlN-ZnO layers.
Highlights
Zinc oxide (ZnO) is a direct group II–VI binary compound semiconductor with a wide energy bandgap of 3.37 eV, which is similar to that of gallium nitride (GaN)
We demonstrated that the conduction type, optical energy bandgap, and crystalline structure of a cosputtered AlN-ZnO film were controllable by altering the theoretical atomic ratios of the Al in the cosputtered film [23,24]
AlN-ZnO film at a theoretical atomic ratio of 20% resulted in the crystal growth from 23 to 29 nm and enhancement of the c-axis preferred growth orientation originating from the release of residual internal stress
Summary
Zinc oxide (ZnO) is a direct group II–VI binary compound semiconductor with a wide energy bandgap of 3.37 eV, which is similar to that of gallium nitride (GaN). The growth in technology and temperature necessary to realize a ZnO film with quality c-axis crystallization is much simpler, such as when done by sputtering, pulse laser deposition, and hydrothermal methods, rather than for the epitaxial GaN layer, resulting in potentially low-cost fabrications and processes for ZnO-based devices [5,6,7,8]. Among these deposition methods, Coatings 2019, 9, 544; doi:10.3390/coatings9090544 www.mdpi.com/journal/coatings
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