The current study deals with the structural, electronic and optical properties of monovalent element-doped ZnO monolayers using density functional theory. Specifically, we have considered structural and optical properties of monovalent (M = Li, Na and K) atom-doped ZnO monolayers. Among these monovalent elements, the substitution of Li with Zn atom maintains the hexagonal planar geometry of the ZnO monolayer, but Na and K elements protrude out from the plane of the ZnO monolayer. The Li atom binds more strongly with the ZnO sheet compared with Na and K atoms. A Li-doped ZnO monolayer shows metallic behaviour whereas Na-and K-doped ZnO monolayers show half metallic magnetic behaviour. The magnetic moment is of the order of 1µ B. The magnetic moment mainly originates from nonbonding O p states. The substitution of an alkali metal element-doped ZnO monolayer leads to a red-shift in optical spectra. The dielectric constant of a monovalent element-doped ZnO sheet increases compared with that of a pristine ZnO sheet. This study provides the basis to develop opto-electronic devices using doped ZnO monolayers.
The electronic, transport, and optical properties of the trilayer of ZnO and GaN heterostructures are investigated using density functional study to understand its role in optoelectronic devices. For layered systems, the Zn over N and Ga over O stacking arrangement of ZnO over GaN is most favorable. The calculated formation energies reflect the energetically favorable ZnO/GaN heterostructures. The GaN/ZnO/GaN is a more energetically favorable stacking arrangement as compared to ZnO/GaN/ZnO. The band gap of trilayer systems decreases as compared to that of bilayer and monolayer. The ZnO/GaN bilayer and ZnO/GaN/ZnO trilayer show direct band gap characteristics with the value of 1.71 and 1.61 eV, respectively. The GaN/ZnO/GaN shows an indirect band gap of 1.47 eV. The higher recombination rate of ZnO/GaN/ZnO is useful to develop a base for optical emission devices. The transport calculations show that the magnitude of current flowing through the system increases with the layers of heterosystems and is specifically higher for GaN/ZnO/GaN heterostructure. The enhanced channel conductance and higher mobility of GaN/ZnO/GaN heterostructure are crucial for the development of high mobility transistors. The improved absorption energy and dielectric properties are observed for trilayer systems as compared to that of the bilayer and monolayer and may be useful for optical devices. The higher optical efficiency is observed for GaN/ZnO/GaN as compared to ZnO/GaN/ZnO heterostructure system and opens up a way toward optical waveguides and reflectors.
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