Epitaxial Growth Of Zinc Oxide-based Semiconductors And Its First-principles Calculations

Zinc oxide(ZnO)is a representative third-generation semiconductor,which has the advantages of wide band gap,high exciton binding energy,mature single crystal growth technology,strong radiation resistance,flexible preparation technology route,and pollution-free.Magnesium zinc oxide(MgZnO)has larger band gap than zinc oxide and broadenthe working waveband,but phase separation and difficulties inp-type doping still restrict its industrialization.We studied MgZnO and Fe&Mg co-doped ZnO(FeMgZnO)systems through density functional theory(DFT)method.They are all ZnO-based semiconductor materials performance and application frontier research.For the MgZnO system,we found that the wurtzite structure is more stable when the Mg content in the crystal is less than 62%,and the rock salt structure is more stable when the Mg content is higher than 62%.The distribution of Mg in MgZnO shows that Mg tends to uniformly distribute in MgZnO crystals.By comparing with other doping systems,we find that the lattice distortion introduced by Mg is the smallest.We calculated the electronic characteristics of the corresponding system.The projected band diagram shows that Mg atoms can adjust the O 2p orbits to move into the low energy region,while Cd has the opposite effect.These results are consistent with the results of the band engineering in the experiment.For the FeMgZnO system,we have studied its structural,electronic,and magnetic properties.The results show that when Mg atoms and Fe atoms are located in the next nearest neighbors of the same crystal plane perpendicular to the(0001)direction,the total energy of the system is the lowest and the lattice distortion is the smallest.The band gap of the system is larger than that of pure ZnO,which indicates that the doping of Mg atoms can regulate the band gap of ZnO.At the same time,due to the co-doping of Fe atoms,the system is magnetic.Our calculation results show that the co-doping of Mg and Fe can well combine the tasks of band engineering and diluted magnetic semiconductors(DMS),which broadens the application scenarios of ZnO-based semiconductors.In the experimental study,we have grown MgZnO thin films with different parameters on a-plane sapphire substrates by molecular beam epitaxy(MBE).When the Mg content is low,the Mg content in the film decreases as the pressure increases.This is due to the higher reaction activity of magnesium than zinc,which is more likely to react with oxygen at low oxygen pressure,resulting in a competitive advantage and a relatively high level of Mg.When the oxygen pressure is high,the metal activity is a secondary factor,the competitive relationship weakens,which eventually leads to a decrease in the Mg content of the film as the oxygen pressure increases.As the temperature of the Mg source increases,the Mg content in the film increases.When the Mg content increases to a certain extent,phase separation occurs,resulting in an increase in the roughness of the film surface and a decrease in crystal quality.Single-phase zinc magnesium is divided into two phases with different Mg contents.And the higher the total magnesium content,the greater the difference between the two-phase magnesium content.