Theoretical Study On Modulating The Transparency And Conductivity Of Gallium Oxide Through Co-doping

Gallium oxide??-Ga₂O₃?has drawn great attention for optoelectronic devices,energy catalysis,gas sensors and solar-blind ultraviolet detectors,due to its stability,ultra-wide band gap?4.9 eV?and excellent physical and chemical properties.Doping is able to change the structure and physical/chemical properties of Ga2O3,and broaden its application fields.In addition,due to the interaction between the co-dopants,the solubility limit in the mono-doping can be broken and the transition level can be further reduced,which improves the conductivity of materials.Therefore,searching for suitable dopants and exploring the most effective doping method are the key to achieve high-performance Ga2O3.First-principles calculations can screen suitable dopants with lower cost,regulate the electronic properties of the materials,and reveal the origin of the superior performance.In this thesis,the effects of co-doping on the optical and electronic properties of Ga2O3,containing different intrinsic defects?oxygen vacancy or gallium vacancy?,were explored with the aid of first-principles;and an effective strategy for obtaining highly conductive p-type Ga2O3was further explored through co-doping.A theoretical basis for experimental researchers to develop high-performance Ga2O3 is provided in this thesis.The main research contents and conclusions are as follows:1.Theoretical calculations explore the achievement of high conductivity and transparency of Ga2O3 via co-doping.The presence of oxygen vacancy?VO?in Ga2O3is inevitable in the manufacturing process.On the one hand,the n-type conductivity of Ga2O3 is improved by the presence of VO.On the other hand,the transparency of Ga2O3in the visible region is reduced due to the impurity levels generated by oxygen vacancy.Thus,balancing the relation between high conductivity and transparency becomes rather urgent.In this thesis,we propose that aluminum?Al?and indium?In?co-doped Ga2O3 with VO?AlGa-InGa-VO?can not only eliminate the opaqueness,but also retain the carriers provided by VO.The calculated results unveil that AlGa-VO doesn't eliminate the undesirable absorption of VO,whereas InGa-VO does.The defect formation energy of InGa-VO is higher than that of AlGa-VO.Intriguingly,AlGa-InGa-VO not only reduces the defect formation energy but also maintains the transparency of perfect Ga2O3.The strong hybridization between In 4d and O 2p induces a down-shift of impurity level,enhancing the optical bandgap.2.Co-doping activates the hole conductivity of gallium vacancy?VGa?in Ga2O3.VGa has attracted attention as a p-type intrinsic defect.However,due to the high defect formation energy and deep transition level of VGa,it is difficult to provide efficient p-type carriers.If the defect formation energy and transition level of VGa can be reduced by co-doping,it will provide new ideas for the research and development of Ga2O3 with high p-type conductivity.In this thesis,we focus on the effect of N/F mono-doping and N-F co-doping on the conductivity of Ga2O3 with VGa.The calculated results show that the introduction of N/F mono-doping or N-F co-doping can reduce the formation energy of the complex defect under Ga-rich conditions,while it is opposite under the Ga-poor conditions.In VGa-NO,the introduction of N is able to reduce the transition level of VGa,activating the holes generated by VGa and increasing the concentration of p-type carriers in Ga2O3.In addition,VGa-NO-FO has a lower defect formation energy than VGa-NO.N-F co-doping can increase the concentration of VGa,thereby increasing the concentration of holes in Ga2O3.It is worth noting that increasing the N content of N-F co-doped Ga2O3 can further reduce the defect formation energy,but the regulation of the transition level is very limited.Comparing and combining bipolar dopants?N/F?,the tuning of different doping methods on the conductivity of Ga2O3 with VGa is discussed in detail,which provides an effective strategy for elevating the VGa concentration.3.Theoretical calculations explore the improvement of the p-type conductivity of Ga2O3 via co-doping.The lack of high conductivity p-type Ga2O3 has always been a bottleneck restricting the development of bipolar optoelectronic devices.Although nitrogen?N?is a common p-type dopant in oxides,the deep acceptor energy level produced by N doping inhibits its conductivity,which is the same in Ga2O3.We propose that Al/In-N co-doping is an effective way to achieve high-conductivity p-type Ga2O3.The calculated results reveal that Al-N co-doping exhibits lower defect formation energy and shallower transition level than that of N mono-doping,originating from the smaller ionic radius of Al and the increasing electron density near the Fermi level.The introduction of In can further reduces the depth of transition level,corresponding to a shallower transition level.More interestingly,the formation energy and transition level can be further reduced by enhancing the ratio of N in the co-doping,which elevates the p-type carrier concentration of Ga2O3.Co-doping with equivalent elements and N proposed in this work is expected to become a way to improve the p-type conductivity of oxides.