First-principles Studies On The Structures And Properties Of ZnO-CdO Under High Pressure

In recent years,zinc oxide has attracted extensive attention as a potential photoelectric device material mainly due to its many advantages over III–V nitrides,such as a wide band gap（3.37 eV）and a large exciton binding energy（60 meV）at room temperature.At present,there are two hotspots in the research of zinc oxide materials:the preparation of p-type ZnO with relatively stable properties and the preparation of ZnO-based alloy materials.A key step in the development of ZnO-based optoelectronic devices is the fabrication of quantum wells and superlattices of ZnO-based semiconductor alloys by mixing with metal oxides with different band gaps.It has been found that the mixture of ZnO and CdO can reduce the band gap of ZnO,so that the CdO-ZnO alloys can be applied to visible light-emitting optoelectronic devices.Therefore,in the paper,we systematically studied the CdO-ZnO alloys under high pressure by first-principles.The Universal Structure Predictor:Evolutionary Xtallography code was used to systematically search the candidate structures of CdO-ZnO alloys under the pressure of 0-80 GPa.It was found that the predicted structures of CdO-ZnO alloys can’t form stable structure,can only form metastable phase,namely,CdZn₃O₄（Pmmn,P2₁/m-A,P2₁/m-B）,CdZn₂O₃（Cmc2₁,P-3m1）,CdZnO₂（Pna2₁,R-3m）,Cd₂ZnO₃（Pmn2₁,P-3m1）,Cd₃ZnO₄（C2,R-3m）.The structural phase transition of the discovered CdO-ZnO alloys under pressure was as follows:CdZn₃O₄possesses a Pmmn structure at ambient conditions.Pmmn CdZn₃O₄ then transforms to the P2₁/m-A phase at 23.13 GPa.Subsequently,at about65.25 GPa,P2₁/m-A CdZn₃O₄ undergoes a structural phase transition to the P2₁/m-B structure.For CdZn₂O₃,the Cmc2₁ and P-3m1 structures are found to be the most stable in pressure ranges of 0-6.19 GPa and 6.19-80GPa,respectively.CdZnO₂ is stable in the Pna2₁ phase under ambient pressure,and transforms into a R-3m phase at 3.77 GPa.For Cd₂ZnO₃,Pmn2₁ transforms into P-3m1 at 2.68 GPa.As for Cd₃ZnO₄,C2 changes to R-3m at about 2.43 GPa.Then we studied the electronic properties of these metastable structures and found that the band gap increases with the increase of pressure and decreases with the increase of Cd concentration,which is similar to the previous research results.In addition,we have modified the band gap by HSE method due to the shortcoming of underestimating the band gap of LDA.