First-principles Study On Thermodynamic Properties Of ?-? Ternary Alloy Semiconductors

It has been a hot topic of the research about ZnO and other II-VI semiconductors,since they have many excellent performances in photovoltaic,piezoelectric,pyroelectric,ferroelectric and other fields.Since the 1990s,Room-temperature ultraviolet(UV)laser emission of ZnO microcrystallite thin films is reported,ZnO as a new type of photoelectric information functional materials caused the research upsurge.However,two difficulties must be solved to realize the application of ZnO in the optoelectronic devices:the band engineering and p-type doping.For band engineering,the substitute of isoelectronic anions or cations can modify the ZnO band gap.For the cations,the substitute of Zn partly can acquire ternary MeZnO alloys(Me-Mg,Be,Cd etc.).For the anions,the substitute of O partly can acquire ternary ZnOX alloys(X = S,Se etc.).Studies have shown that there are phase separation and limited solubility for these alloys.There are few reports to determine the solubility limitation for ?-? ternary alloys.In this work,we studied the thermodynamic properties and the solid solution properties of CdxZn1-xO,BexZn1-xO,MgxZn1-xO,CdO1-xSx and ZnS1-xSex alloys by first-principles calculations based on density functional theory(DFT)combined with the cluster expansion approach.The main research contents are as follows:First,we studied the thermodynamic properties of CdxZn1-xO alloys of both wurtzite(WZ)and rock salt(RS)structures.It is found that,for both WZ-CdxZni-xO and RS-CdxZ1-xO alloys,the ECIs involve pair,triplet and quadruplet interactions;the pair interactions are dominant and contribute mostly to the formation energy.The formation energy of most structures is positive in the whole composition range,except for WZ-CdxZn1-xO with Cd concentrations of 1/3 and 2/3.Thus,the crystal and electronic band structures of the metastable WZ-Cd1/3Zn2/3O and WZ-Cd2/3Zn1/3O are investigated.It turns out that,both lattice constants a and c increase while the value of c/a and the bond angle of O-Zn(Cd)-O decrease with increasing Cd concentration in the WZ-CdxZn1-xO alloys.Analysis of the band structure WZ-CdxZn1-xO reveals that the VBM rises while the CBM declines,leading to decrease of the bandgap of WZ-CdxZn1-xO with increasing Cd concentration.The phase diagrams of WZ-CdxZn1-xO and RS-CdxZn1-xO have been derived based on the Gibbs free energy.At 1600 K,the solubility of Cd in WZ-ZnO amounts to 0.13,while the solubility of Zn in RS-CdO limits to only 0.01,indicating that it is much easier to incorporate Cd into WZ-ZnO than incorporating Zn into RS-CdO.Second,a theoretical study on the thermodynamics of BexZn1-xO alloys has been carried out.For both WZ and ZB phases of BexZn1-xO alloys,the formation energy of all structures is positive for the whole range of composition.This implies the miscibility gap when BeO and ZnO form alloy.It is found that,the ECIs involve pair,triplet and quadruplet interactions.For WZ-BexZn1-xO alloys,the pair and quadruplet interactions are dominant and contribute mostly to the formation energy.For ZB-BexZn1-xO alloys,the pair interactions are dominant and contribute mostly to the formation energy.Then,the phase diagram of BexZn1-xO has been investigated.Including the phonon contributions to the cluster expansion Hamiltonian appears to influence the solubility of Be-and Zn-rich alloys.Third,a theoretical study on the thermodynamics of MgxZn1-xO alloys has been carried out with and without phonon contribution.The negative formation energy of most structures for the whole range of composition implies good miscibility for MgO and ZnO.The ECIs involve pair,triplet and quadruplet interactions and the pair interactions are dominant and contribute mostly to the formation energy.The calculated phase diagram of MgxZn1-xO has been investigated.Due to phase change between WZ phase and RS phase,a miscibility gap occurs between two phases.Fourth,a theoretical study on the thermodynamics of CdO1-xSx alloys has been carried out.The positive formation energy of all structures for the whole range of composition implies the miscibility gap when CdO and CdS form alloys.The ECIs involve pair and triplet interactions and the pair interactions are dominant and contribute mostly to the formation energy.The calculated phase diagram of CdO1-xSx indicates that it is much easier to incorporate O into WZ-CdS than incorporating S into RS-CdO.Fifth,a theoretical study on the thermodynamics of ZnS1-xSex alloys has been carried out.The formation energy of all structures is positive for the whole range of composition.This implies ZnS and ZnSe are difficult to form alloys.There is a miscibility gap when ZnS and ZnSe form alloys.It is found that,the ECIs involve only pair interactions;the pair interactions are dominant and contribute mostly to the formation energy.The calculated phase diagram of ZnS1-xSex indicates that the solid solubility of S into ZnSe is substantially equal to the solubility of Se into ZnS.The effective cluster interactions(ECIs)fitted formation energy agrees well with the DFT-calculated formation energy for different compositions and structures,validating the cluster expansion approach in calculations of the formation energy for CdxZn1-xO,BexZn1-xO,MgxZn1-xO,CdO1-xSx and ZnS1-xSex alloys.