First-Principles Study On Oxide Semiconductor Alloy

The emergence and application of oxide semiconductor materials play a substantial role in the development of the entire material science.A research fever has gripped the whole world with the discovery of the unique photoelectric information function of ZnO,another highly efficient oxide semiconductor material right following GaN.However,before applying ZnO in the new energy industry on a large scale,people must solve two key problems of ZnO,that is,energy band engineering(bandgap adjustment)and p-type doping.Meanwhile,Cu2O,as a monovalent oxide of copper,is an oxide semiconductor material with narrow bandgap.Abundant on the earth's surface,this material features high chemical stability,low price,and high sunlight absorptivity,causing no pollution.In recent years,widespread interests of scientific researchers are also aroused in Cu2O due to its potential applications to highly efficient solar cells and photocatalysis.An in-depth study is conducted in this paper on electronic band structures,state densities,band offsets,formation energies,phonon vibrations,phase diagrams,and other properties of ZnO1-xSx and ZnO1-xSex ternary alloys by carrying out first-principles calculations and cluster expansions.?sol method is used to correct the bandgaps of the alloys obtained through generalized gradient approximation.The band offsets of the alloys are determined by taking Branch-point energy as the reference point,and the lattice vibration effects of the alloys are calculated using the bond length-bond stiffness model.In addition,a study is performed on the band structures,impurity formation energy,and other properties of Group ? metal elements(Be,Mg,Ca,Sr,and Ba)and Group ?transition metal elements(Zn,and Cd)after n-type doping.The main findings of this paper are as follows:First,ZnO1-xSx ternary alloy is a direct-bandgap semiconductor no matter it is of a wurtzite structure or a zinc blende structure,and its bandgap does not monotonously vary with the content of S.With a low content of S,the bandgap of the alloy decreases with the increase of the content.When the content of S exceeds 50%,the bandgap of the alloy increases with its increase,which is consistent with the experimental results.The reason is that the doping of a small amount of S in ZnO leads to a rapid rise of the valence-band maximum,thereby narrowing the bandgap.However,with a high content of S,valence-band maximum only changes slightly but conduction-band bottom rises sharply,thus broadening the bandgap.An analysis of the formation energy of the alloy shows that ZnO1-xSx does not have a stable intermediate structure and a solubility gap is generated when an alloy is formed by ZnO and ZnS.As shown in the x-T phase diagram,phase separation transition temperature of the wurtzite(WZ)structure is 3946 K and that of the zinc blende(ZB)structure is 3894 K when the vibrational free energy of the lattice is not considered;and if the engery is taken into consideration,the transition temperature of wurtzite decreases by 17.1%,thus increasing its solid solubility,while that of zinc blende increases by 4.4%.An analysis of the formation energy's changes with temperature reveals that at a high temperature,ZnO1-xSx of the zinc blende structure is more stable than that of the wurtzite structure.Secondly,Zn1-xSex ternary alloy is a direct-bandgap semiconductor despite of the content of Se,and the bandgap of the alloy is greatly curved as the content of Se is increasing.According to an analysis of the bandgap offset,the valence-band maximum significantly increases when ZnO is doped with a small amount of Se while the conduction-band bottom greatly decreases when ZnSe is doped with a small amount of O.This is mainly because of the major differences in ion radius and electronegativity between O and Se.An analysis of the formation energy of the alloy shows that a solubility gap will also emerge when ZnO and ZnSe form an alloy.As shown in x-T phase diagram,provided that the vibrational energy of the lattice is not taken into account,phase separation transition temperature of the wurtzite(WZ)structure is 6350 K and that of the zinc blende(ZB)structure is 6571 K;after the vibrational energy of the lattice is considered,the phase separation transition temperature of the wurtzite structure is 5626 K,namely,an 11.4%decrease while that of the zinc blende structure decreases to 5063 K by 22.9%,both of which see an increase in the solid solubility of Se.Thirdly,divalent IIA metals such as Be,Mg,Ca,Sr,Ba and transition ?B metals such as Zn,Cd were investigated as possible n-type dopants into the Cu2O theoretically.Then by systematical analyses of the various doping systems,it is revealed that Ca,Sr,Ba and Be are more suited for n-type doing of Cu2O as shallow donors,compared to Mg which introduces a relatively deep donor level in Cu2O.Meanwhile,Zn and Cd can hardly be doped into Cu2O due to positive formation energy of relevant defects.