| The perovskite metal oxide heterojunction,which is an example of LaAlO3/SrTiO3heterojunction,has attracted people's attention due to the appearance of two-dimensional electron gas?2DEG?with novel behavior such as good conductivity,magnetic properties,and superconductivity.In this paper,first-principles calculations based on density functional theory?DFT?are used to study the relationship between different termination of LaAlO3 and temperature or oxygen pressure,and the reason why LaAlO3/SrTiO3 heterojunction has good conductivity,and defect structure to different termination surfaces.Effects of Two-Dimensional Electron Gas on n-type and p-type Heterojunction Interfaces.First,the stability of the different termination surfaces on the cubic LaAlO3?001?surface was investigated,and the effect of oxygen pressure and temperature on the surface stability was revealed.The three surface structures of?LaO-LaO?n,?LaO-AlO2?m,and?AlO2-AlO2?n with different end faces of LaAlO3?001?were constructed and their surface energies were calculated.It was found that the ultra-high vacuum(p=10-12 mbar),the low-temperature region AlO2 end surface is a stable structure;with the temperature increases,the beginning of the mixing end surface;when the temperature reaches 1250K,LaO termination surface is more stable.And as the oxygen pressure rises,the critical temperature at which the final surface of the mixture appears is also getting higher.This result provides a theoretical basis for experimentally obtaining accurate end surface information.Next,the electronic structures of n-type and p-type LaAlO3/SrTiO3?001?heterojunction interfaces with different defect planes were calculated to further understand the formation mechanism of two-dimensional electron gas.The results show that:?1?The n-type interface band gap of the AlO2 termination surface gradually decreases as the thickness of LaAlO3increases,and the built-in electric field causes electrons to transfer to the interface.When the thickness of LaAlO3 reaches 4u.c.the interface exhibits metallic conductivity,which is basically consistent with the experimentally observed phenomenon;the n-type interface structure of LaO termination surface is non-stoichiometric and is affected by the excess surface effect of LaO.The built-in electric field of the heterojunction is canceled.Therefore,as the thickness of LaAl O3 increases,the heterojunction band gap hardly changes.The state density results show that the Fermi level enters the conduction band and the density of electronic states is high,and the interface exhibits n-type conductivity.?2?The p-type interface with LaO as the termination surface is a stoichiometric structure with an almost zero electronic structure,but the heterojunction is still nonconductive due to the very low carrier concentration.The AlO2 is the p-type structure of the termination surface.The hole concentration gradually increases with the increase of the thickness of LaAlO3.When the thickness reaches 4u.c.,the interface is p-type conductive.Finally,four kinds of interface models for oxygen vacancy defects?VO?or metal vacancy defects?VAl or VLa?for n-type LaO,Al O2 and p-type LaO,AlO2 different termination surfaces were constructed respectively,and the defect was calculated with position and concentration changes.The formation energy,and analysis of the changes in the density of states,reveals the effect of defects on the interface of two-dimensional electron gas.The results show that:?1?n-type heterojunction model with AlO2 as the termination surface,under the reducing conditions,VO formation energy is lower and easier to form on the surface than metal defects;under the oxidation conditions,VLa defects It is easier to form than VO and can be made lower at the interface.The interstitial metal vacancy defect acts as an acceptor,compensating the interface carrier concentration and increasing the bandgap;the presence of VO destroys the built-in electric field generated by LaAlO3,and the Fermi level is much more.In the conduction band,the higher the VO concentration is,the stronger the interface conductivity is.This result can explain the experimental phenomenon that the oxygen vacancy defect enhances the interface conductivity.?2?The n-type interface with LaO as the termination surface.Under oxidizing conditions,metal vacancy defects are more stable than oxygen vacancy defects.No matter the reduction or oxidation conditions,the formation energy of VLa is always lower than that of VAl,and the formation energy of interfacial metal vacancy defects is the smallest.The presence of VLa reduces the concentration of two-dimensional electron gas at the interface,resulting in heterojunction insulation in this case.?3?With AlO2 as the termination surface p-type interface,the formation energy of Vo is always less than the formation energy of VAl or VLa,and VO is most easily formed at the interface.VO compensates the holes in the interface,reduces the carrier concentration at the interface,and makes the p-type interface non-conductive.?4?At the p-type interface with LaO as the termination surface,VO is more easily formed at the interface under the reducing conditions,and the heterojunction is not conductive due to donor compensation;under the oxidizing conditions,the surface VLa is easily spontaneously formed,and the concentration is higher.Theoretically,the interface can be p-type conductivity.However,in fact,the formation of the LaO termination surface requires high temperature and low oxygen pressure conditions,which is inconsistent with the fact that the metal vacancy defects in this model can spontaneously form under the oxidizing conditions.Therefore,it is difficult to experimentally obtain a conductive p-type interface.The calculations in this paper can help to further understand the effect of vacancy defect structures on the two-dimensional electron gas interface of the LaAlO3/Sr TiO3perovskite oxide heterojunction interface,and further understand the reason for the non-conductivity of the p-type interface,and the physical mechanism of the influence of the La/Al ratio variation and the oxygen pressure conditions on the heterojunction conductivity is revealed. |
PDF Full Text Request