第一性原理的广义梯度近似＋U方法的纤锌矿Zn(1-x)MgxO极化特性与Zn(0.75)Mg(0.25)O/ZnO界面能带偏差研究

Polarization properties of wurtzite structure Zn_(1-x)Mg_xO and band offset at Zn_(0.75)Mg_(0.25)O/ZnO interfaces:A GGA+U investigation

在纤锌矿结构Zn_(1-x)Mg_xO/ZnO异质结构中发现了高迁移率的二维电子气(2DEG),2DEG的产生很可能是由于界面上存在不连续极化,而且2DEG通常也被认为是由极化电荷产生的结果.为了探索2DEG的形成机理及其产生的根源,研究Zn_(1-x)MgxO合金的极化特性与ZnO/Zn_(1-x)Mg_xO超晶格的能带排列是非常必要的.基于第一性原理广义梯度近似+U方法研究了Zn_(1-x)Mg_xO合金的自发极化随Mg组分x的变化关系,其中极化特性的计算采用Berry-phase方法.由于ZnO与Zn_(1-x)Mg_xO面内晶格参数大小相当,ZnO与Zn_(1-x)Mg_xO的界面匹配度优良,所以ZnO/Zn_(1-x)Mg_xO超晶格模型较容易建立.计算了Mg_(0.25)Zn_(0.75)O/ZnO超晶格静电势的面内平均及其沿着Z(0001)方向上的宏观平均.(5+3)Mg_(0.25)Zn_(0.75)O/ZnO超晶格拥有较大的尺寸,确保远离界面的Mg_(0.25)Zn_(0.75)O与ZnO区域与块体计算情况一致.除此之外,基于宏观平均为能量参考,计算得到Mg_(0.25)Zn_(0.75)O/ZnO超晶格界面处价带偏差为0.26 eV,并且导带偏差与价带偏差的比值处于合理区间,这与近来实验上报道的结果相符.除了ZnO在[0001]方向上产生自发极化外,由于在ZnO中引入Mg杂质会产生应变应力,导致Mg_xZn_(1-x)O层产生额外的极化值.这样必然会在Mg_(0.25)Zn_(0.75)O/Zn界面处产生非连续极化现象,促使单极性电荷在界面处积累,从而在Mg_(0.25)Zn_(0.75)O/Zn超晶格中产生内在电场.此外,计算了Mg_(0.25)Zn_(0.75)O/ZnO超晶格的能带排列,由于价带偏差△Ev=0.26 eV与导带偏差△Ec=0.33 eV,表明能带遵循I型排列.Mg_(0.25)Zn_(0.75)O/ZnO的这种能带排列方式足以让电子与空穴在势阱中产生禁闭作用.2DEG在电子学与光电子学领域都有重要应用,本文的研究结果将对Mg_(0.25)Zn_(0.75)O/ZnO界面2DEG的设计与优化中起到重要作用,并且可以作为研究其他Mg组分的Mg_xZn_(1-x)O/ZnO超晶格界面电子气特性的参考依据.

Two-dimensional（2D） electron gas with high-mobility is found in wurtzite ZnO/Zn（Mg）O heterostructure,which probably arises from the polarization discontinuity at the ZnO/Zn（Mg）O interface,and the 2D electron gas in the heterostructure is usually also regarded as resulting from polarization-induced charge.In order to explore both the formation mechanism and the origin of the 2D electron gas in ZnMgO/ZnO heterostructure,it is necessary to study the polarization properties of Zn（1-x）MgxO alloy and energy band alignment of ZnO/Zn（1-x）MgxO super-lattice.In this paper,we study the polarization properties of Zn（1-x）MgxO alloy with different Mg compositions by using first-principles calculations with GGA＋U method,and the polarization properties are calculated according to Berryphase method.Owing to the excellent match between the in-plane lattice constants of ZnO and Zn（1-x）MgxO,the lattice constants of the ZnO and Zn（1-x）MgxO interface are similar,ZnO/Zn（1-x）MgxO super-lattice could be constructed easily.The planar-averaged electrostatic potential for the Mg（0.25）Zn（0.75）O/ZnO super-lattice and the macroscopically averaged potential along Z（0001） direction are calculated.The large size of（5＋3） Mg（0.25）Zn（0.75）O/ZnO super-lattice ensures the convergence of potential to its bulk value in the region of the ZnO layer and Mg（0.25）Zn（0.75）O layer far from ZnO/Zn（1-x）MgxO interface.Besides,the valence band offset at the Mg（0.25）Zn（0.75）O/ ZnO interface is calculated to be0.26 eV based on the macroscopically averaged potential mentioned above,and the ratio of conduction band offset（△EC）to valence band offset（△EV） is in a reasonable range,and this is in substantial agreement with the values reported in recent experimental results.Because strain induces additional piezoelectric polarization in MgxZn（1-x）O,which is introduced by Mg dopant,the lack of inversion symmetry and the bulk ZnO induce its spontaneous polarization in the[0001]direction.The polarization discontinuity at the Mg（0.25）Zn（0.75）O/ZnO interface leads to the charge accumulation in the form of interface monopoles,giving rise to built-in electric fields in the super-lattice.In addition,energy alignment determination of the Mg（0.25）Zn（0.75）O/ZnO super-lattice is performed,which shows a type-Ⅰ band alignment with △EV=0.26 eV and △EC=0.33 eV.The determination of the band alignment indicates that the Mg（0.25）Zn（0.75）O/ZnO super-lattice is competent to the confining of both electron and hole.These findings will be useful for designing and optimizing the 2D electron gas at Mg（0.25）Zn（0.75）O/ZnO interface,which can be regarded as an important reference for studying the 2D electron gas at MgxZn（1-x）O/ ZnO super-lattices for electronics and optoelectronics applications.