第一性原理的广义梯度近似+U方法的纤锌矿Zn1-xMgxO极化特性与Zn0.75Mg0.25O/ZnO 界面能带偏差研究

在纤锌矿结构Zn_1-xMg_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-xMg_xO合金的极化特性与ZnO/Zn_1-xMg_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-xMg_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-xMg_xO 面内晶格参数大小相当, ZnO 与Zn_1-xMg_xO 的界面匹配度优良, 所以ZnO/Zn_1-xMg_xO 超晶格模型较容易建立. 计算了Mg_0.25Zn_0.75O/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_0.25Zn_0.75O/ZnO超晶格拥有较大的尺寸, 确保远离界面的Mg_0.25Zn_0.75O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_0.25Zn_0.75O/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-xO层产生额外的极化值. 这样必然会在Mg_0.25Zn_0.75O/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_0.25Zn_0.75O/Zn超晶格中产生内在电场. 此外, 计算了Mg_0.25Zn_0.75O/ZnO超晶格的能带排列, 由于价带偏差Δ E_V=0.26 eV与导带偏差ΔE_C=0.33 eV, 表明能带遵循I型排列. Mg_0.25Zn_0.75O/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_0.25Zn_0.75O/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-xO/ZnO超晶格界面电子气特性的参考依据.

Polarization properties of wurtzite structure Zn1-xMgxO and band offset at Zn0.75Mg0.25O/ZnO interfaces: A GGA+U investigation

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-xMg_xO alloy and energy band alignment of ZnO/Zn_1-xMg_xO super-lattice. In this paper, we study the polarization properties of Zn_1-xMg_xO alloy with different Mg compositions by using first-principles calculations with GGA+U method, and the polarization properties are calculated according to Berry-phase method. Owing to the excellent match between the in-plane lattice constants of ZnO and Zn_1-xMg_xO, the lattice constants of the ZnO and Zn_1-xMg_xO interface are similar, ZnO/Zn_1-xMg_xO super-lattice could be constructed easily. The planar-averaged electrostatic potential for the Mg_0.25Zn_0.75O/ZnO super-lattice and the macroscopically averaged potential along Z(0001) direction are calculated. The large size of (5+3) Mg_0.25Zn_0.75O/ZnO super-lattice ensures the convergence of potential to its bulk value in the region of the ZnO layer and Mg_0.25Zn_0.75O layer far from ZnO/Zn_1-xMg_xO interface. Besides, the valence band offset at the Mg_0.25Zn_0.75O/ZnO interface is calculated to be 0.26~eV based on the macroscopically averaged potential mentioned above, and the ratio of conduction band offset (ΔE_C) to valence band offset (ΔE_V) 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 Mg_xZn_1-xO, 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_0.25Zn_0.75O/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_0.25Zn_0.75O/ZnO super-lattice is performed, which shows a type-I band alignment with ΔE_V=0.26 eV and ΔE_C=0.33 eV. The determination of the band alignment indicates that the Mg_0.25Zn_0.75O/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_0.25Zn_0.75O/ZnO interface, which can be regarded as an important reference for studying the 2D electron gas at Mg_xZn_1-xO/ZnO super-lattices for electronics and optoelectronics applications.