Control of crystal polarity in a wurtzite crystal: ZnO films grown by plasma-assisted molecular-beam epitaxy on GaN
Control of crystal polarity in a wurtzite crystal: ZnO films grown by plasma-assisted molecular-beam epitaxy on GaN
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DOI：
10.1103/physrevb.65.115331
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发表时间：
2002-03-15
影响因子：
3.7
通讯作者：
Terauchi, M
中科院分区：
文献类型：
--
作者：
Hong, SK;Hanada, T;Terauchi, M
ZnO/GaN异质界面的设计，以控制通过等离子体辅助分子束外延生长的Ga极性GaN模板上的ZnO薄膜的极性。用同轴碰撞离子散射谱（CAICISS）和会聚束电子衍射（CBED）确定了ZnO薄膜的极性。极性反转可以通过插入具有对称中心的界面层来实现，因为极性来自对称中心的缺失。通过在界面处插入Ga 2 O3层，可以在Ga极性（阳离子极性）GaN上生长O极性（阴离子极性）ZnO膜，而在GaN上生长Zn极性ZnO而不形成界面层。一个单晶单斜Ga 2 O3层，它有一个对称的中心，形成由O-等离子体预曝光的Ga极性GaN表面之前，ZnO的生长，而ZnO/GaN界面没有任何额外的层形成由Zn预曝光。ZnO、Ga 2 O3和GaN之间的取向关系被确定为[2-1-10] ZnO平行于[010]（Ga 2 O3）平行于[2-1-10]（GaN）和（0001）（ZnO）平行于（001）（Ga 2 O3）平行于（0001）（GaN）。CAICISS结果显示，O-等离子体预暴露的GaN上的O-极性的ZnO薄膜的生长，而Zn-极性的ZnO薄膜的Zn-预暴露的GaN上。在极角相关的CAICISS光谱中观察到的特征的起源可以通过考虑由入射离子形成的Zn和O原子的阴影锥和散射离子的阴影和聚焦效应来分析。方位角相关的CAICISS光谱揭示了锌和O-极性ZnO薄膜的表面作为c和c/2平面的混合物，比例约为50：50。具有Ga 2 O 3界面层的ZnO膜显示出晶体质量的劣化，如由X射线摇摆曲线的加宽所证明的。O-等离子体预暴露样品的CBED结果揭示了O-等离子体预暴露样品的Ga极性GaN和O极性ZnO，这直接证实了从阳离子极性到阴离子极性的极性反转。另一方面，从在Zn-预暴露的Ga-极性GaN上生长的ZnO膜获得Zn-极性ZnO CBED图案，这表明ZnO/GaN界面具有相同的阳离子极性，而没有形成界面层。注意，没有形成平面或刻面反转域边界来反转极性（从Ga极性到O极性）。这表明我们可以通过工程界面来控制极性。
ZnO/GaN heterointerfaces are engineered to control the polarity of ZnO films grown by plasma-assisted molecular beam epitaxy on Ga-polar GaN templates. The polarity of ZnO films is determined both by coaxial impact collision ion scattering spectroscopy (CAICISS) and by convergent beam electron diffraction (CBED). Polarity inversion can be achieved by inserting an interface layer with a center of symmetry, because the polarity comes from a lack of the center of symmetry. An O-polar (anion-polar) ZnO film can be grown on Ga-polar (cation-polar) GaN by inserting a Ga2O3 layer at the interface, while Zn-polar ZnO is grown on GaN without forming an interface layer. A single-crystalline monoclinic Ga2O3 layer, which has a center of symmetry, is formed by O-plasma preexposure on the Ga-polar GaN surface prior to ZnO growth, while the ZnO/GaN interface without any extra layer is formed by Zn preexposure. The orientation relationship between ZnO, Ga2O3, and GaN is determined as [2-1-10]ZnOparallel to[010](Ga2O3)parallel to[2-1-10](GaN) and (0001)(ZnO)parallel to(001)(Ga2O3)parallel to(0001)(GaN). The CAICISS results reveal the growth of an O-polar ZnO film on O-plasma-preexposed GaN, while a Zn-polar ZnO film on Zn-preexposed GaN. The origin of the observed features in polar-angle-dependent CAICISS spectra can be analyzed by considering the shadow cones of Zn and O atoms formed by incident ions and shadowing and focusing effects of scattered ions. Azimuthal-angle-dependent CAICISS spectra reveal the surfaces of both Zn- and O-polar ZnO films as mixture of c and c/2 planes with a ratio of about 50:50. The ZnO film with a Ga2O3 interface layer shows a degradation in the crystal quality as evidenced by a broadening of the x-ray rocking curves. The CBED results for the O-plasma-preexposed samples reveal Ga-polar GaN and O-polar ZnO for the O-plasma-preexposed samples, which directly confirms polarity inversion from cation to anion polar. On the other hand, Zn- polar ZnO CBED patterns are obtained from ZnO films grown on Zn- preexposed Ga-polar GaN, which indicates the same cation polarity for a ZnO/GaN interface without the formation of an interface layer. It is noted that no planar or faceted inversion domain boundaries are formed to invert the polarity (from Ga polar to O polar). This indicates that we can control the polarity by engineering interfaces.