SCANNING TUNNELING SPECTROSCOPY OF MOTT-HUBBARD STATES ON THE 6H-SIC(0001)3 X 3 SURFACE
SCANNING TUNNELING SPECTROSCOPY OF MOTT-HUBBARD STATES ON THE 6H-SIC(0001)3 X 3 SURFACE
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6H-SIC(0001)3 X 3 表面莫特-哈伯德态的扫描隧道光谱
DOI：
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发表时间：
1999
期刊：
影响因子：
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通讯作者：
R. Feenstra
中科院分区：
文献类型：
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作者：
V. Ramachandran;R. Feenstra
在6 H-SiC（0001）SiO 3 × SiO 3表面上测量了p型和n型材料的扫描隧道谱。随着使用非常低的隧道电流，隧道光谱揭示了不同的带的空和填充状态，分开2.0 eV。这些状态位于相同的空间位置，从而支持预测Mott-Hubbard型态密度的硅吸附原子模型。碳化硅是一种宽带隙半导体，其特性使其在高温电子和其他领域非常有用。SiC晶体以原子平面的各种堆叠排列存在;特别感兴趣的是立方3C多型体和六方4 H和6 H多型体。在表征6 H-SiC（0001）表面作为适合于外延和器件开发的表面方面已经做了很多工作。一些工作人员已经使用扫描隧道显微镜（STM）研究了表面的原子结构[1，2]。已经发现了一组Si终止的（0001）表面的重构，其中之一，103 × 103，引起了许多实验[3-5]和理论[6-9]的兴趣。从理论上讲，这种重建的最低能量模型包括在T4位置上的Si终止的块状晶体上的Si吸附原子。在这里，三个Si吸附原子键合轨道背键到Si原子。第四成键轨道延伸到真空中，其中只有一个电子。对该结构模型的局域密度泛函计算[6，8]预测了半填充的，因此是由悬挂键产生的金属带。在光电发射[4]和逆光电发射[5]光谱可用后，进行了更精细的计算[7]，采用了二维哈伯德模型。这些计算表明，该表面的能级由填充带和空带组成，由U=1.6 eV的Hubbard间隙分开，从而产生半导体态密度（DOS）。这样的表面在每个位置都有一个局域电子，形成一个二维自旋系统，在三角形晶格上可以取两个值。这个问题在理论上很有意义，因为这样的系统可能会被破坏并形成2-D自旋玻璃。这种结构在文献中受到质疑，并提出了几种替代方案[1，3，10，11]，但迄今为止的实验结果无法区分这些模型。在n型6 H-SiC（0001）晶片上进行的光电子能谱（PES）实验显示，6 H-SiC（0001）晶片的低能电子衍射（LEED）图案显示出费米能级以下1.2 eV的完全填充带[4]。在同一个表面上的逆光电子能谱（IPES）[5]显示了一个空的表面态，比费米能级高1.1 eV。这两个结果都与最近对该表面的Mott-Hubbard基态的理论预测相一致[7]。
Scanning tunneling spectra have been measured on the 6H-SiC(0001)√3 × √3 surface for both pand n-type material. With the use of exceptionally low tunnel currents, the tunneling spectra reveal distinct bands of empty and filled states, separated by 2.0 eV. The states are located at the same spatial position, thereby supporting a silicon adatom model which predicts a Mott-Hubbard type density of states. Silicon carbide is a wide bandgap semiconductor with properties that make it useful in high temperature electronics and other areas. SiC crystals exist with various stacking arrangements of the atomic planes; of particular interest are the cubic 3C polytype and hexagonal 4H and 6H polytypes. Much work has been done on characterizing the 6H-SiC (0001) surface as a surface suitable for epitaxy and device development. Several workers have studied the atomic structure of the surface using the scanning tunneling microscope (STM) [1,2]. A set of reconstructions of the Si terminated (0001) surface has been discovered, one of which, the √3 × √3, has evoked much experimental [3-5] and theoretical [6-9] interest. Theoretically, the lowest energy model for this reconstruction consists of Si adatoms at T4 positions on a Si terminated bulk crystal. Here, three of the Si adatom bonding orbitals are backbonded to Si atoms. The fourth bonding orbital extends into vacuum with only one electron in it. Local density functional calculations for this structural model [6,8] predict a half-filled, and hence metallic band arising from the dangling bond. More refined computations [7] performed after the photoemission [4] and inverse photoemission [5] spectra became available, employed a two-dimensional Hubbard model. These calculations indicate that the energy levels of this surface consist of a filled and an empty band, separated by a Hubbard gap of U=1.6 eV, thus producing a semiconducting density of states (DOS). Such a surface would have at each site a single localized electron, forming a 2-D system of spins which can take two values on a triangular lattice. This problem is of great theoretical interest because such systems can be frustrated and form 2-D spin glasses. This structure has been questioned in the literature, and several alternatives have been proposed [1,3,10,11] but experimental results to date are unable to distinguish between these models. Photoemission spectroscopy (PES) experiments on n-type 6H-SiC (0001) wafers which show a √3 × √3 low energy electron diffraction (LEED) pattern reveal a fully-filled band 1.2 eV below the Fermi level [4]. Inverse photoemission spectroscopy (IPES) [5] on the same surface shows an empty surface state 1.1 eV above the Fermi level. Both these results are in moderate agreement with recent theoretical predictions of a Mott-Hubbard ground state for this surface [7]