Surface states and annihilation characteristics of positrons trapped at the (100) and (111) surfaces of silicon
Surface states and annihilation characteristics of positrons trapped at the (100) and (111) surfaces of silicon
复制标题
硅(100)和(111)面俘获正电子的表面态和湮没特性
DOI：
10.1103/physrevb.70.165309
复制
发表时间：
2004
影响因子：
3.7
通讯作者：
A. Weiss
中科院分区：
文献类型：
--
作者：
N. G. Fazleev;J. L. Fry;A. Weiss
最近用正电子湮没诱导俄歇电子能谱(PAES)对Si(100)和Si(111)进行的研究表明，对于元素半导体Clean Si的两面，表面俘获正电子与相应的Si核能级电子的实验湮没几率显著不同。这些实验结果是通过对理想终止、未重构和重构表面的“像势”正电子表面态和表面俘获正电子与相应的Si核能级电子的湮没特性的计算从理论上得到的。计算的正电子表面结合能表明它们对表面最顶层的特定原子结构很敏感，当与正电子功函数相比时，所有被研究的Si(100)和Si(111)表面上的正电子表面态的稳定性。发现正电子表面态波函数局域在两个未重构半导体表面真空面的势垒中。Si(100)表面的结构使得正电子表面态波函数在原子偏离其理想终止位置的区域扩展到晶格中。对Si(100)表面正电子结合能的理论和实验比较表明，用非对称二聚体模型描述重构表面时，符合得最好。计算表明，正电子表面态波函数在重建表面的角空洞区的三个维度上都是局域化的。这种局域化解释了以前的实验，这些实验未能显示正电子表面态在表面平面内离域时，电子-正电子对动量密度分布的各向异性。计算了每个表面的正电子湮没特性，并与实验正电子能谱数据进行了比较。这些计算揭示了正电子湮没特性对清洁硅晶面的强烈依赖性，而在清洁金属表面上发现的面依赖性要小得多。与重建表面的结果相比，表面俘获正电子与核能级电子的湮没几率明显减小，这与实验的PAES数据一致。这些结果表明，与原子核湮没几率成正比的PAES强度对元素半导体的晶面和表面结构很敏感。
Recent studies of Si(100) and Si(111) using positron annihilation induced Auger-electron spectroscopy (PAES) reveal that experimental annihilation probabilities of surface trapped positrons with relevant Si core-level electrons differ significantly for two faces of clean Si, an elemental semiconductor. These experimental results are investigated theoretically by performing calculations of the “image-potential” positron surface states and annihilation characteristics of the surface trapped positrons with relevant Si core-level electrons for the ideally terminated, nonreconstructed and reconstructedandsurfaces. Computed positron surface binding energies demonstrate their sensitivity to the specific atomic structure of the topmost layers of surfaces, and, when compared to positron work functions, the stability of positron surface states on all studied Si(100) and Si(111) surfaces. The positron surface state wave function was found to be localized in a potential well on the vacuum side at both nonreconstructed semiconductor surfaces. Thereconstruction of the Si(100) surface causes the positron surface state wave function to extend into the lattice in the regions where atoms are displaced away from their ideal terminated positions. A comparison of theoretical and experimental positron surface binding energies for Si(100) shows that the best agreement is achieved when the reconstructedsurface is described within the asymmetric dimer model. Calculations indicate that the positron surface state wave function is localized in all three dimensions in the corner hole regions of the reconstructedsurface. This localization provides an explanation for previous experiments that failed to show the anisotropy in the electron-positron pair momentum density distribution expected for a positron surface state delocalized in the plane of the surface. Positron annihilation characteristics are calculated for each surface and compared with experimental positron spectroscopy data. These calculations reveal strong dependence of positron annihilation characteristics on the crystal face of clean Si in contrast to the much smaller face dependence found on clean metal surfaces. Annihilation probabilities of surface trapped positrons with- and-core-level electrons are found to be significantly smaller for the reconstructedsurface when compared with the results for the reconstructedsurface, in agreement with experimental PAES data. These results indicate that PAES intensities, which are proportional to core annihilation probabilities, are sensitive to the crystal face and surface structure of an elemental semiconductor.