Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22901, USA.
J Phys Condens Matter. 2013 Aug 7;25(31):315801. doi: 10.1088/0953-8984/25/31/315801. Epub 2013 Jul 9.
The interaction of Mn with Ge quantum dots (QD), which are bounded by {105} facets, and the strained Ge wetting layer (WL), terminated by a (001) surface, is investigated with scanning tunneling microscopy (STM). These surfaces constitute the growth surfaces in the growth of Mn-doped QDs. Mn is deposited on the Ge QD and WL surface in sub-monolayer concentrations, and subsequently annealed up to a temperature of 400 ° C. The changes in bonding and surface topography are measured with STM during the annealing process. Mn forms flat islands on the Ge{105} facet, whose shape and position are guided by the rebonded step reconstruction of the facet. Voltage-dependent STM images reflect the Mn-island interaction with the empty and filled states of the Ge{105} reconstruction. Scanning tunneling spectra (STS) of the Ge{105} facet and as-deposited Mn-islands show a bandgap of 0.8 eV, and the Mn-island spectra are characterized by an additional empty state at about 1.4 eV. A statistical analysis of Mn-island shape and position on the QD yields a slight preference for edge positions, whereas the QD strain field does not impact Mn-island position. However, the formation of ultra-small Mn-clusters dominates on the Ge(001) WL, which is in contrast to Mn interaction with unstrained Ge(001) surfaces. Annealing to T < 160 °C leaves the Mn-clusters on the WL unchanged, while the Mn-islands on the Ge{105} facet undergo first a ripening process, followed by a volume gain which can be attributed to the onset of intermixing with Ge. This development is supported by the statistical analysis of island volume, size and size distribution. Increasing the annealing temperature to 220° and finally 375 ° C leads to a rapid increase in the Mn-surface diffusion, as evidenced by the formation of larger, nanometer size clusters, which are identified as germanide Mn5Ge3 from a mass balance analysis. This reaction is accompanied by the disappearance of the original Mn-surface structures and de-wetting of Mn is complete. This study unravels the details of Mn-Ge interactions, and demonstrates the role of surface diffusion as a determinant in the growth of Mn-doped Ge materials. Surface doping of Ge-nanostructures at lower temperatures could provide a pathway to control magnetism in the Mn-Ge system.
使用扫描隧道显微镜（STM）研究了 Mn 与由{105}面限域的 Ge 量子点（QD）以及由（001）面终止的应变 Ge 覆盖层（WL）之间的相互作用。这些表面构成了 Mn 掺杂 QD 生长的表面。在亚单层浓度下，将 Mn 沉积在 Ge QD 和 WL 表面上，然后在高达 400°C 的温度下进行退火。在退火过程中，使用 STM 测量键合和表面形貌的变化。Mn 在 Ge{105}面形成了平坦的岛，其形状和位置由面的重新键合台阶重构来引导。与 Ge{105}重构的空态和满态相互作用的电压依赖 STM 图像反映了 Mn 岛。Ge{105}面和沉积 Mn 岛的扫描隧道谱（STS）显示带隙为 0.8eV，Mn 岛谱的特征是在约 1.4eV 处存在一个空态。对 QD 上 Mn 岛的形状和位置进行统计分析表明，边缘位置略有偏好，而 QD 应变场对 Mn 岛位置没有影响。然而，在 Ge(001)WL 上，超小 Mn 簇的形成占主导地位，这与 Mn 与未应变 Ge(001)表面的相互作用形成对比。在 T<160°C 以下退火时，WL 上的 Mn 簇保持不变，而 Ge{105}面的 Mn 岛首先经历一个成熟过程，然后体积增加，这可以归因于与 Ge 的混合开始。岛体积、大小和大小分布的统计分析支持这一发展。将退火温度提高到 220°C，最后提高到 375°C，导致 Mn 表面扩散迅速增加，这表现为更大的纳米尺寸簇的形成，从质量平衡分析确定为 Mn5Ge3 。该反应伴随着原始 Mn 表面结构的消失，并且 Mn 完全去湿。这项研究揭示了 Mn-Ge 相互作用的细节，并证明了表面扩散作为决定 Mn 掺杂 Ge 材料生长的因素的作用。在较低温度下对 Ge 纳米结构进行表面掺杂可能为控制 Mn-Ge 系统中的磁性提供途径。
