Self-organized growth controlled by charge states of magnetic impurities
Self-organized growth controlled by charge states of magnetic impurities
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DOI：
10.1038/nmat1721
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发表时间：
2006-09
期刊：
影响因子：
41.2
通讯作者：
Tomasz Dietl
中科院分区：
文献类型：
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作者：
Tomasz Dietl
利用异质结构界面上晶格失配产生的应变场，已成功地应用于量子点的自组织生长中。在这个方向上的进一步进展可以促进高密度三维存储器和空间光调制器的发展，用于先进的光子应用。我们认为半导体合金成分之间的库仑力也可以用来控制纳米晶体的沉积。某些合金相图表现出溶解度间隙，不可能均匀生长，导致独立分解成低浓度和高浓度的特定成分区域。在其中一种成分的浓度很小的情况下，这可能导致在多数相中形成相干纳米晶体。例如，这种spinodal分解已知发生在外延生长的（Ga, in） N中，其中富in量子点区域嵌入在贫in矩阵中。van Schilfgaarde和Mryasov3以及其他人开创性的从头计算揭示了稀释磁性半导体（DMS）形成非随机合金的强烈倾向。Spinodal分解不涉及另一晶体相的沉淀，因此不容易在实验中检测到。然而，在（Ga, Mn） As的电子透射显微镜下观察到，相干富锰（Mn, Ga） As纳米晶体5,6导致了360 K的表观居里温度。此外，同步辐射微探针检测显示（Ga, Mn） N的六方结构中存在富锰纳米晶体，因此有理由推测具有高浓度磁性成分的相干纳米团簇可以解释（Ga, Mn） N和相关体系中较高的表观居里温度。这可以解释长期以来关于稀释磁性半导体和氧化物铁磁响应起源的难题，其中没有检测到外来晶体相，但磁性离子的平均浓度远远低于最近邻耦合的渗透极限，同时自由载流子密度太低，无法调解有效的远程交换相互作用。值得注意的是，铁磁纳米晶体的存在导致磁光和磁输运响应增强。这为这种混合系统的各种应用打开了大门，包括前面提到的用于体积记录的磁光空间光调制器，前提是可以找到控制纳米晶体生长特性的方法。从过渡金属的开壳层得到的能级通常驻留在主半导体的带隙中。这一特性已经被用于半绝缘材料的制造，在半绝缘材料中，磁性杂质的中间间隙水平捕获来自残留杂质或缺陷的载流子。我们论证的要点是观察到这种俘获改变了磁性离子的电荷状态，从而影响了它们之间的库仑相互作用。在载流子浓度小的材料中，这些相互作用不会被屏蔽掉，因此我们认为，由于最近邻键的作用，位间库仑斥力过度补偿了自由能的降低。这阻碍了旋多分解，并稳定了均匀合金的生长，即使组分的浓度在等电子化合物的溶解度间隙内。因此……
To the editor–The exploitation of strain fields, generated by the lattice mismatch at interfaces of heterostructures, has been successfully used in the self-organized growth of quantum dots1. Further progress in this direction could boost the development of high-density threedimensional memories and spatial light modulators for advanced photonic applications2. We argue here that Coulomb forces between the constituents of semiconductor alloys may also be used to control the deposition of nanocrystals. Certain alloy phase diagrams exhibit a solubility gap, where no homogenous growth is possible, leading to spinodal decomposition into regions with low and high concentrations of a particular constituent. In cases where the concentration of one of the constituents is small, this could lead to the formation of coherent nanocrystals within a majority phase. For instance, such spinodal decomposition is known to occur in epitaxially grown (Ga, In) N, where In-rich quantum-dot-like regions are embedded within an In-poor matrix. Pioneering ab initio calculations by van Schilfgaarde and Mryasov3 as well as others4 have revealed a particularly strong tendency of diluted magnetic semiconductors (DMS) to form non-random alloys. Spinodal decomposition does not involve the precipitation of another crystallographic phase, and therefore is not easy to detect experimentally. Nevertheless, it has been observed in electron transmission microscopy in (Ga, Mn) As, where coherent zincblende Mn-rich (Mn, Ga) As nanocrystals5, 6 led to an apparent Curie temperature of 360 K. Furthermore, a synchrotron radiation microprobe examination7 revealed the presence of Mn-rich nanocrystals in the host hexagonal structure of (Ga, Mn) N. It is therefore reasonable to speculate that coherent nanoclusters with a large concentration of the magnetic constituent could account for the high apparent Curie temperatures in (Ga, Mn) N and related systems. This could explain the longstanding puzzle concerning the origin of the ferromagnetic response of diluted magnetic semiconductors and oxides, in which no foreign crystallographic phases are detected but the average concentration of magnetic ions is far below the percolation limit for the nearest-neighbour coupling, and at the same time the free-carrier density is too low to mediate an efficient long-range exchange interaction. Remarkably, the presence of ferromagnetic nanocrystals leads to an enhanced magnetooptical6 and magnetotransport8 response. This opens the door for various applications of such hybrid systems, including aforementioned magnetooptical spatial light modulators for volumetric recording, provided that methods for controlling the properties of the nanocrystal growth could be found.It is known that the energy levels derived from the open d shells of transition metals reside usually in the bandgap of the host semiconductor. This property is already exploited for the fabrication of semi-insulating materials, in which mid-gap levels of magnetic impurities trap carriers originating from residual impurities or defects. The essential point of our argument is the observation that such a trapping alters the charge state of the magnetic ions and hence affects their mutual Coulomb interactions. These interactions are not screened out in materials with small carrier concentrations, and we argue therefore that the intersite Coulomb repulsion overcompensates the lowering of the free energy due to the nearest-neighbour bonding. This impedes the spinodal decomposition and stabilizes the growth of a uniform alloy even if the concentration of the constituents lies within the solubility gap for the isoelectronic compound. Accordingly …