Development of MBE grown CrSe for spintronics applications

电子是既具有自旋又具有电荷的粒子，像电荷一样，自旋的方向可以用来存储和传输信息。这导致了自旋电子学领域的迅速发展。目前成功的自旋电子器件都是基于包含磁性和非磁性金属交替层的结构，迄今为止，在开发成功的包含半导体的自旋电子器件方面存在相当大的问题。这种装置需要一种磁性材料，这种材料可以与半导体形成结，以选择性地注入或移除具有特定自旋方向的电子。理想情况下，这种材料应该在室温下工作良好，不与半导体发生化学反应，并且只注入一个自旋方向。到目前为止，还没有材料满足所有三个标准。半金属是一种磁性材料，作为潜在的半导体触点引起了相当大的兴趣。在半金属中，两种不同自旋电子的能带具有完全不同的能量，因此，费米能量在一个自旋方向的导带底部以上，而在另一个方向的导带底部以下。对于第一个自旋方向，材料表现为金属，而对于另一个方向，它表现出所有典型的半导体特征，例如带隙。最近有研究表明，某些含有元素周期表V族或VI族元素的过渡金属化合物在闪锌矿晶体结构中生长时是半金属，这也意味着它们可以作为更传统半导体多层结构的一部分生长。然而，所有这些化合物通常不是在这种晶体结构中生长的，这意味着它们必须以更高的能量、亚稳态的形式生长。在这个提议中，我们已经确定了这些材料中最有前途的一种，即化合物硒化铬。它具有许多吸引人的特性，应该满足良好接触材料的所有三个标准。尽管如此，闪锌矿CrSe以前从未被种植过。这一原理证明方案的目的是通过分子束外延（MBE）在闪锌矿晶体结构中生长CrSe。结构性质（晶格常数，泊松比等）将在Heriot-Watt大学进行测量，样品也将提供给马尔堡菲利普斯大学的Wolfram Heimbrodt教授，他将对层的磁性进行基本评估。如果这个提议证明可以用锌闪锌矿晶体结构生产CrSe，并且正如预测的那样，薄层在室温以上是铁磁性的，那么随后的完整提议将会产生，其目的是在一系列自旋电子器件中利用CrSe的特性。

Electrons are particles which have spin as well as charge, and like charge the orientation of the spin can be used to store and transport information. This has given rise to the rapidly developing area of spintronics. Currently the successful spintronic devices are all based on structures which contain alternating layers of magnetic and non-magnetic metals, and to date there have been considerable problems in developing successful spintronic devices containing semiconductors.Such devices require a magnetic material which can form a junction with a semiconductor to selectively either inject or remove electrons with one particular spin orientation. Ideally, the material should work well at room temperature, not react chemically with the semiconductor and only inject one spin orientation. To date there is no material with fulfils all three criteria.Half metals are types of magnetic material which have attracted considerable interest as potential semiconductor contacts. In a half metal, the bands for the two different spin electrons have quite different energies, so that the Fermi energy lies above the bottom of the Conduction Band for one spin orientation, but is below it for the other. For the first spin orientation the material behaves as a metal, while for the other it demonstrates all the typical semiconducting features, such as a bandgap.Recently it has been shown that compounds of certain transition metals with elements from group V or VI of the periodic table are half metallic when grown in the zinc blende crystal structure, which also means they can be grown as part of multilayer structures with more conventional semiconductors. However all of these compounds do not normally grow in this crystal structure which means that they have to be grown in a higher energy, metastable form.In this proposal we have identified one of the most promising of these materials, which is the compound chromium selenide. This has many attractive properties and should satisfy all three criteria for a good contact material.Despite this, zinc blende CrSe has never been grown before. The aim of this proof of principle proposal is to demonstrate that CrSe can be grown in the zinc blende crystal structure by molecular beam epitaxy (MBE). The structural properties (lattice constant, Poisson's ratio, etc) will be measured at Heriot-Watt University and samples will also be provided to Prof. Wolfram Heimbrodt at Philipps Universitaet, Marburg who will provide a basic assessment of the magnetic properties of the layers.If this proposal demonstrates that CrSe can be produced with the zinc blende crystal structure and, as predicted thin layers are ferromagnetic above room temperature, then a subsequent full proposal will then be produced which will aim to exploit the properties of CrSe in a range of spintronic devices.