Development of MBE grown CrSe for spintronics applications

Lead Research Organisation: Heriot-Watt University
Department Name: Sch of Engineering and Physical Science

Abstract

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.

Planned Impact

This programme is a proof-of-principle proposal to be followed, after successful demonstration of CrSe layers with the appropriate characteristics (zinc blende crystal structure and ferromagnetic with a Curie temperature well above room temperature) by a full proposal to develop and exploit the material in spintronic devices. There are two stages of impact; that arising in the short term from this initial phase of the project, and that generated from the subsequent stage.
In the initial stage there are clear impacts arising from the development of a new material system, the samples generated and the training given. The RA will be given a good training in all aspects of project management from the very start and in growth and characterisation of a challenging materials system. This will equip this person with a wide range of skills which are always in demand both in academia and industry. For PhD students within the MBE group, the challenge of working with such demanding materials means that they are trained to the highest level and are in demand at other laboratories.
The results of the growth and characterization of this material will be of interest to the MBE and II-VI communities as they provide an example of a new type of material with many applications. This information will be disseminated to these communities via presentations at International Conferences and publications in high impact journals.
Other research communities also have an interest in the development of these compounds, including ab initio theorists calculating materials properties. There is already an extensive literature on the calculated physical and electronic properties of CrSe but this work will provide the first experimental data with which the models can be tested. Another community is those investigating the magnetic properties of materials. Semiconducting half metals are rare and CrSe will be a welcome addition to the limited number currently available.
The other major impact of the first stage will be generated by the samples themselves. The material produced will be unique, as epitaxial zinc blende CrSe layers have never been produced before. After the routine analysis required for this project they will be used in projects both at PhD level and in undergraduate projects at Heriot-Watt and will also be provided to our colleagues at Phillips Uni., Marburg. After the routine measurements we require for this project, the magnetic properties will be studied in detail by the consortium of Universities headed by Prof Heimbrodt. In addition, as is our normal practice, samples will also be made available to other groups.
The strategic importance of half metals in the long term development of spintronics was described in the International Technology Roadmap for Semiconductors. Successful demonstration of zinc blende CrSe will make this material an interesting attractive alternative to the more conventional Heusler materials. However, in the second phase of the project, the successful demonstration of spintronic devices incorporating CrSe will generate a significantly greater impact with target device structures including spin injectors, spin valves and spin extractors, leading subsequently to memristors.
There is a clear application route which involves spin injection into III-V devices, as II-VI layers can be deposited after growth onto III-V structures lattice matched to GaAs. This would be of enormous interest to groups working on a variety of III-V systems, for example quantum dots and high mobility 2D electron gases.
The second project stage includes the development of novel devices with considerable potential for commercialisation. Heriot-Watt has an excellent record of attracting and working with commercial partners and the MBE group has a sound track record in this area, with previous industrial support for II-VI device development. Commercialisation of devices will be through the University's Technology Research Services.

Publications

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Moug R (2014) Growth of metastable ZB CrSe Growth of metastable ZB CrSe in physica status solidi (c)

 
Description This was a short grant to demonstrate that thin films of chromium selenide (CrSe), could be grown by molecular beam epitaxy (MBE)and to supply samples to prof Wolfram Heimbrodt at Philipps Uni., Marburg for testing of their magnetic properties. This material is of interest as it was predicted to be ferromagnetic up to room temperature and was identified as being a potential half-metal. This is a material in which electrons behave in different ways depending on the orientation of their spins - for one type the material is a metal, while for the other it is a semiconductor.
Growing chromium compounds by MBE is extremely challenging, requiring special material sources and our group has now developed considerable expertise through this grant which can be extended to other related materials. Analysis of the ultra-thin films produced was also not straightforward and in doing so we have improved our understanding of the use of X-ray techniques on such samples.
During the period of the grant CrSe samples were successfully produced and supplied to Prof Heimbrodt. Extensive characterization of this material is still in progress, but preliminary results show that the layers are ferromagnetic, but with indications of multiple ferromagnetic phases. This behavior is not predicted by theory.
Exploitation Route The growth of these materials is of interest to crystal growers currently attempting to develop growth techniques for more challenging materials.
the magnetic properties of CrSe, once explored in detail, will be of considerable interest to the magnetic materials community, and in particular those theorists who routinely use the results from our materials to refine their computations.
Our initial analysis of the material is that alloying CrSe with another element, either Mn, S or Te will modify the properties and allow the production of both half metals and room temperature ferromagnetic devices which can be used for spintronic applications.
As a result, we have recently submitted a grant proposal on extending this work together with Prof Heimbrodt and colleagues at Bath University.
Sectors Electronics

 
Description Samples from this project have been a) supplied to other Universities for optical and magnetic analysis (Marburg, Bath) and b) used within student projects at (structural characterization) Heriot-Watt University at both PhD and Undergraduate levels. c) Details of Analysis of samples, together with the growth method have been presented at an International Conference and a National UK meeting. d) Analysis of the magnetic properties of CrSe has allowed us to design structures using CrSe and its alloys with other elements to produce novel structures which can be incorporated in spintronic devices. This is the next stage of the program after this preliminary proposal which has recently been submitted to EPSRC.
First Year Of Impact 2013
Sector Education,Electronics
Impact Types Societal,Economic