Up to 100% spin injection and detection in heterostructures of graphene and hBN

In our first “Behind the Paper” post, we cover a paper recently published in Nature Communications by Mallikarjuna Gurram et al., entitled “Bias induced up to 100% spin-injection and detection polarizations in ferromagnet/bilayer-hBN/graphene/hBN heterostructures”. We have interviewed Mallikarjuna Gurram, who is currently working towards a PhD at the Zernike Institute for Advanced Materials, University of Groningen,  under the supervision of Prof Bart van Wees.

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Nature Communications

Could you briefly outline the key findings of your paper?

M. Gurram: “A poor injection and detection of spins in graphene has been a bottleneck in realizing graphene spintronic devices for practical purposes. We address this issue in a fully hexagonal boron nitride (hBN) encapsulated graphene spin valve device which demonstrated the possibility to inject and detect spins in graphene with differential spin injection and detection polarizations up to 100% by applying a bias across the cobalt/2L-hBN/graphene/hBN contacts at room temperature.
Surprisingly, we also found a unique sign inversion of the spin polarization, at a small voltage bias, which is unique only to two-layers of hBN tunnel barrier. Besides, we also demonstrate two-terminal spin valve with a record magnitude of the spin signal up to 800 Ohms and magnetoresistance ratio of 2.7%.”

What is your role in this work?

M. Gurram: “I, along with my colleague Siddhartha Omar and my PhD advisor Prof. Bart van Wees conceived the experiments. I carried out the sample fabrication and measurements. All authors carried out the analysis, discussed the results and the manuscript.

What was the genesis of this paper?  How did you come to this particular problem?

M. Gurram: “Nearly a decade ago, our group first reported the electrical spin injection and transport in graphene, published in Nature in 2007. Since then, most research on graphene spintronics is focused on finding a better substrate and better tunnel barrier in order to realize graphene’s full potential in a clean environment. In 2010, hBN was found to be a better substrate for studying spin transport in graphene. However, the conventional oxide tunnel barriers were still limiting the spin injection efficiencies and spin lifetimes due to the presence of unwanted spin sinks and spin scatterers at the interface. Later, it was discovered that hBN can also act as a tunnel barrier in its thin form with an atomically smooth surface and no signature of spin sinks.

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Nature Communications

In principle, the problems associated with the commonly used SiO2 substrate and the conventional oxide tunnel barriers can be overcome by using hBN material. Therefore, in our study, we prepared a fully hBN encapsulated device where the bottom thick-hBN acts as a substrate and the top thin monolayer-hBN acts as a tunnel barrier for electrical spin injection and detection, published in Phys. Rev. B in 2016. Herein, we reported a low spin injection efficiency due to the atomically thin nature of hBN tunnel barrier causing a well-known conductivity mismatch problem between the ferromagnet and graphene. To overcome this mismatch and to achieve large spin injection efficiency, a recent theoretical study, published by Q. Wu, et al. in Phys. Rev. Appl. in 2014, suggests to use bi or tri layers of hBN. In a follow up experiment in this paper, we used a similar device geometry as before but with bilayer-hBN as a tunnel barrier for spin injection and detection.

What is the most empowering implication of your results?

M. Gurram: “Our results have some interesting implications. Firstly, large spin polarization achieved in this type of graphene spin valve devices can be used in spin-transfer torque memory devices (random access memory, RAM) technology wherein the magnetization orientation of various magnetic materials can be changed. Secondly, we found sign reversal of spin polarization just by applying electrical bias across the ferromagnet/bilayer-hBN/graphene/hBN interface. The implication of this finding is that it avoids the necessity of external magnetic field to change the orientation of spins in graphene. Other promising applications include magnetic sensors, spin logic gates, and spin transistors.”

How have 2D materials been uniquely instrumental to enabling these results?

M. Gurram: “Without 2D materials our results would not have been achieved. Firstly, graphene is the first 2D material to demonstrate spin transport at room temperature, and instrumental in enlightening 2D spintronics research field. Finding a good tunnel barrier for electrical spin injection and detection in graphene has been the basis for most of the graphene spintronics research in the past decade. For this, we need a very thin, few atoms think, ultra-smooth, insulating, and pinhole free material. hBN, an isomorph of graphene and belonging to 2D materials family, has been found to satisfy these requirements. Moreover, multilayer hBN acts as a flat and neutral dielectric substrate for achieving large electron mobility in graphene. Besides their individual properties of graphene and hBN, one unique characteristics of the 2D materials is the possibility of making hBN-graphene-hBN heterostructures by utilizing the van der Waals forces attraction at their interfaces.”

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Mallikarjuna Gurram

Can you describe the main challenges associated to the preparation of this manuscript? Any anecdotes you’d like to share with us?

M. Gurram: “It may sound trivial but finding a thin layer of hBN tunnel barrier flake was challenging for making the device. We regularly use a SiO2/Si substrate with oxide thickness of 300 nm to exfoliate and find the graphene flakes. However, it is not optimal for providing good optical contrast to find the hBN flakes down to monolayer. Now we started to use 90 nm SiO2/Si substrate which gives better colour contrast for “transparent” hBN. However, we still face the problem to find an optimal thin bilayer of hBN flake for tunnel barrier.”

Anything that stroke you as particularly surprising, unexpectedly pleasant/unpleasant during the peer review process?

M. Gurram: “During the review process, I was very glad that the reviewers were highly positive and enthusiastic about the results and the paper publishing went smoothly.  One useful comment, pointed by a referee was the concern about relatively lower mobility and spin transport properties of our sample in spite of minimizing the chances of impurity inclusion during the fabrication process. We still tend to believe that thin hBN flakes such as mono-bi layers are not optimal for complete encapsulation of graphene and one needs to find alternate routes to optimize the cleanliness of these samples.

What is your favourite 2D paper published in 2016/2017, and why?

M. Gurram: “The paper published by Huang B. et al., Nature 546, 270(2017) is quite interesting to me. The paper reported for the first time, an intrinsic ferromagnetism in a 2D material, CrI3. Different 2D materials with distinct physical properties have been found since the discovery of graphene. These can be broadly categorized into conductors, semiconductors, and insulators. Whereas, this paper becomes the first one to report a 2D material, CrI3 which is ferromagnetic down to monolayer. Magnetic materials are essential building blocks of spintronics devices. Especially graphene spin valves need a ferromagnetic material for electrical spin injection and detection. However, the conventional ferromagnetic materials used in graphene spinvalves include cobalt and permalloy, need to be grown in bulk amounts on top of the device. Our spin valve device, ferromagnet/bilayer-hBN/graphene/hBN consists of 2D materials except the cobalt ferromagnet. The use of 2D ferromagnet in place of the conventional ferromagnet creates a possibility of making ultra-thin graphene spin valve devices completely out of 2D materials. Of course, 2D ferromagnets hold promise for many more opportunities for spintronics as outlined in this paper.”

Which is the development in the field of 2D materials that you would like to see in the next 10 years?

M. Gurram: “Currently, academic research is done using micro scale flakes which is great to explore the interesting physics. But for the practical applications, these materials should be produced in bulk with high quality and with a possibility of growing heterostructures of desired layers.”

And now, what’s next?

M. Gurram: “From our results, we do not understand what is causing the unusual behaviour of spin polarization as a function of bias applied across ferromagnet/bilayer-hBN/graphene/graphene. Currently we are working on understanding the underlying mechanism.”

Behind the Paper

We will soon launch a new section of the blog entitled “Behind the Paper”, where we aim to highlight recently published articles across the Nature journals representing important advances of significance to researchers working on 2D materials.

We hope to engage with both young scientists and established group leaders, and we will encourage our authors to share the real story behind their papers, from genesis to publication, the highs and the lows.

We have chosen an interview format, where authors of relevant Nature journal publications will be periodically invited to answer a number of questions covering the motivations, context, key findings, and implications of their work.

Stay tuned!

Silvia Milana (Nature Communications)

2D materials devices: Challenges in device fabrication

Semiconducting 2D materials such as transition metal dichalcogenides are beginning to generate a lot of interest as a candidate for ultra thin body electronics and optoelectronics. There have been significant advances in the last 2 to 3 years and a shift from papers reporting fundamental properties of these materials towards proof of concept devices and high quality, larger scale synthesis. In technology speak one could say the field has graduated into a Technology Readiness Level of 3-4 where scientists all over the world are conducting research to understand feasibility of using these materials in electronics and optoelectronic components.

This is rapid progress indeed but the jump to TRL 5-6 will require clear understanding of all facets of the device fabrication and integration in order to preserve the figures of merit such as the mobility, subthreshold swing, responsivity and luminescence quantum yield.  R&D would therefore need to focus on optimizing each step of device fabrication :

  1. Growth/deposition of 2D materials

This is the first and main step of fabrication and has therefore received the most attention. There are numerous papers reporting the large area deposition of MoS2 and other 2D materials using powder precursors in a furnace.  While this is a great first step in producing high quality crystals, there are several unanswered questions regarding the continuity, grain boundary structure and scalability of these films. Metal organic precursor based techniques such as MOCVD and ALD present an attractive alternative to this technique and optimization of film properties via this method would be critical to the progress of this technology. An aspect in this realm that has not received much attention is the importance of substrates and the influence of stresses in the resulting films on the aforementioned figures of merit.

  1. Contacts

Scalable deposition of high quality Ohmic contacts is still an unsolved problem that needs careful study to minimize parasitic effects.  Successful strategies in the lab involve the deposition pure metal contacts such as Au and Pd under ultra high vacuum or patterning/surface treatment of the area under the contact to create stronger interaction between the inert 2D surface and the metal. This in turn has implications for high frequency operation (fmax) etc.  In order to set the challenge, best values of contact resistance on MoS2 are on the order of ~700-900 Ω-μm while state-of-the-art Si-MOSFETs with values as low 80 Ω-μm have been reported. An improvement in this figure will certainly go a long way in affecting the next TRL jump.

  1. Gate dielectric and passivation layers.

One of the reasons for the rise of silicon as ubiquitous material in everyday electronics is the stability of its oxide this is not true of Germanium which was the material used for Shockley’s transistor!  More recently the ability to deposit thin, high-k dielectrics by techniques such as ALD with extremely low interface density states and high breakdown voltages not only extended the scaling limits for Si based devices but also enabled the commercialization of III-V based devices such as HEMTs.  The inert nature of Van der Waals layers endow them with properties conducive to these applications but this by it’s very nature causes homogeneity and quality problems for the deposition of high-k dielectrics. Several workarounds have been reported ranging from the deposition of a metallic seed layer to functionalization of the 2D layers which show great promise.

  1. Etching of 2D materials

While it seems trivial due to its atomically thin structure controlled etch of 2D materials is critical to achieve sophisticated device structures such as LEDs, tunnel transistors or for side contacts to 2D materials. The relatively mature etch technology can therefore be easily tuned to etch stacks of these layers with profiles dictated by the device architecture. At the limit of controlled etching is the emerging field of Atomic Layer Etching (ALE).  Significant advances in ALE methods of both conventional and 2D materials would enable further advances in atomic scale devices.

  1. Metrology and in-line quality control

Important advances in the solutions of the above challenges requires non destructive characterization techniques both in-line and post fabrication. Techniques such as Raman spectroscopy and ThZ spectroscopy show great promise in providing significant insights into the structural and electronic properties.  Correlation of material quality by evaluating device performance with spectral features obtained using these techniques would aid in standardization. This definition would enable scientists across the world to work towards the same goal and thereby further accelerate development of this exciting new technology.

 

 

Dr Ravi S Sundaram

Market Manager: Research and Emerging Technologies

Oxford Instruments Plasma Technology

North End, Yatton,

BS49 4AP

http://bit.ly/2rtQ4kt

https://www.linkedin.com/in/ravi-shankar-sundaram-86241727

Nature Materials: Focus on 2D materials beyond graphene

One of the many things that were revealed with the isolation of graphene, was the pursuit of atomically thin forms of other materials: semiconductors, boron nitride and, more recently, Xenes, are offering endless possibilities not only to explore fundamental physics, but also to demonstrate improved or even entirely novel applications.

2D materials have a lot to offer in terms of optoelectronics applications and in a wide range of wavelengths (microwave to the visible),  as they exhibit something that graphene does’t have: a bandgap. Tony Low, Frank Koppens and colleagues review the physics and applications of different kinds of polaritons (exciton-, plasmon- and phonon) in layered 2D materials.

Alessandro Molle, Deji Akinwande and collaborators offer a critical overview of the issues that remain open when it comes to Xenes, where atoms from the group IVA elements are organized into a single layered, honeycomb-like lattice. Several issues remain still under debate, such as their stability, while various theoretical studies are predicting a plethora of interesting topological properties.

A unique characteristic of 2D materials, however, is the possibility to easily form (horizontal or vertical) van der Waals heterostructures, following the stacking of layers of different materials and thicknesses. Such heterostructures however are not only limited to combinations of 2D materials; Deep Jariwala, Tobin Marks and Mark Hersam explain in their Review that 2D materials can be combined with non-2D materials, such as organic molecules and quantum dots (0D), carbon nanotubes (1D) and bulk Si, Ge, III–V and II–VI semiconductors (3D), that adhere primarily through non-covalent interactions.

 

Maria Maragkou (Nature Materials)

 

Nature Reviews Materials: Focus on 2D materials

The applications of 2D materials are numerous and diverse, ranging from electronics to catalysis, and from information storage to medicine.

A Focus Issue just published in Nature Reviews Materials covers the synthesis and fundamental properties of sFocus on 2D materialseveral 2D materials, as well as the devices they enable, combining Reviews, Comments and Research Highlights. In particular, a Review by Manish Chhowalla explores the use
of graphene, hexagonal boron nitride, transition metal dichalcogenides (TMDCs), phosphorene and silicene as channels in field-effect transistors. The emerging field of valleytronics and its implementation in graphene and TMDCs is the topic of a Review by John Schaibley, Xiaodong Xu and colleagues. Beyond electronics, graphene (and, more in general, carbon-based materials) is attracting growing interest as a low-cost catalyst for renewable energy production and storage: the use of heteroatom-doped graphene as a metal-free catalyst is the topic of a Review by Xien Liu and Liming Dai. In another Review, by Castro Neto and colleagues, the synthesis, properties and applications of phosphorene are discussed.

There is clearly a lot of fundamental research being carried out on 2D materials; however, it is also necessary to address their translation into commercial or medical devices. This problem is analyzed in two Comments, one by Seongjun Park and one by Kostas Kostarelos. A common theme emerging from these opinion pieces is the need for stronger collaboration between academia and industry or medical professionals.

As it was argued in other posts in this blog, the road to the commercialization of 2D-materials-based products is still long, and many challenges lie ahead. But numerous exciting developments in the field of 2D materials are keeping researchers busy, and we hope that the overview provided in this Focus Issue will be a useful tool for the community to explore them.

 

Giulia Pacchioni (Nature Reviews Materials)

 

Graphene commercialization: a voice from industry

The commercialization of graphene-based products is a recurring theme in this blog. Why after much talking about graphene being a wonder material the most high-tech graphene-based product we can buy is still a tennis racket? Nature Reviews Materials asked this question to Seongjun Park, an engineer working in Samsung and studying graphene. In a Comment piece, he reminded the readers that the commercialization of new materials and technologies always takes time, often decades — optical memory devices and phase-change memories are good examples, as it took more than 30 years to take them to the market. Compared to them, graphene is still a young technology: it is only 12 years that scientists and engineers are playing with it and tweaking its properties.

Park likens the process of commercialization to a jigsaw puzzle, in which many pieces need to fit together in order to produce a recognizable image. Many studies are carried out on graphene, but they often focus on one specific property, whereas for creating a graphene-based device multiple properties have to be optimized at the same time, and multiple engineering challenges have to be addressed. Currently, two types of applications are under development: those that are easy to develop and promise low reward, and those that are very challenging but are potential game-changers. The way to go, Park reckons, is to develop the first kind of products while we wait for the second kind to mature. One factor that is slowing down the process of graphene commercialization is the fact that academics tend to focus on research lines that are likely to lead to high-profile publications, expecting engineers in industry to develop commercial products on their own; papers reporting results from industry are often judged incremental and of little interest. A stronger link between academia and industry is thus needed to speed up graphene commercialization.

It is often said that in the Gartner hype cycle graphene has passed the peak of inflated expectations. Now is time to take a more realistic approach to what researchers and engineers can develop in the short term. It is very early to lose faith in the potential of graphene to enable new, revolutionary products.

 

Giulia Pacchioni (Nature Reviews Materials)

 

Celebrating Nature Nanotechnology

Nature Nanotechnology has recently turned 10. To celebrate this milestone, a number of experts from different areas of nanotechnology have been invited to describe how the field has evolved in the last ten years. The ever growing demand for improved functionalities and nanoscale miniaturisation of electronic devices, in addition to the approaching limits of current silicon-based technology, has driven the quest for materials enabling alternative technological solutions. In this context, two-dimensional materials have significantly shaped the nanotechnology landscape over this decade. In the feature entitled “Nano on reflection”, Dr Silvia Milana, Associate editor at Nature Communications, outlines the development of two-dimensional materials, highlighting both promising achievements and associated challenges.

2D goes 3D

Van der Waals heterostructures

Two-dimensional layered materials and van der Waals heterostructures. From Nature Reviews Materials 16042 (2016) “Van der Waals heterostructures and devices

If you are reading this blog, you probably already think that 2D materials are awesome. However, stacks combining several 2D materials could be even better — they open almost endless possibilities for new properties and devices, as they draw from a wide library of 2D materials with different electronic properties, ranging from insulating to metallic, conductive and superconductive, which can be mixed and matched to create hybrid structures with unique functionalities. Xiangfeng Duan and colleagues bring us on an inspiring journey to discover van der Waals heterostructures in a newly published Review in Nature Reviews Materials. Flexible and transparent electronic and optoelectronic devices based on van der Waals stacks have already been demonstrated, including tunneling transistors, vertical field-effect transistors, wearable electronics and innovative solar cells.
The ‘ingredients’ for the heterostructures feature graphene as the most common component, but they can also include boron nitride, transition metal dichalcogenides, phosphorene and other materials. Thanks to the fact that the interlayer interactions are van der Waals in nature, highly disparate materials can be integrated without limitations imposed, for example, by lattice mismatches. Because all the components of a device can be integrated in a single membrane without needing to incorporate a substrate, flexible and adaptable devices can be obtained.
There are still important challenges that lie ahead; namely, the difficulty of developing fabrication methods that are both scalable and precise, and the need to produce reliable contacts. But van der Waals heterostructures promise to enable amazing functionalities in electronic and optoelectronic devices — maybe it is this growth in the third dimension that will realize the full potential of 2D materials.

 

Giulia Pacchioni (Nature Reviews Materials)