University of Groningen
Graphene heterostructures for spin and charge transport
Zomer, Paul Joseph
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Publication date: 2019
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Zomer, P. J. (2019). Graphene heterostructures for spin and charge transport. University of Groningen.
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9
Conclusion
Abstract
This chapter will conclude this thesis. After looking back at the main accomplishments, namely the development of fabrication methods for heterostructeres and demonstration of how these can be employed to fabricate high quality graphene devices for charge and spin transport studies, it will be shown how this is of importance for the emerging research on other layered materials.
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118 9. Conclusion
9.1
Conclusion
The isolation of single layer graphene for research in 2004 has been a very impor-tant step[1, 2]. And the developments since then have illustrated this[3]. The ease with which one can obtain a graphene is remarkable, certainly considering the rich physics that can be studied in this truly 2-dimensional material. However, to access graphenes true potential and experimentally investigate the intrinsic properties of monolayer flakes, another step was necessary. Ultimately graphene is made up of only surface area and thus it can always be influenced by extrinsic elements in its environment. And not in the last place by the SiO2substrate.
In this thesis two approaches were presented to isolate graphene by means of an-other substrate: hexagonal boron nitride (h-BN). This does not completely cancel out extrinsic factors, as can be clearly seen in figure 5.3 a), where additional resistance peaks show up due to the Moir´e pattern. However, the negative influence of for example charge impurities and substrate roughness are overcome this way[4]. The method explained in chapter 4 deals with this[5]. Importantly, commercially avail-able h-BN was used to demonstrate high electronic quality devices. The polymer used here has a low molecular weight, making it easier to remove it after the h-BN graphene heterostructure has been made. However, in this lies also the downside of this transfer method.
Besides the bottom side of the graphene that is in contact with the substrate, the topside also suffers from extrinsic elements. This is mainly due to polymer residue, unavoidable in the fabrication steps required to make a device, but also adsorbates play a role. Thus, ideally the graphene flake is protected from the bottom as well as the top side. The method presented in chapter 5 provides an elegant approach[6]. Rather than building a multilayer stack on the substrate one flake at a time, the stack is built by picking up flakes from the substrate. A major advantage is that the flakes in the stack will have no polymer remains between them, but also the process is much faster as no cleaning steps are needed after every single layer. Graphene ap-pears to be a difficult material to lift from the substrate, making it difficult to directly contact it[7]. This problem is overcome here.
The usefulness of the transfer methods is further demonstrated in chapter 6, where quantum Hall measurements are used to probe the capacitance profile at the graphene edge[8]. Essentially, the effective capacitance depends on the spatial posi-tion of an quantum Hall edge channel, with a higher capacitance for channels that are closer to the edge. High quality devices are needed to access the quantum Hall regime at relatively low magnetic fields.
Another field that may profit from improved quality is that of spintronics. The expectations for spin transport in graphene have been very high. And while the first results were impressive, with room temperature spin transport reported to have a relaxation length of 2 µm micron[9], theoretical predictions were even more
opti-9
mistic, expecting relaxation lengths to be much larger[10]. Using h-BN graphene het-erostructures the effect of the substrate could be studied for the first time, as shown in chapter 7[11]. Interestingly little influence was found where the spin relaxation time was considered. The diffusion time, and with this the spin relaxation time, in-creased as expected for higher mobility devices. This led to long range non-locally detected spin transport, over distances even up to ∼20 µm. Looking at which spin relaxation mechanism was dominating it was best explained as a combination of the Elliott-Yafet and Dyakonov-Perel mechanisms. Comparing the relaxation rates for these mechanisms to a standard SiO2based graphene device, similar relaxation rates
were found. So, while heterostructure spintronic devices clearly improve transport characteristics, the underlying relaxation mechanism apparently remains in place.
However, for practical applications this is not the main concern. More impor-tantly is the robustness of spintronic devices and this is what chapter 8 is concerned with. Single layer graphene has been intensely studied when it comes to spin trans-port, but few layer graphene (FLG) should not be disregarded[12]. In fact, this sys-tem profits from the fact that the outer layer effectively protects the interior. FLG devices were intentionally damaged using high energy proton irradiation[13]. The damage is directly apparent when looking at the charge transport characteristics, but spin transport is surprisingly robust. It is only at the highest doses that no spin transport can be observed.
In conclusion, methods to fabricate high quality graphene devices have been demonstrated here. These improvements are an important step in order to study the intrinsic properties of graphene. The methods have been employed for charge and spin transport studies. Charge transport measurements mainly dealt with the quantum Hall effect, the onset of which is achieve for lower magnetic fields when the sample quality is improved. Where graphene spin transport is considered, a step forward has been taken in improving the spin relaxation length. However, this only appears due to an improved device mobility. On the other hand, intentionally decreasing the device quality of few layer graphene devices indicates that spin trans-port persists here as well. This is an imtrans-portant realization where the applicability of (few layer) graphene for spintronics is considered.
9.2
Outlook
The ability to accurately transfer single or multi-layer flakes has been a very im-portant step in the further development of graphene research. But it has not been limited to this field. In fact, there exists a vast amount of other materials that share an interesting property with graphene. Namely their van der Waals bound layered character[14]. Soon after graphene research was sparked, other materials such as molybdenum disulfide (MoS2)[15] or tungsten diselenide (WSe2)[16] started
receiv-9
120 9. Conclusion
ing attention. And this development continues to gain momentum, with more and more layered materials being researched[17]. The interesting aspect is that these other crystals give access to other properties, such as direct or indirect band gaps, magnetic moments, superconductivity or variation in spin orbit coupling strength. In particular the existence of magnetic layered materials such as CrI3, CrBr3, Cr2Ge2Te6
or Fe3GeTe2is of relevance for spintronics[18–21]. Inclusion of these into
heterostruc-tures is a viable approach to fully 2 dimensional spintronic devices. And by using the methods for heterostructure fabrication presented here, one can indeed create in-teresting layered structures that combine material specific properties. This provides near limitless possibilities in term of device tailoring.
It is therefore no surprise that this is exactly what has been happening. While graphene has been widely explored over the years, research groups are now also using their know-how built while studying graphene to explore new directions. This led to the development of a whole research field based on van der Waals materials, that can go in any direction, be it electronics, spintronics or optics[17]. This also means that transfer methods as discussed here have become an indispensable tool in the device fabrication process. And clearly their application has progressed well beyond simply safeguarding a graphene flake from its environment.
Regarding graphene spintronics, follow-up research was done on partially en-capsulated devices [22, 23] using the fabrication method from chapter 5. One com-plication here is that the non-encapsulated regions contribute to relaxation, as was also shown for suspended graphene spintronic devices[24]. Despite this it is clear that the h-BN encapsulation improves the spin transport characteristics considerably, with spin relaxation times of 2 ns and relaxation lengths of 12 µm being reported for single layer graphene. Furthermore the encapsulated device architecture allows for the easy inclusion of a top gate that can be used to influence the spin transport pa-rameters. Another use of hBN in graphene spintronics is to replace the contact tun-nel barrier[25–29]. Mono-crystalline sheets of h-BN should provide perfect barriers without pin holes.
9
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