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Sub-Nanometer Atomic Layer Deposition for Spintronics in Magnetic Tunnel Junctions Based on Graphene Spin-Filtering Membranes Marie-Blandine Martin, Bruno Dlubak, Robert S Weatherup, Heejun Yang, Cyrile Deranlot, Karim Bouzehouane, Frédéric Petroff, Abdelmadjid Anane, Stephan Hofmann, John Robertson, Albert Fert, and Pierre Seneor ACS Nano, Just Accepted Manuscript • DOI: 10.1021/nn5017549 • Publication Date (Web): 02 Jul 2014 Downloaded from http://pubs.acs.org on July 14, 2014
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Sub-Nanometer Atomic Layer Deposition for Spintronics in Magnetic Tunnel Junctions Based on Graphene Spin-Filtering Membranes Marie-Blandine Martin1, Bruno Dlubak1,2, Robert S. Weatherup2, Heejun Yang1,3, Cyrile Deranlot1, Karim Bouzehouane1, Frédéric Petroff1, Abdelmadjid Anane1, Stephan Hofmann2, John Robertson2, Albert Fert1 and Pierre Seneor1,* 1 - Unité Mixte de Physique CNRS/Thales, 91767 Palaiseau, France, and University of ParisSud, 91405 Orsay, France 2 - Department of Engineering, University of Cambridge, Cambridge CB21PZ, United Kingdom 3 - Center for Integrated Nanostructure Physics, Institute of Basic Science, Sungkyunkwan University, Suwon 440-746, South Korea
KEYWORDS: atomic layer deposition ; spintronics ; spin filter ; graphene ; dielectrics ; magnetic tunnel junction ;
We report on the successful integration of low cost, conformal and versatile atomic layer deposited (ALD) dielectric in Ni-Al2O3-Co magnetic tunnel junctions (MTJ) where the Ni is
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coated with a spin-filtering graphene membrane. The ALD tunnel barriers, as thin as 0.6 nm, are grown layer-by-layer in a simple, low vacuum, ozone-based process which yields high quality electron-transport barriers as revealed by tunneling characterization. Even under these relaxed conditions, including air exposure of the interfaces, a significant tunnel magnetoresistance is measured highlighting the robustness of the process. The spin filtering effect of graphene is enhanced leading to almost fully inversed spin polarization for the Ni electrode with -42%. This unlocks the potential of ALD for spintronics with conformal, layer-by-layer control of tunnel barriers in magnetic tunnel junctions towards low-cost fabrication and down-scaling of tunnel resistances.
Atomic layer deposition (ALD) is a key technology for applications requiring ultra-thin, uniform, high quality, and conformal films. Owing to the chemically surface-limited growth mechanisms involved, ALD enables atomic control of the film thickness even for high aspect ratio substrates while being scalable and compatible with other industrial techniques such as rollto-roll processing.1-3 These advantages are already exploited in a number of application, including for instance multilayer stacks in thin film electroluminescent flat panel displays,4,5 high quality dielectrics in dynamic random access memory (DRAM) capacitors,6,7 conformal passivation coatings for organic electronics e.g. organic light-emitting diodes (OLED),8,9 and conformal functionalization for porous biomaterials and bioelectronic devices.10,11 Furthermore, ALD grown ultra-thin, high-k gate dielectrics were industrially adopted for complementary metal oxide semiconductor (CMOS) logic technologies for high volume manufacturing as early as the 45 nm node,12 and ALD is particularly well adapted for the high aspect ratio architectures such as FinFET and TriGate transistors used in the current 22 nm node.13,14
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To date, despite a number of pioneering attempts15-19 with promising results, the advantages of ALD have not been widely employed in the field of magnetic tunnel junctions for spintronics, which has instead had to rely on more complex physical deposition setups. Spin-valves, the active elements in technologies such as hard-drive read-heads and magnetic random-access memories (MRAM),20 would appear to be prime candidates to benefit from the use of ALD:15-17 spin devices rely heavily on ultra-thin films such as nanometer-scale oxide tunnel barriers, whose quality and atomic composition crucially impact the electron’s spin polarization. A key issue has however remained: performing ALD directly on metallic spin sources (usually ferromagnets like Ni, Co, Fe and their alloys) leads to their oxidation during the fabrication and thus a quenching of their spintronic performances. This has drastically limited the range of processes that can be used to fabricate spin-valve devices. Hence, although promising in terms of thickness and composition control, the application of water or ozone based ALD processes to magnetic tunnel junctions has so far been hindered by the associated ferromagnet oxidation.15-17
Here we demonstrate the successful realization of graphene coated Ni/Al2O3/Co magnetic tunnel junctions based on sub-nanometer, ozone based ALD grown tunnel barriers. We are thus able to take advantage of the layer-by-layer thickness control, inherent to ALD, to down-scale the thickness and the resistance×area product of tunnel barriers. Measurements of large inverse tunnel magneto-resistance (TMR) signals highlight the almost full reversal of the graphene coated Ni spin polarization (P=-42%) compared to the max reported value (P=+46% for the highest quality Ni/Al2O3 interfaces21) and demonstrate the quality of the ALD barrier.
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RESULTS AND DISCUSSION In figures 1 and 2, we present the technological steps employed to integrate ALD grown tunnel barriers in vertical spin valves. Ni bottom electrodes are defined by a standard lift-off step and a subsequent chemical vapor deposition (CVD) process22,23 covers these electrodes with a continuous, few layer graphene coating. This yields a reduced and passivated Ni surface.24 This direct graphene growth method on patterned ferromagnetic electrodes is readily scalable, and avoids graphene transfer or exfoliation steps otherwise typical for the fabrication of graphene devices.25,26 A 1 µm2 junction is then defined in patterned resist (see figure 1). Next, after exposing the bottom electrode to air, ALD is performed with cycles of ozone and Tri[Methyl]Aluminum (TMA) pulses to grow ultra-thin Al2O3 layers (see methods). This fabrication flow results in electrodes fully covered with an Al2O3 thin film. We note that the growth of ultra-thin dielectrics on sp2-bonded graphene coatings has previously been considered challenging:27,28 both the preservation of the pristine graphene’s sp2 structure and the homogeneous wetting of the dielectric require careful engineering which has been particularly discussed for films deposited by evaporation or water-based ALD. In particular, ALD on graphene has been shown to require surface seeding (such as modified ALD cycles, inorganic clusters or molecular layers such as PTCA…) to increase wetting as required for ultra-low film thicknesses.27-29 Interestingly, the unseeded ozone-based ALD (O3-ALD) process used here is shown to be non-detrimental to the graphene layer (see figure 3), whilst yielding Al2O3 films homogeneous enough to be used as a tunnel barriers (see figures 4 and 5).
As ozone is a powerful oxidant, its use in the process might be expected to result in significant oxidation of the graphene coating. We have thus investigated the impact of the whole O3-ALD
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growth process on CVD graphene monolayers using Raman spectroscopy, for 3 different growth temperatures (Tch): 80 °C, 150 °C and 200 °C (see figure 3). At the highest growth temperature of 200 °C, a strong Raman defect-enabled D peak (1350 cm-1) appears in the spectra, with an amplitude even exceeding that of the graphitic G peak (1600 cm-1) indicating a heavily degraded sp2 structure.30 Clearly at this high temperature, ozone reacts with the graphene layer introducing defects and forming functional groups such as epoxides on carbon sites.31,32 Remarkably, however, this reactivity decreases sharply with lower growth temperature. In particular, Raman spectra measured after the 80 °C growth of Al2O3 remain unchanged compared to those measured on bare as-grown CVD graphene (see figure 3): graphene appears to be inert to ozone during the 80 °C ALD process. These observations relate to the fact that ozone physisorption occurs on the graphene surface whilst further chemical bonding between the oxygen and carbon atoms requires a non-negligible activation energy barrier to be overcome.31,32 In the case of our 80 °C O3-ALD process, the kinetics of this reaction are quenched for the exposure times considered. This inert process is thus considered in the following to grow ultra-thin Al2O3 films in complete magnetic tunnel junctions.
Given the low wettability of the carbon sp2 structure, another challenge is the formation of continuous, ultra-thin, pinhole-free tunnel barriers on graphene. Despite it so far not being achievable by evaporation,33 we previously reported that sputtering led to high quality Al2O3 films on graphene down to a thickness of 1 nm.34 However, growing Al2O3 films thinner than 1 nm on graphene by sputtering did not yield continuous layers. In particular, AFM characterisation of a 0.6 nm tunnel barrier, produced by sputtering a 0.4 nm film of metallic Al that was further oxidized in a 50 Torr O2 atmosphere (as for the high quality 1 nm tunnel
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barrier34), reveals a particularly high roughness (RMS>0.7 nm, even in excess of the targeted nominal film thickness). In fact, at these lower thicknesses the coalescence of Al islands is not achieved following the sputtering process, leading to severely pinholed films. In stark contrast, 0.6 nm of Al2O3 obtained by the O3-ALD process on graphene shows significantly higher homogeneity and no observable pinholes, as confirmed by AFM. The measured root mean square (RMS) roughness of 0.25 nm for the 0.6 nm ALD film is comparable to what is achieved for the thicker, high quality, 1 nm, sputtered films. These characteristics underline that the inherent attributes of ALD are well-suited for the needs of spintronics devices in terms of producing conformal, ultra-thin, high quality, films. Indeed, in the following the
