Abstract
Magnetic tunnel junctions (MTJ) with MgO/Fe based interfaces and out-of-plane spin direction form the basis of present-day spin-transfer-torque magnetic random-access memory (STT-MRAM) devices. They are a leading type of nonvolatile memory due to their very long endurance times and lack of reliability problems. Many semiconductor devices, such as the field effect transistor or nonvolatile memories, have undergone fundamental changes in materials design as dimensional scaling has progressed. Here, we consider how the future scaling of the MTJ dimensions might affect materials choices and compare the performance of different tunnel barriers, such as 2D materials like h-BN with the existing MgO tunnel barriers. We first summarize key features of MgO-based designs of STT-MRAM. We then describe general aspects of the deposition of 2D materials and h-BN on metals. We compare the band structures of MgO and h-BN with their band gaps corrected for the GGA band error. The different absorption sites of h-BN on Fe or Co are compared in terms of physisorbtive or chemisorbtive bonding sites and how this affects their spin-polarized bands and the transmission magneto-resistance (TMR). The transmission magneto-resistance is found to be highest for the physisorptive sites. We look at how these changes would affect the overall TMR and how scaling might progress.
Highlights
Si over that of 2D semiconductors is being lost, so that 2D semiconductors are being considered for FET channel thicknesses below $2 nm[1,2] (Fig. 1)
Hexagonal boron nitride is studied as a possible tunnel barrier in magnetic tunnel junctions
It is argued that the present chemical vapor deposition process of hexagonal boron nitride (h-BN) should ideally be replaced by atomic layer deposition to lower the growth temperature, to a value that minimizes B dissolution in the metal electrodes
Summary
Si over that of 2D semiconductors is being lost, so that 2D semiconductors are being considered for FET channel thicknesses below $2 nm[1,2] (Fig. 1). Other 2D materials like hexagonal boron nitride (h-BN) are being studied for some areas in electronics,[3] such as low K dielectrics by Hong et al.[4] or as the nonlinear dielectrics in resistive random-access memories.[5,6] This suggests that h-BN could be considered as a tunnel barrier layer in STT-MRAM, despite what seems as the overwhelming advantage of the present design. Patterned FeCo electrodes were found to possess sufficient perpendicular magnetic anisotropy (PMA)[13,14,15] and MgOjFeCo stacks had enough interfacial PMA14–17 to orient the spin direction perpendicular to the electrodes, which allowed the continued increase in TMR with decreasing bit size The chemical thermodynamics and materials selection of this design are very favorable.[15,16,17]
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