Influence of Pinholes on MgO-Tunnel Junction Barrier Parameters Obtained from Current–Voltage Characteristics

Abstract

Magnetic tunnel junctions (MTJs) with thin barriers are already used as read sensors in recording media. However, the presence of pinholes across such few A thick barriers cannot be excluded and one needs to investigate their effect on the MTJ-transport properties. By applying large electrical currents we could change the electrical resistance of the studied MgO MTJs (due to pinhole-size variations), and study how pinholes influence the barrier parameters (thickness t and height phi) obtained by fitting current-voltage characteristics to Simmons' model. We found that, with decreasing resistance, the barrier thickness (height) decreases (increases). These results were well reproduced by a model of parallel-resistances, allowing us to estimate pinhole-free barrier parameters.

High Tunneling Magnetoresistance in Magnetic Tunnel Junctions with Subnanometer Thick Al2O3 Tunnel Barriers Fabricated Using Atomic Layer Deposition

“…Thus, exposure of metal surfaces to ambient conditions before ALD TB growth implies that a native oxide IL forms before nucleation of the ALD TB. ,,,,− The IL has a significant impact on the quality of the ALD TB growth on top of it because it is typically defective with oxygen vacancies and pinholes that leads to the defective growth continue to ALD TBs, which explains the significant increase in leakage with ex situ ALD TBs at thickness below 2–5 nm. − However, the control over the formation of a native oxide IL, which can even form in an in situ ALD process, has been optimized in our previous work. , The defective ALD TBs obtained in ex situ ALD processes are illustrated in the low E b together with soft dielectric breakdown. In the in situ ALD processes, the IL can be reduced to a negligible level ∼ 0.1–0.2 Å, which not only enables sub-nm think pinhole-free ALD TBs to be achieved but also reduces the defect concentration in ALD-Al 2 O 3 TBs, as illustrated in the E b of 1.31–1.47 eV that is significantly higher than the previously reported E b ∼ 0.3–0.6 eV for AlO x TBs. ,, This work therefore illustrates the critical importance in controlling the FM/ALD TB interface and the potential of the in situ ALD process for the fabrication of ultrathin, high-quality TBs for MTJs. − , Finally, it is worth mentioning that the ALD-Al 2 O 3 TBs are highly uniform, as reflected in the negligible TB thickness dependence of E b values. − This is in contrast to the strong TB thickness dependence of the TB height in the AlO x TBs obtained via oxygen diffusion into Al or Al alloys …”

“…In addition, the E b ∼ 1.31 ± 0.03 eV of the 0.55 nm thick ALD-Al 2 O 3 TB on Fe is significantly better than the low E b ∼ 0.3−0.6 eV for thermal TB-based devices. 5,8,48 This suggests that the defect concentration in ALD-Al 2 O 3 TBs is much reduced, as compared to the thermal AlO x TB case. This argument is supported by the significantly lower leakage current and harder dielectric breakdown observed in the former in contrast to the latter.…”

“…The current state-of-the-art MTJ fabrication uses an optimized FM electrode, an antiFM pinning layer, and magnetic thermal annealing, and the best figure-of-merit TMR is in the range of 10–70% for MTJs with AlO x TBs − , and 200–350% for MTJs having epitaxial MgO TBs. − However, further reduction of these TBs to a sub-nm thickness range remains challenging because of defects, such as grain boundaries and oxygen vacancies, within the dielectric TBs, resulting in increased leakage current and incoherent tunneling in both AlO x - or MgO-based MTJs. − ,− This is in agreement with the observed reduced E b in the range of 0.3–0.6 eV for thermal TB-based devices. ,, …”

Change Character of Conductivity in Metal–Insulator–Metal Thin Films

“…Formulas (2) and 3show that for a 2D structure the energy dependence of the density of states is linear, whereas for the 1D structure it is a descending half-parabola. To calculate the tunneling current in MIM systems, one should use formulas that do not include the density of states, [30,31] such as the model proposed by Simmons. [32] However, the current in tunneling microscopy is considered to be proportional to the density of states.…”

Investigation on Etch Characteristics of Nanometer-Sized Magnetic Tunnel Junction Stacks Using a HBr/Ar Plasma

“…MRAM device may be employed as an effective replacement for static random-access memory (SRAM) device, dynamic random-access memory (DRAM) device, flash memory device, and their combinations, to provide highspeed operation, low operating voltage, and nonvolatile storage. [1][2][3][4][5] CoFeB thin films have been used of late as free layers in MTJ stacks, instead of the conventional NiFe and CoFe films, to improve the electrical properties of MTJ stacks. In this study, the etch characteristics of CoFeB films, among the various magnetic thin films, were examined because they are currently used as the key layers in MTJ stacks.…”