Paper TF+EM+MI-TuM12
Precision Defect Engineering of Metal/Insulator/Metal (MIM) Diodes using Localized ALD Transition Metal Impurities in Al₂O₃ Tunnel Barriers

Tuesday, October 22, 2019, 11:40 am, Room A122-123

Thin film MIM tunnel diodes are receiving increased interest for high-speed applications such as THz detection and rectenna based energy harvesting. Traditionally, current density vs. field (J-ℰ) asymmetry (ƒ_asym = J^-/J⁺) with MIM diodes has been achieved through metal work function differences (ΔΦ_M). Recently, nanolaminate insulator tunnel barrier MIIM diodes enabled by ALD showed improved ƒ_asym, non-linearity, and responsivity at low voltage by step tunneling through the wider bandgap (E_G) insulator to the conduction band of the narrow E_G insulator.¹ Intrinsic defects present in narrow E_G insulators were later demonstrated to further improve low ℰ asymmetry via defect enhanced direct tunneling, when paired with an insulator dominated by tunneling.^2,3 In this work, we investigate the impact of localized extrinsic defects by using ALD to intentionally introduce Ni at precise intervals in an Al₂O₃ tunnel barrier.

ALD of Al₂O₃ on TiN was performed at 200 °C using TMA and H₂O. Five samples were prepared in which a 100 cycle Al₂O₃ ALD sequence was interrupted by two cycles (c) of Ni(^tBu2DAD)₂ and O₃ after 25, 50, 75, and every 25 c of Al₂O₃. As-deposited MIM devices were tested with bias applied to an Al top electrode (Fig. 1).

DC J-ℰ sweeps of the 100 c device show Fowler-Nordheim tunneling (FNT) at high ℰ, with ƒ_asym > 1 due to ΔΦ_M ≈ 0.2 eV (Fig. 1). The addition of Ni cycles in all cases leads to an increase in J at low ℰ vs. the 100 c Al₂O₃ device, suggesting defect related conduction. At high ℰ, however, J of all Ni devices is lower than the 100 c device, suggesting suppression of FNT. The 25/2/75 and 75/2/25 (Al₂O₃/Ni/Al₂O₃) devices show ƒ_asym opposite of the 100 c device, while the 50/2/50 and 25/(2/25)x3 devices are roughly symmetric (Fig. 1). The greater reduction in J at large negative ℰ, ƒ_asym reversal, and reduced J-ℰ slope for the 25/2/75 and 75/2/25 devices suggest that FNT is suppressed more for emission from the smaller Φ_M electrode (Al) than for TiN. FNT suppression appears greatest for the 75/2/25 device in which Ni is closest to the Al, pointing to an increase in effective barrier height, likely due to negative charge in the Al₂O₃. Capacitance (C) vs. ℰ sweeps (Fig. 2) reveal a positive voltage shift in C_min for all Ni devices, consistent with negative charge.

The asymmetry reversal demonstrates the possibility of precision defect engineering of MIM tunnel devices using ALD. An in-depth discussion of J-ℰ and C-ℰ, temperature-IV, frequency-CV, other impurities, and annealing will be presented.

1. Alimardani et al., APL 102 143501 (2013).

2. Alimardani et al., JAP 116, 024508 (2014).