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Pinhole-free and defect-free ultrathin dielectric tunnel barriers (TBs) is a key to obtaining high tunnelling magnetoresistance (TMR) and efficient switching in magnetic tunnel junctions (MTJs). Among others, atomic layer deposition (ALD) provides a unique approach for the fabrication of ultrathin TBs with several advantages including an atomic-scale control on the TB thickness, conformal coating, and low defects density. Motivated by this, this work explores fabrication and characterization of spin-valve Fe/ALD-Al2O3/Fe MTJs with ALD-Al2O3 TB thickness of 0.55 nm using in situ ALD. Remarkably, high TMR values of ~77% and ~ 90% have been obtained respectively at room temperature and at 100 K, which are comparable to the best reported values on MTJs having thermal AlOx TBs with optimized device structures. In situ scanning tunnelling spectroscopy characterization of the ALD-Al2O3 TBs has revealed a higher tunnel barrier height (Eb) of 1.33±0.06 eV, in contrast to Eb~0.3-0.6 eV for their AlOx TB counterparts, indicative of significantly lower defect concentration in the former. This first success of the MTJs with sub-nm thick ALD-Al2O3 TBs demonstrates the feasibility of in situ ALD for fabrication of pinhole-free and low-defect ultrathin TBs for practical applications and the performance could be further improved through device optimization.
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In situ atomic layer deposition and electron tunneling characterization of monolayer Al2O3 on Fe for magnetic tunnel junctions
Magnetic tunnel junctions (MTJs), formed through sandwiching an ultrathin insulating film (so-called tunnel barrier or TB), with ferromagnetic metal electrodes, are fundamental building blocks in magnetoresistive random access memory (MRAM), spintronics, etc. The current MTJ technology employs physical vapor deposition (PVD) to fabricate either amorphous AlOx or epitaxial MgO TBs of thickness around 1 nm or larger to avoid leakage caused by defects in TBs. Motivated by the fundamental limitation in PVD in, and the need for atomically thin and defect-free TBs in MTJs, this work explores atomic layer deposition (ALD) of 1-6 Å thick Al2O3 TBs both directly on Fe films and with an ultrathin Al wetting layer. In situ characterization of the ALD Al2O3 TB was carried out using scanning tunneling spectroscopy (STS). Despite a moderate decrease in TB height Eb with reducing Al wetting layer thicknesses, a remarkable Eb of ∼1.25 eV was obtained on 1 Å thick ALD Al2O3 TB grown directly on an Fe electrode, which is more than twice of that of thermal AlOx TB (∼0.6 eV). Achieving such an atomically thin low-defect TB represents a major step towards improving spin current tunneling in MTJs.
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- PAR ID:
- 10597229
- Publisher / Repository:
- American Institute of Physics
- Date Published:
- Journal Name:
- AIP Advances
- Volume:
- 8
- Issue:
- 12
- ISSN:
- 2158-3226
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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