Abstract
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.
Highlights
Magnetic Tunnel Junction (MTJs) are the building blocks for non-volatile magnetoresistive random access memory (MRAM) and are created by sandwiching an ultrathin insulator between two ferromagnetic electrodes.[1,2,3,4] MRAM has significant performance and power consumption advantages over standard dynamic random access memory or flash memory with fast read, write times and a memory state which is retained without power draw.[1,2,5] Magnetic tunnel junctions (MTJs) operate with a differing spin-polarized electron tunneling resistance for parallel and antiparallel magnetization of the ferromagnetic layers
We report the first success in growth of atomically thin (1-6 Å in thickness), lowdefect ALD Al2O3 tunnel barrier (TB) through atomic layer deposition directly on ferromagnetic Fe electrode
In situ STS studies have confirmed that high Eb up to 1.25 eV can be obtained on the 1 Å thick ALD Al2O3 TBs grown directly on Fe, which is more than twice of that of the thermal AlOx TBs
Summary
Magnetic Tunnel Junction (MTJs) are the building blocks for non-volatile magnetoresistive random access memory (MRAM) and are created by sandwiching an ultrathin insulator between two ferromagnetic electrodes.[1,2,3,4] MRAM has significant performance and power consumption advantages over standard dynamic random access memory or flash memory with fast read, write times and a memory state which is retained without power draw.[1,2,5] MTJs operate with a differing spin-polarized electron tunneling resistance for parallel and antiparallel magnetization of the ferromagnetic layers. The tunneling current decreases exponentially with the tunnel barrier (TB) thickness and can be further affected due to scattering from defects in TBs. The current MTJ technology based on physical vapor deposition (PVD) employs either amorphous AlOx or epitaxial MgO TBs of thickness around 1 nm or larger to avoid leakage caused by defects in TBs. The TMR values are around 10-70% for AlOx TBs6–8 and at least a factor of three higher for crystalline MgO TBs due to enhanced coherent spin current tunneling.[5,9,10,11,12,13,14,15] It should be realized that TBs with a smaller thickness approaching atomic scale, that are pinhole-free with low defect concentration are the key to high TMR values desirable for achieving greater signal to noise ratio, lower power consumption, higher speed, and large design margin for device fabrication.
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