Coherent Resonant Tunneling through Double Metallic Quantum Well States

International audience; Study of resonant tunneling through multi-metallic quantum well (QW) structure is not only important for the fundamental understanding of quantum transport but also for the great potential to generate advanced function-alities of spintronic devices. However, it remains challenging to engineer such a structure due to the short electron phase coherence length in metallic QW system. Here, we demonstrate the successful fabrication of double-QW structure in a single fully epitaxial magnetic tunnel junction (MTJ) heterostructure, where two Fe QW layers are sandwiched between three MgAlO x tunnel barriers. We show clear evidence of the coherent resonant tunneling through the discrete QW states in the two QWs. The coherent resonant tunneling condition is fulfilled only when the middle barrier between the two QWs is thin enough and available QW states are present simultaneously in both QWs under a certain bias. Compared to the single QW structure, the resonant tunneling in double-QW MTJ produces strong conductivity oscillations with much narrower peak width (about half) owing to the enhanced energy filtering effect. This study presents a comprehensive understanding of the resonant tunneling mechanism in MTJ with multiple QWs, which is essential for future development of new spintronic devices operating in the quantum tunneling regime. R esonant tunneling in quantum well (QW) structures has been extensively studied because of its importance in the field of nanoelectronic science and technology. 1 Recently, in the metallic QW system the combination of the tunneling magnetoresistance (TMR) 2,3 effect with resonant tunneling through metallic QW states in magnetic tunnel junctions (MTJs) has triggered considerable interest in the new functionality of spintronic devices. 4− 8 In these structures, the discrete spin-dependent QW states can be formed in the middle metallic QW layer sandwiched between two barriers. The QW potential barrier can be formed either by metallic layer using the symmetry-dependent Bloch state orbital band structure, 9− 12 or by a double oxide tunneling barriers with a much higher barrier height for better electron confinement. 13− 15 Because the QW states are often formed in the majority spin channel, 4,8 the conductance in parallel magnetic configuration can be enhanced when the injector Fermi level is aligned with the QW state at resonance, resulting in an enhanced TMR ratio. 4 A record high resonant TMR (≈ 4000%) has been reported for a double barrier MTJ measured with point-contact technique at 4.2 K. 16 In semiconductor materials, multi-QW structures have already been extensively studied, 17 such as resonant-tunneling diode, 18 multi-quantum well light-emitting diode and laser, 19 and so forth. When the thickness of the barrier and QW is thin enough, the multi-QWs form the superlattice structure and the