Investigation of In Vacuo Atomic Layer Deposition of Ultrathin MgAl₂O₄ Using Scanning Tunneling Spectroscopy

Recently, disordered spinel MgAl₂O₄ as insulating tunnel barriers for perpendicular magnetic tunnel junctions has attracted interest due to their observed high tunneling magnetoresistance (TMR) and excellent voltage response. Motivated by this, we report the first success in the synthesis of ultrathin films (0.33–4.29 nm) of MgAl₂O₄ using in vacuo atomic layer deposition (ALD) on Fe and Al electrodes. The electronic properties of samples were evaluated using in situ scanning tunneling spectroscopy. Intriguingly, the sequence of the ALD Al₂O₃ and ALD MgO was found to dramatically impact the electronic structure of the ALD MgAl₂O₄, which may be attributed to the different initial adsorption mechanisms of ALD MgO and ALD Al₂O₃, as revealed in the molecular dynamics simulation. The optimum sequence for the first unit cell (or supercycle) of MgAl₂O₄ is two ALD Al₂O₃ cycles followed by one ALD MgO cycle. At three supercycles (0.99 nm), a much higher conduction band minimum (CBM) of 1.71 eV was observed, in contrast to 1.58 or 1.45 eV, which were observed when beginning the supercycles with 1 cycle of Al₂O₃ (0.11 nm) followed by 1 cycle of MgO (0.11 nm) or only 1 cycle of MgO, respectively. Decreasing the number of supercycles from 3 (∼0.99 nm) to 1 supercycle (∼0.33 nm) resulted in a monotonic decrease in CBM from 1.71 to 1.49 eV, showing some frustration of growth during earlier atomic layer deposition cycles. Additionally, growth on a Fe layer showed a moderate CBM of 1.25 eV. Nevertheless, the observed CBM in the ultrathin ALD MgAl₂O₄ greatly exceeds that of thermally oxidized AlO_x barriers (∼0.6 eV) and is similar to that of high-quality ALD-grown Al₂O₃ (∼1.7 eV) and MgO grown with an Al₂O₃ seed layer (∼1.50 eV) of comparable total thickness in the ultrathin range. The high CBM values are indicative of a low defect concentration in the ultrathin ALD MgAl₂O₄, which is supported by a high dielectric constant of 8.85 (comparable to that of the crystalline MgAl₂O₄ bulk) observed for a 4.3 nm thick ALD MgAl₂O₄ film capacitor.