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 (similar to 0.99 nm) to 1 supercycle (similar to 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 AlOx barriers (similar to 0.6 eV) and is similar to that of high-quality ALD-grown Al2O3 (similar to 1.7 eV) and MgO grown with an Al2O3 seed layer (similar to 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.