We report scanning tunneling spectroscopy studies of the electronic structure of 1.5 to 3 nm (001) textured MgO layers grown on (001) Fe. Thick MgO layers exhibit a bulk-like band gap, ~ 5-7 eV, and sparse, localized defect states with characteristics attributable to oxygen and, in some cases, Mg vacancies. Thin MgO layers exhibit electronic structure indicative of interacting defect states forming band tails which in the thinnest case extend to ~ ±0.5 V of the Fermi level. These vacancy defects are ascribed to compressive strain from the MgO/Fe lattice mismatch, accommodated as the MgO grows.

We present results from an X-ray photoemission spectroscopy (XPS) study of CoFeB/MgO bilayers where we observe process-dependent formation of B, Fe, and Co oxides at the CoFeB/MgO interface due to oxidation of CoFeB during MgO deposition. Vacuum annealing reduces the Co and Fe oxides but further incorporates B into the MgO forming a composite MgB x O y layer. Inserting an Mg layer between CoFeB and MgO introduces an oxygen sink, providing increased control over B content in the barrier.

We present X-ray photoelectron spectroscopy data which show that the chemisorbed oxygen previously observed to be on the surface of thin AlO x layers formed by room temperature thermal oxidation is bound by oxygen vacancies in the oxide. Increasing the electric field across the oxide, either by over-coating with a metallic electrode, or by electron bombardment, drives this surface chemisorbed oxygen into the vacancy sites. Due to the low bonding energies of these oxygen sites, subsequent oxygen exposures draw these

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We report scanning tunneling microscopy and ballistic electron emission microscopy studies of the electronic states of the uncovered and chemisorbed-oxygen covered surface of AlOx tunnel barrier layers. These states change when chemisorbed oxygen ions are moved into the oxide by either flood gun electron bombardment or by thermal annealing. The former, if sufficiently energetic, results in locally well defined conduction band onsets at ∼ 1 V, while the latter results in a progressively higher local conduction band onset, exceeding 2.3 V for 500 and 600 C thermal anneals.The prevalence of aluminum oxide layers formed by room temperature oxidation as the barrier in Josephson junctions (JJ) 1 and magnetic tunnel junctions (MTJs) 2 continues to motivate efforts to better understand and control its electronic structure. While bulk, stoichiometric Al 2 O 3 has a band gap of ∼ 8.8 eV, 3 for amorphous AlO x films grown at ∼ 20 C it is a much smaller. This is beneficial as thin, transparent barriers provide the high critical current densities (JJs) and low specific impedance levels (MTJs) required by many applications, but band tails, localized states, and spatial inhomogeneities that may also be found in amorphous AlO x 4,5,6 can be very detrimental for high performance, low noise applications. 7,8 Indeed, conducting atomic force microscopy studies of AlO x layers have shown inhomogeneous current distributions at the nanoscale, attributed to either a variation in local barrier heights 9 or in barrier thickness 10 . However, a serious challenge for such surface spectroscopy studies of the electronic properties of AlO x is that the surface is invariably covered, even in ultra-high vacuum (UHV), with chemisorbed oxygen bound by positively charged oxygen vacancies in the oxide, with the degree of coverage depending on oxide thickness. 6,11 We report the use of scanning tunneling microscopy (STM) and ballistic electron emission microscopy (BEEM) to examine the density of states (DOS) of the AlO x surface, and to determine how these states change when chemisorbed oxygen ions are moved into the oxide by either flood-gun electron-bombardment (FGEB), or by thermal annealing. Both treatments greatly reduce, if not eliminate, low energy band tail states and narrow the DOS distribution over an oxide area. However FGEB, which we argue has similarities in effect to depositing a metallic over-layer with a high work function φ, causes different changes in the DOS than annealing. The former, if sufficiently energetic, results in locally well defined conduction band onsets at ∼ 1 V, while the latter results in a progressively higher local conduction band onset, exceeding 2.3 V for 500 and 600 C anneals.We fabricated the samples for this study via thin film thermal evaporation and post-growth processing in UHV. For most samples, we deposited 12 nm of Au on hydrogen terminated (111) Si to form a high quality Schottky barrier (SB) to serve as the BEEM detector. This was followed by a 1.2 nm buffer layer of Cu, 1.2 nm Co, and finally 1 nm Al,...