Ultrathin Aluminum Oxide Tunnel Barriers

Abstract

Ballistic electron emission microscopy is used to study the formation of ultrathin tunnel barriers by the oxidization of aluminum. An O2 exposure, approximately 30 mTorr sec, forms a uniform tunnel barrier with a barrier height straight phi(b) of 1.2 eV. Greater O2 exposure does not alter straight phi(b) or the ballistic transmissivity of the oxide conduction band. Tunneling spectroscopy indicates a broad energy distribution of electronic states in the oxide. With increasing O2 dose the states below 1.2 eV gradually become localized, but until this localization is complete these states can provide low-energy single-electron channels through the oxide.

Observation of fluctuation-induced tunneling conduction in micrometer-sized tunnel junctions

“…In a series of ballistic electron emission microscopy (BEEM) and scanning tunneling spectroscopy (STS) studies, Buhrman and coworkers 3,29 have systematically shown that in an ultrathin ( 1 nm) and not fully oxidized AlO x layer, there could possibly exist low-energy electron channels that could provide low-voltage "leakage" through the barrier. In the present study, our AlO x layers are fully oxidized by the O 2 glow discharge and relatively thicker (1.5-2 nm on average), as mentioned in Sec.…”

“…It also lies at the heart of numerous solid-state devices, such as tunnel diodes, 1 Coulomb blockade thermometers, 2 Josephson junctions, 3 and memory elements based on magnetic tunnel junctions. 4 The functionality of a tunnel device relies heavily on the material properties of the intermediate thin insulating layer.…”

Atomic and electronic structure of ultra-thin Al/AlOx/Al interfaces

“…Above the chemisorbed oxygen we find a 2/3-filled Al layer (Al2 atoms, hexagonal structure without the centers) forming with a full oxygen layer and 1/3-filled Al layer (Al1 atoms) an oxide AlO x with stoichiometry x = 1.0. The geometrical parameters are given in the table (1). The width of the interface, which we define as the vertical distance between the first non-bulk layer (Al1) and the last bulk-like layer (Al3) is d tot A = 4.53Å.…”

“…Several recent attempts to characterize such an ultra-thin barrier has been recently published. Rippard et al [1] studied the dependence of the transmission spectrum of a ultra-thin interface (0.6 − 1.5nm) on the oxidation, finding a conduction band within the oxide only 1.2eV above the Fermi energy (effective barrier), which has been argued to origin from a disorder in the barrier. Regions of similar barriers, referred to as "hot spots", were also observed by Gloos et al [2].…”

Epitaxial aluminum nitride tunnel barriers grown by nitridation with a plasma source

“…8 It has been demonstrated that a major problem of amorphous AlO x barriers is that they are laterally inhomogeneous. 9,10 We take this into account by using a distribution of transparencies T n by writing for the voltageindependent normal conductance,…”