Growth of Self‐Passivating Oxide Layers on Aluminum—Pressure and Temperature Dependence

Abstract

Optical transmission spectroscopy is used to monitor the in situ oxidation of granular aluminum films over a wide range of temperatures and pressures. By fitting the time evolution of the transmission to adequate geometric models, the rising transparency of the sample is converted into oxide growth during reaction. The observed oxidation rates increase steeply with temperature, but depend on pressure only below 100 mbar O 2 and become pressure-independent above. The Al oxidation proceeds in two regimes. The initial fast one is compatible with the Cabrera-Mott mechanism of an electric-field-driven process and self-terminates at 1-2 nm oxide thickness depending on oxidation conditions. The subsequent slow regime exhibits a two times larger activation energy and is relevant only for hightemperature oxidation. It is assigned to the thermally activated transport of Al ions through the emerging oxide layer. In contrast to the initial Cabrera-Mott process, it shows no self-passivation and enables oxide thickening beyond 2 nm.

Partial Measurements of Quantum Systems

“…This prediction has been recently measured directly with in situ optical-transmission measurements during aluminum oxidation [96]. Using constants fit to this data, the inverse-logarithmic behavior of Equation [2.12] is shown in Figure 2.1.…”

“…Figure 2.11(a) shows the resistance of JJs with a variable number (N layer ) of total oxide layers. Each layer after the first (N layer ≥ 2) is created with a 0.5 nm filler layer aluminum, so we expect the film thickness to be (1.5 + 0.5N layer ) [nm], based on an initial oxide layer's thickness [95,82,96]. Despite the predicted exponential scaling, we see significantly subexponential scaling.…”

“…The resulting dependence of oxide thickness on time is logarithmic and self-limited. The model is commonly used to describe the growth of aluminum oxide [93,94,95,82,96].…”

“…We then oxidized the filler under the same conditions. 300 seconds ensures the full layer is oxidized [95,82,96]. We add as many fully oxidized filler layers to achieve the desired critical current, typically three.…”

Optical Direct Write of Dolan--Niemeyer-Bridge Junctions for Transmon Qubits

“…However, the side-face JJ has an area comparable to eBL JJs: its dimensions are 30 nm (the thickness of the bottom electrode) by 1.5 µm (the length of the Dolan bridge). The side-face JJ is relatively thin, due to the self-limited growth of aluminum oxide 35,41,42 . Because the normal resistance (and the inductance) of a tunnel barrier scales exponentially with the barrier's thickness 20,[43][44][45] , the participation of the thick, large-area JJ is ∼ 1% 46 .…”