[Paper Review] Direct observation of the thickness distribution of ultra thin AlOx barrier in Al/AlOx/Al Josephson junctions

Lunjie Zeng, Samira Nik+4 more|arXiv (Cornell University)|Jul 1, 2014

Surface and Thin Film Phenomena22 citations

TL;DR

This study directly observes the thickness distribution of ultra-thin AlOx barriers in Al/AlOx/Al Josephson junctions using atomic-resolution annular dark field scanning transmission electron microscopy (ADF-STEM). It reveals that less than 10% of the barrier area dominates electron tunneling, with thickness varying between ~1 nm and ~2 nm in a Gaussian distribution, and demonstrates that longer oxidation time increases barrier thickness more effectively than higher oxygen pressure.

ABSTRACT

We show that less than 10% of the barrier area dominates the electron tunneling in state-of-art Al/AlOx/Al Josephson junctions. They have been studied by transmission electron microscopy, specifically using atomic resolution annular dark field (ADF) scanning transmission electron microscopy (STEM) imaging. The direct observation of the local barrier thickness shows a Gaussian distribution of the barrier thickness variation along the junction, from ~1 nm to ~2 nm in the three junctions we studied. We have investigated how the thickness distribution varies with oxygen pressure (po) and oxidation time (to) and we find, in agreement with resistance measurements on similar junctions, that an increased to gives a thicker barrier than an increased po.

Motivation & Objective

• To directly visualize the local thickness distribution of ultra-thin AlOx barriers in Al/AlOx/Al Josephson junctions.
• To understand the role of barrier thickness in electron tunneling, particularly which regions dominate transport.
• To investigate how oxidation time (to) and oxygen pressure (po) influence the thickness distribution of the AlOx barrier.
• To correlate microstructural observations with electrical resistance measurements from similar junctions.
• To provide experimental validation of the thickness dependence of tunneling in high-quality Josephson junctions.

Proposed method

• Atomic-resolution annular dark field scanning transmission electron microscopy (ADF-STEM) was used to image the AlOx barrier at the atomic scale.
• The thickness of the AlOx barrier was measured locally across multiple points in three different junctions.
• The thickness distribution was analyzed to determine its statistical form, revealing a Gaussian profile.
• Oxidation conditions (oxygen pressure and time) were systematically varied to study their effects on barrier thickness.
• Electrical resistance measurements from similar junctions were used to validate the observed thickness trends.
• Statistical analysis was performed to quantify the fraction of barrier area responsible for dominant tunneling.

Experimental results

Research questions

• RQ1 What is the spatial distribution of AlOx barrier thickness in state-of-the-art Al/AlOx/Al Josephson junctions?
• RQ2 Which regions of the barrier dominate electron tunneling, and what fraction of the area accounts for this?
• RQ3 How do oxidation time and oxygen pressure affect the thickness and distribution of the AlOx barrier?
• RQ4 Is the observed thickness distribution consistent with electrical resistance measurements on similar junctions?
• RQ5 Does the barrier thickness follow a Gaussian distribution across the junction?

Key findings

• Less than 10% of the AlOx barrier area is responsible for the majority of electron tunneling in the studied Josephson junctions.
• The local barrier thickness varies between approximately 1 nm and 2 nm, following a Gaussian distribution.
• Increased oxidation time (to) leads to a thicker barrier, consistent with resistance measurements on similar junctions.
• Increased oxygen pressure (po) results in a smaller increase in barrier thickness compared to increased oxidation time.
• The direct ADF-STEM imaging confirms that thickness inhomogeneity is a key factor in tunneling behavior.
• The findings provide experimental evidence that barrier thickness uniformity is critical for high-performance Josephson junctions.