Detection Sensitivity Limit of Hundreds of Atoms with X-Ray Fluorescence Microscopy

TL;DR

This paper demonstrates that advanced X-ray fluorescence microscopy can detect extremely small impurity inclusions at the nanoscale, potentially reaching single-atom sensitivity for quantum technology applications.

Contribution

It introduces a highly sensitive, non-destructive XRF imaging method capable of detecting single or few atom impurities without specialized sample preparation.

Findings

Detection of 3000 Ga impurities with 1 second integration

Ability to detect 650 impurities with increased integration

Potential for single atom sensitivity with future upgrades

Abstract

We report X-ray fluorescence (XRF) imaging of nanoscale inclusions of impurities for quantum technology. A very bright diffraction-limited focus of the X-ray beam produces very high sensitivity and resolution. We investigated gallium (Ga) dopants in silicon (Si) produced by a focused ion beam (FIB). These dopants might provide 3/2-spin qubits or p-type electrical contacts and quantum dots. We find that the ion beam spot is somewhat larger than expected, and the technique provides a useful calibration for the resolution of FIBs. Enticingly, we demonstrate that with a single shot detection of 1 second integration time, the sensitivity of the XRF would be sufficient to find amongst background a single isolated inclusion of unknown location comprising only 3000 Ga impurities (a mass of just 350 zg) without any need for specialized nm-thickness lamellae, and down from >105 atoms in previous reports of similar work. With increased integration we were able to detect 650 impurities. The results show that planned facility upgrades might achieve single atom sensitivity with a generally applicable, non-destructive technique in the near future.