Element-specific, non-destructive profiling of layered heterostructures

TL;DR

This paper introduces a non-destructive, element-specific X-ray reflectometry technique that accurately profiles ultra-thin layers in heterostructures, enabling detailed 3D characterization of quantum devices and layered materials.

Contribution

It demonstrates the first application of resonant-contrast X-ray reflectometry for sub-nanometer profiling of delta-doped layers in semiconductors.

Findings

Successfully profiles single arsenic delta-layers in silicon

Achieves sub-nanometer chemical thickness resolution

Enables 3D characterization of quantum and layered materials

Abstract

Fabrication of semiconductor heterostructures is now so precise that metrology has become a key challenge for progress in science and applications. It is now relatively straightforward to characterize classic III-V and group IV heterostructures consisting of slabs of different semiconductor alloys with thicknesses of $\sim$5 nm and greater using sophisticated tools such as X-ray diffraction, high energy X-ray photoemission spectroscopy, and secondary ion mass spectrometry. However, profiling thin layers with nm or sub-nm thickness, e.g. atomically thin dopant layers ($\delta$-layers), of impurities required for modulation doping and spin-based quantum and classical information technologies is more challenging. Here, we present theory and experiment showing how resonant-contrast X-ray reflectometry meets this challenge. The technique takes advantage of the change in the scattering factor of atoms as their core level resonances are scanned by varying the X-ray energy. We demonstrate the capability of the resulting element-selective, non-destructive profilometry for single arsenic $\delta$-layers within silicon, and show that the sub-nm electronic thickness of the $\delta$-layers corresponds to sub-nm chemical thickness. In combination with X-ray fluorescence imaging, this enables non-destructive three-dimensional characterization of nano-structured quantum devices. Due to the strong resonances at soft X-ray wavelengths, the technique is also ideally suited to characterize layered quantum materials, such as cuprates or the topical infinite-layer nickelates.