The formation of a Sn monolayer on Ge(1 0 0) studied at the atomic scale

The growth of multi-layer germanium-tin (GeSn) quantum wells offers an intriguing pathway towards the integration of lasers in a CMOS platform. An important step in growing high quality quantum well interfaces is the formation of an initial wetting layer. However, key atomic-scale details of this process have not previously been discussed. We use scanning tunneling microscopy combined with density functional theory to study the deposition of Sn on Ge(1 0 0) at room temperature over a coverage range of 0.01 to 1.24 monolayers. We demonstrate the formation of a sub-2% Ge content GeSn wetting layer from three atomic-scale characteristic ad-dimer structural components, and show that small quantities of Sn incorporate into the Ge surface forming two atomic configurations. The ratio of the ad-dimer structures changes with increasing Sn coverage, indicating a change in growth kinetics. At sub-monolayer coverage, the least densely packing ad-dimer structure is most abundant. As the layer closes, forming a two-dimensional wetting layer, the more densely packing ad-dimer structure become dominant. These results demonstrate the capability to form an atomically smooth wetting layer at room temperature, and provide critical atomic-scale insights for the optimization of growth processes of GeSn multi-quantum-wells to meet the quality requirements of optical GeSn-based devices

High-energy photoemission final states beyond the free-electron approximation

Three-dimensional (3D) electronic band structure is fundamental for understanding a vast diversity of physical phenomena in solid-state systems, including topological phases, interlayer interactions in van der Waals materials, dimensionality-driven phase transitions, etc. Interpretation of ARPES data in terms of 3D electron dispersions is commonly based on the free-electron approximation for the photoemission final states. Our soft-X-ray ARPES data on Ag metal reveals, however, that even at high excitation energies the final states can be a way more complex, incorporating several Bloch waves with different out-of-plane momenta. Such multiband final states manifest themselves as a complex structure and added broadening of the spectral peaks from 3D electron states. We analyse the origins of this phenomenon, and trace it to other materials such as Si and GaN. Our findings are essential for accurate determination of the 3D band structure over a wide range of materials and excitation energies in the ARPES experiment

Nanoscale imaging of mobile carriers and trapped charges in delta doped silicon p–n junctions

Integrated circuits and certain silicon-based quantum devices require the precise positioning of dopant nanostructures, and hydrogen resist lithography can be used to fabricate such structures at the atomic-scale limit. However, there is no single technique capable of measuring the three-dimensional location and electrical characteristics of these dopant nanostructures, as well as the charge dynamics of carriers and trapped charges in their vicinity. Here, we show that broadband electrostatic force microscopy can be used for non-destructive carrier profiling of atomically thin n-type (phosphorus) and p-type (boron) dopant layers in silicon, and their resulting p–n junctions. The probe has a lateral resolution of 10 nm and a vertical resolution of 0.5 nm, and detects the capacitive signature of subsurface charges in a broad 1 kHz to 10 GHz frequency range. This allows the bias-dependent charge dynamics of free electrons in conducting channels and trapped charges in oxide–silicon interfaces to be investigated

Research data supporting "Atomic-Scale Patterning of Arsenic in Silicon by Scanning Tunneling Microscopy"

Research data supporting the publication: Stock, T. J. Z, et. al., 2020, Atomic-Scale Patterning of Arsenic in Silicon by Scanning Tunneling Microscopy, ACS Nano, https://dx.doi.org/10.1021/acsnano.9b08943Research data supporting the publication: Stock, T. J. Z, et. al., 2020, Atomic-Scale Patterning of Arsenic in Silicon by Scanning Tunneling Microscopy, ACS Nano, https://dx.doi.org/10.1021/acsnano.9b08943