Non‐Destructive X‐Ray Imaging of Patterned Delta‐Layer Devices in Silicon

The progress of miniaturization in integrated electronics has led to atomic and nanometer-sized dopant devices in silicon. Such structures can be fabricated routinely by hydrogen resist lithography, using various dopants such as P and As. However, the ability to non-destructively obtain atomic-species-specific images of the final structure, which would be an indispensable tool for building more complex nano-scale devices, such as quantum co-processors, remains an unresolved challenge. Here, X-ray fluorescence is exploited to create an element-specific image of As dopants in Si, with dopant densities in absolute units and a resolution limited by the beam focal size (here ≈1 µm), without affecting the device's low temperature electronic properties. The As densities provided by the X-ray data are compared to those derived from Hall effect measurements as well as the standard non-repeatable, scanning tunneling microscopy and secondary ion mass spectroscopy, techniques. Before and after the X-ray experiments, we also measured the magneto-conductance, which is dominated by weak localization, a quantum interference effect extremely sensitive to sample dimensions and disorder. Notwithstanding the 1.5 × 10^{10} Sv (1.5 × 10^{16} Rad cm^{−2}) exposure of the device to X-rays, all transport data are unchanged to within experimental errors, corresponding to upper bounds of 0.2 Angstroms for the radiation-induced motion of the typical As atom and 3% for the loss of activated, carrier-contributing dopants. With next generation synchrotron radiation sources and more advanced optics, the authors foresee that it will be possible to obtain X-ray images of single dopant atoms within resolved radii of 5 nm

Low-temperature magneto-transport and X-ray metrology of ultra-thin group V doping layers in silicon: platforms for quantum science and technology

In 1947 the first field-effect-transistor was invented, since then the miniaturisation of semiconductor electronics has enabled the sustained exponential growth in the density of transistors in integrated circuits, know as Moore's law. The advent and refinement of the transistor is arguably one of, if not the most, important inventions of the past century. So far the challenges in reducing transistors' dimensions were mostly related to fabrication techniques, rather than fundamental physics. However, as the industrial transistors approach the atomic limit, with industrial designs at the 5~nm scale, quantum physical effects become dominant, making conventional electronics inadequate. Thus, it becomes indispensable to understand and take advantage of the quantum effects governing nano-scale electronics. This considerable task encompasses a wide variety of research directions, from developing quantum computing to discovering and understanding quantum effects in new low-dimensional materials (i.e. materials that are confined to less than three-dimensions). A central long-standing problem in the field of low-dimensional conductors is the metal-to-insulator transition (MIT). It was observed in a wide variety of materials, that are conductive at high carrier densities and insulating at low carrier densities (in the limit of 0~K). The existence and the understanding of such a transition in two-dimensional materials is the focus of many research centres and remains an open question. The main focus of this work was on low-temperature magneto-transport experiments on novel two-dimensional (2D) arsenic-doped silicon ∂-layers, which are proposed as a platform for silicon-based quantum technologies. The doping density was controlled to create a series of samples ranging from metallic to almost insulating, allowing for studies of the two-dimensional MIT in a highly disordered half-filling Hubbard model. Owing to the unprecedented thinness and low density of our arsenic ∂-layers, we had access to an unexplored regime of strong electron-electron interaction in a highly disordered 2D electron liquid. We showed that the enhanced interaction strength reduces the weak-localisation effects typically observed in doped silicon ∂-layers, and causes the Zeeman effect to become dominant in the low temperature magneto-transport. In light of these effects, we developed a procedure to distinguish the two effects, and to extract the relevant electron characteristics. Furthermore, we found that the low temperature phase in dilute dopant layers is that of an inhomogeneous conductor. This phase manifests anomalous transverse voltages V_xy with an even response to the applied magnetic field, and hysteresis in the longitudinal and transverse magneto-resistance. We argue that the inhomogeneous phase is characterised by percolation of insulating and conducting regions that can be tuned by a magnetic field, in which the current is forced into meandering conduction channels resulting in the observed anomalous transverse voltages V_xy. Moreover, we argue that the electron localisation length, responsible for the conductivity's temperature dependence, is cut-off by inelastic scattering in the insulating regions causing the conductivity to saturate at low temperatures; offering an explanation to the general inability to thermalise disordered 2D electron layers at low temperatures. In order to confirm the structure of our samples we developed X-ray imaging methods and showed that they are non-destructive. Specifically, we used X-ray fluorescence to detect the position and element-species of the atoms in our samples, and verified with magneto-transport that the dopants' position remained unmoved within 0.2 Angstroms. To obtain the depth distribution and thickness of the buried dopant atom layers in silicon, we showed that resonant X-ray reflectometry measurement can be used for atom specific nanometer resolution depth measurements. The findings of this research should stimulate further investigation of dilute-doping layers near the MIT in semiconductors to elucidate the exact mechanisms driving the inhomogeneous phase and its associated anomalous Hall effect and hysteresis

Study Protocol National policies, legislations and regulations for production, sale, and consumption of food in European and 4P CAN consortium countries: a scoping review

Study protocol of a scoping revie

Non-Destructive X-Ray Imaging of Patterned Delta-Layer Devices in Silicon

ISSN:2199-160

Momentum‐Space Imaging of Ultra‐Thin Electron Liquids in δ‐Doped Silicon

Abstract Two‐dimensional dopant layers (δ‐layers) in semiconductors provide the high‐mobility electron liquids (2DELs) needed for nanoscale quantum‐electronic devices. Key parameters such as carrier densities, effective masses, and confinement thicknesses for 2DELs have traditionally been extracted from quantum magnetotransport. In principle, the parameters are immediately readable from the one‐electron spectral function that can be measured by angle‐resolved photoemission spectroscopy (ARPES). Here, buried 2DEL δ‐layers in silicon are measured with soft X‐ray (SX) ARPES to obtain detailed information about their filled conduction bands and extract device‐relevant properties. This study takes advantage of the larger probing depth and photon energy range of SX‐ARPES relative to vacuum ultraviolet (VUV) ARPES to accurately measure the δ‐layer electronic confinement. The measurements are made on ambient‐exposed samples and yield extremely thin (< 1 nm) and dense (≈1014 cm−2) 2DELs. Critically, this method is used to show that δ‐layers of arsenic exhibit better electronic confinement than δ‐layers of phosphorus fabricated under identical conditions

Effect of the COVID-19 pandemic on surgery for indeterminate thyroid nodules (THYCOVID): a retrospective, international, multicentre, cross-sectional study

Background: Since its outbreak in early 2020, the COVID-19 pandemic has diverted resources from non-urgent and elective procedures, leading to diagnosis and treatment delays, with an increased number of neoplasms at advanced stages worldwide. The aims of this study were to quantify the reduction in surgical activity for indeterminate thyroid nodules during the COVID-19 pandemic; and to evaluate whether delays in surgery led to an increased occurrence of aggressive tumours. Methods: In this retrospective, international, cross-sectional study, centres were invited to participate in June 22, 2022; each centre joining the study was asked to provide data from medical records on all surgical thyroidectomies consecutively performed from Jan 1, 2019, to Dec 31, 2021. Patients with indeterminate thyroid nodules were divided into three groups according to when they underwent surgery: from Jan 1, 2019, to Feb 29, 2020 (global prepandemic phase), from March 1, 2020, to May 31, 2021 (pandemic escalation phase), and from June 1 to Dec 31, 2021 (pandemic decrease phase). The main outcomes were, for each phase, the number of surgeries for indeterminate thyroid nodules, and in patients with a postoperative diagnosis of thyroid cancers, the occurrence of tumours larger than 10 mm, extrathyroidal extension, lymph node metastases, vascular invasion, distant metastases, and tumours at high risk of structural disease recurrence. Univariate analysis was used to compare the probability of aggressive thyroid features between the first and third study phases. The study was registered on ClinicalTrials.gov, NCT05178186. Findings: Data from 157 centres (n=49 countries) on 87 467 patients who underwent surgery for benign and malignant thyroid disease were collected, of whom 22 974 patients (18 052 [78·6%] female patients and 4922 [21·4%] male patients) received surgery for indeterminate thyroid nodules. We observed a significant reduction in surgery for indeterminate thyroid nodules during the pandemic escalation phase (median monthly surgeries per centre, 1·4 [IQR 0·6-3·4]) compared with the prepandemic phase (2·0 [0·9-3·7]; p&lt;0·0001) and pandemic decrease phase (2·3 [1·0-5·0]; p&lt;0·0001). Compared with the prepandemic phase, in the pandemic decrease phase we observed an increased occurrence of thyroid tumours larger than 10 mm (2554 [69·0%] of 3704 vs 1515 [71·5%] of 2119; OR 1·1 [95% CI 1·0-1·3]; p=0·042), lymph node metastases (343 [9·3%] vs 264 [12·5%]; OR 1·4 [1·2-1·7]; p=0·0001), and tumours at high risk of structural disease recurrence (203 [5·7%] of 3584 vs 155 [7·7%] of 2006; OR 1·4 [1·1-1·7]; p=0·0039). Interpretation: Our study suggests that the reduction in surgical activity for indeterminate thyroid nodules during the COVID-19 pandemic period could have led to an increased occurrence of aggressive thyroid tumours. However, other compelling hypotheses, including increased selection of patients with aggressive malignancies during this period, should be considered. We suggest that surgery for indeterminate thyroid nodules should no longer be postponed even in future instances of pandemic escalation. Funding: None