crystals Review Mechanism of Electronegativity Heterojunction of Nanometer Amorphous-Boron on Crystalline Silicon: An Overview Paolo Sberna 1 , Piet X. Fang 2,3 , Changming Fang 4 and Stoyan Nihtianov 2, *   Citation: Sberna,P.; Fang, P.X.; Fang, C.; Nihtianov, S. Mechanism of Electronegativity Heterojunction of Nanometer Amorphous-Boron on Crystalline Silicon: An Overview. Crystals 2021, 11, 108. https://doi.org/10.3390/cryst11020108 Academic Editor: Riccardo Camattari Received: 30 November 2020 Accepted: 20 January 2021 Published: 26 January 2021 Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Li- censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/ licenses/by/4.0/). 1 Else Kooi Laboratory, Faculty of Electrical Engineering, Mathematics and Computer Science, TU Delft, Feldmannweg 17, 2628 CT Delft, The Netherlands; p.m.sberna@tudelft.nl 2 Electronic Instrumentation Lab, Faculty of Electrical Engineering, Mathematics and Computer Science, TU Delft, Mekelweg 4, 2628 CD Delft, The Netherlands; Lele.fang@gmail.com 3 High Field Magnet Laboratory (HFML-EMFL), Radboud University, Toernooiveld 7, 6525 ED Nijmegen, The Netherlands 4 BCAST, Brunel University London, Uxbridge, Middlesex UB8 3PH, UK; Changming.Fang@brunel.ac.uk * Correspondence: S.Nihtianov@tudelft.nl Abstract: The discovery of the extremely shallow amorphous boron-crystalline silicon heterojunction occurred during the development of highly sensitive, hard and robust detectors for low-penetration- depth ionizing radiation, such as ultraviolet photons and low-energy electrons (below 1 keV). For many years it was believed that the junction created by the chemical vapor deposition of amor- phous boron on n-type crystalline silicon was a shallow p-n junction, although experimental results could not provide evidence for such a conclusion. Only recently, quantum-mechanics based mod- elling revealed the unique nature and the formation mechanism of this new junction. Here, we review the initiation and the history of understanding the a-B/c-Si interface (henceforth called the “boron-silicon junction”), as well as its importance for the microelectronics industry, followed by the scientific perception of the new junctions. Future developments and possible research directions are also discussed. Keywords: rectifying junction; photodiode; chemical vapor deposition; first principle molecular dynamics; electronegativity 1. Introduction: Initiation and History of Boron-Silicon Junctions and Importance of Si-Based Junctions/Diodes in Microelectronics The first report about an ultra-shallow rectifying junction (diode) created by a pure boron atmospheric/low-pressure chemical vapor deposition (AP/LPCVD) on crystalline n-type silicon surface was published in 2006 [1]. Initially, the application which led to the development of this novel rectifying junction was: a linear and high Q-factor varactor diode designed for the capacitance tuning of frequency in RF circuits [2,3]. The demon- strated good performance in the varactor application did not attract the expected attention. Fortunately, in 2006 it became clear that a different field of applications would benefit even more from the excellent electrical properties of this extremely shallow junction. This junction would prove useful as an accurate, stable and reliable detector for low-penetration depth radiation such as UV light and low-energy electrons, which are applied in UV optical lithography and scanning electron microscopes. Since 2006 a significant amount of research has been completed in the following directions: (1) optimization of the critical junction creation process, i.e., the chemical vapor deposition (CVD) of amorphous boron on n-type crystalline silicon (in a method called the “PureB” process); (2) device characterization and design optimization for a variety of applications; and (3) rendering the PureB process CMOS-compatible. Initially it was believed that the excellent electrical properties of the junction—especially the very low saturation currents which are typical for deep p-n junctions—were defined by the p + Crystals 2021, 11, 108. https://doi.org/10.3390/cryst11020108 https://www.mdpi.com/journal/crystals