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 amorphous 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 modelling 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.

A new generation of radiation detectors relies on the crystalline Si and amorphous B (c-Si/a-B) junctions that are prepared through chemical vapor deposition of diborane (B₂H₆) on Si at low temperature (∼400 C). The Si wafer surface is dominated by the Si{0 0 1}3 1 domains that consist of two different Si species at low temperature. Here we investigate the geometry, stability and electronic properties of the hydrogen passivated Si{0 0 1}3 1 surfaces with deposited BH_n (n = 0 to 3) radicals using parameter-free first-principles approaches. Ab initio molecular dynamics simulations using the density functional theory (DFT) including van der Waals interaction reveal that in the initial stage the BH₃ molecules/radicals deposit on the Si(-H), forming (-Si)BH₄ radicals which then decompose into (-Si)BH₂ with release of H₂ molecules. Structural optimizations provide strong local relaxation and reconstructions at the deposited Si surface. Electronic structure calculations reveal the formation of various defect states in the forbidden gap. This indicates limitations of the presently used rigid electron-counting and band-filling models. The attained information enhances our understanding of the initial stage of the PureB process and the electric properties of the products.

Abstract Deposition of a thin B layer via decomposition of B2H6 on Si (PureB process) produces B-Si junctions which exhibit unique electronic and optical properties. Here we present the results of our systematic first-principles study of BHn (n=0-3) radicals on Si(100)2x1:H surfaces, the initial stage of the PureB process. The calculations reveal an unexpectedly high stability of BH2 and BH3 radicals on the surface and a plausible atomic exchange mechanism of surface Si atoms with B atoms from absorbed BHn radicals. The calculations show strong local structural relaxation and reconstructions, as well as strong chemical bonding between the surface Si and the BHn radicals. Electronic structure calculations show various defect states in the energy gap of Si due to the BHn absorption. These results shed light on the initial stages of the complicated PureB process and also rationalize the unusual electronic, optical and electrical properties of the deposited Si surfaces.