X-ray photoemission spectroscopy of nonmetallic materials : electronic structures of boron and BᵪOᵧ

Abstract

Although an increasing volume of x-ray photoemission spectroscopic (XPS) data has been accumulated on boron and boron-rich compounds because of their unusual properties, including a unique three-center, two-electron bonding configuration, their common nonmetallic nature has been overlooked. Typically, the measured energy-state data are not clarified by surface Fermi level positions of these nonmetallic samples, which compromises the scientific contents of the data. In the present study, we revisited the XPS studies of sputter-cleaned β-rhombohedral boron (βᵣ-B), the oxidized surface of βᵣ-B, B₆O pellet, and polished B₂O₃, to illustrate the impact and resolution of this scientific issue. These samples were chosen because βᵣ-B is the most thermodynamically stable polytype of pure boron, B₂O₃is its fully oxidized form, and B₆O is the best known superhard family member of boron-rich compounds. From our XPS measurements, including those from a sputter-cleaned gold as a metal reference, we deduced that our βᵣ-B had a surface Fermi level located at 0.7±0.1 eV from its valence-band maximum (VBM) (referred as E [sub FL]) and a binding energy for its B 1s core level at 187.2 eV from VBM (E [sub b,VBM]). The latter attribute, unlike typical XPS binding energy data that are referenced to a sample-dependent Fermi level (E[sub b,FL]), is immune from any uncertainties and variations arising from sample doping and surface charging. For bulk B₂O₃, we found an E[sub b,VBM] for its B 1s core level at 190.5 eV and an E[sub b,FL] at 193.6 eV. For our βᵣ-B subjected to a surface oxidation treatment, an overlayer structure of ~1.2 nm B₂O₃/ ~2 nmB₂O/B was found. By comparing the data from this sample and those from βᵣ-B and bulk B₂O₃, we infer that the oxide overlayer carried some negative fixed charge and this induced on the semiconducting βᵣ-B sample an upward surface band bending of ~0.6 eV. As for our B₆O sample, we found an E[sub FL] of ~1.7 eV and two different chemical states having E[sub b,VBM] of 185.4 and 187.2 eV, with the former belonging to boron with no oxygen neighbor and the latter to boron with an oxygen neighbor. The methodology in this work is universally applicable to all nonmetallic samples.