No abstract

Beta gallium oxide (β-Ga 2 O 3 ) is an ultrawidebandgap semiconductor with one of the highest-known breakdown strengths (∼8 MV/cm) making it a strong candidate for highefficiency power electronics, deep-ultraviolet photodetectors, and transparent electronic devices. Bare β-Ga 2 O 3 surfaces exhibit a strong upward band bending and electron depletion that is reduced by the formation of a hydroxyl termination on exposure to the atmosphere, although this effect varies with exposure conditions and the processing history of different samples. This work investigates the covalent modification of (2̅ 01) and ( 010) β-Ga 2 O 3 surfaces with organic layers, as a mechanism for more effectively controlling their surface band bending. We compare the different electronic effects of grafted layers formed by the electrochemical reduction of 4-nitrobenzenediazonium ions and the spontaneous grafting of octadecylphosphonic acid (ODPA). Atomic force microscopy, synchrotron X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure spectroscopy confirmed the presence of few-nanometer layers of covalently attached nitrophenyl (NP) and ODPA molecules. Valence band XPS showed that NP modification produced upward band bending shifts of +0.51 and +0.53 eV on (2̅ 01) and (010) β-Ga 2 O 3 , respectively, with further increases of +0.35 and +0.20 eV after X-ray-induced reduction of the nitro substituent to aminolike moieties. In contrast, ODPA modification produced downward shifts in surface band bending of −0.36 and −0.07 eV on (2̅ 01) and (010) β-Ga 2 O 3 , respectively. Our study demonstrates that covalently bound NP and ODPA molecules can be used to significantly modify the electronic properties of β-Ga 2 O 3 surfaces, a finding that should prove useful for electronic applications of this material.

Nanotechnology has delivered an amazing range of new materials such as nanowires, tubes, ribbons, belts, cages, flowers, and sheets. However, these are usually circular, cylindrical, or hexagonal in nature, while nanostructures with square geometries are comparatively rare. Here, a highly scalable method is reported for producing vertically aligned Sb‐doped SnO2 nanotubes with perfectly‐square geometries on Au nanoparticle covered m‐plane sapphire using mist chemical vapor deposition. Their inclination can be varied using r‐ and a‐plane sapphire, while unaligned square nanotubes of the same high structural quality can be grown on silicon and quartz. X‐ray diffraction measurements and transmission electron microscopy show that they adopt the rutile structure growing in the [001] direction with (110) sidewalls, while synchrotron X‐ray photoelectron spectroscopy reveals the presence of an unusually strong and thermally resilient 2D surface electron gas. This is created by donor‐like states produced by the hydroxylation of the surface and is sustained at temperatures above 400 °C by the formation of in‐plane oxygen vacancies. This persistent high surface electron density is expected to prove useful in gas sensing and catalytic applications of these remarkable structures. To illustrate their device potential, square SnO2 nanotube Schottky diodes and field effect transistors with excellent performance characteristics are fabricated.