Deposition of undoped and H doped WOx (x ≤ 3) films in a hot-wire atomic layer deposition system without the use of tungsten precursors

Abstract

A hot-wire atomic layer deposition system is used for the synthesis of tungsten oxide films with various stoichiometries, undoped and hydrogen doped. For this synthesis, no precursors are used and only a W wire is heated at 660°C at a base pressure of 0,1Torr set by the flow of various gases (O2, N2, H2) or gas mixtures (N2–O2 10% in H2, forming gas, FG) and the pulsed injection of O2 or H2. Four classes of hot-wire tungsten oxide films were synthesized: i) stoichiometric (hwWO3), ii) oxygen deficient (sub-stoichiometric, hwWOx with x<3), iii) stoichiometric and hydrogen doped (hwWO3:H) and iv) sub-stoichiometric and hydrogen doped (hwWOx:H). During deposition, due to the pulsed injection of O2, the W wire re-oxidizes creating continuously WO3 vapors and so films deposited do not suffer by thickness limitations. Moreover, they are highly porous. Each one of the four classes of the deposited tungsten oxides exhibits its own optical properties, which indicates that each class has its own electronic structure. So, hwWO3 films were semiconducting, exhibiting a band gap near 3eV. Sub-stoichiometric hwWOx deposited with up to 2 injections of O2 were semi-metalic, exhibiting some features of the electronic structure of the pure metal, while further injection of O2 leads to stoichiometric films. The injection of H2 during deposition leads to the formation of atomic H, which dopes the films. Depositions carried out with base pressure set by O2 or N2 and with injection of H2, lead to the formation of hwWO3:H films that exhibit optical properties similar to Na lightly-doped WO3 films. In base pressure set by H2 or FG and with H2 injection, substoichiometric, hydrogen-doped films were obtained, exhibiting optical properties corresponding to a metal substantially different than W. Fourier transform infrared spectroscopy and spectroscopic ellipsometry measurements indicated that the hwWOx films grown in the presence of hydrogen in the deposition ambience contain H bonded with the O ions. When these films are doped with hydrogen, protons are bonded not only with the oxygen but also with the tungsten ions.