Alternate-Target Layer-By-Layer Pulsed Laser Deposition of Epitaxial BiVO4 Thin Films

Abstract

Bismuth vanadate (BiVO4) has emerged as one of the highest performing metal oxide photoelectrodes for solar energy-to-fuels applications [1, 2]. This achievement has been largely attributed to the development of high-quality synthesis techniques. Specifically, epitaxial synthetic routes to producing lattice-matched, near single-crystalline quality BiVO4 with minimal extrinsic defects have been essential to understand the fundamental and PEC properties of BiVO4 that are inaccessible by conventional bulk synthesis techniques. In addition, epitaxial synthesis may also be employed as a strategy to further improve the PEC properties of BiVO4 through, for instance, altering the band structure and enhancing the carrier dynamics by strain engineering. Hence, the ability to produce high-quality single-phase epitaxial BiVO4 films is desirable.To date, molecular beam epitaxy (MBE), chemical vapor deposition (CVD), and pulsed laser deposition (PLD) [3] have been used to fabricate epitaxial BiVO4 thin films, with PLD having the advantage of relatively simple experimental setup and material versatility. However, an oft-noted drawback in conventional PLD using single oxide targets is off-stoichiometry in the deposited films; this is especially so for BiVO4 which requires a Bi-rich compound target to achieve a stoichiometric BiVO4 film [4]. This problem is also expected in general for multinary oxides composed of elements with very different ablation properties. To alleviate the limitations of conventional PLD, we employ for the first time alternate-target layer-by-layer PLD as a more elegant approach to produce high quality epitaxial BiVO4 films. Briefly, constituent oxide targets are alternately ablated by an excimer laser (λ= 248 nm) to build the desired film one unit cell thick at a time. Stoichiometry is controlled by the number of shots corresponding to a specified laser fluence for the constituent oxide target.To demonstrate the method, we deposited epitaxial BiVO4 films onto (001)-oriented yttrium-stabilized zirconia (YSZ) using 4N-pure Bi2O3 and V2O5 targets. Films grown are single phase BiVO4 as shown by x-ray diffraction. Out-of-plane diffraction peaks indexed to BiVO4(00l) suggests epitaxy of the film onto the substrate; reciprocal space maps of the asymmetric BiVO4(208) and YSZ(204) peaks further confirm the BiVO4(001) || YSZ(001) epitaxial relationship (top figure). Rocking curves of the BiVO4 (004) peak (FWHM ~ 0.015-0.041) indicate high crystalline perfection of the BiVO4 film, almost approaching that of the YSZ substrate (FWHM ~ 0.010). Thickness dependent-rocking curve studies reveal that BiVO4 films are strained to the substrate for film thicknesses under 22 nm; above this critical thickness, film relaxation ensues. In turn, the resulting optoelectronic properties of the BiVO4 film is dictated by its relaxation state. The optical band gap narrows with film relaxation as observed with spectroscopic ellipsometry. Moreover, steady-state photoluminescence emission spectroscopy reveals a sub-bandgap state (A 2, bottom figure) associated with strained BiVO4 films, on top of a state consistent with band-to-band recombination (B1 ). A higher energy sub-bandgap state (A 1, bottom figure) develops as the film relaxes. The implications of the relaxation state on the charge carrier dynamics and photoelectrochemical properties of BiVO4 will be discussed. Figure: (top) reciprocal space maps of the asymmetric BiVO4(208) and YSZ(204) peaks; (bottom) photoluminescence emission spectra of strained and relaxed BiVO4 films.