Growth and characterization of off-stoichiometric $\mathrm{La}{\mathrm{VO}}_3$ thin films

$\mathrm{La}{\mathrm{VO}}_3$ (LVO) has been proposed as a promising material for photovoltaics because its strongly correlated $d$ electrons can facilitate the creation of multiple electron-hole pairs per incoming photon, which would lead to increased device efficiency. In this paper, we intentionally grow off-stoichiometric LVO films by changing the growth conditions such as laser fluence. Our aim is to study how deviating La:V stoichiometries affect the electronic properties of LVO thin films. We find that the off-stoichiometry clearly alters the physical properties of the films. Structural characterization shows that both La-rich and V-rich films have different levels of structural distortion, with La-rich (V-rich) films showing a larger (smaller) out-of-plane lattice parameter compared to what one would expect from epitaxial strain effects alone. Both types of films show deviation from the behavior of bulk LVO in optical measurement, i.e., they do not show signatures of the expected long range orbital order, which can be a result of the structural distortions or the presence of structural domains. In transport measurements, La-rich films display clear signatures of electronic phase separation accompanying a temperature induced metal-insulator transition, while V-rich films behave as Mott insulators. The out-of-plane lattice parameter plays a crucial role in determining the transport properties, as the crossover from Mott-insulating to disorder-induced phase-separated behavior occurs around a lattice parameter value of 3.96 Å, quite different from what has been previously reported.

Figure 1

(a) Two possible orbital configurations in the $R{\mathrm{VO}}_3$ family. Left, G-type OO; right, C-type OO; for LVO, these have corresponding peaks in absorption spectra at 1.8 and 2.4 eV, respectively. (b) Optical measurement setup to detect types of orbital ordering. A sample stage that is angled at ${45}^{\circ }$ with regard to the incoming light is located inside of a cryostat. A horizontally polarized white light source is used to illuminate the samples. The transmitted light intensity is measured by a visible-near infrared CCD detector.

Figure 2

AFM and TEM images for LVO films with different stoichiometry. Top row: V-rich film LVO-V1. (a) AFM image showing a surface with unit cell terraces (RMS roughness: 2.8 Å). (b) Low magnification bright field TEM image of the cross-section view. The streaky lines inside the film are contrast from the antiphase domains. (c) Atomic resolution HAADF-STEM image. Insets: Fast Fourier transforms (FFTs) from the [001] domain (left) and [110] domain (right). Bottom row: La-rich film LVO-L1. (d) AFM image showing a surface with unit cell terraces (RMS roughness: 3.5 Å). (e) Low magnification bright field TEM image of the cross-section view. (f) Atomic resolution HAADF-STEM image. Inset: FFTs from domain 1 (left) and domain 2 (right).

Figure 3

Reciprocal space mapping for LVO-V2 and LVO-L1. (a), (b) RSM for LVO-V2 around its (103) and (013) peaks, respectively. (c), (d) RSM for LVO-L1 around its (103) and (013) peaks, respectively. The STO and LVO peaks are labeled in each panel. The white cross-hairs indicate the peak position for stoichiometric bulk LVO. The red “x” marks the position of maximum intensity for each film peak.

Figure 4

Summary of the out-of-plane lattice constants for the films discussed in the paper. Each film was grown using nominally the same growth conditions, except for the laser fluence, which was set to $J\sim 0.6$ and $1{\mathrm{J}/\mathrm{cm}}^2$ (or higher), for La-rich and V-rich films, respectively. The La:V stoichiometry was determined using EDS (La:V = 55:45 and 45:55) or inferred from the growth conditions. The film thickness was around 200 nm unless indicated otherwise, i.e., L3: 350 nm, as determined from TEM and from the number of pulses used during growth. The dashed line is the expected lattice parameter based on a Poisson ratio of $\nu =0.398$ [30, 31] and epitaxial strain of ${\varepsilon }_{\left|\right|}=-0.5\%$.

Figure 5

Half order peaks of LVO-L1. With the diffraction peaks shown, we are able to determine an $a^{+}{a}^{-}{c}^{-}$ tilting pattern in our films.

Figure 6

Temperature-dependent absorption spectra on (a) LVO-L2 (integration time: 250 ms) and (b) LVO-V2 (integration time: 400 ms) at 250, 150, and 25 K. Insets are Gaussian fittings of the 25-K spectra.

Figure 7

Resistance measurement on a representative V-rich film LVO-V2 measured with 0.001 mA, showing that its low temperature resistance can be well fitted to the Mott variable range hopping model of a three dimensional material ($\approx T^{-1/4}$); the resistance at high temperature can be explained using the thermal activation model (see inset).

Figure 8

Temperature dependent resistance measurement for a representative La-rich film LVO-L3 measured with 0.001 mA. There are two turnovers in the full temperature range scan. Part of its metallic region can be fitted to Fermi-liquid model $\rho \sim T^2$ (see inset).

Figure 9

The upturn in resistance at low $T$ can be suppressed by increasing driving current. Inset: A scheme showing the insulating phase (colored in red) is not formed homogeneously, allowing current to percolate when higher currents are applied (colored in green).