Nature of the metal-insulator transition in few-unit-cell-thick LaNiO3 films

The nature of the metal-insulator transition in thin films and superlattices of LaNiO3 only a few unit cells in thickness remains elusive despite tremendous effort. Quantum confinement and epitaxial strain have been evoked as the mechanisms, although other factors such as growth-induced disorder, cation non-stoichiometry, oxygen vacancies, and substrate–film interface quality may also affect the observable properties of ultrathin films. Here we report results obtained for near-ideal LaNiO3 films with different thicknesses and terminations grown by atomic layer-by-layer laser molecular beam epitaxy on LaAlO3 substrates. We find that the room-temperature metallic behavior persists until the film thickness is reduced to an unprecedentedly small 1.5 unit cells (NiO2 termination). Electronic structure measurements using X-ray absorption spectroscopy and first-principles calculation suggest that oxygen vacancies existing in the films also contribute to the metal-insulator transition.


Supplementary Note 1: Oxide target preparation
To grow LaNiO3 from separate oxide targets, we need La2O3 and NiO targets. NiO target was prepared by pressing and sintering high purity powder. For La2O3, the process is more complicated as La2O3 target is not stable in the ambient atmosphere due to quick hydration and carbonation of La2O3 upon exposure to air and moisture. To avoid this, the commercial La2O3 powder (purity 99.99%) was pre-sintered at 600°C for 24 hours as La(OH)3 decomposes to La2O3 at this temperature. We then ground and pressed the powder into a dense pallet. The pallet was sintered at 1300°C for 24 hours in air and then cooled down to 300°C at 1°C/min cooling rate. After taking the target out of the furnace at about 200°C, it was transferred into vacuum chamber quickly and remained in vacuum.

Supplementary Note 2: LaAlO3 buffer layer
Studies on the structure of 100 surfaces of LaAlO3 single crystal shows that surface termination can be single LaO layer, single AlO2 layer or the mixture of both depends on temperature and oxygen vacancies. Following the recipe by Ohnishi 1 AlO2 terminated surface was achieved by HCl etching of single crystal substrate followed by annealing at 750°C in flowing oxygen.
Supplementary Fig. 1a shows the Atomic Force Microscopy (AFM) image of the substrate after this treatment. The line profile of the scan shows flat traces with 1 u.c. high steps ( Supplementary Fig. 1b). Homoepitaxial LaAlO3 buffer layer (5-10 u.c.) was then deposited on the single terminated substrate in atomic layer by layer manner from La2O3 and Al2O3 targets.
LaAlO3 buffer layer were grown right before growth of LaNiO3 film in the same growth chamber and identical growth parameters (oxygen pressure, substrate temperature, and laser spot size). Starting from the La2O3 target, a full atomic layer of LaO deposited followed by switching to Al2O3 target for the growth of a full AlO2 atomic layer to complete 1 u.c. of LaAlO3. We continue this until RHEED intensity oscillation shows the pattern characteristic of stoichiometry and full surface coverage. Supplementary Fig. 1c shows RHEED specular spot intensity oscillation during the growth of the last two LaAlO3 layers followed by a layer of LaNiO3. As it can be seen in the figure, an LaO layer brings RHEED intensity to a minimum while AlO2 and 2 NiO2 atomic layers bring RHEED intensity to maximize intensity. Sharp diffraction spots confirm the desired growth mode and surface quality ( Supplementary Fig. 1d). Following the growth of homoepitaxial LaAlO3 buffer layer with an AlO2 surface termination, a LaO layer will be first grown by ablating from a La2O3 target. RHEED intensity was monitored as the deposition took place so that the ablation was stopped when a full layer of LaO was deposited. The target will then be switched to NiO to grow the NiO2 layer. Again, the RHEED intensity was monitored to ensure that one full layer of NiO2 was deposited. The steps were repeated to grow a LaNiO3 film of desired thickness and surface termination.

Supplementary
To calibrate the growth rate (number of pulses needed to complete one atomic layer), we first grow 40 u.c. LaNiO3 film on LaAlO3 substrate. The RHEED intensity oscillation was used as the primary tool to control the growth the mode. The as-grown 40 u.c. films were characterized by xray reflection for thickness measurement and by x-ray diffraction for the lattice constant measurement as well as for phase purity (Supplementary Fig. 3). Using the result of the LaNiO3 film characterization, adjustments were made to the number of pulses if needed. As a more precise way to test the number of pulses required for La-Ni stoichiometry, we used the number of pulses necessary for each atomic layer of LaO and NiO2 from the LaNiO3 calibration to grow La2NiO4, the Ruddlesden-Popper phase with n = 1. It has been shown that the growth of the Ruddlesden-Popper phase is more sensitive to stoichiometry and full layer coverage. We use the ablation sequence of La2O3-La2O3-NiO for each u.c. Supplementary Fig. 4 shows RHEED intensity oscillation and XRD θ-2θ scan for the growth of La2NiO4 film. The persistent RHEED oscillation confirms the atomic layer-by-layer growth mode and the sharp peaks in XRD scan shows the phase purity of our film. We found out that even with the most precise calibration, in-situ monitoring and control were needed for each film. We found that the transport properties of LaNiO3 films were strongly dependent on the concentration of oxygen vacancies in the films. To reduce the oxygen vacancies, oxygen pressure was set to the maximum achievable in our chamber without affecting the RHEED spot quality. We have also investigated the effect of post annealing oxygen pressure on the transport properties of the films. Supplementary Fig. 5 shows the temperature dependent resistivity for 1.5 u.c. films grown at the same condition but post-annealed at 2 different oxygen pressures. We can see a slight improvement in transport properties of the film post-annealed at a higher oxygen pressure.  Photon Energy (eV) 4.5 u.c.
Supplementary Figure 6, Orbital polarization in the LaNiO3 films. XAS spectra at Ni L2-edge for Ec (red) and E∥c (black) polarizations LaNiO3 films with different thicknesses.