CaMnO₃ Thin Films: Tuning Electrical Transport with Oxygen

Calcium manganese oxide (CaMnO₃, or CMO) is a perovskite oxide being studied as a catalyst for turning biofuel into diesel, a photocatalyst for splitting water, a fuel cell electrode and a thermoelectric material. What makes it useful in all of these is oxygen: fully oxygenated CMO is an insulator, but removing some oxygen (CaMnO₃₋ᵧ) mixes Mn³⁺ and Mn⁴⁺ ions, and electrons can hop between them, so the resistance drops. Earlier work in the lab found that CMO films grown at 80 mTorr on LaAlO₃ get less resistive as they get thinner, because strain from the substrate creates oxygen vacancies. My project asked: how do the oxygen pressure during growth and a fluorination treatment change the film's structure and resistance?

A set of CaMnO₃ epitaxial thin films on LaAlO₃ substrates, each grown under a different oxygen environment, plus a fluorinated version of one film to compare against: - CMOLAO 067: 80 mTorr oxygen - CMOLAO 068: vacuum - CMOLAO 069: 1 mTorr - CMOLAO 070: 10 mTorr - CMOLAO 067 fluorinated (SCF), with an as-grown sample and a control I measured every sample's crystal structure with X-ray diffraction and its resistance with two-probe measurements, then compared how growth conditions changed both.

Pulsed laser deposition (PLD) A laser pulses onto a solid CMO target inside a vacuum chamber. Each pulse blasts off a plume of material that lands on a heated single-crystal substrate and builds the film layer by layer. We controlled the oxygen pressure, the pulse energy and the number of pulses. Fluorination I mixed 18.7% PVDF (polyvinylidene fluoride) with 81.3% DMF (dimethylformamide), spin-coated one drop onto the 067 film, and left it in the furnace overnight so fluorine could work into the film. X-ray diffraction (XRD) X-rays reflect off the planes of atoms, and they only add up into a peak at angles where Bragg's law holds (nλ = 2d sin θ). Comparing the 002 and 202 peaks tells you the spacing of the crystal lattice, so you can see how growth conditions stretch or squeeze the film. Resistance Two-probe measurements on every sample, done in an ISO 7 clean room environment.

Four CMO films grown on LaAlO₃ at different oxygen pressures: CMOLAO 067 · 80 mTorr · resistance OL · a = 3.757 Å · c = 3.704 Å CMOLAO 068 · vacuum · resistance 10–12 MΩ · a = 3.732 Å · c = 3.729 Å CMOLAO 069 · 1 mTorr · resistance OL · a = 3.770 Å · c = 3.706 Å CMOLAO 070 · 10 mTorr · resistance OL · a = 3.774 Å · c = 3.700 Å Fluorinating film 067 took its resistance from OL (out of range) to 0.3–0.4 MΩ. Presented as a research poster at Towson University, summer 2025.

- Oxygen pressure during growth really does change the film. The vacuum-grown film (068) was the only as-grown sample with a measurable resistance (10–12 MΩ); the others read out of range (OL). It also had a larger c-lattice constant (3.729 Å vs. about 3.70 Å for the others), which fits with more oxygen vacancies. - Fluorination lowered the resistance of the 067 film from out-of-range to 0.3–0.4 MΩ, and its 002 XRD peak shifted slightly lower than the as-grown and control samples. - 067 (80 mTorr) and 068 (vacuum) gave more outlier results than 069 and 070, which behaved alike. That matches their growth conditions, which were the two extremes. - Hands-on lab skills: PLD, spin coating, XRD analysis, resistance measurements and clean room practice, and how to connect a structural measurement to an electrical one.

- Fluorinate the 068, 069 and 070 films too and compare their XRD results - Use four-probe measurements, which take the contact resistance out of the reading, for more accurate resistance values - Vary film thickness and substrate as well, to separate strain effects from oxygen effects