Deposition
Sample films of Bi2O3were sputtered from a metallic Bi target (>99.99 %) using an active
plasma of argon and oxygen onto silicon wafers (p type, 0.01Ω-cm, WRS Materials) with a thermal SiO2buffer layer. The substrate was unheated during sputtering, and the Bi2O3 was amorphous as
deposited (Extended Data Fig. 1). The thermal SiO2 buffer layer between the silicon wafer and
Bi2O3was used to prevent melt alloying with the silicon wafer in the center of the laser scans where
temperatures exceed the 830°C Bi2O3 melting temperature. Melt alloying of Bi2O3 and SiO2 is
known to precipitate a stable body centered cubic bismuth silicate phase with a melting point near 900°C at low silicon concentrations[87, 121]; this phase was not observed for melt anneals in this study. On these time scales, the SiO2 buffer layer appears to successfully prevent alloying
of the Bi2O3 films even during melt. Samples used for structural analysis had a 100 nm thermal
SiO2buffer, while samples used for electrical conductivity measurements had a thinner, 5 nm SiO2
buffer layer. Production of phase pure δ-Bi2O3in the devices used for electrical measurements was
verified by XRD (Extended Data Fig. 2). After laser annealing and X-ray analysis, and prior to device patterning, an additional 5 nm of SiO2 was deposited using electron beam evaporation from an SiO2target as a capping layer on top of the Bi2O3film; this protected films from solvents during
photolithography, enabling the patterning of electrical contacts. Thermal evaporation was used to deposit adhesion layers (1 nm Cr) and gold top and bottom contacts (30 nm).
Laser Annealing
Laser annealing was performed by scanning a continuous 10.6 µm wavelength line- focused CO2laser (as described previously[13]) across the sample, as shown in Figure 1a. Dwell
ning direction divided by the scanning speed. Anneals were performed in air. Temperature evolu- tion as a function of time across the entire scan width was measured as described previously[13]. The peak temperature experienced across the laser scan width is approximately Gaussian with a maximum slope of 2°C m-1 and FWHM of 680 m. The Bi
2O3 thickness dependent perturbation
to surface temperature was calibrated by Bi2O3and Au dot melts[13]. Laser scans create a line of
annealed material where annealing conditions are invariant along the scan length but vary across the scan width. This allows the use of high aspect ratio rectangular areas for both X-ray diffraction and ion conduction measurements, with large areas of comparable annealing conditions despite the steep temperature gradient across the scan width. A characteristic annealing time, referred to as the dwell, is defined as the FWHM of the line-focused laser in the scanning direction divided by the scanning speed. On the same sample, multiple separated scans were made with dwells of 0.15 – 10 ms and peak temperatures in the scan centers ∼900°C and center-center separations of 3 mm. This allowed high throughput measurements of annealing times and temperature conditions on a single sample using spatially resolved measurements.
Phase Identification
X-ray diffraction measurements were conducted at station G1 of the Cornell High En- ergy Synchrotron Source (CHESS). The X-ray beam was reduced to a 25x200 µm rectangle using a standard three-slit setup. The sample was mounted in a near-grazing-incidence configuration (ω=3°) and aligned such that the temperature gradient across the laser scans was orthogonal to the incident beam, allowing diffraction measurements to be spatially indexed to precise temperature (averaging over a 0-50°C temperature range) and dwell annealing conditions. Diffracted X-rays were collected on a Pilatus 3-300K pixel array detector (Dectris, Inc.). The incident beam energy was 9.81 keV and a typical sample-detector distance was 20 cm; the precise sample-detector dis- tance was calibrated using copper tape adhered to each end of each sample measured to ensure precision. Radial integration was performed using the Nika package for Igor Pro[99]. The fraction
of polymorphs present in each diffraction pattern was determined by multi-peak fitting and scaled relative to phase-pure patterns. For Figure 3a, if a single phase has more than 25 % higher signal than all other phases it is defined as dominant, if two phases have intensities within 25 % then it is mixed phase. Fits in Figure 2d were developed using even polynomials which forced symmetry of the fit for equivalent annealing conditions on either side of the laser scan, and were transformed into temperature for Figure 2e using lgLSA calibrations. 3D surfaces used to create contour plots in Figure 3b,c,d were developed using fitting shown in Figure 2e. The reported limit on coherent scattering domain size was determined by the Debye-Scherrer equation assuming zero instrumen- tal broadening and, as such, represents a lower limit on domain size. Prolonged anneals above the equilibrium melt resulted in damage in the form of an amorphous structure with a shifted amor- phous peak compared to the as-deposited amorphous film, likely due to the well reported instability of Bi2O3in low oxygen partial pressure environments at high temperatures[208].
Scanning Electron Microscopy
The morphology of the annealed areas was examined using a Tescan Mira3 scanning electron microscope (SEM) using an in-lens detector and an accelerating voltage of 15 kV. Figure 1d gives an SEM image of a 1 ms dwell 900°C peak temperature laser scan.
Electrical Measurements and Devices
Devices had a Bi2O3 contact area of 0.328 mm2 were constructed on areas of 160 nm
thick δ-Bi2O3 generated by anneal to a peak temperature of 850°C with a 0.25 ms dwell (Fig. 4a).
Conductivity was measured using impedance spectroscopy using a 4284A LCR meter (Hewlett- Packard Inc) at frequencies from 25 kHz to 1 MHz at room temperature. The impedance of the Bi2O3 layer was extracted by fitting the measured curves using a model consisting of parallel
circuit (SiO2 buffer layers) and series resistor (substrate). Conductivity measurement error was calculated from errors of fit and device dimensions. DC measurements were conducted using handheld multimeters and confirmed by 10 Hz measurements with the 4284A LCR meter.