Supplemental Information for

An all-perovskite p-n junction based on transparent conducting p-La1-xSrxCrO3 epitaxiallayers

Yingge Du,*,1Chen Li,2 Kelvin H. L. Zhang,1,3 Martin E. McBriarty,1 Steven R. Spurgeon,1 Hardeep S. Mehta,4 Di Wu,2 Scott A. Chambers*,1

1Physical Sciences Division, Physical & Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, USA

2Department of Materials Science and Engineering, College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, China

3 Department of Chemical and Biochemical Engineering, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China4Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, USA

(Received by APL, ??March 2017)

Film Growth

Nb-doped SrTiO3(001) (Nb:STO, 0.05% doping, CrysTec GmbH) substrates were prepared by HF etching and annealing to achieve TiO2-terminated surfaces. Following insertion into the MBE chamber, the substrates were cleaned at ~650oC for 20 min in an oxygen partial pressure of 6.0×10−6Torr prior to film growth. Epitaxial La1−xSrxCrO3 (LSCO) films with 0.12 < x < 0.15 and thicknesses of 2 - 30 nm were grown on Nb:STO substrates by MBE. La, Sr, and Cr were evaporated from high-temperature effusion cells, and evaporation rates were calibrated prior to each growth using a quartz crystal oscillator positioned at the substrate position. The substrate temperature was set to 625◦C, and the O2 partial pressure was kept at ∼3.0 ×10-6Torr during growth. Reflection high-energy electron diffraction (RHEED) was used to monitor the overall growth rate and surface structure. After deposition, the substrate temperature was lowered at a rate of 50oC/min while the background O2 was pumped out. Film characterization data, including RHEED, XPS, and X-ray diffraction, have been reported elsewhere.1,2 Figs. S1(a) and (b) show atomic force microscopy (AFM) images of a clean Nb:STO(001) substrate and a 30 nm LSCO film on Nb:STO, respectively. Step terraces with a minimum height difference of0.4 nm can be clearly seen in both cases.

XPS Measurements

XPS measurements were performed in a UHV analytical chamber coupled to the MBE chamber via a UHV transfer line using a VG/Scienta R3000 analyzer and a monochromatic AlK x-ray source with an energy resolution of ~0.55 eV. The binding energy scale was calibrated using the Ag 3d5/2 core level (368.21 eV) and the Fermi level from a polycrystalline Ag foil. All samples were grown on conductive Nb:STO substrates, so the reported binding energies are accurate in an absolute sense to ±0.02 eV. We selected the La 4d, Sr 3d5/2 and Ti 2p3/2 core levels for band offset measurements because they exhibit the narrowest full widths at half maximum (FWHM) values for each element.

STEM-EELS

TEM samples were prepared using a standard lift out method on an FEI Helios DualBeam focused ion beam (FIB) microscope. Cuts were made at a 4-7o incidence angle, using an ion beam energy of 30 keV, which was gradually reduced to 0.5 keV for final polishing to minimize surface amorphization. STEM measurements were performed using an aberration-corrected JEOL ARM 200-CF microscope operating at 200 keV. The microscope is equippedwith both a JEOL Centurio silicon drift detector (SDD) and a Gatan Quantum GIF for EDSand EELS analysis, respectively.All images were acquired along the [100] zone-axis with a ~1Å probe size and a 27.5 mrad inner convergence semi-angle, yielding an approximate probe current of ~130 pA. STEM-EELS mapping was conducted at an 83 mrad inner collection semi-angle. Composition maps were acquired using a 1 eV ch-1 dispersion, with 130× vertical and 4× horizontal spectrometer binning, a 108×45 px map size, and a dwell time of 10 ms px-1. A power law background was fit over each edge and 60 eV signal windows were integrated for the Ti L23 and La M45 edges and a 200 eV signal window for the Sr L23 edge. Zero-loss peak thickness measurements show that the sample is ~50 nm thick in the mapped regions.

ElectricalMeasurement

A Keithley 2636 SourceMeter was used to measure the I-V characteristics and resistanceswitching properties of the LSCO/Nb:STO samples. The electrical measurements wereperformed ona Lake Shore CRX-4K probe station by positioning BeCu probes on the electrodes. Measurement temperatures rangedfrom 20 K to 360 K, with a step of 20 K. At every temperature point, two test signals ((1) DC voltage from -2.5 to 2.0 V with a step of 0.1 V; (2) DC voltage from -2.5 V to 2.0 V, then immediately back to -2.5 V, with a step of 0.1V) were applied sequentially at several different Cr/Au electrodes on the Nb:STO substrate.

Reverse IV Measurement

Fig. S2 shows typical I-V characteristics of a 10nm LSCO/Nb:STO heterojunction at two different temperatures (200K and 300K). The I-V curves show no change when the voltage sweep direction is reversed, indicating no resistive switching effect. This further proves that the effect of oxygen vacancies at the interface can be neglected.

Impact of Substrate Defects on Film Growth

It is widely recognized that the quality of oxide substrates are still far behind that of traditional semiconductors. On chemically etched STO(001) surfaces, defects associated with step bunching, etchpit formation, Sr surface segregation, and nanostructuring are commonly found.3,4We have foundthat both film quality and I-V measurements can be significantly affected by substrate defects. For example, Fig. S3 shows STEM-HAADF images that indicate local film amorphization in a 5 u.c. LSCO film deposited on substrates with extensive step bunching or etch pits. In this case, local defect structures dominate transport properties, and I-V measurements indicated Ohmic behavior.

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