Quantitative Determination of Native Point‐Defect Concentrations at the ppm Level in Un‐Doped BaSnO₃ Thin Films

Abstract

The high-mobility, wide-bandgap perovskite oxide BaSnO 3 is taken as a model system to demonstrate that the native point defects present in un-doped, epitaxial thin films grown by hybrid molecular beam epitaxy can be identified and their concentrations at the ppm level determined quantitatively. An elevatedtemperature, multi-faceted approach is shown to be necessary: oxygen tracer diffusion experiments with secondary ion mass spectrometry analysis; molecular dynamics simulations of oxygen-vacancy diffusion; electronic conductivity studies as a function of oxygen activity and temperature; and Hall-effect measurements. The results indicate that the oxygen-vacancy concentration cannot be lowered below 10 17.3 cm −3 because of a background level of barium vacancies (of this concentration), introduced during film growth. The multi-faceted approach also yields the electron mobility over a wide temperature range, coefficients of oxygen surface exchange and oxygen-vacancy diffusion, and the reduction enthalpy. The consequences of the results for the lowest electron concentration achievable in BaSnO 3 samples, for the ease of oxide reduction and for the stability of reduced films with respect to oxidation, are discussed.

Surface Space‐Charge Potential and Oxygen Surface Exchange Coefficient of SrTiO3: The Effect of Surface Orientation

“…[ 70,71 ] Surface exchange data for a number of other perovskite oxides [BaTiO 3 , LaAlO 3 , (La,Sr)(Ga,Mg)O 3 , (La,Sr)FeO 3 , BaSnO 3 ] under dry conditions are consistent with this overall behaviour. [ 52,53,72,73 ] Furthermore, BaTiO 3 , BaSnO 3 and LaAlO 3 , all exhibit $$\Delta {H_{k_{\rm{O}}^*}} \approx 3$$ eV in the high temperature regime. [ 52,53,73 ] And in the low‐temperature regime, $$\Delta {H_{k_{\rm{O}}^*}}$$ drops to (1.35 ± 0.07) eV for BaTiO 3 [ 52 ] and to (1.85 ± 0.22) eV for LaAlO 3 .…”

“…[ 52,53,72,73 ] Furthermore, BaTiO 3 , BaSnO 3 and LaAlO 3 , all exhibit $$\Delta {H_{k_{\rm{O}}^*}} \approx 3$$ eV in the high temperature regime. [ 52,53,73 ] And in the low‐temperature regime, $$\Delta {H_{k_{\rm{O}}^*}}$$ drops to (1.35 ± 0.07) eV for BaTiO 3 [ 52 ] and to (1.85 ± 0.22) eV for LaAlO 3 . [ 53 ]…”

“…eV in the high temperature regime. [52,53,73] And in the low-temperature regime, O * ∆H k drops to (1.35 ± 0.07) eV for BaTiO 3 [52] and to (1.85 ± 0.22) eV for LaAlO 3 . [53] Now, having described the data (Figure 5), and with the explanation for the change in…”

Hydration Entropy and Enthalpy of a Perovskite Oxide from Oxygen Tracer Diffusion Experiments

“…One possibility for detecting low defect concentrations is to perform a tracer diffusion experiment. Low defect concentrations (ppm or lower) give rise to measurable diffusion coefficients. − The measured tracer diffusion coefficient of species i can be expressed in terms of the tracer correlation coefficient , the diffusion coefficient of the defects responsible for diffusion D def , and the concentrations of defects [def] and species [ i ], Thus, with a knowledge of , D def , and [ i ], the defect concentration can be obtained. In the present case, there are two problems: first, [def] and [ i ] are identical for interstitial hydrogen so that the measurement of D H * gives D def ; second, true tracer diffusion experiments for hydrogen are impossible because the labeled ( 2 H) and standard ( 1 H) species are not chemically identical (isotope effects are expected for both the concentrations of defects and their mobilities).…”

Doping the Undopable: Hybrid Molecular Beam Epitaxy Growth, n-Type Doping, and Field-Effect Transistor Using CaSnO3

“…Therefore, in the future, techniques to minimize hot-carrier trapping, including the use of a fieldplate geometry and surface passivation, 38 will be important to assess the ultimate high-voltage capabilities of CaSnO 3 . Additionally, the role of ionic conduction in stannates 39 under high applied bias may also become an important factor and should be investigated.…”