[Paper Review] Oxygen vacancy driven mobility enhancement in epitaxial La-doped BaSnO3 from vacuum annealing

Hai Jun Cho, Takaki Onozato+3 more|arXiv (Cornell University)|Aug 31, 2018

Electronic and Structural Properties of Oxides37 references18 citations

TL;DR

This study demonstrates that vacuum annealing of epitaxial La-doped BaSnO3 (LBSO) films significantly enhances carrier mobility by inducing oxygen vacancies, which simultaneously increase carrier concentration and promote lateral grain growth. The mobility improvement—unachievable under air annealing—results from defect engineering via controlled oxygen vacancy formation.

ABSTRACT

Wide bandgap (~3.1 eV) La-doped BaSnO3 (LBSO) has attracted increasing attention as one of the transparent oxide semiconductors since its bulk single crystal shows a high carrier mobility (~320 cm2 V-1 s-1) with a high carrier concentration (~10^20 cm-3). For this reason, many researchers have fabricated LBSO epitaxial films thus far, but the obtainable carrier mobility is substantially low compared to that of single crystals due to the formation of the lattice/structural defects. Here we report that the mobility suppression in LBSO films can be lifted by a simple vacuum annealing process. The vacuum annealing of the LBSO films on MgO substrate increased the carrier concentrations due to the oxygen vacancy formation, which leads to simultaneous lateral grain growth. As a result, the carrier mobility was greatly improved by the vacuum annealing, which does not occur after heat treatment in air. These results expand our current knowledge on the point defect formation in epitaxial LBSO films and show that vacuum annealing is a powerful tool for enhancing the mobility values of LBSO films.

Motivation & Objective

• To address the persistent mobility suppression in epitaxial La-doped BaSnO3 (LBSO) films compared to bulk single crystals.
• To investigate the role of oxygen vacancies in modulating electronic and structural properties of LBSO films.
• To determine whether vacuum annealing can enhance carrier mobility by promoting defect-mediated grain growth.
• To compare the effects of vacuum versus air annealing on carrier concentration and mobility in LBSO films.
• To establish vacuum annealing as a viable post-growth treatment for optimizing LBSO film performance.

Proposed method

• Epitaxial LBSO films were grown on MgO substrates via pulsed laser deposition.
• Vacuum annealing was performed at elevated temperatures to induce oxygen vacancy formation.
• Carrier concentration and mobility were measured using Hall bar geometry and van der Pauw methods.
• Structural evolution was monitored via in situ X-ray diffraction to track grain growth and lattice changes.
• Comparative analysis with air-annealed samples was conducted to isolate the role of oxygen vacancies.
• The relationship between oxygen vacancy concentration and mobility enhancement was analyzed using defect chemistry principles.

Experimental results

Research questions

• RQ1 How does vacuum annealing affect oxygen vacancy concentration in epitaxial LBSO films?
• RQ2 What is the impact of oxygen vacancies on carrier concentration and mobility in LBSO films?
• RQ3 Does vacuum annealing induce lateral grain growth in LBSO films, and if so, how does it affect transport properties?
• RQ4 Why is mobility enhancement observed in vacuum-annealed LBSO but not in air-annealed samples?
• RQ5 Can defect engineering via oxygen vacancy formation be used to bridge the mobility gap between LBSO films and bulk single crystals?

Key findings

• Vacuum annealing increased the carrier concentration in LBSO films due to the formation of oxygen vacancies.
• The mobility of LBSO films was significantly enhanced after vacuum annealing, with no such improvement observed under air annealing.
• Oxygen vacancy formation led to concurrent lateral grain growth, reducing grain boundary scattering and improving carrier transport.
• The mobility enhancement was attributed to the combined effects of increased carrier concentration and reduced grain boundary density.
• The results demonstrate that vacuum annealing is a powerful post-growth treatment for improving the electronic properties of epitaxial LBSO films.
• The study establishes a direct link between oxygen vacancy concentration and enhanced mobility in epitaxial LBSO, offering a pathway to achieve mobility values closer to those of bulk single crystals.