Control of phase formation of (Al<sub>x</sub>Ga<sub>1 − x</sub>)<sub>2</sub>O<sub>3</sub> thin films on c-plane Al<sub>2</sub>O<sub>3</sub>
Anna Hassa, Charlotte Wouters, Max Kneiß, Daniel Splith, Chris Sturm, Holger von Wenckstern, M. Albrecht, Michael Lorenz, Marius Grundmann
Abstract
Abstract In this paper, the growth of orthorhombic and monoclinic (Al x Ga 1 − x ) 2 O 3 thin films on (00.1) Al 2 O 3 by tin-assisted pulsed laser deposition is investigated as a function of oxygen pressure p (O 2 ) and substrate temperature <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>T</mml:mi> <mml:mrow> <mml:mi mathvariant="sans-serif">g</mml:mi> </mml:mrow> </mml:msub> </mml:math> . For certain growth conditions, defined by <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>T</mml:mi> <mml:mrow> <mml:mi mathvariant="sans-serif">g</mml:mi> </mml:mrow> </mml:msub> </mml:math> ≥ 580 ° C and p (O 2 ) ≤ 0.016 mbar, the orthorhombic κ -polymorph is stabilized. For <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>T</mml:mi> <mml:mrow> <mml:mi mathvariant="sans-serif">g</mml:mi> </mml:mrow> </mml:msub> </mml:math> = 540 ° C and p (O 2 ) ≤ 0.016 mbar, the κ -, and the β -, as well as the spinel γ -polymorph coexist, as illustrated by XRD 2 θ - ω -scans. Further employed growth parameters result in thin films with a monoclinic β -gallia structure. For all polymorphs, p (O 2 ) and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>T</mml:mi> <mml:mrow> <mml:mi mathvariant="sans-serif">g</mml:mi> </mml:mrow> </mml:msub> </mml:math> affect the formation and desorption of volatile suboxides, and thereby the growth rate and the cation composition. For example, low oxygen pressures lead to low growth rates and enhanced Al incorporation. This facilitates the structural engineering of polymorphic, ternary (Al,Ga) 2 O 3 via selection of the relevant process parameters. Transmission electron microscopy (TEM) studies of a κ - (Al 0.13 Ga 0.87 ) 2 O 3 thin film reveal a more complex picture compared to that derived from x-ray diffraction measurements. Furthermore, this study presents the possibility of controlling the phase formation, as well as the Al-content, of thin films based on the choice of their growth conditions.