Investigating the ranges of (meta)stable phase formation in (InxGa1−x)2O3

Abstract

We investigate the phase diagram of the heterostructural solid solution (In x Ga 1-x ) 2 O 3 both computationally, by combining cluster expansion and density functional theory, and experimentally, by means of transmission electron microscopy (TEM) measurements of pulsed laser deposited (PLD) heteroepitaxial thin films. The shapes of the Gibbs free energy curves for the monoclinic, hexagonal, and cubic bixbyite alloy as a function of composition can be explained in terms of the preferred cation coordination environments of indium and gallium. We show by atomically resolved scanning TEM that the strong preference of indium for sixfold coordination results in ordered monoclinic and hexagonal lattices. This ordering impacts the configurational entropy in the solid solution and thereby the (In x Ga 1-x ) 2 O 3 phase diagram. The resulting phase diagram is characterized by very limited solubilities of gallium and indium in the monoclinic, hexagonal, and cubic ground state phases, respectively, but exhibits wide metastable ranges at realistic growth temperatures. On the indium rich side of the phase diagram a wide miscibility gap up to temperatures higher than 1400 K is found, which results in phase separated layers. The experimentally observed indium solubilities in the PLD samples are in the range of x = 0.45 and x = 0.55 for monoclinic and hexagonal single-phase films, while for phase separated films we find x = 0.5 for the monoclinic phase, x = 0.65-0.7 for the hexagonal phase and x 0.9 for the cubic phase. These values are consistent with the computed metastable ranges for each phase.

The electrical conductivity of cubic (In1−x Ga x )2O3 films (x ≤ 0.18): native bulk point defects, Sn-doping, and the surface electron accumulation layer

“…Experimentally, a decrease of the Hall electron concentration and mobility of MOCVD-grown (In 1−x Ga x ) 2 O 3 films grown on well-matched ZrO 2 :Y (YSZ) substrates has been earlier demonstrated by Kong et al 31) for increasing x up to 0.9. However, limited information is given on the phase purity: the films of this study surprisingly remain cubic up to x = 0.5, which contradicts current knowledge about the transition from cubic to monoclinic phase 32) and may be explained by a lower incorporation of Ga in the cubic phase than the given total Ga content. Recent investigations by Nagata et al 33) and Swallow et al 34) on films grown using pulsed layer deposition (PLD) show the existence of a SEAL up to approximately x = 0.4.…”

Bandgap widening and behavior of Raman-active phonon modes of cubic single-crystalline (In,Ga)$_2$O$_3$ alloy films

“…5 Accordingly, the knowledge about the miscibility of the binary compounds for a specific crystal structure is of crucial importance. [6][7][8][9] Furthermore, material properties such as the electronic band structure and the behavior of phonon modes in (In,Ga) 2 O 3 alloys are of fundamental interest. In particular for monoclinic alloys with Ga contents above 60%, experimental results regarding physical properties like the composition-dependent electronic band gap and phonon frequencies can be found in the literature for films grown by various methods.…”

Formation energy crossings in Ga2O3-Al2O3 quasibinary system: ordered structures and phase transitions in (Al x Ga1−x )2O3

“…This implies a relatively higher configurational entropy for α-(Al x Ga 1−x ) 2 O 3 which may stabilize the αphase structure by lowering the free energy of formation of the system. 45) 3.3. Comparison of (Al x Ga 1−x ) 2 O 3 phases By considering multiple combinatorial configurations for the same alloy composition, it is possible to see how the arrangement of Group-III atoms in (Al x Ga 1−x ) 2 O 3 affects phase transition from β to α polymorphs.…”