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Register a new userLow In solubility and band offsets in the small-$x$ $beta$-Ga$_2$O$_3$/(Ga$_{1-x}$In$_x$)$_2$O$_3$ system
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Added by
Vincenzo Fiorentini
Publication date
2014
fields
Physics
and research's language is
English
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Based on first-principles calculations, we show that the maximum reachable concentration $x$ in the (Ga$_{1-x}$In$_x$)$_2$O$_3$ alloy in the low-$x$ regime (i.e. In solubility in $beta$-Ga$_2$O$_3$) is around 10%. We then calculate the band alignment at the (100) interface between $beta$-Ga$_2$O$_3$ and (Ga$_{1-x}$In$_x$)$_2$O$_3$ at 12%, the nearest computationally treatable concentration. The alignment is strongly strain-dependent: it is of type-B staggered when the alloy is epitaxial on Ga$_2$O$_3$, and type-A straddling in a free-standing superlattice. Our results suggest a limited range of applicability of low-In-content GaInO alloys.
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$beta$-Ga$_2$O$_3$ is a promising ultra-wide bandgap semiconductor whose properties can be further enhanced by alloying with Al. Here, using atomic-resolution scanning transmission electron microscopy (STEM), we find the thermodynamically-unstable $gamma$-phase is a ubiquitous defect in both $beta$-(Al$_x$Ga$_{1text{-}x}$)$_2$O$_3$ films and doped $beta$-Ga$_2$O$_3$ films grown by molecular beam epitaxy. For undoped $beta$-(Al$_x$Ga$_{1text{-}x}$)$_2$O$_3$ films we observe $gamma$-phase inclusions between nucleating islands of the $beta$-phase at lower growth temperatures (~400-600 $^{circ}$C). In doped $beta$-Ga$_2$O$_3$, a thin layer of the $gamma$-phase is observed on the surfaces of films grown with a wide range of n-type dopants and dopant concentrations. The thickness of the $gamma$-phase layer was most strongly correlated with the growth temperature, peaking at about 600 $^{circ}$C. Ga interstitials are observed in $beta$-phase, especially near the interface with the $gamma$-phase. By imaging the same region of the surface of a Sn-doped $beta$-(Al$_x$Ga$_{1text{-}x}$)$_2$O$_3$ after ex-situ heating up to 400 $^{circ}$C, a $gamma$-phase region is observed to grow above the initial surface, accompanied by a decrease in Ga interstitials in the $beta$-phase. This suggests that the diffusion of Ga interstitials towards the surface is likely the mechanism for growth of the surface $gamma$-phase, and more generally that the more-open $gamma$-phase may offer diffusion pathways to be a kinetically-favored and early-forming phase in the growth of Ga$_2$O$_3$.
Using density-functional ab initio theoretical techniques, we study (Ga$_{1-x}$In$_x$)$_2$O$_3$ in both its equilibrium structures (monoclinic $beta$ and bixbyite) and over the whole range of composition. We establish that the alloy exhibits a large and temperature-independent miscibility gap. On the low-$x$ side, the favored phase is isostructural with $beta$-Ga$_2$O$_3$; on the high-$x$ side, it is isostructural with bixbyite In$_2$O$_3$. The miscibility gap opens between approximately 15% and 55% In content for the bixbyite alloy grown epitaxially on In$_2$O$_3$, and 15% and 85% In content for the free-standing bixbyite alloy. The gap, volume and band offsets to the parent compound also exhibit anomalies as function of $x$. Specifically, the offsets in epitaxial conditions are predominantly type-B staggered, but have opposite signs in the two end-of-range phases.
Using density-functional ab initio calculations, we provide a revised phase diagram of (Ga$_{1-x}$In$_{x})_2$O$_3$. Three phases --monoclinic, hexagonal, cubic bixbyite-- compete for the ground state. In particular, in the $x$$sim$0.5 region we expect coexistence of hexagonal, $beta$, and bixbyite (the latter separating into binary components). Over the whole $x$ range, mixing occurs in three disconnected regions, and non-mixing in two additional distinct regions. We then explore the permanent polarization of the various phases, finding that none of them is polar at any concentration, despite the possible symmetry reductions induced by alloying. On the other hand, we find that the $varepsilon$ phase of Ga$_2$O$_3$ stabilized in recent growth experiments is pyroelectric --i.e. locked in a non-switchable polarized structure-- with ferroelectric-grade polarization and respectable piezoelectric coupling. We suggest that this phase could be used profitably to produce high-density electron gases in transistor structures.
I use first principles calculations to investigate the thermal conductivity of $beta$-In$_2$O$_3$ and compare the results with that of $alpha$-Al$_2$O$_3$, $beta$-Ga$_2$O$_3$, and KTaO$_3$. The calculated thermal conductivity of $beta$-In$_2$O$_3$ agrees well with the experimental data obtain recently, which found that the low-temperature thermal conductivity in this material can reach values above 1000 W/mK. I find that the calculated thermal conductivity of $beta$-Ga$_2$O$_3$ is larger than that of $beta$-In$_2$O$_3$ at all temperatures, which implies that $beta$-Ga$_2$O$_3$ should also exhibit high values of thermal conductivity at low temperatures. The thermal conductivity of KTaO$_3$ calculated ignoring the temperature-dependent phonon softening of low-frequency modes give high-temperature values similar that of $beta$-Ga$_2$O$_3$. However, the calculated thermal conductivity of KTaO$_3$ does not increase as steeply as that of the binary compounds at low temperatures, which results in KTaO$_3$ having the lowest low-temperature thermal conductivity despite having acoustic phonon velocities larger than that of $beta$-Ga$_2$O$_3$ and $beta$-In$_2$O$_3$. I attribute this to the fact that the acoustic phonon velocities at low frequencies in KTaO$_3$ is less uniformly distributed because its acoustic phonon branches are more dispersive compared to the binary oxides, which causes enhanced momentum loss even during the normal phonon-phonon scattering processes. I also calculate thermal diffusivity using the theoretically obtained thermal conductivity and heat capacity and find that all four materials exhibit the expected $T^{-1}$ behavior at high temperatures. Additionally, the calculated ratio of the average phonon scattering time to Planckian time is larger than the lower bound of 1 that has been observed empirically in numerous other materials.
We report results of the dielectric and pyroelectric measurements on solid solutions of Ga$_2$$_-$$_x$Fe$_x$O$_3$ with x = 0.75, 1.0 and 1.25. These systems exhibit dipolar cluster glass behavior in addition to the spin glass behavior making them belong to a class of few systems showing multiglass behavior. Presence of two contributing relaxations in dielectric data are observed possibly due to the flipping and breathing of polar nano-clusters. Further, emergence of polarization in these systems can be understood in terms of thermally stimulated depolarization current (TSDC) effect caused by defect dipoles possibly associated with charged oxygen vacancies rather than the intrinsic ferroelectric behavior.
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