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Register a new userFirst principles study of thermal conductivity of In$_2$O$_3$ in relation to Al$_2$O$_3$, Ga$_2$O$_3$, and KTaO$_3$
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Alaska Subedi
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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.
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We determine the anisotropic dielectric functions of rhombohedral $alpha$-Ga$_2$O$_3$ by far-infrared and infrared generalized spectroscopic ellipsometry and derive all transverse optical and longitudinal optical phonon mode frequencies and broadening parameters. We also determine the high frequency and static dielectric constants. We perform density functional theory computations and determine the phonon dispersion for all branches in the Brillouin zone, and we derive all phonon mode parameters at the Brillouin zone center including Raman-active, infrared-active, and silent modes. Excellent agreement is obtained between our experimental and computation results as well as among all previously reported partial information from experiment and theory. We also compute the same information for $alpha$-Al$_2$O$_3$, the binary parent compound for the emerging alloy of $alpha$-(Al$_{x}$Ga$_{1-x}$)$_2$O$_3$, and use results from previous investigations [Schubert, Tiwald, and Herzinger, Phys. Rev. B 61, 8187 (2000)] to compare all properties among the two isostructural compounds. From both experimental and theoretical investigations we compute the frequency shifts of all modes between the two compounds. Additionally, we calculate overlap parameters between phonon mode eigenvectors and discuss the possible evolution of all phonon modes into the ternary alloy system and whether modes may form single mode or more complex mode behaviors.
$beta$-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$_3$.
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.
In all archetypical reported (001)-oriented perovskite heterostructures, it has been deduced that the preferential occupation of two-dimensional electron gases is in-plane $d_textrm{xy}$ state. In sharp contrast to this, the investigated electronic structure of a spinel-perovskite heterostructure $gamma$-Al$_2$O$_3$/SrTiO$_3$ by resonant soft X-ray linear dichroism, demonstrates that the preferential occupation is out-of-plane $d_textrm{xz}$/$d_textrm{yz}$ states for interfacial electrons. Moreover, the impact of strain further corroborates that this anomalous orbital structure can be linked to the altered crystal field at the interface and symmetry breaking of the interfacial structural units. Our findings provide another interesting route to engineer emergent quantum states with deterministic orbital symmetry.
The search for new wide band gap materials is intensifying to satisfy the need for more advanced and energy efficient power electronic devices. Ga$_2$O$_3$ has emerged as an alternative to SiC and GaN, sparking a renewed interest in its fundamental properties beyond the main $beta$-phase. Here, three polymorphs of Ga$_2$O$_3$, $alpha$, $beta$ and $varepsilon$, are investigated using X-ray diffraction, X-ray photoelectron and absorption spectroscopy, and ab initio theoretical approaches to gain insights into their structure - electronic structure relationships. Valence and conduction electronic structure as well as semi-core and core states are probed, providing a complete picture of the influence of local coordination environments on the electronic structure. State-of-the-art electronic structure theory, including all-electron density functional theory and many-body perturbation theory, provide detailed understanding of the spectroscopic results. The calculated spectra provide very accurate descriptions of all experimental spectra and additionally illuminate the origin of observed spectral features. This work provides a strong basis for the exploration of the Ga$_2$O$_3$ polymorphs as materials at the heart of future electronic device generations.
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