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Register a new user$gamma$-phase Inclusions as Common Defects in Alloyed $beta$-(Al$_x$Ga$_{1text{-}x}$)$_2$O$_3$ and Doped $beta$-Ga$_2$O$_3$ Films
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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$_3$.
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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.
Recent breakthroughs in bulk crystal growth of the thermodynamically stable beta phase of gallium oxide ($beta$-Ga$_2$O$_3$) have led to the commercialization of large-area beta-Ga$_2$O$_3$ substrates with subsequent epitaxy on (010) substrates producing high-quality films. Still, metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), and processing of the (010) $beta$-Ga$_2$O$_3$ surface are known to form sub-nanometer scale facets along the [001] direction as well as larger ridges with features perpendicular to the [001] direction. A density function theory calculation of the (010) surface shows an ordering of the surface as a sub-nanometer-scale feature along the [001] direction. Additionally, the general crystal structure of $beta$-Ga$_2$O$_3$ is presented and recommendations are presented for standardizing (010) substrates to account for and control the larger-scale ridge formation.
We introduce a deep-recessed gate architecture in $beta$-Ga$_2$O$_3$ delta-doped field effect transistors for improvement in DC-RF dispersion and breakdown properties. The device design incorporates an unintentionally doped $beta$-Ga$_2$O$_3$ layer as the passivation dielectric. To fabricate the device, the deep-recess geometry was developed using BCl$_3$ plasma based etching at ~5 W RIE to ensure minimal plasma damage. Etch damage incurred with plasma etching was mitigated by annealing in vacuum at temperatures above 600 $deg$C. A gate-connected field-plate edge termination was implemented for efficient field management. Negligible surface dispersion with lower knee-walkout at high V$_mathrm{DS}$, and better breakdown characteristics compared to their unpassivated counterparts were achieved. A three terminal off-state breakdown voltage of 315 V, corresponding to an average breakdown field of 2.3 MV/cm was measured. The device breakdown was limited by the field-plate/passivation edge and presents scope for further improvement. This demonstration of epitaxially passivated field effect transistors is a significant step for $beta$-Ga$_2$O$_3$ technology since the structure simultaneously provides control of surface-related dispersion and excellent field management.
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.
Point defects in crystalline materials often occur in multiple charge states. Although many experimental methods to study and explore point defects are available, techniques to explore the non-equilibrium dynamics of the charge states of these defects at ultrafast (sub-nanosecond) time scales have not been discussed before. We present results from ultrafast optical-pump supercontinuum-probe spectroscopy measurements on $beta$-Ga$_2$O$_3$. The study of point defects in $beta$-Ga$_2$O$_3$ is essential for its establishment as a material platform for high-power electronics and deep-UV optoelectronics. Use of a supercontinuum probe allows us to obtain the time-resolved absorption spectra of material defects under non-equilibrium conditions with picosecond time resolution. The probe absorption spectra shows defect absorption peaks at two energies, $sim$2.2 eV and $sim$1.63 eV, within the 1.3-2.5 eV probe energy bandwidth. The strength of the absorption associated with each peak is time-dependent and the spectral weight shifts from the lower energy peak to the higher energy peak with pump-probe delay. Further, maximum defect absorption is seen for probe polarized along the crystal c-axis. The time-dependent probe absorption spectra and the observed dynamics for all probe wavelengths at all pump-probe delays can be fit with a set of rate equations for a single multi-level defect. Based on first-principles calculations within hybrid density functional theory we attribute the observed absorption features to optical transitions from the valence band to different charge states of Gallium vacancies. Our results demonstrate that broadband ultrafast supercontinuum spectroscopy can be a useful tool to explore charge states of defects and defect dynamics in semiconductors.
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