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تسجيل مستخدم جديدPerspective: Towards the predictive discovery of ambipolarly dopable ultra-wide-band-gap semiconductors: the case of rutile GeO$_2$
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Ultrawide-band-gap (UWBG) semiconductors are promising for fast, compact, and energy-efficient power-electronics devices. Their wider band gaps result in higher breakdown electric fields that enable high-power switching with a lower energy loss. Yet, the leading UWBG semiconductors suffer from intrinsic materials limitations with regards to their doping asymmetry that impedes their adoption in CMOS technology. Improvements in the ambipolar doping of UWBG materials will enable a wider range of applications in power electronics as well as deep- UV optoelectronics. These advances can be accomplished through theoretical insights on the limitations of current UWBG materials coupled with the computational prediction and experimental demonstration of alternative UWBG semiconductor materials with improved doping and transport properties. As an example, we discuss the case of rutile GeO$_2$ (r-GeO$_2$), a water-insoluble GeO$_2$ polytype which is theoretically predicted to combine an ultra-wide gap with ambipolar dopability, high carrier mobilities, and a higher thermal conductivity than b{eta}-Ga$_2$O$_3$. The subsequent realization of single-crystalline r-GeO$_2$ thin films by molecular beam epitaxy provides the opportunity to realize r-GeO$_2$ for electronic applications. Future efforts towards the predictive discovery and design of new UWBG semiconductors include advances in first-principles theory and high-performance computing software, as well as the demonstration of controlled doping in high-quality thin films with lower dislocation densities and optimized film properties.
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GeO$_2$ has an $alpha$-quartz-type crystal structure with a very wide fundamental band gap of 6.6 eV and is a good insulator. Here we find that the stable rutile-GeO$_2$ polymorph with a 4.6 eV band gap has a surprisingly low $sim$6.8 eV ionization p
otential, as predicted from the band alignment using first-principles calculations. Because of the short O$-$O distances in the rutile structure containing cations of small effective ionic radii such as Ge$^{4+}$, the antibonding interaction between O 2p orbitals raises the valence band maximum energy level to an extent that hole doping appears feasible. Experimentally, we report the flux growth of $1.5 times 1.0 times 0.8$ mm$^3$ large rutile GeO$_2$ single crystals and confirm the thermal stability for temperatures up to $1021 pm 10~^circ$C. X-ray fluorescence spectroscopy shows the inclusion of unintentional Mo impurities from the Li$_2$O$-$MoO$_3$ flux, as well as the solubility of Ga in the r-GeO$_2$ lattice as a prospective acceptor dopant. The resistance of the Ga- and Mo-codoped r-GeO$_2$ single crystals is very high at room temperature, but it decreases by 2-3 orders of magnitude upon heating to 300 $^circ$C, which is attributed to thermally-activated p-type conduction.
Rutile germanium dioxide (r-GeO$_2$) is a recently predicted ultrawide-band-gap semiconductor with potential applications in high-power electronic devices, for which the carrier mobility is an important material parameter that controls the device eff
iciency. We apply first-principles calculations based on density functional and density functional perturbation theory to investigate carrier-phonon coupling in r-GeO$_2$ and predict its phonon-limited electron and hole mobilities as a function of temperature and crystallographic orientation. The calculated carrier mobilities at 300 K are $mu_{text{elec},perp vec{c}}$=244 cm$^2$ V$^{-1}$ s$^{-1}$, $mu_{text{elec},||vec{c}}$=377 cm$^2$ V$^{-1}$ s$^{-1}$, $mu_{text{hole},perp vec{c}}$=27 cm$^2$ V$^{-1}$ s$^{-1}$, and $mu_{text{hole},||vec{c}}$=29 cm$^2$ V$^{-1}$ s$^{-1}$. At room temperature, carrier scattering is dominated by the low-frequency polar-optical phonon modes. The predicted Baliga figure of merit of n-type r-GeO$_2$ surpasses several incumbent semiconductors such as Si, SiC, GaN, and $beta$-Ga$_2$O$_3$, demonstrating its superior performance in high-power electronic devices.
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