GeO$_2$ has an $α$-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$6.8 eV ionization potential, as predicted from the band alignment using first-principles calculations. Because of the short O$-$O distances in the rutile structure containing cations of small effective ionic radii such as Ge$^{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.
The hyperfine structure of the interstitial muonium (Mu) center in rutile (TiO$_2$, weakly $n$-type) has been identified by means of muon spin rotation technique. The angle-resolved hyperfine parameter has a tetragonal anisotropy within the $ab$ plane and axial anisotropy along the $c$ axis, strongly suggesting that Mu simulates the known local structure of interstitial hydrogen (H) located at an off-center position within a channel along $c$ axis, and the electron wave function bound to Mu is highly delocalized (~1.5 nm along $c$ axis, ~0.8 nm for $a$ axis). The ionization energy of Mu ($\rig…
The behavior of electrons in the conduction band of TiO2 and other transition-metal oxides is key to the many applications of these materials. Experiments seem to produce conflicting results: optical and spin-resonance techniques reveal strongly localized small polarons, while electrical measurements show high mobilities that can only be explained by delocalized free electrons. By means of hybrid functional calculations we resolve this apparent contradiction and show that small polarons can actually coexist with delocalized electrons in the conduction band of TiO2, the former being energetical…
Rutile GeO$_2$ is a visible and near-ultraviolet-transparent oxide that has not been explored for semiconducting applications in electronic and optoelectronic devices. We investigate the electronic and optical properties of rutile GeO$_2$ with first-principles calculations based on density functional theory and many-body perturbation theory. Our band-structure calculations indicate a dipole-forbidden direct band gap at $Γ$ with an energy of 4.44 eV and effective masses equal to $m^*_{e\perp}$ =0.43$m_0$ , $m^*_{e||}$ =0.23$m_0$ , $m^*_{h\perp}$ =1.28$m_0$ , and $m^*_{h||}$=1.74$m_0$ . In contr…
A combination of Scanning Tunneling Microscopy/Spectroscopy and Density Functional Theory (DFT+U) is used to characterize excess electrons in TiO$_2$ rutile and anatase, two prototypical materials with identical chemical composition but different crystal lattices. In rutile, excess electrons can localize at any lattice Ti atom, forming a small polaron, which can easily hop to neighboring sites. In contrast, electrons in anatase prefer a free-carrier state, and can only be trapped near oxygen vacancies or form shallow donor states bound to Nb dopants. The present study conclusively explains the…
The hyperfine structure of the interstitial muonium (Mu) in rutile (TiO$_2$, weakly $n$-type) has been identified by means of a muon spin rotation technique. The angle-resolved hyperfine parameters exhibit a tetragonal anisotropy within the $ab$ plane and axial anisotropy with respect to the $\langle 001\rangle$ ($\hat{c}$) axis. This strongly suggests that the Mu is bound to O (forming an OH bond) at an off-center site within a channel along the $\hat{c}$ axis, while the unpaired Mu electron is localized around the neighboring Ti site. The hyperfine parameters are quantitatively explained by …
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 efficiency. 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 $μ_{\text…
Ultra-wide-band-gap (UWBG) semiconductors have tremendous potential to advance electronic devices as device performance improves superlinearly with increasing gap. Ambipolar doping, however, has been a major challenge for UWBG materials as dopant ionization energy and charge compensation generally increase with increasing band gap and significantly limit the semiconductor devices that can currently be realized. Using hybrid density functional theory, we demonstrate rutile germanium oxide (r-GeO2) to be an alternative UWBG (4.68 eV) material that can be ambipolarly doped. We identify SbGe, AsGe…
Ultrawide-band-gap (UWBG) semiconductors are promising for fast, compact, and energy-efficient power-electronics devices. Their wider band gaps result in higher breakdown electric fields that enable high-power switching with a lower energy loss. Yet, the leading UWBG semiconductors suffer from intrinsic materials limitations with regards to their doping asymmetry that impedes their adoption in CMOS technology. Improvements in the ambipolar doping of UWBG materials will enable a wider range of applications in power electronics as well as deep- UV optoelectronics. These advances can be accomplis…
Band alignment effects of anatase and rutile nanocrystals in TiO$_2$ powders lead to an electron hole separation, increasing the photo catalytic efficiency of these powders. While size effects and types of possible alignments have been extensively studied, the effect of interface geometries of bonded nanocrystal structures on the alignment is poorly understood. In order to allow conclusive studies of a vast variety of bonded systems in different orientations, we have developed a new density functional tight binding parameter set to properly describe quantum confinement in nanocrystals. Applyin…
A first-principles atomic orbital-based electronic structure method is used to investigate the low index surfaces of rutile Titanium Dioxide. The method is relatively cheap in computational terms, making it attractive for the study of oxide surfaces, many of which undergo large reconstructions, and may be governed by the presence of Oxygen vacancy defects. Calculated surface charge densities are presented for low-index surfaces of TiO$_2$, and the relation of these results to experimental STM images is discussed. Atomic resolution images at these surfaces tend to be produced at positive bias, …
The detailed analysis of the structural variations of three GeO$_2$ and SiO$_2$ polymorphs ($α$-quartz, $α$-cristobalite, and rutile) under compression and expansion pressure is reported. First-principles total-energy calculations reveal that the rutile structure is the most stable phase among the phases of GeO$_2$, while SiO$_2$ preferentially forms quartz. GeO$_4$ tetrahedras of quartz and cristobalite GeO$_2$ phases at the equilibrium volume are more significantly distorted than those of SiO$_2$. Moreover, in the case of quartz GeO$_2$ and cristobalite GeO$_2$, all O-Ge-O bond angles vary w…
The adsorption profiles and electronic structures of Pt$_n$ (n = 1--4) clusters on stoichiometric, reduced and reconstructed rutile TiO$_2$(110) surfaces were systematically studied using on site d-d Coulomb interaction corrected hybrid density functional theory calculations. The atomic structure of small Pt cluster adsorbates mainly depend on the stoichiometry of the corresponding titania support. The cluster shapes on the bulk terminated ideal surface look like their gas phase low energy structures. However, for instance, they get significantly distorted on the reduced surfaces with increasi…
