Oxide muonics: II. Modelling the electrical activity of hydrogen in wide-gap and high-permittivity dielectrics

Abstract

Following the prediction and confirmation that interstitial hydrogen forms shallow donors in zinc oxide, inducing electronic conductivity, the question arises as to whether it could do so in other oxides, not least in those under consideration as thin-film insulators or high-permittivity gate dielectrics. We have screened a wide selection of binary oxides for this behaviour, therefore, using muonium as an accessible experimental model for hydrogen. New examples of the shallow-donor states that are required for n-type doping are inferred from hyperfine broadening or splitting of the muon spin rotation spectra. Electron effective masses are estimated (for several materials where they are not previously reported) although polaronic rather than hydrogenic models appear in some cases to be appropriate. Deep states are characterized by hyperfine decoupling methods, with new examples found of the neutral interstitial atom even in materials where hydrogen is predicted to have negative-U character, as well as a highly anisotropic deep-donor state assigned to a muonium-vacancy complex. Comprehensive data on the thermal stability of the various neutral states are given, with effective ionization temperatures ranging from 10 K for the shallow to over 1000 K for the deep states, and corresponding activation energies between tens of meV and several eV. A striking feature of the systematics, rationalized in a new model, is the preponderance of shallow states in materials with band-gaps less below 5 eV, atomic states above 7 eV, and their coexistence in the intervening threshold range, 5-7 eV.

Intrinsic point-defect equilibria in tetragonal ZrO2: Density functional theory analysis with finite-temperature effects

“…We also found out that oxygen interstitials exhibit negative-U behavior with a U value of -0.838 eV. Although the negative-U behavior indicates that the intermediate charge state is never stable thermodynamically, signatures of this banned charge state were observed in electron spin resonance spectra 59 in the context of studying hydrogen defects in wide band gap oxides. This was explained by metastability due to sufficient isolation of the charged defect.…”

“…This was explained by metastability due to sufficient isolation of the charged defect. 59 However, there is no transparent way to quantify the concentration of the defects that are in a metastable charge state and at any case it is expected that this concentration We turn now to compare our results of the formation energies with the values available in the literature. The detailed comparison is in the Supplemental Material.…”

Isolated hydrogen configurations in zirconia as seen by muon spin spectroscopy andab initiocalculations

“…Muonium spectroscopy has the particular advantage of corresponding to the high-dilution limit for the muonium impurity, which can thus quite generally be considered isolated and is affected only indirectly by other defects and impurities (including hydrogen impurities) through the overall Fermi energy. Extensive studies have thus been carried out to characterize the muonium centers formed in different semiconductors and oxides [12,13,[19][20][21] and the respective results for the muonium configurations compare well with those obtained with protons, for the very few cases allowing comparison [9,[22][23][24][25].…”

“…From a theory standpoint, hydrogen states in zirconia have been studied in the past by first-principles calculations; nonetheless with the exception of an earlier attempt by μSR [19] a detailed experimental confirmation and analysis is still largely lacking. More specifically, DFT-based calculations determined the type of hydrogen configurations and defect electrical levels for hydrogen in the monoclinic [26,27], the ideal cubic [28,29] and tetragonal [30], and the cubic yttria-stabilized [31] phases.…”

The new high field photoexcitation muon spectrometer at the ISIS pulsed neutron and muon source

“…In some circumstances implanted muons capture an electron to form a muonium (Mu = µ + + e − ), which is an analogue of hydrogen atom, and thus is an important probe of the behavior of hydrogen in various material systems such as semiconductors and insulators. [5][6][7][8] There are two types of muon sources with different time structures: quasi-continuous and pulsed. The continuous sources, such as PSI in Switzerland and TRIUMF in Canada, provide a quasi-continuous beam of muons with weak modulation at the RF frequency of the accelerator, typically 50 MHz.…”