R. C. Vilão

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

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.

Shallow versus deep hydrogen states in ZnO and HgO

The muonium states mimicking interstitial hydrogen in ZnO and HgO are compared. Whereas in ZnO a theoretically predicted shallow donor state is confirmed, in HgO we find a considerably deeper state. The respective ionization temperatures are around 40 K and 150 K and the donor ionization energies are 19±1 and 136±3 meV, deduced from the temperature dependence of the µSR (muon spin-rotation) signal amplitudes. The µSR spectra provide a comprehensive characterization of the undissociated paramagnetic states: the hyperfine parameters, which measure the electron spin density on and near the muon, differ by a factor of ~30. These define a hydrogenic radius of 1.1 nm in ZnO but indicate a much more compact electronic wavefunction in HgO, more akin to those of Mu* and the AA9 centre in Si. These data should largely carry over to hydrogen as a guide to its electrical activity in these materials.

The first 25 years of semiconductor muonics at ISIS, modelling the electrical activity of hydrogen in inorganic semiconductors and high-κ dielectrics

Early muonium studies provided the very first atomistic pictures of interstitial hydrogen in semiconductors. By the time ISIS muons came on line, the main crystallographic sites, and the electronic structures for the neutral centres, were established in archetypal materials such as Si and GaAs. The results were quite unanticipated, and raised awareness of this deceptively simple defect system. This paper marks contributions to the subject made using ISIS muon beams, in the first 25 years of their operation since 1987. By this time, hydrogen was understood to be a significant and unavoidable impurity in all electronic grade material, and attention was turning to the interaction with charge carriers, revealing an equally unanticipated interplay of site and charge state. In particular, muonium spectroscopy now provides a model for hydrogen in dozens of materials where hydrogen itself is difficult or impossible to study directly, and is able to predict its effect on the electronic properties of new materials, such as those envisaged for optoeletronic or dielectric applications. Donor, acceptor and so-called pinning levels are known in a good many of these materials, revealing intriguing systematics and providing severe tests and challenges to current theory. Progress and prospects are summarized in this report, addressing the obvious questions such as ‘why, how and what next?’

Double-resonance determination of electron g-factors in muonium shallow-donor states

The discovery and significance of weakly bound muonium states with low hyperfine constants in a number of compound semiconductors of the II–VI and III–V (nitride) families are briefly reviewed. With ionization energies of several tens of meV, these imply that their hydrogen counterparts would act as shallow donors and that hydrogen could, either as an impurity or a deliberate dopant, be a source of electronic conductivity in the relevant materials. We examine whether, in their neutral undissociated states, the electron orbitals can be described in the effective-mass approximation and are correspondingly dilated, made up of conduction-band states. The best evidence that this is so comes from novel double-resonance measurements of the electron g-factors, devised for the ISIS pulsed muon source, and so far undertaken for ZnO, CdS, CdSe and CdTe. The respective values are |g| = 1.97, 1.86, 0.51 and 1.68; these results discount orbitally quenched compact states and are fully consistent with literature values for known shallow dopants in these compounds. They also illustrate the potential for µSR detection and characterization of such states in new electronic materials where hydrogen-induced conductivity is suspected or predicted.

Shallow donor versus deep acceptor state in II–VI semiconductor compounds

Abstract Information on the properties of the possible muonium states in II–VI semiconductor compounds is obtained in this study. In these materials, muonium may either form a shallow donor state, which is characterized by a small hyperfine interaction and a level-energy close to the conduction band, or an acceptor state, which corresponds to muonium at an interstitial site with a tightly bound electron and a hyperfine interaction close to that of free muonium. We show here that in CdS, CdSe and ZnO muonium preferentially forms a donor state whereas the acceptor state is preferred in ZnS and ZnSe. In CdTe both states are observed, indicating that the level energies are similar.

Probing the shallow-donor muonium wave function in ZnO and CdS via transferred hyperfine interactions

Abstract The assignment of muon spin rotation spectra to muonium counterparts of hydrogen shallow–donor states is reviewed in four II–VI widegap semiconductors, CdS, CdSe, CdTe and ZnO. The existence of extended electronic orbitals is argued from the muon–electron hyperfine parameters and supported by the new muon spin repolarization data for CdS and ZnO, characterizing the superhyperfine parameters on the sparse Cd and Zn dipolar nuclei. The possibility of a more tightly bound electron occupying a compact orbital is reasonably excluded in these materials, contrasting with the muonium state in HgO.

Muon diffusion and trapping in chalcopyrite semiconductors

The diffusion parameters of diamagnetic muons in chalcopyrites CuInSe2, CuInS2, CuInTe2, CuGaTe2 and (Ag0.25Cu0.75)InSe2 were obtained by mSRmethods. The variations among the different compositions were found to validate the anion-antibonding localization model. The application of a two-state model to the zero-field data revealed muon trapping by defects. The dipolar width at the trap and the number of jumps before trapping were determined. The Cu vacancy is identified as the trapping center in CuInSe2 and the energy depth of the trap has been determined. r 2002 Elsevier Science B.V. All rights reserved.

Defect levels and hyperfine constants of hydrogen in beryllium oxide from hybrid-functional calculations and muonium spectroscopy

Abstract The atomistic and electronic structures of isolated hydrogen states in BeO were studied by ab initio calculations and muonium spectroscopy (SR). Whereas standard density-functional theory with a semi-local GGA functional led to a detailed probing of all possible minimum-energy configurations of hydrogen further calculations with the hybrid HSE06 functional provided improved properties avoiding band-gap and self-interaction errors. Similarly to earlier findings for the other wide-gap alkaline-earth oxide, MgO, hydrogen in BeO is also predicted to be an amphoteric defect with the pinning level, E(), positioned in the mid-gap region. Both donor and acceptor levels were found too deep in the gap to allow for hydrogen to act as a source of free carriers. Whereas, hydrogen in its positively-charged state, , adopts exclusively hydroxide-bond OH configurations, and instead show a preference to occupy cage-like interstitial sites in the lattice. in particular displays a multitude of minimum-energy configurations: its lowest-energy ground state resembles closely a trapped-atom state with a nearly spherical spin-density profile. In contrast to the strongly ionic MgO, in BeO was further found to stabilise in additional higher-energy elongated-bond OH configurations whose existence should be traced to a partial covalent character of the Be–O bonding. Calculations of the proton-electron hyperfine coupling for all states showed that the ground-state interstitial configuration is dominated by an isotropic hyperfine interaction with a magnitude very close to the vacuum value, a finding corroborated by the SR-spectroscopy data.

High-field study of muonium states in HfO2 and ZrO2

We present high-transverse field measurements, as a function of temperature, in monoclinic ZrO2 and HfO2. In monoclinic zirconia and hafnia, a diamagnetic component had been previously reported in low-transverse-fields, but a significant fraction of the total muon polarization was missing in these experiments. We now characterize this missing fraction using the high-field capabilities at TRIUMF: a high relaxation component (above 100 μs−1 in monoclinic ZrO2, about 10 μs−1 in HfO2) is observed, which we relate to the formation of compact muonium in these materials. A model for the formation of muonium in these materials is presented.

Muonium states in Cu2ZnSnS4 solar cell material

We investigated bulk and thin-film samples of the quaternary p-type semiconductor Cu2ZnSnS4 (CZTS) by μSR, in order to characterize the existing muonium signals. We find that the majority of the implanted muons form a diamagnetic state broadened by an interaction with the Cu nuclear moments, which we interpret as Mu+ bound to sulphur. A paramagnetic fraction is also present at low temperatures and the ratio between the two muon charge states, Mu+ and Mu0, varies between 20 and 40% prior to the onset of muon diffusion, which occurs at around 150 K. The fraction of Mu0 is found to be sensitive to the defect content of the sample. The paramagnetic fraction has two different contributions and their origin is discussed and related with the muon role as a probe for charge carriers in the material.