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?'

Using the lessons gained from implementing the St Vincent Declaration for Diabetes (1990), this declaration details general goals, 5 years targets, performance and outcome measures for the detection and management of early psychosis. Its successful implementation requires integrated partnerships working locally, nationally and internationally to: educate and raise awareness about psychosis; plan, deliver and audit care; promote recovery and social inclusion; promote and apply research. General goals: timely access to evidence-based interventions sensitive to age, gender and culture; a ‘recovery’ paradigm; eliminating stigma and ensuring rights to citizenship and social inclusion. Five-year targets: a workforce (resourced for skills, leadership and manpower) which delivers comprehensive EI programmes; greater consumer independance, self sufficiency and socialequity; greater consumer service satisfaction; raised awareness among the general population and care professionals; developed centres of excellence for research and training performance and outcome measures which: monitor: DUP, relapse rates, legal detention use, police involvement, suicide/untoward death; audit: pathways into care personal, social, educational and vocational outcomes consumer and carer satisfaction.

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.

In a survey of muonium spectroscopy in some 30 binary oxides, we have discovered 12 new examples of shallow-donor states of the type already known in ZnO, with hints of several others. Their occurrence shows a striking correlation with band-gap. The shallow states are favoured when the gap is less than about 5eV, whereas only deep atomic states (possibly the neutral states of deep acceptors) occur when the gap is greater than 7eV. Most remarkably, there appears a coexistence of deep and shallow states in the intervening region, 5–7eV, that will require substantial revision of current theoretical treatments. A parameterized model is proposed to aid understanding of the systematics. Our survey illustrates the potential of the methodology, which we term muonics, to model the electronic structure and electrical activity of hydrogen impurity in new electronic materials.

A new survey of muonium states brings the total of binary non-magnetic oxides studied to 30, with normal muonium—the interstitially trapped atomic state—found in 15 of these. The number of shallow-donor states of the type known in ZnO now also totals 15, but there are hints of several others. Tantalizingly, the shallow-donor and deep-atomic states are found to coexist in several of the candidate high-permittivity dielectrics. Highly anisotropic states, resembling anomalous muonium in semiconductors and including examples of muonium trapped at oxygen vacancies, complete a spectrum of hyperfine parameters covering five powers of ten. Effective ionization temperatures range from 10 K for shallow to over 1000 K for deep states, with corresponding activation energies between several meV and several eV. The oxide band gap emerges as a parameter controlling the systematics of the deep-to-shallow transition for muonium and, by inference, monatomic hydrogen.

The diffusion dynamics of the neutral muonium state found in HgO is addressed in this work. We propose a hopping model for the diffusion, and use it to analyse time-domain mu SR data. It is found that the diffusion is an incoherent quantum process, with an activation energy of 5.2(2)meV. The analysis also points to the anti-bonding site as the best-suited candidate for the muon's localisation. (c) 2005 Elsevier B.V. All rights reserved.

The theoretical suggestion that hydrogen might form shallow donor states in zinc oxide - and hence account for the n-type conductivity normally found in undoped samples - has been confirmed by experimental studies using mounium, a light pseudo-isotope of hydrogen. Characteristic frequencies in muon spin rotation experiments yield a hyperfine constant that is similar to 10(4) times smaller than that of vacuum-state muonium, indicating an extended orbital and a shallow centre. Temperature-dependence studies yield an ionization energy of about 30 meV. Band-offset diagrams and measurements on other semiconductors suggest that hydrogen forms shallow donor centres when the electron affinity of the host material is more than about 4 eV; otherwise the hydrogen level lies deep in the energy gap.

The nature of the neutral, monatomic states of interstitial hydrogen in a wide range of oxides, both semiconducting and dielectric, has been surveyed experimentally via μSR spectroscopy of their muonium counterparts. The states fall into three categories: shallow donor, deep donor and trapped atom — these latter probably the neutral state of deep acceptors. New examples are found in all three categories, giving an emerging picture of the systematics, for comparison with current theoretical models of the electrical activity of hydrogen impurity.

Muon spin relaxation measurements in the III–V nitrides have provided data on the motion of each of the three charge‐states of the muonium ‘isotope’ of hydrogen. Motional parameters are established for Mu+ in InN, Mu0 in AlN, and Mu− in GaN, with qualitative results in the other materials. Both Mu+ and Mu0 show tunneling below 300 K and thermal diffusion at higher temperatures leading to interactions with other impurities or extended defects.

Muon spin depolarization measurements in a zero applied magnetic field confirm the existence of a shallow muonium (Mu) donor in InN, consistent with earlier transverse-field muon spin precession results. The zero-field data imply two Mu+ centres in InN, similar to those associated with channel and cage sites in GaN. The zero-field results provide strong evidence that the shallow Mu0 ionizes to Mu+ at its lowest-energy location. We argue that data on Mu in the III–V nitrides, taken in total, support the assignment of a wurtzite channel location to the shallow hydrogen donor in InN. The present results yield 15.4 ± 2 meV below the conduction band edge for the donor level depth.

Sites and dynamics obtained for muonium (Mu) defect centres provide a good experimental model for the behaviour of the equivalent hydrogen impurities. We discuss the dynamic properties of Mu centres in the III–V nitrides focusing on features common to the three materials. Muon spin depolarization data in zero magnetic field provide motional dynamics for the Mu+ and Mu− charge states and field dependent longitudinal relaxation rates probe motion of Mu0 centres. The data also show dynamics associated with metastable locations, either intrinsic to the wurtzite structure or defect related, including trapping and release transitions. A general picture of the behaviour of H in the III–V nitrides is developed from these measurements for comparison to theoretical results.

The nature of the electron states associated with hydrogen in InN has been inferred by studying the behavior of positive muons, which mimic protons when implanted into semiconductors. The muons capture electrons below 60 K, forming paramagnetic centers with a binding energy of about 12 meV. Together with an exceedingly small muon-electron hyperfine constant indicative of a highly delocalized electron wave function, the results confirm the recently predicted shallow-donor properties of hydrogen in InN.

Zero-field muon spin depolarization measurements on heavily Zn-doped p-type GaP reveal several different states related to the Mu+ defect center. We find that the MuBC+ ground state becomes mobile near 200K. At slightly higher temperature Mu+ motion involves a second site, suggested to be TP, which is relatively long lived up to 270K. Zn–Mu complexes are formed at both low and high temperatures by reaction of Zn with a mobile Mu center, MuT0 below 100K and MuT+ above 400K. The high-temperature complex dissociates above 700K with a characteristic energy of 1.3eV.

Hydrogenated amorphous GaP films have been prepared by reactive rf sputtering. Infrared spectroscopy, optical transmission and reflection, photothermal deflection spectroscopy and dc conductivity have been studied to investigate the local bonding configurations, the optical absorption edges and the temperature dependence of the conductivity as a function of hydrogen content. The results are discussed and compared with the effects of hydrogenation on amorphous Si.

The nature of hydrogen defect centres in II-VI semiconductors has been inferred by studying implanted positive muons, used to mimic interstitial protons. Neutral paramagnetic centres formed by electron capture fall into three distinct categories. CdS, CdSe, CdTe and ZnO show shallow-donor states, implying that hydrogen may act as an n-type dopant in these materials. ZnS, ZnSe, MgO, BeO and SrO show the more normal trapped-atom states, whilst HgO exhibits a state intermediate between the two extremes, reminiscent of the deep-donor hydrogen state in Si. Preliminary indications of shallow states in CdO and HgS and of a deep state in ZnTe remain to be clarified. In a search for a predictive model and in the light of recent theoretical notions of a common hydrogen pinning level, the systematics of the shallow-to-deep instability are correlated with the depths of the conduction-band minima.

This paper reviews recent work on the properties of hydrogen defect centres in two group III nitrides, AlN and GaN, and relevant studies by µSR spectroscopy, i.e. muon spin rotation, relaxation and resonance. We highlight, especially, results obtained by a form of nuclear quadrupole resonance. Implanted positive muons are used to mimic and model the behaviour of interstitial protons. The resultant defect centres exhibit both metastability and bistability. In AlN, they remain as positive ions but partition themselves between a highly mobile species and one that is trapped and immobilized to temperatures as high as 800 K in cage-like sites adjacent to nitrogen. The barrier to escape from the cage is 0.86 eV. In n-type GaN, the cage-site positive ions are stable only up to 200 K; above this temperature they capture electrons to convert to negatively charged centres, analogues of hydride ions, relocating to sites antibonding to gallium. These latter escape from the cage sites around 600 K with an activation energy of 1.5 eV to join more mobile negative ions diffusing via channel sites with an activation energy of 0.65 eV. Data on the neutral paramagnetic centre suggest that hydrogen can act as a shallow-donor in at least one other member of this family of materials, namely InN.

Experimental data on shallow donor muonium states in the II-VI semiconductor compounds CdS, CdSe, CdTe, and ZnO are presented. These are characterized by very weak hyperfine interactions amounting to approximately 10(-4) of the free-atom value, and by donor levels with binding energies comparable to those calculated on the hydrogenic model of the shallow centers. The data are discussed in terms of a model generalizing the knowledge of muonium and hydrogen states in tetrahedrally bonded semiconductors. Within this model the shallow muonium state is attributed to muonium bound to the anion of the II-VI compound and located at the antibonding site or close to it in the bond direction.

Extended X-ray absorption fine structure (EXAFS) measurements have been made on thin films of a-GexS1-x with 0.2 < x < 1. The results show that the Ge-Ge, Ge-Se and Se-Se bond lengths have values 2.45, 2.37 and 2.32 (all +/-0.03) A, respectively and are independent of film composition. The partial atomic coordination numbers of Ge and Se suggest that the films have a predominantly chemically ordered 4-2 coordinated covalent bond network structure throughout the whole composition range, as found by others for Ge-Se bulk glasses with x < 0.4. EXAFS data for a-GexSey Zn-z films show that the lengths of Ge-Zn and Se-Zn bonds are 2.57 +/- 0.05 and 2.44 +/- 0.03 Angstrom, independent of the composition. Results on the atomic coordinations reveal that Zn is fourfold coordinated and preferentially bonds to Se atoms. (C) 2002 Elsevier Science B.V. All rights reserved.