Muonium Centers Research Articles

Hyperfine Parameters for Muonium in Copper (I), Silver (I) and Cadmium Oxides

Muonium centres in Cu2O, Ag2O and CdO show hyperfine parameters spanning four orders of magnitude. They exemplify the three different categories of hydrogen defect centre in semiconducting and dielectric solids, with very different electronic structure and electrical activity, namely quasi-atomic (possibly deep acceptor), deep donor and shallow donor.

Formation and dynamics of muonium centres in semiconductors—a new approach

Various processes of muonium atom formation in semiconductorsvia electron capture by a positive muon have been studied usingμSR techniques, including those with applied electric field. Experiments in GaAs,GaP and CdS suggest that the electron is initially captured into a highlyexcited state, from which the cascade down to the muonium ground state goesthrough an intermediate weakly bound state determined by the electron effectivemass and the dielectric constant of the host. The electronic structure of thisweakly bound state is shown to be hydrogenic. The nature of the final (on theμSR timescale) muonium state depends on the energy releasing mechanisms in the cascadeprocess. We suggest that muonium dynamics in semiconductors (including the effects ofelectric and magnetic fields and temperature) reflect the electron dynamics in weaklybound muonium state(s) in which the electron is delocalized over distances of about 100 Å.

Charge dynamics of muonium centers in Si revealed by photoinduced muon spin relaxation

Muonium dynamics in crystalline Si has been studied in both p-type and n-type samples using the muon spin-relaxation technique combined with photoexcitation. Controlling the excess carrier density in this way has revealed the dynamics of charge-exchange cycles and transfer processes involving ${\mathrm{Mu}}_{\mathrm{BC}}^{0/+}$ and ${\mathrm{Mu}}_{\mathrm{T}}^{\ensuremath{-}/0}.$ In particular, photoexcitation has provided access to processes involving hole capture not attained by conventional experimental techniques. As a consequence, strong evidence was found for the delayed formation of electrically inactive muonium states in both types of Si, suggesting a process of diffusion-limited direct interaction of muonium with dopant impurities (i.e., muonium passivation).

Structure of isolated positively charged muonium/hydrogen in GaAs

We have investigated the structure of isolated positively charged muonium centers (Mu+), as an analog for the interstitial hydrogen ion, in heavily doped p-type crystalline GaAs. At temperatures below 200K, Mu+ is static on the timescale of microseconds. In this regime, detailed information on the structure of Mu+ can be obtained by probing the dipolar and muon-induced quadrupolar interactions of Mu+ and the neighboring Ga and As host nuclei. These studies establish that isolated Mu+ is located between a Ga–As bond. Furthermore, the distances between Mu+ and the nearest-neighbor host atoms can be estimated. These results are compared to existing theoretical calculations on the structure of muonium/hydrogen in GaAs.

Frequency shifts and local spin susceptibility of muonium in heavily-doped Si and GaAs

We report high transverse field μSR measurements in heavily doped p-type Si as well as p-type and n-type GaAs. In all these samples, the precession frequencies due to diamagnetic Mu + and Mu − are within ≈10 ppm of the Larmor frequency of the bare muon. In contrast to heavily doped n-type Si, no metastable neutral muonium centers are observed.

Short range electron transport in GaAs

We report direct evidence that the anomalous muonium (Mu BC 0) center in GaAs is formed via transport to the muon of excess electrons that are initially more than 3×10 −6 cm away.

First-principles calculation of electronic structures and hyperfine interactions for muonium centers in ZnO

The first-principles Hartree–Fock cluster method is applied to investigate the possible positions and associated hyperfine properties of the muonium centers in the II–VI semiconductor ZnO. Two clusters are used to simulate the local environments around the anti-bonding (AB) ∥ and bond-center (BC) ∥ muonium with axial symmetry along the c-axis, to try to understand the natures of the two centers whose axially symmetric hyperfine tensors have been observed experimentally. Our results show that the muonium center at the anti-bonding site close to oxygen (AB O) ∥ is more stable and its charge state is +0.44, which indicates a donor state in the semiconductor, but the associated hyperfine parameters are only in reasonable order of magnitude agreement with the experiments. The results for the (BC) ∥ center show a smaller binding energy than for the (AB O) ∥ center, and hyperfine parameters three orders of magnitude larger than the experiment but still an order of magnitude smaller than for free muonium.

Muonium in semiconductors: new discoveries and new questions

Muonium centres in semiconductors, especially those newly reported, show widely different hyperfine constants and binding energies. Three reasonably distinct categories can be discerned, with very different implications for the electrical activity of hydrogen impurity or the use of hydrogen in the control of conductivity. The factors which might influence this variation in behaviour are considered; methods of determining the spatial extent of the electronic wave function are discussed, and a variety of new experiments proposed. This paper is dedicated to the memory of MCR Symons (1925–2002).

Electronic structure of the muonium center as a shallow donor in ZnO.

The electronic structure and the location of muonium centers (Mu) in single-crystalline ZnO were determined for the first time. Two species of Mu centers with extremely small hyperfine parameters have been observed below 40 K. Both Mu centers have an axial-symmetric hyperfine structure along with a <0001> axis, indicating that they are located at the antibonding (AB(O, parallel )) and bond-center (BC( parallel )) sites. It is inferred from their small ionization energy ( approximately 6 and 50 meV) and hyperfine parameters ( approximately 10(-4) times the vacuum value) that these centers behave as shallow donors, strongly suggesting that hydrogen is one of the primary origins of n type conductivity in as-grown ZnO.

Shallow donor state due to nitrogen-hydrogen complex in diamond.

Based on an ab initio calculation, we propose a possible shallowing of a nitrogen (N) donor in diamond, in contrast to the traditional thinking that it is very deep. A complex defect of N and hydrogen (H), N-H-N, should be much shallower than an isolated N donor. A qualitative scenario for formation of the N-H-N defects is presented. The existence of this complex is strongly suggested by a recent discovery of a new muonium center in N-rich diamond.

Positively charged muonium centers in aluminum and gallium nitrides

The Mu + defect centers in AlN and GaN are studied using muon nuclear-quadrupole level-crossing resonance (QLCR) and zero-field muon spin depolarization measurements. The dominant Mu + center is highly mobile, but QLCR spectra identify a second static Mu + state at AB ||(N) sites in both nitrides. This state becomes mobile above 800 K in AlN, but converts to a Mu − above 200 K in GaN. Zero-field data characterize local fields and motional properties for this state in AlN. Above 1100 K the mobile Mu + in AlN shows strong interactions and traps at another defect.

Recent Progress in the Study on Muonium Centers in Semiconductors

The electronic structures and the locations of muonum centers (Mu) were investigated in single crystalline ZnO. Mu centers with extremely small hyperfine parameters have been observed below 40 K. It is inferred from their small ionization energy and hyperfine parameters that these centers behave as shallow donors, strongly suggesting that hydrogen is one of the primary origins of n type conductivity in as-grown ZnO.

Oxygen-related muon species in crystalline silicon

A new muonium centre, termed Mu X , with anisotropic hyperfine interaction has been discovered recently in crystalline silicon in transverse-field muon spin rotation experiments (Schefzik et al., Philos. Mag. B 79 (1999) 1561). Its hyperfine tensor is very close to that of bond centred muonium (Mu BC). The signature of Mu X is only observable in samples containing oxygen in a concentration which is higher than that of the donors or acceptors. This indicates that Mu X is related to an oxygen–muonium centre. In the present work we have carried out a study of the temperature behaviour of the Mu X signal on a sample showing a very pronounced Mu X signature. The observations clearly show that the Mu X signal, on the one hand, and the Mu BC signals, on the other, have an identical temperature dependence.

A vacancy-related muon species in crystalline silicon

A new muonium centre, termed Mu V, with weak hyperfine interaction has been discovered recently in crystalline silicon in longitudinal field quenching μSR (LFQ) experiments (Schefzik et al., Solid State Commun. 107 (1998) 395). The signatures of this species were found in intrinsic, but not in doped samples (dopant concentration larger than 10 16 cm −3 ). The centre is not formed promptly but results from a reaction in which normal muonium transforms into the novel species. Since from LFQ experiments one can obtain only a rough estimate of the hyperfine tensor of Mu V it is now determined from zero-field (ZF) μSR experiments. The hyperfine interaction is found to be axially symmetric around the 〈1 1 0〉 crystallographic axis with small hyperfine parameters. Properly rescaled it essentially agrees with the hyperfine tensor of the hydrogen centre VH, which has been discovered recently by Bech Nielsen et al. (Phys. Rev. Lett. 79 (1997) 1507) and which has been attributed to hydrogen trapped in vacancies. Accordingly Mu V is interpreted as muonium trapped in a vacancy, in agreement with the interpretation given on the basis of the earlier LFQ data.

First-principles theoretical investigation of neutral vacancy-associated muonium center in silicon

To understand the nature of the recently discovered vacancy-associated center Mu V in silicon and its H counterpart V H, we have carried out a first-principles Hartree–Fock cluster investigation mainly on the cluster Si 4H 12Mu involving the muonium and the four silicon neighbors nearest to the vacancy in a neutral vacancy-associated environment, including lattice relaxation effects. Our results demonstrate that the neutral vacancy-associated model is able to explain the non-axial symmetry of the Mu V center through Jahn–Teller distortion and provides reasonable quantitative agreement with experimental hyperfine data.

Shallow-level muonium centre in CdS

A new type of muonium defect centre has been observed in undoped CdS at low temperatures ( T<20 K). The hyperfine parameters are A ∥=335 (7) kHz and A ⊥=199 (6) kHz (approximately 10 −4 of the vacuum value) with the symmetry axis along the Cd–S bond direction. The disappearance of the centre at around 20 K is in agreement with the binding energy of a shallow centre. The extremely weak hyperfine interaction implies that the electron is only weakly bound to the muon and is distributed over a large complex of atoms in the neighborhood of the muon resembling the dilated hydrogen-like wave functions of shallow centres in a dielectric medium. Comparing the hyperfine interaction with that of free muonium, a Bohr radius of a d=26 a 0=1.4 nm is calculated for this defect, with a 0 being the Bohr radius for the free muonium.

Muon Korringa relaxation

Significant muon spin-lattice relaxation is found in a number of non-magnetic semimetals and metals. Measured in longitudinal magnetic field, the relaxation rate is independent of field up to several kilogauss and generally increases monotonically with temperature. This suggests a form of Korringa relaxation, originating in the hyperfine interaction between the implanted muons and the conduction electrons. Bearing in mind that NMR data on Korringa relaxation refers chiefly to the host nuclei, the muon offers a probe of conduction–electron encounter at an interstitial site, linking the topic to the nature of defect screening in metals, to relaxation by spin-density fluctuations in magnetic materials and to spin- and charge-exchange on paramagnetic muonium centres in semiconductors. Data are presented for C (graphite), Bi, Pb and Cd and compared with the Korringa predictions using known values of the muon Knight shift. Control experiments are described on Zn and Cu, both pure and deliberately doped with magnetic impurity. For graphite, an interpretation is given in terms of charge-exchange on a molecular radical formed by the chemical reaction of interstitial muonium.

Negatively charged muonium states in gallium nitride

Muon nuclear-quadrupole level-crossing resonances (QLCR) and zero-field muon spin depolarization functions are identified for negatively charged muonium (Mu−) centers in wurtzite-structured GaN.The QLCR spectra imply that Mu− occupies interstitial locations next to Ga atoms, consistent with the two inequivalent Ga anti-bonding sites. Mu− related depolarization shows an increase in average dipolar fields near 200K and indicates motion above 500K. Transitions suggested by relaxation features are briefly discussed.

Oxygen-related anisotropic muonium centres in crystalline silicon

Abstract A new muonium centre, termed Mux, with anisotropic hyperfine interaction has been discovered in crystalline silicon in transverse-field muon spin rotation experiments. Its hyperfine tensor is very close to that of bond centred muonium (MuBC). The new muonium centre has been observed only in some of the many samples investigated. A correlation between dopant and oxygen concentration in the investigated samples and the observation of Mux has been found, which indicates that Mux might be related to a oxygen-muoniuni centre. According to the hyperfine tensor of Mux the new muonium species is attributed to an oxygen-MuBC complex. The Mux signal is affected by applied electrostatic fields in contrast with the signal of all other known muon species.

Muonium Formation via Electron Transport in Silicon

We report muon spin rotation measurements in $n$-type silicon from 10 to 310 K in electric fields up to 5 kV/cm. These experiments reveal the nature of the partition of neutral muonium centers into normal muonium $({\mathrm{Mu}}_{T}^{0})$ and anomalous muonium $({\mathrm{Mu}}_{\mathrm{BC}}^{0})$: Formation of ${\mathrm{Mu}}_{T}^{0}$ is consistent with epithermal processes prior to thermalization, while ${\mathrm{Mu}}_{\mathrm{BC}}^{0}$ is formed via electron transport to a thermalized positive muon. The spatial distribution of the excess electrons created in the ${\ensuremath{\mu}}^{+}$ ionization track is shown to be anisotropic with respect to the muon.