Comments on the stability of charge states and locations for hydrogen and muonium in semiconductors

This paper brings together some current concepts concerning hydrogen states in semiconductors (illustrated with reference to silicon but quite generally applicable) and highlight certain findings of recent quantum electronic structure calculations for defect centres involving hydrogen and its isotopes. As regards muonium, the situation which held in the early μSR literature [1] is entirely reversed: the location and structure of Mu^* are now well established [2], whereas the precise location and the nature of the metastability of Mu′ have become open questions! In fact, neither state is stable in the presence of other electrically active impurities, which undoubtedly accounts for the difficulty in detecting paramagnetic protium. The implications for the interpretation of existing data, and for various possible future experiments, are examined.     

Theory of H sites in undoped crystalline semiconductors

The equilibrium sites of atomic hydrogen and muonium in both doped and undoped elemental semiconductors, as investigated by means of several theoretical methods and experimental techniques, are outlined. A particular emphasis is given to the undoped c-Si results. Two controversial points regarding the H and muonium equilibrium sites have been carefully discussed. The bond centered model has been favored with respect to the vacancy associated one. The T site quite reasonably results to be a metastable site and the most favored candidate for the normal muonium center.     

Theoretical aspects of hydrogen in crystalline semiconductors

Hydrogen in semiconductors displays behavior quite different from any other impurity, causing effects which can be either beneficial or detrimental to the electronic properties. Theoretical calculations have been able to elucidate many puzzling aspects of this behavior, by providing a microscopic picture of the interactions of hydrogen with the host lattice and with other impurities. I will critically review the available theoretical techniques, and address the following topics: microscopic structure of isolated interstitial hydrogen, as well as various hydrogen-impurity complexes; electronic structure, including the relative stability of different charge states; diffusion; and H motion around impurities.     

Large binding due to dispersive screening and bloch function interference in many-valley semiconductors

We derive here a generalization of the effective mass equation which includes the intervalley mixing for many-valley semiconductors. This equation is numerically solved with a model impurity potential for donors in silicon. The results show an extreme sensitivity to the short-range impurity potential and a shallow-deep instability. The combined effect of dispersive screening and many-valley interference gives a deep ground state. This seem to be in agreement with the experimental situation for hydrogen and muonium impurities, to which the chosen model potential applies.     

Muonium defect centers in semiconductors

After a brief summary of the properties of the muonium defect centers observed in the elemental group IV semiconductors, the status of studies of muonium centers in semiconductors at the time of the last μSR conference in 1983 will be compared with what is currently known. With the introduction of new experimental techniques, such as high-transverse-field μSR and level-crossing spectroscopy, many new results are or soon will be available on muonium centers. These, combined with new theoretical studies, should lead to rapidly increased insight into a subject which has been both puzzling and resistant to clarification.     

Muonium as an experimental model of hydrogen point defects in semiconductors

Hydrogen is an important defect in semiconductors which can significantly alter the electrical and optical properties of the host. Information on isolated interstitial hydrogen is virtually nonexistent. Fortunately, complementary data can be obtained by studying muonium. In this article, we describe recent experimental examples which illustrate how μSR can be used to investigate the electronic structure and dynamics of muonium in semiconductors and discuss the relevance of these measurements for isolated hydrogen.     

Electronic structure and dynamics of muonium in semiconductors

In this paper, a selection of recent results on muonium in semiconductors is presented. These are primarily taken from Si and GaAs and encompass the electronic structure of the diamagnetic centers, charge state cycling, spin‐exchange scattering and interconversion between muonium states. These experiments illustrate the power of μSR for investigating the behavior of muonium and, by analogy, the technologically relevant isolated hydrogen centers in semiconductors. This revised version was published online in August 2006 with corrections to the Cover Date.     

Temperature dependence of muon-decay positron channeling in semiconductors

Planar channeling data ofμ ⁺-decay positrons in various semiconductors are reported. Together with the extensive spectroscopic data supplied by transverse μSR, the location of the different states of the hydrogen pseudo-isotopeμ ⁺ e⁻ (muonium) can be identified by means of planar simulations. In high purity silicon as well as in gallium arsenide a thermally activated site transition is observed which can be assigned to a transition between different muonium states.     

Electronic structure, dynamics, and metastability of muonium in semiconductors

Muonium centers are light hydrogen-like centers formed when positive muons are stopped in crystalline semiconductors. Detailed information on the hyperfine structure, dynamics and metastability of muonium are obtained using a combination of muon spin rotation or relaxation, muon level-crossing resonance and related methods. The expected close similarity to hydrogen, especially with regard to electronic structure, is important since the equivalent information on isolated hydrogen is either less detailed or completely absent. There are also interesting differences between muonium and hydrogen. In particular muonium dynamics are expected to exhibit enhanced quantum mechanical effects since the muon has only 1/9th the proton mass. In this paper we review the current status of experiments.     

μSR in diamond

It has recently become possible to take advantage of the interesting properties of diamond, as the synthesis of diamond has reached a point that material with impurities in the low the part per billion range and residual strain in the order of ten nano-radians has been realized. Hyperfine interactions have played a key role in the study of diamond and the emergence of novel applications. This relates to the characterization of defects and the development of engineered few spin systems. A particular aspect of the defect studies is the elusive hydrogen defect. In μSR studies, muonium is considered a light isotope of hydrogen with very similar chemical properties, but with very interestingly different dynamical properties, due to its much lighter mass. It offers a unique opportunity to study the behaviour of hydrogen in diamond at very low concentrations. The studies have revealed details of the elementary muonium atom as well as a molecule involving muonium in the diamond lattice. The dynamics of the muonium, which include quantum diffusion and ionization have also been studied. This contribution reviews μSR in diamond in the context of diamond as a modern material hosting advanced applications.     

Hydrogen in semiconductors: The roles of μSR and theory

Hydrogen is an unavoidable impurity in all semiconductors. It interacts with intrinsic defects (from monovacancies to dislocations), with impurities (shallow dopants, deep centers, even electrically inactive impurities), with the crystal, and with otherH interstitials. These interactions profoundly affect the electrical and optical properties of the host. Conventional experimental techniques used to study the properties of hydrogen (EPR, IR or Raman spectroscopy, etc.) have provided information on a number ofH-related defects. Theory has played a major role in these studies, not only by confirming the models proposed on the basis of experimental data, but often by explaining the data altogether or predicting new features. So far,SR has provided fundamental information on isolated hydrogen-like species in many semiconductors. It would be wonderful if thespectroscopic signature of muonium-impurity pairs could be identified or ifquantitative information regarding the stability of the various charge states of muonium could be obtained.     

Influence of impurities on short range electron transport in GaAs

The influence of Cr impurities on muonium atom formation in GaAs has been studied using muon spin relaxation techniques with alternating electric fields. The results suggest that electron transport to and capture by the muon is suppressed by capture/scattering on intervening Cr centers. The length scale involved is estimated to be about 3x10(-6) cm. This offers an opportunity to study electron transport to positive centers in semiconductors on a microscopic scale.     

Powder Pattern Hyperfine Spectroscopy of Shallow- Donor Muonium Centres

The hyperfine spectroscopy of muonium in II–VI semiconductors is reviewed, suggesting that whereas hydrogen is a deep-level defect in ZnS, ZnSe and ZnTe, it constitutes a shallow donor in ZnO, CdS, CdSe and CdTe. Shallow and deep states coexist in CdTe. Using new data for ZnO, it is shown that the principal values of the muonium hyperfine tensor may be obtained with equal facility from measurements in longitudinal or in transverse magnetic field, and from samples that are polycrystalline powders or single crystals. Spin density on the central muon in the shallow states correlates with the electron binding energy or donor depth. This revised version was published online in September 2006 with corrections to the Cover Date.     

Electronic structure of bond-centred hydrogen and muonium in III–V compound semiconductors: Ab initio cluster calculations

The molecular orbital model for bond-centred hydrogen or muonium in the III–V compound semiconductors is developed with the help of ab initio cluster calculations. The influence of the loss of symmetry in going from the elemental (group IV) to the compound (III–V) materials on the electronic structure is studied. The equilibrium configurations, potential energy surfaces and electronic structures of hydrogen or muonium near the bond-centred site in GaAs, GaP and InP are calculated at the ab initio HF level in the clusters Ga₄As₄H₁₈ and the corresponding one for GaP and InP using a split-valence basis set and ab initio pseudopotentials for the core orbitals. First results of the calculations using a large Ga₂₂As₂₂H₄₂ cluster are discussed. Preliminary results for InP indicate that ionization of the bond-centre defect may be considerably easier than in the other III–V compounds. which would explain why μSR-signals corresponding to the neutral Mu^* state have not been detected in this material.     

Hydrogen defect-level pinning in semiconductors: the muonium equivalent

We have determined locations for the donor and acceptor levels of muonium in six semiconductor materials (Si, Ge, GaAs, GaP, ZnSe, and 6H-SiC) as a test of defect-level pinning for hydrogen. Within theoretical band alignments, our results indicate a common energy for the equilibrium charge-transition level Mu(+/-) to within experimental uncertainties. However, this is nearly 0.5 eV higher than the energy at which the equivalent level for hydrogen was predicted to be pinned. Corrections for zero-point energy account for only about 10% of this difference. We also report experimental results for the (negative-U) difference between donor and acceptor levels for Mu to be compared with calculated values for H impurities in the same materials.     

Muonium in semiconductors: Recent experimental developments

Recent experiments covering a range of problems including the nuclear hyperfine structure of bond-centered muonium in diamond and GaP, charge-cycling reactions of muonium in Si at high temperatures, muonium state dynamics in Si probed by RF-SR, and endohedral muonium in semiconducting C₆₀ compounds, are discussed. These examples show that as traditionalSR techniques are continually being refined and new methods are being developed,SR is becoming an increasingly powerful tool to investigate the behavior of muonium and the in many respects analogous, and technologically relevant, hydrogen centers in semiconductors.     

Effects of electronically neutral impurities on muonium in germanium

Low-temperature measurements of muonium parameters in various germanium crystals have been performed. We have measured crystals with different levels of neutral impurities, with and without dislocations, and with different annealing histories. The most striking result is the apparent trapping of Mu by silicon impurities in germanium.     

Hydrogen-mediated spin-spin interaction in ZnCoO

The effect of hydrogen impurities on the electronic and magnetic properties of ZnO-based diluted magnetic semiconductors is examined through first-principles pseudopotential calculations. We suggest that interstitial H can mediate a strong short-ranged ferromagnetic spin-spin interaction between neighboring magnetic impurities through the formation of a bridge bond. Results based on first-principles total-energy calculations and Monte Carlo simulations indicate that such H-mediated spin-spin interactions can lead to high temperature ferromagnetism.