O. Kormann

μ-SR investigations in silicon

Abstract Results on the temperature dependence of the residual polarization of negative muons in silicon with phosphorus ( 4.5×10 18 , 2.3×10 15 , and 3.2×10 12 cm −3 ) and aluminium (2.4×1018 and 2×10 14 cm −3 ) impurities are presented. The muon spin rotation (μSR) experiments were carried out in a magnetic field of 0.2 T and in the temperature range 4.2–300 K. In all investigated samples a relaxation of the muon spin and a shift of the spin-precession frequency were observed. The frequency shift (relative to the room-temperature value) amounts to 7×10−3 at 15 K. In the sample with a high concentration of phosphorus impurity ( 4.5×10 18 cm −3 ) damped and undamped components of the muon spin polarization were observed at T K . Hyperfine interaction between the magnetic moments of the muon and that of the electron shell of the muonic atom (acceptor centre – μAl) is estimated on the basis of the muon spin precession frequency shift data. The temperature dependence of the spin-lattice relaxation rate of the magnetic moment of the shallow acceptor centre in silicon in the absence of external stress is determined for the first time. It is found that the relaxation rate is well approximated by the power function ν(T)=CTq, where the parameter q lies between 2 and 3.

Evidence for a novel muon species in crystalline silicon

Abstract Investigations of positive muons (μ+) in crystalline silicon employing the longitudinal field-quenching (LFQ) technique give strong evidence for the existence of a novel paramagnetic muon species with a small anisotropic hyperfine interaction. It adds to the list of known muon species, i.e., normal and anomalous muonium and the diamagnetic muon species. The signatures of the novel species are found in intrinsic but not in doped samples (dopant concentration about 1016 cm−3). The novel species is not formed promptly but results from a reaction in which normal muonium transforms into the novel species. The reaction rate constant at 10 K was found to be about 106 s−1. The hyperfine coupling of the novel paramagnetic muon species corresponds, after rescaling, to that of a hydrogen center termed 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. The LFQ data are tentatively interpreted in terms of trapping of normal muonium in vacancies that are created during the slowing-down of implanted muons close to the end of their stopping tracks.

µ−spin rotation study of the temperature-dependent relaxation rate of acceptor centers in silicon

Temperature-dependent remanent polarization of negative muons in a silicon crystal doped with phosphorus (3.2 × 1012, 2.3 × 1015, and 4.5 × 1018 cm−3) and aluminum (2 × 1014 and 2.4 × 1018 cm−3) was examined. Measurements were made over the temperature range 4–300 K in a magnetic field of 2000 G perpendicular to the muon spin. Temperature dependence of the relaxation rate was determined for the magnetic moment of a shallow Al acceptor center in a nondeformed silicon sample, and the hyperfine interaction constant was estimated for the interaction between the magnetic moments of muon and electron shell of the muonic mAl atom in silicon.

Magnetic moment relaxation of a shallow acceptor center in heavily doped silicon

Results of studying the temperature dependence of the residual polarization of negative muons in crystalline silicon with germanium (9×1019 cm−3) and boron (4.1×1018, 1.34×1019, and 4.9×1019 cm−3) impurities are presented. It is found that, similarly to n-and p-type silicon samples with impurity concentrations up to ∼1017 cm−3, the relaxation rate ν of the magnetic moment of a μAl acceptor in silicon with a high impurity concentration of germanium (9×1019 cm−3) depends on temperature as ν∼Tq, q≈3 at T=(5–30) K. An increase in the absolute value of the relaxation rate and a weakening of its temperature dependence are observed in samples of degenerate silicon in the given temperature range. Based on the experimental data obtained, the conclusion is made that the spin-exchange scattering of free charge carriers makes a significant contribution to the magnetic moment relaxation of a shallow acceptor center in degenerate silicon at T≲30 K. Estimates are obtained for the effective cross section of the spin-exchange scattering of holes (σh) and electrons (σe) from an Al acceptor center in Si: σh∼10−13 cm2 and σe∼8×10−15 cm2 at the acceptor (donor) impurity concentration na(nd)∼4×1018 cm−3.

Measurements of the magnetic moment of the negative muon in the bound state in various atoms

The magnetic moment of the negative muon in the 1s state was measured in carbon, oxygen, magnesium, silicon, sulfur, and zinc. The attained precision of measurements allowed the dependence of the relativistic correction to the magnetic moment of the bound muon on the charge of the nucleus to be verified.

The relaxation rate of the magnetic moment of a shallow acceptor center as a function of impurity concentration in silicon

A study is made into the temperature dependence of residual polarization of negative muons in crystalline silicon with the concentration of impurity of the n-and p-types ranging from 8.7×1013 to 4.1× 1018 cm−3. The measurements are performed in a magnetic field of 1000 G transverse to the muon spin, in the temperature range from 4.2 to 300 K. The form of the temperature dependence of the relaxation rate v of the magnetic moment of the μAl0 acceptor in silicon is determined. For a nondegenerate semiconductor, the relaxation rate depends on temperature as v ∝ Tq (q ≈ 3). A variation in the behavior of the temperature dependence and a multiple increase in the relaxation rate are observed in the range of impurity concentration in excess of 1018 cm−3. The importance of phonon scattering and spin-exchange scattering of free charge carriers by an acceptor from the standpoint of relaxation of the acceptor magnetic moment is discussed. The constant of hyperfine interaction in an acceptor center formed by an atom of aluminum in silicon is estimated for the first time: |Ahf (Al)/2π| ∼ 2.5×106s−1.

Radio-frequency μSR investigations on paramagnetic muonium centres in crystalline silicon

Abstract Radio frequency (RF) muon-spin resonance (RFμSR) experiments on the paramagnetic muonium centres MuT (muonium located at tetrahedral sites) and MuBC (bond-centred muonium), present in crystalline silicon, were carried out at applied magnetic fields B0 up to 6 mT and a constant irradiated frequency νRF of approximately 80 MHz. In the magnetic-field dependence of the RF asymmetry aRF, i.e. the RF induced change of the muon-decay asymmetry, two resonances were observed, one corresponding to MuT, the other to MuBC. Systematic RFμSR measurements were performed on intrinsic silicon single crystals grown by the Czochralski method (oxygen concentration 2×10 17 cm −3 ) and the float-zone method (oxygen concentration 15 cm −3 ). No MuT signal has been observed in transverse field (TF) μSR, below T=50 K in the Czochralski-grown sample whereas a MuT signal was found with RFμSR. This RFμSR result proves that the MuT species in this sample undergo dynamic processes, in which presumably oxygen is involved.

A novel time-differential μSR data-acquisition system

Abstract A versatile time-differential μSR data-acquisition set-up for quasi-continuous muon beams is described, which is based on a recently available NIM time-to-digital converter. The set-up is very compact, easy to use and, in contrast to the traditional hardware method, the full data selection is performed by software. The set-up proved to be reliable and extremely stable, currently it is regularly used at the Paul Scherrer Institut by several groups.

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.

Oxygen-related muon species in crystalline silicon

Abstract A new muonium centre, termed MuX, 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 (MuBC). The signature of MuX is only observable in samples containing oxygen in a concentration which is higher than that of the donors or acceptors. This indicates that MuX is related to an oxygen–muonium centre. In the present work we have carried out a study of the temperature behaviour of the MuX signal on a sample showing a very pronounced MuX signature. The observations clearly show that the MuX signal, on the one hand, and the MuBC signals, on the other, have an identical temperature dependence.