Muonium spectroscopy in ZnSe: Metastability and conversion R. C. Vilão,* H. V. Alberto, J. Piroto Duarte, J. M. Gil, A. Weidinger, and N. Ayres de Campos Physics Department, University of Coimbra, P-3004-516 Coimbra, Portugal R. L. Lichti Physics Department, Texas Tech University, Lubbock, Texas 79409-1051, USA K. H. Chow Department of Physics, University of Alberta, Edmonton, Canada T6G 2J1 S. F. J. Cox ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, United Kingdom and Condensed Matter Physics, University College London, WC1E 6BT, United Kingdom Received 7 July 2005; revised manuscript received 14 September 2005; published 14 December 2005 High-precision spectroscopic information is obtained on the muonium states in ZnSe by high-field transverse SR measurements. At low temperatures, two muonium states Mu I and Mu II are observed with isotropic hyperfine parameters of A I =3283.63±0.51 MHz and A II =3454.26±0.02 MHz 74% and 77% of the vacuum value, respectively. State I is thermally unstable and converts to state II at approximately 40 K. State II is stable up to 300 K, at least. We assign Mu II to the cation interstitial tetrahedral site and discuss the possibility that Mu I may correspond either to muonium at the same site but in the unrelaxed lattice or to the anion interstitial tetrahedral site. The temperature dependence of the hyperfine interaction was fitted with a local vibrational model giving an oscillator energy of approximately 8 meV. The amplitudes and the depolarization rates are measured over the entire temperature range and are discussed in the text. DOI: 10.1103/PhysRevB.72.235203 PACS numbers: 61.72.Vv, 71.55.Gs, 76.75.i I. INTRODUCTION Muonium, consisting of a positive muon and a bound electron, can be considered as a light isotope of hydrogen and can be used to study the properties of hydrogen-related defects in semiconductors. 1,2 Of note, a hydrogenic, i.e., ef- fective mass, donor state was located via muonium spectros- copy in CdS and ZnO 3,4 and has also been seen in several other compounds. 5–9 The shallow donor state in ZnO was subsequently confirmed by IR and electron paramagnetic resonance EPR/electron-nuclear double resonance EN- DOR studies on hydrogen in the same material. 10–12 In theoretical papers on this subject, 13,14 a systematic be- havior of hydrogen defect states over a larger class of semi- conductors was reported on. The main prediction is that the stability of the shallow hydrogen donor state depends on the bottom of the conduction band being below a fixed level of about 4.5 eV with respect to the vacuum. This distinguishing level appears to remain constant for a wide range of materi- als. ZnSe is, together with ZnO, a prominent representative of the wide band gap II-VI semiconductors, which are under intensive research for potential use in electronic and opto- electronic devices. 15 Unlike ZnO, ZnSe is an example of the second class of compounds, in which the effective mass do- nor does not occur but instead a deep-level muonium con- figuration is observed. This is consistent with the predicted amphoteric behavior of hydrogen in ZnSe. 16 The existence of this deep defect state in ZnSe is known from earlier experiments, 17,18 but the spectroscopic information is to date rather scarce because of the small amplitude of the signal. The earlier measurements were made only below about 50 K and did not show that the second atomiclike muonium state, observed in the current experiment, is also present at low temperatures. The current experiments were performed at high external magnetic field 7T where precise information on the fre- quencies and linewidths can be obtained. The primary infor- mation from the present study is that a second muonium state exists in ZnSe at low temperature. This state was not seen in the earlier experiments because its linewidth increases with decreasing field and becomes so large at low fields that the spin precession signal is unobservable. The hyperfine parameters of this second muonium center are very similar to those of the previously known state. Both have an isotropic hyperfine interaction that is a significant fraction of the vacuum value 74% and 77%. A difference is that the muonium state, which we label as Mu I , becomes unstable with increasing temperature and converts to Mu II at about 40 K. A detailed study of the muonium signals was performed in the temperature range between 2 and 300 K, with emphasis on the conversion region. II. EXPERIMENTAL DETAILS ZnSe single crystals obtained commercially from Alpha- Aesar and from Crystec were used. The crystals are nomi- nally undoped and cut on the main crystallographic orienta- tions 100, 110, and 111. The Alpha-Aesar sample was used in the temperature dependence study and the Crystec samples in the orientation dependence study. PHYSICAL REVIEW B 72, 235203 2005 1098-0121/2005/7223/2352036/$23.00 ©2005 The American Physical Society 235203-1