VOLUME 86, NUMBER 12 PHYSICAL REVIEW LETTERS 19 MARCH 2001 Experimental Confirmation of the Predicted Shallow Donor Hydrogen State in Zinc Oxide S. F. J. Cox, 1,2 E. A. Davis, 3 S. P. Cottrell, 1 P. J. C. King, 1 J. S. Lord, 1 J. M. Gil, 4 H. V. Alberto, 4 R. C. Vilão, 4 J. Piroto Duarte, 4 N. Ayres de Campos, 4 A. Weidinger, 5 R. L. Lichti, 6 and S. J. C. Irvine 7 1 ISIS Facility, Rutherford Appleton Laboratory, Chilton OX11 0QX, United Kingdom 2 Department of Physics and Astronomy, University College London, London WCE 6BT, United Kingdom 3 Department of Physics and Astronomy, University of Leicester, Leicester LEI 7RH, United Kingdom 4 Physics Department, University of Coimbra, P-3004-516 Coimbra, Portugal 5 Hahn-Meitner Institut Berlin, Glienicker Strasse 100, D-14109 Berlin, Germany 6 Physics Department, Texas Tech University, Lubbock, Texas 79409-1051 7 Chemistry Department, University of Wales, Bangor, Gwynedd LL57 2UW, United Kingdom (Received 10 October 2000) We confirm the recent prediction that interstitial protium may act as a shallow donor in zinc oxide, by direct spectroscopic observation of its muonium counterpart. On implantation into ZnO, positive muons — chemically analogous to protons in this context — form paramagnetic centers below about 40 K. The muon-electron contact hyperfine interaction, as well as the temperature and activation energy for ionization, imply a shallow level. Similar results for the cadmium chalcogenides suggest that such shallow donor states are generic to the II-VI compounds. The donor level depths should serve as a guide for the electrical activity of interstitial hydrogen. DOI: 10.1103/PhysRevLett.86.2601 PACS numbers: 61.72.Vv, 71.55.Gs, 76.75.+i This Letter is in response to the very recent theoretical result by Van de Walle [1], that interstitial hydrogen can provide a donor level just below the conduction band in ZnO and therefore act as a source of n-type conductivity in this material. We have detected and characterized the muonium coun- terpart of this state, adding to the theoretical predictions precise values of ionization temperature, donor level, and spin density at the donor site in the paramagnetic (undis- sociated) state. We also report the systematics of these parameters among the cadmium chalcogenides, further to our first observation of such a state in CdS [2]. This com- bination of experimental and theoretical evidence for a particular compound, together with data for three others, establishes the general importance of the shallow donor state in the II-VI compounds and raises the question of how widespread such behavior of hydrogen and its iso- topes might be among other wide-gap semiconductors. Isolated hydrogen atoms commonly form deep-level defects in semiconductors, with donor and acceptor states involving different interstitial locations. The paramagnetic neutral centers are especially difficult to study spectro- scopically for two reasons. First, hydrogen defects typi- cally constitute a negative-U system, so that the neutral states are only metastable. Calculations for another wide-gap semiconductor, GaN [3], illustrate this behavior and may be compared with those for ZnO [1]. Second, hydrogen is so mobile and reactive that it quickly pairs with and passivates other defects or dopants, removing their electrically active levels from the energy gap. To obtain an atomistic picture of the behavior of the isolated centers, studies of the analogous states of muonium are at a considerable advantage [4,5]. In these experiments, muonium is treated as a light isotope of hydrogen: Mu  m 1 e 2 , with m Mu m H  19. In their vacuum states, Mu and H have the same Bohr radius and binding energy to within a fraction of a percent. With due regard for differences in zero-point energy when constrained interstitially or chemically bound, muonium provides a good model for hydrogen, adopting the same sites and an essentially identical local electronic structure. It may be detected and characterized readily by the muon spin rotation (mSR) technique, thanks to the special properties of muon production and decay, which give this spectroscopy a particulary high sensitivity per spin. Following muon implantation in the sample, the evolution of spin polarization is displayed for up to 5–10 muon lifetimes (t m  2.2 ms) — usually before the muonium encounters or reacts with other defects. Prior to our own studies of CdS [2], muonium centers in semiconductors had all been found to have energy levels deep in the energy gap, with tightly localized electron wave functions. Thus in Si, for which the correspondence be- tween Mu and H is best documented [6,7], muonium pro- vides a donor level at about 200 meV below the conduction band, defined by its ionization energy at the bond-center site [6]. The negative-U issue is not fully resolved but the acceptor level, involving a change to the tetrahedral cage site, is believed to lie deeper still [7]. A similar metastabil- ity of neutral muonium and bistability of its charged states is seen in compound semiconductors such as GaAs [5]. According to Van de Walle’s calculations for H in ZnO [1], interstitial protons could be sited either antibonding to oxygen in the wurtzite-type lattice or roughly midway between adjacent O and Zn atoms. The former may be imagined as the hydroxyl site which is common in oxides and the latter is analogous to the bond-center site which is established in the more covalent semiconductors with 0031-9007 01  86(12)  2601(4)$15.00 © 2001 The American Physical Society 2601