Acceptor level of interstitial muonium in ZnSe and ZnS

Features observed below 300 K in muon-spin rotation and resonance measurements on low-concentration n-type ZnSe and ZnS samples are interpreted as electron capture by muonium to form Mu^-(μ⁺e^-e^-), the analog of negatively charged hydrogen H^-, and subsequent emission of an electron from Mu^- to the conduction band. In this model, the emission step yields the binding energy for the second electron to place the muonium acceptor levels at 0.10(2) and 0.33(5) eV below the conduction-band edge for ZnSe and ZnS, respectively. An approximate constancy of the muonium electron affinity for Zn and Cd semiconductor compounds is suggested and the consequences for the observed muonium states are discussed.