Muonium in Silicon Germanium Alloys

We report observations of muonium defect centers in bulk, single crystalline Silicon Germanium alloys. Analysis of both bond-centered and interstitial T-site muonium gives a test for predictions of isolated hydrogen defect levels in Si$_{1-x}$Ge$_{x}$ alloys across the full alloy composition range. Temperature dependent amplitudes for neutral Mu$_{BC}$ and Mu$_{T}$ from high transverse field muon spin rotation (HTF-$\mu $SR) measurements yield ionization energies for these muonium centers. The hyperfine parameter for Mu$_{BC}$ varies linearly with alloy composition, whereas for Mu$_{T}$ it varies non-linearly, perhaps due to faster motion among germanium T-sites compared to silicon. Our HTF-$\mu $SR analysis of Si$_{0.16}$Ge$_{0.84}$ shows that the diamagnetic muonium species grows around 130 K with an activation energy of 102(1) meV. Similar data for Si$_{0.10}$Ge$_{0.90}$ give an activation energy of 67(3) meV. Both HTF-$\mu $SR and RF resonance results show two distinct Mu$_{T}$ signals for x $\ge $ 0.84. These states are tentatively assigned to T-sites with all Ge neighbors versus those with a Si neighbor.