Charge-state transitions of muonium in germanium

Abstract

Muonium defect centers in ultrapure germanium show evidence of cyclic charge-state transitions above approximately 220 K. Detailed analysis of the magnetic-field and temperature dependences of longitudinal muon spin depolarization rates reveals contributions from both bond-centered and tetrahedral cage-centered interstitial ${\mathrm{Mu}}^{0}$ species involved in separate ionization/capture charge cycles. An additional diamagnetic state, observed as a nonrelaxing fraction, is identified as muonium bound in a slowly formed complex which dissociates to join these cycles above 350 K. Capture cross sections and ionization energies are extracted and the most likely identification of states and processes responsible for each transition feature are discussed. A consistent picture requires a reinterpretation of earlier data as bidirectional site changes for ${\mathrm{Mu}}^{0}$ which become active well below 100 K, and indicates a very shallow potential-energy surface for ${\mathrm{Mu}}^{0}$ in germanium as compared to silicon.