Muonium defect centers in aluminum nitride and silicon carbide



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Texas Tech University


We report the results of ƒÝSR measurements on Aluminum Nitride (AlN) and Silicon Carbide (SiC). The importance of studying muonium states comes from its analogy to atomic hydrogen making it an excellent source of information on isolated hydrogen impurities in various materials. Neutral muonium exists in AlN to high temperatures, a large hyperfine constant of ~4450 MHz with a small temperature-dependent dipolar contribution indicating weak anisotropy is obtained from decoupling curves. Tentative site assignments and results on the diffusion of these Mu0 centers along with the associated conversion rates are presented. The low-energy location of neutral muonium in AlN lies off-axis in the unblocked c-axis channels at sites anti-bonding to Aluminum. Motion of Mu0 at low temperatures is due to tunneling and is dominated by thermally activated processes at high temperatures. Diffusion-limited conversion out of the mobile Mu0 state is observed in both low and high temperature regimes.

All electrical types, high-resistivity, n-type and p-type, of the hexagonal 4H and 6H polytypes of SiC were studied. Two isotropic Mu0 states were found in 4H-SiC and a total of four Mu0 states were seen in the 6H-SiC samples. Temperature dependence of the hyperfine constant (AHF) for each state is discussed. Data on the hyperfine interactions imply isotropic atomic-like states with no hint of any bond-centered Mu0 species in SiC. Temperature and field dependences of signal amplitudes and relaxation rates were studied. Tentative assignments for locations and some of the dynamical characteristics of the muonium centers have been reached; however, more work is needed to fully understand the nature of these centers in SiC.