UBC Theses and Dissertations
Spin dynamics and electronic structure of muonium and its charged states in silicon and gallium arsenide Chow, K. H.
This thesis describes recent fiSH measurements on muonium (Mu=μ⁺e⁻) centers in crystalline Si and GaAs. Spin-exchange scattering and charge-exchange of Mu with conduction electrons are found to be important dynamical processes in these systems. Longitudinal muon spin relaxation (1/T₁) measurements in intrinsic Si and p-type Si:B from 350 K to 900 K can be explained within a model where Mu cycles rapidly between its positive and neutral charge states via interaction with conduction electrons. The average muon-electron hyperfine parameter in the neutral state is consistent with Mu at the tetrahedral (T) interstitial site. This indicates that at the highest temperatures measured neutral Mu spends significant time away from the bond center site, the calculated minimum in the adiabatic potential energy surface. Measurements of the 1/ T₁ rates were made in intermediate doped n-type Si and heavily doped n-type GaAs under conditions where coherent spin precession of Mu is unobservable. A peak in 1/ T₁ as a function of magnetic field is observed and shown to be characteristic of neutral bond-centered Mu (Mu⁰[sub BC]) undergoing spin-exchange scattering with free carriers. These results establish that neutral Mu⁰[sub BC] does not convert to Mu⁻ , the expected stable state in n-type Si, at temperatures below approximately 200 K. Furthermore, we conclude Mu⁰[sub BC] is present in heavily doped n-type GaAs below ≈ 30 K. At very high spin-exchange rates, such as in heavily doped Si:P, and under the application of a large magnetic field, Mu⁰[sub BC is "spin-polarized" and a frequency shift from the Larmor precession frequency of a free muon is observed. Muon level-crossing resonance and muon spin rotation measurements on heavilydoped n-type GaAs:Si and GaAs:Te show that the majority of positive muons implanted at room temperature form an isolated diamagnetic Mu center located at a high-symmetry site with Ga neighbors along the (111) direction(s). The experimental results, together with simple theoretical considerations, imply that the center is Mu⁻ located at or near a T[sub Ga] site. These studies on Mu have bearing on research on hydrogen in semiconductors. In particular, the spin dynamics taking place for Mu should also be occuring with hydrogen. The importance of such processes is usually not taken into account by researchers of hydrogen diffusion and related dynamics. The characterization of Mu⁻ is the first experimental determination of the detailed local structure of a charged isolated hydrogen or Mu center in any semiconductor. This is especially interesting since the existence of isolated H⁻ in semiconductors is still being debated.
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