Charged point defects in GaAs crystals evaluated by nuclear-magnetic-resonance spin echo

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Abstract
Separation of the quadrupolar interaction in GaAs is of practical importance since the interaction magnitude is proportional to the density of charged point defects in the crystal. For this purpose, nuclear-magnetic-resonance spin-echo experiments were carried out and the results were analyzed with a theory extended from a conventional spin-echo theory. Spin echoes were obtained for 75As under a static magnetic field H0=11.7 T at room temperature. The analysis included effects of incoherent spins which are not in phase at the formation of echoes. With a combination of a Gaussian-type relaxation function for the dipolar interaction and an exp(−Kt3/2) type for the quadrupolar interaction, the theoretical fitting in the analysis showed an excellent agreement with the experiment. A pair of fitting parameters thus obtained corresponded to the strengths of the above two interactions. Each of the two parameters showed consistent orientation dependence with theoretical predictions: A+Bf(θ) for the dipolar parameter and A+Bf(θ)3/4 for the quadrupolar parameter, where f(θ)=4 cos2 θ−3 cos4 θ with θ the angle between H0 and the [110] direction of the crystal. Considering that the anisotropic amplitude B in the quadrupolar parameter represents the virtual strength of the quadrupolar interaction, the charged point-defect density in the crystal was obtained and compared with the values of optical measurements.