Nuclear Magnetic Resonance in Indium Antimonide I. The Effect of Impurities

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Abstract
A study has been made of the effect of impurities on the 115In nuclear magnetic resonance in indium antimonide. In n-type (Te-doped) and p-type (Zn-doped) samples, the amplitude and width of the resonance are very sensitive to the presence of donors or acceptors, a concentration of 1019 cm−3 reducing the maximum of the derivative of the absorption curve by a factor of 10. This is ascribed to broadening arising from the interaction of the nuclear quadrupole moment with the Coulomb field of the ionized impurity atoms. The effect is slightly less pronounced in p-type than in n-type samples, which can be explained by the fact that holes are more effective than electrons in screening the ionic charges, because of their greater effective mass. Assuming the field gradient at a distance r from an ion to be given by ∂2 V/∂r 2=2βe/r 3, β must be ∼ 350 to fit the experiments, which implies an'anti-shielding factor’ of ∼ 1000. It is suggested that this large value is associated with the smallness of the forbidden gap in InSb. The replacement of a small fraction of In atoms by Ga in an InSb-GaSb mixed crystal has a qualitatively similar effect on the 115In resonance, but the concentration of Ga atoms must be about 30 times that of Te or Zn for the same reduction in resonance amplitude. Since the Ga atoms are electrically neutral, their effect is attributed to strain set up in the InSb lattice by the disparity between the sizes of the Ga and In atoms. A search for a possible Knight shift in n- and p-type InSb containing 1019 carriers cm−3 revealed no deviation in either case from the 115In resonance frequency in intrinsic material, within an experimental accuracy of I in 104. The negative result in n-type material is attributed to the small effective mass of the electrons, which reduces the Pauli susceptibility, while the p-type result provides possible evidence for the p-character of hole wave functions.

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