NMR Study of Si:As and Si:P near the metal-insulator transition

Results of pulsed NMR measurements on the ²⁹Si spin system in Si:As and Si:P are reported for samples spanning the metal-insulator (M-I) transition. The ranges of donor concentrations nD are 1.6×{}10¹⁸ to 4×{}10¹⁹ cm^-3 for Si:As and 1.1×{}10¹⁷ to 6.9×{}10¹⁹ cm^-3 for Si:P. The measurements, made at 4.3 K and primarily in a resonant field of 58.5 kG, show that nonzero values of peak Knight shifts and a Korringa-like spin relaxation are observed for values of nD/nc as low as 0.4. Knight-shift distributions for Si:As and Si:P at a given value of nD appear to coincide at values of nD near 10²⁰ cm^-3, but differ near the respective critical concentrations for the two systems. Values of local Knight shifts change smoothly through the M-I transition. Knight-shift distributions determine the inhomogeneous linewidth. Spin-echo measurements give a determination of the homogeneous, dipolar width. The nuclear spin system does not relax with a single value of T₁. Interpretation of the data emphasizes the importance of focusing attention on the distribution of values of Knight shift and T₁ in this disordered system, rather than attempting to characterize the system with a single value of K or T₁.