NUCLEAR-SPIN ECHOES IN GAAS-ZN AND GAAS-IN

We present a study of the spin echoes of Ga-69, Ga-71, and As-75 in Ga As single crystal with high concentrations of defects (1X10(18) to 2X1 0(20) cm(-3)). The observed echoes have two distinctively resolved fea tures: a rapidly decaying narrow component and a slowly decaying broad component. These two features correspond, respectively, to the broad satellite transitions and narrow central transition in the frequency d omain. The satellite transitions are broadened by the quadrupolar inte ractions between the spins and the electric field gradients (EFG's) in duced by defects. Defects produce the EFG's through lattice distortion (strain effect) and electric fields, if the defects are charged (char ge effect). In order to separate these two effects, we used GaAs sampl es ''doped'' with indium or zinc. The widths and amplitudes of the ech oes were investigated as functions of the orientation of the magnetic field and the defect concentration. We mapped the short-ranged strain held in terms of quadrupolar broadening of Ga-69 resonances. Within on e lattice constant (a(0)) from the defect, the strain-induced EFG's ar e so large that the satellite transitions are several MHz broad. At di stances of a(0) to similar to 3a(0) from the defect, the EFG's are lar ge enough to spread out the satellite transitions from the central tra nsitions. The satellite transitions of the nuclei in this region are u ndetectable in the free-induction decay. At distances of 3a(0) to simi lar to 5a(0) from the defect, the satellite transitions are only sligh tly broadened. At distances further away (>5a(0)), the strain effect i s negligible. For charged defects, in addition to the strain effect, E FG's are also produced by the electric fields through the perturbation of the valence electronic states. The charge effect is long ranged. T he orientational dependence of the quadrupolar broadening is found to be in good agreement with earlier theoretical calculations, and the br oadening is proportional to the defect concentration for defect concen trations as high as 10(19) cm(-3). Finally, we point out that the earl ier calculations of the defect concentration by the second-moment meth od were flawed because the strain effects were neglected.