Nuclear-magnetic-resonance detection of charge defects in gallium arsenide

Pulsed NMR was used to detect charged-defect concentrations as small as 2 × ${10}^{14}$ ${\mathrm{cm}}^{-3\mathrm{}}$. An undoped $n$-type single crystal of GaAs was cut into several samples, and NMR second moments were obtained as a function of magnetic field orientation for ${}_{}{}^{71}{}_{}{}^{}\mathrm{Ga}$, ${}_{}{}^{69}{}_{}{}^{}\mathrm{Ga}$, and ${}_{}{}^{75}{}_{}{}^{}\mathrm{As}$. The orientationally dependent part of the ${}_{}{}^{71}{}_{}{}^{}\mathrm{Ga}$ second moment is one-half that expected from the dipolar interaction. An explanation is given based on interference between the dipolar and the negative pseudodipolar interactions. There is a contribution to the second moment due to the electric quadrupole interaction between nuclei and the electric field gradient associated with point-charge-like defects, and it is proportional to the defect concentration. Because there are three isotopes, we can separately identify the quadrupolar and pseudodipolar second moments and preferred defect site. In order to introduce crystalline defects, samples were held at a constant temperature (500, 550, 600, and 700°C) in evacuated tubes and quenched to room temperature. The increase in second moment is due to the increased quadrupolar contribution, and it establishes the defect density for each damaged sample. The pseudodipolar interaction is observed to be independent of damage. Our data indicate that the defects are As monovacancies. No recovery from the damage was observed. The agreement between the predicted ionized As monovacancy concentration and our measurement is good.