INTERACTIONS OF IN-111-]CD-111 PROBE ATOMS ON GAAS(111)B RECONSTRUCTED SURFACES MEASURED USING PERTURBED-ANGULAR-CORRELATION SPECTROSCOPY

We have used In-111 --> Cd-111 perturbed-angular-correlation (PAC) spectroscopy to measure hyperfine interactions at surface sites on GaAs(111)B (As-terminated) surfaces, The (111)B surfaces form two reconstructions, 2 x 2, which is formed under As-rich conditions and is stable below similar to 700 K, and root 19 x root 19 R23.4 degrees, which is formed under Ga-rich conditions above this temperature. These nearly-atomically-flat surfaces were grown using molecular beam epitaxy. During each experiment, the In-111 probe atoms were effused onto the GaAs surface, and a series of thermal anneals was performed. A laboratory-temperature PAC measurement was made after each anneal. The PAC measurements performed on the 2 x 2 reconstructed surfaces show two well-defined nuclear electric-quadrupole interactions that occur at two inequivalent Ga-sites on the surface. The corresponding electric-field gradients (EFGs) are large and asymmetric: V-zz approximate to 8.6 x 10(17) V cm(-2), and eta approximate to 0.47 and V-zz approximate to 16.5 x 10(17) V cm(-2) and eta approximate to 0.85; the respective site fractions are similar to 50% and similar to 30%. The measurements performed on the root 19 x root 19 R23.4 degrees reconstructed surfaces show one well-defined interaction that involved similar to 82% of the probes. The corresponding EFG is also large and asymmetric: V-zz approximate to 16.7 x 10(17) V cm(-2) and eta approximate to 0.8, and the EFG z-axis is oriented essentially perpendicular to the surface. Annealing experiments were performed to convert one reconstruction to the other acid then to reconvert it back to the original reconstruction, e.g., 2 x 2 --> root 19 x root 19 R23.4 degrees --> 2 x 2. The PAC measurements that followed each annealing step show the characteristic frequencies for each reconstruction. This result indicates that these probe sites are thermodynamically stable. Additionally, at high annealing temperatures and during these thermal-cycling experiments, a small fraction of the probes appear to diffuse into the bulk crystal. The interpretation of the probe-site assignments does not agree with a model of the root 19 x root 19 R23.4 degrees reconstructed surface developed using scanning tunneling microscopy. These experiments represent the first measurements of group III bonding symmetries on a compound III-V semiconductor surface. The corresponding analysis indicates that these symmetries differ qualitatively from the conventional picture.