Surface Phase Transitions Studied with Low Energy Electron Diffraction
Abstract
Using lowenergy electron diffraction, we have studied the phase transition of three different surface systems. We have determined the coverage at which overlayers of Cl adsorbed on Ag(100) order into a c(2 x 2) structure for sample temperatures ranging from 300 to 500 K. We find no evidence for a variation in the critical coverage theta_{rm c}. The addition of modest thirdneighbor repulsions to the model improves agreement of transfer matrix scaling calculations with experimental measurements. We have also studied the disordering of the (sqrt{3} x sqrt{3})R30^ circ structure of Al/Si(111). If this transition were continuous, it would be governed by the critical exponents of the threestate Potts model. However, for Al coverages close to 1/3 of a monolayer on a substrate on which no Al has been previously deposited, we find the effective specific heat exponent obtained from measurements much larger than that of the threestate Potts model. Therefore, the transition appears to be firstorder. A roughness induced finitesize effect is proposed for a previous report that the transition is secondorder. We have designed and built a high resolution LEED instrument for specific application to studies of critical phenomena, with a transfer width of at least 500 A in the energy range from 30 eV to 100 eV. We have applied this instrument to study the behavior of silicon surfaces misoriented by approximately 1^circ from the (111) plane towards the (211) and (110) directions. Below approximately 850^circ, these surfaces phase separation into uniformly stepped regions and regions of (111) facets exhibiting the (7 x 7) reconstruction. This phenomenon has been interpreted as the formation of a sharp edge in the equilibrium crystal shape of Si. This implies that the shape of the equilibrium crystal shape near the (111) facet can be mapped by studying shallow miscut angles. Theory has predicted this shape to be of a power law form, with the exponent given by that of the ProkrovskyTalapov phase transition. For samples misoriented in the (211) direction, we measure an exponent consistent with this prediction. In the (110) direction, we do not observe asymptotic behavior. We have also seen that small amounts of carbon profoundly change the observed behavior.
 Publication:

Ph.D. Thesis
 Pub Date:
 1989
 Bibcode:
 1989PhDT.......108H
 Keywords:

 PHASE TRANSITIONS;
 ELECTRON DIFFRACTION;
 Physics: Condensed Matter