SURFACE MELTING OF GALLIUM SINGLE-CRYSTALS

Optical properties of the Ga(010), Ga(111), and Ga(112) surfaces have been studied. Single crystals have been grown at UHV conditions and in situ ellipsometric techniques have been applied. On the Ga(112) surfa ce the formation of a quasiliquid layer has been detected at temperatu res below the bulk melting point T(m). The optical properties of the l ayer are close to the ones of the crystal bulk. The thickness of the q uasiliquid layer increases with increasing temperature T. The function al character of the growth law of the quasiliquid layer of the Ga(112) surface is in agreement with the prediction of mean-field theory. In the temperature range 0.5 < T(m) - T < 3.5 K the increase in layer thi ckness can be described by a logarithmic growth law with a correlation length xi = 0.8+/-0.2 nm. For temperatures 0 < T(m) - T < 0.2 K the i ncrease in layer thickness can be described by a power-law growth with a Hamaker constant W = (4.8+/-2.0)X10(-18) mJ. There is a crossover f rom a logarithmic to a power-law growth of the layer in the temperatur e range 0.2 < T(m) - T < 0.5 K. The crossover thickness is comparable with the correlation length within the disordered or quasiliquid layer : l(c) = 0.7+/-0.1 nm. Ga(010) and Ga(111) surfaces are stable agains t thermal disordering up to T(m). The highest stability has been obser ved at the Ga(010) surface. On the Ga(111) surface, changes in the ref ractive index have been detected while cycling the temperature up to t he bulk melting temperature, whereas the extinction coefficient remain s constant. Close to the melting point no drastical changes in n or k have been observed indicating that no enhanced disordering takes place close to T(m).