Disordering of the (111) surface of germanium crystal near its bulk melting temperature

Abstract

The reversible disordering transition of the Ge(111) surface near 1050 K (160 K below the bulk melting temperature) has been studied by low-energy electron diffraction (LEED). Measurements were made using a position-sensitive detection system with the following characteristics: maximum speed, ${10}^{5}$ electrons/sec; resolution, 256\ifmmode\times\else\texttimes\fi{}256 channels with up to ${2}^{16}$ counts per channel. LEED peak and total (integrated) intensities I were recorded for varying electron energy E [I(E) plots] or varying crystal temperature T [I(T) plots]. Angular intensity profiles and intensity contour plots were also recorded. The I(E) plots are interpreted to indicate that layerlike crystalline order is preserved in the transition, up to but possibly not including the outermost double layer. The angular intensity profiles are interpreted to rule out thermal roughening as a disordering mechanism. The ratios of nonspecular beam intensities to the specular beam intensity for T>1050 K are found to be incompatible with a surface-melting mechanism like that described by molecular-dynamics (MD) simulations. The I(T) plots exhibit features, such as ranges of positive slope, which cannot be explained by any previously proposed mechanism of surface disordering. The intensity contour plots for T near 1050 K reveal satellite peaks which also have no conventional explanation. A domain-disordering mechanism is proposed to explain the observations qualitatively. In this mechanism, the domains are laterally strained to a depth of one double layer of crystalline Ge(111). The disordering is described as a loss of registry between the strained domains and the substrate. The possible role of intrinsic lateral compressive stress at the Ge(111) surface is discussed with reference to the observations, the disordering mechanism, and relevant MD simulations. Experimental results for Ge crystal films and a theoretical treatment of LEED from strained domains are presented in the Appendixes.