Jumping Atoms At The Surface Of A Metallized Semiconductor

Schematic diagram showing a one-dimensional cut through the potential energy profile which Ge adatoms (shaded circles) experience on the surface of a Ge single crystal. At temperatures below 1050 K the atoms are localized at sites indicated by T4. At high temperatures when the underlying surface becomes metallic and the barriers between sites are reduced the atoms are able to jump rapidly from site to site. Full size image available through contact

Physicists Alexei Glebov and Stefan Vollmer in the group of Professor Peter Toennies at the Max Planck Institute for Fluid Dynamics in Goettingen have succeeded in gaining the first insight into the dynamical behavior of single adatoms on a semiconductor surface at temperatures close to its melting point. The results of their experiments, reported in the April 19 issue of Physical Review Letters [82, 3300 (1999)], unraveled the long-time debated nature of the high temperature phase transition at the germanium (111) surface and provided for the first time a microscopic description of atomic processes occurring at the surface of a semiconductor at temperatures above 1000 K.

Microelectronic devices are currently undergoing a rapid transition from the sub-micron to the nanometer scale. At these smaller length scales, the dynamics of atomic processes at surfaces, such as atomic diffusion and defect formation, begin to play a major role in determining semiconductor material properties, especially during their high-temperature processing. Therefore, a full understanding of these microscopic processes is of profound importance for the further miniaturization of integrated circuits.

At high temperatures the surface of Ge exhibits a complex behavior in particular at temperatures around 1050 K at which the last phase transition takes place before complete melting of the crystal occurs at 1211 K. This behavior so far has eluded a clear description, since previous experimental techniques were insensitive to processes occurring at the surface, especially at high temperatures. In particular the fate of the Ge-adatom defects on the (111) surface and the nature of the surface structure at these high temperatures have been intensively debated. Moreover, it was not known whether the surface and the subsurface region of Ge are melted (disordered), quasi-melted (partly disordered) or perfectly ordered.

The scientists from Goettingen used the scattering of very low energy helium (He) atoms to probe the surface of Ge at these high temperatures. Being too large to penetrate into the crystal, the He atoms bounce back from the very top layer of the target crystal and, therefore, provide information exclusively from the surface. Furthermore the He atoms, when scattered from other atoms diffusing at the surface, undergo a slight change in their velocities as a result of the Doppler effect (quasi-elastic scattering). This phenomenon has been exploited in the past for studying diffusion inside the bulk of crystals with neutrons, but the neutron technique is insensitive to processes occurring at the surface. The high-resolution helium scattering apparatus developed in Goettingen is so sensitive that it is able to detect and analyze quantitatively the small velocity changes of the scattered He atoms.

The results of the experiments presented by the Max Planck researchers clearly indicate that at a temperature of 1050 K the Ge-surface undergoes a structural phase transition from an ordered phase to another highly ordered phase. The following scenario of the phase transition has been inferred from the new scattering data: At 1050 K the surface of germanium becomes metallic and, as a result of the greater mobility of the metal electrons, the corrugation of the surface becomes smoother. At the same time a small number of atoms is released from the top surface layer to become highly mobile surface adatom defects. Their large mobility is a result of the reduction in the surface corrugation together with the reduction of the energetic barrier for the diffusion of the Ge adatoms. These start hopping very rapidly from one adsorption site to another, in a process which is called jump diffusion. Not surprisingly, a quantitative analysis of the data demonstrates that the adatom diffusion coefficient is similar to that of liquid germanium, which is known to be a metal. These experiments provide the first microscopic insight into the high temperature behavior of a semiconductor surface and its transition to a metallic state.