MPISF - Göttingen
Abt. Molekulare Wechselwirkungen, Projekt 407

Structure, Phonons, and Diffusion on the Surfaces of Ultra-thin Films


Investigation of film growth using helium atom scattering

Ultra-thin films play an important role in modern technology. It is, e.g., essential, to master the growth, the characterization, and the handling of these systems in order to achieve an even denser integration on computer chips or the media that are used to store digital data.

We are using helium atom scattering as a tool to control the growth of ultra-thin alkali films and to obtain information about the thickness and the amount of order in these systems. These film are being prepared by depositing up to 15 monolayers of alkali atoms on appropriately chosen mono-crystalline metal substrates using the MBE (molecular beam epitaxy) technique in an ultra-high vacuum environment. For all the different alkali atoms it was possible to find suitable substrates which warrant a periodic structure of the films and a layer-by-layer growth. We have thus been able to prepare well ordered films of K and Rb on Ni(001) surfaces, of Na and Cs on Cu(111), and of Li on W(110).

Detailed information about the growth mechanisms, film thickness and film surface morphology is provided by monitoring the development of the specularly reflected helium beam intensity while deposting the alkali atoms. The specular reflectivity is extremely sensitive to the presence of defects which diffusely scatter helium atoms and lead thus to a decrease in reflectivity. Also, incompleted monolayers lead to terraces, separated by steps, which cause destructive interferences of the reflected de Broglie waves that are detected in a changing reflectivity. Figure 1. shows these changes in the specular intensity as a function of time during deposition. The oscillations indicate layer by layer growth where the maxima in this curve refer to a situation at which the highest degree of order in the film surface is achieved, usually on completing another alkali monolayer.

Fig.1
K/Ni(001):Oscillations in the specularly reflected helium beam intensity vs. deposition time for a constant deposition rate. The maxima in the curve indicate the times at which another monolayer of adsorbates is being completed.

Organ pipe modes and film surface morphology

Time of flight spectrocopy of the scattered helium atoms provides yet another possibility to gain insight into the roughness of the prepared film surfaces. The vibrational spectrum of the alkali films studied here is dominated by resonances which originate from the presence of the interface between film and substrate: reflections of the elastic waves in the film from this interface and from the film surface result in the formation of standing waves of longitudinal polarization in the direction of the surface normal, with nodes at the interface and anti-nodes at the surface. The wavelength is thus determined by the thickness of the alkali layer and the frequency is that of the corresponding longitudinal bulk phonons in the film. Such vibrational modes were discovered some time ago in this institute and were given the name 'organ pipe modes' because their origin is the same as that of vibrations in an organ pipe.

If the energy spectrum of the scattered helium atoms contains peaks that are fingerprints of more than one of these modes, it is an indication that the film consists of domains of different thicknesses. On monitoring the change in the intensity of these inelastic peaks one can also study the annealing process by raising the surface temperature and even, as shown in Fig. 2., layer by layer desorption.


Abb.2.
The series of time-of-flight spectra is obtained by scattering helium atoms with a kinetic energy of 12 meV from a potassium film on Ni(001) at different temperatures of the film. At 148K the film was 'nominally' 4 monolayers thick, but the two inelastic organ pipe peaks reveal the presence of 4- and 3 monolayer patches. The intensity of these peaks changes on raising the temperature until they disappear altogether and organ pipe modes related to a thinner film appear, indicating the layer-wise depletion of the film by evaporating the layer material.
Original literature:
N.S. Luo, P. Ruggerone and J.P. Toennies, Phys. Rev. B 54, 5051 (1996)

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last revision: E. Hulpke, Dec. 12, 2000; e-mail to Webmaster: mailto:Webmaster