Spot profile analysis and lifetime mapping in ultrafast electron diffraction: Lattice excitation of self-organized Ge nanostructures on Si(001).

T Frigge,M Horn-Von Hoegen,T Witte,B Hafke,V Tinnemann

doi:10.1063/1.4922023

T Frigge, M Horn-Von Hoegen + Show 3 more

Open Access

https://doi.org/10.1063/1.4922023

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Abstract

Ultrafast high energy electron diffraction in reflection geometry is employed to study the structural dynamics of self-organized Germanium hut-, dome-, and relaxed clusters on Si(001) upon femtosecond laser excitation. Utilizing the difference in size and strain state the response of hut- and dome clusters can be distinguished by a transient spot profile analysis. Surface diffraction from {105}-type facets provide exclusive information on hut clusters. A pixel-by-pixel analysis of the dynamics of the entire diffraction pattern gives time constants of 40, 160, and 390 ps, which are assigned to the cooling time constants for hut-, dome-, and relaxed clusters.

Highlights

The ultrafast structural dynamics of impulsively excited non-equilibrium processes at surfaces and in solids have recently attracted much attention
Ultrafast high energy electron diffraction in reflection geometry is employed to study the structural dynamics of self-organized Germanium hut, dome, and relaxed clusters on Si(001) upon femtosecond laser excitation
The cooling time constants for the three different cluster types are in qualitative agreement with the theory of nanoscale heat transport in hetero structures: the smallest structure exhibits the shortest cooling time s / h/rtbc with rtbc the thermal boundary conductance between Ge and Si

Summary

INTRODUCTION

The ultrafast structural dynamics of impulsively excited non-equilibrium processes at surfaces and in solids have recently attracted much attention. We demonstrate in detail how timeresolved electron diffraction can be used to distinguish between the transient contributions from three different cluster types upon impulsive excitation by an intense fs laser pulse. The formation of steeper facets allows for a more efficient reduction of lattice mismatch induced strain than in the case of huts.32–34 With respect to their special shape, these larger clusters are usually denoted as dome clusters. Both cluster types are free of lattice mismatch relieving defects and dislocations.. The generation of defects and dislocations accommodates the lattice mismatch and causes the formation of large and fully relaxed 3D islands Such islands do not grow any longer in a self-organized and kinetically self-limited way. Those relaxed clusters exhibit a very broad size distribution.

EXPERIMENTAL

Composition of the diffraction pattern

Transient response of the diffraction pattern upon heating

Lifetime map and statistical analysis

CONCLUSIONS AND SUMMARY