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  • Fundamental Research on Compliant Ge / Si Structures

Strain engineered Ge micro- and nanostructures

Strain engineered micro- and nanostructures on patterned wafers

 

The major “stumble block” is given by a large lattice mismatch of 4.2 % between Ge and Si, resulting in too high defect densities [4] as well as complex, strain driven SiGe interdiffusion phenomena [5]. The Ge thin film quality in terms of defect density (scattering and recombination centres) and purity (SiGe alloys adopt Si optoelectronic properties at rather small Si interdiffusion concentration) is of utmost importance.  In this respect, Si CMOS compatible Ge heteroepitaxy approaches for high quality Ge micro- and nanostructure growth with reasonable thermal budget must be developed. Such techniques like direct deposition with cyclic annealing [6], SiGe [7] and oxide buffers [8], selective growth in mesa windows [9] or aspect ratio trapping [10] were investigated in the past to realize high quality Ge on Si substrates but limits in terms of defects and purity still exist. In the present Nanoheteroepitaxy for fully coherent Ge / Si nanostructures project, alternative advanced Si CMOS compatible Ge heteroepitaxy approaches are investigated to push the quality of Ge micro- and nanostructures on Si(001) beyond the state-of-the-art.

A fundamentally different approach with respect to the above mentioned studies is given by nanoheteroepitaxy. This is true because it does not rely on defect filtering approaches but aims to completely suppress plastic relaxation. In consequence, the vision is not only to remove threading dislocations in the Ge film from active device areas but also to completely suppress the formation of a misfit dislocation network at the Ge / Si interface. Certainly, all devices (like Tunnel Field Effect Transistors (TFETs)), relying on current transport across the Ge / Si heterointerface or where the nanoscaled device dimensions do not allow anymore to discriminate between active and passive film areas (like nano-scaled photodetector arrays) would thus greatly benefit. The theory is based on the compliant behavior of Ge/Si nanostructures: In contrast to the classical Ge deposition on bulk Si substrates, the strain energy, accumulated in the Ge epilayer. is partially transferred to the free-standing Si nanostructure. This strain partitioning phenomenon is at the very heart of the compliant substrate effect and, if strain energy levels are correctly balanced, offers the vision to grow defect-free nanostructures of lattice mismatched semiconductors on Si [11].

Fig. 1: Left: Scanning and transmission electron microscope images of Ge nanoclusters on free-standing Si(001) nanopillars on bulk Si(001); right: Transmission electron microscope images of Ge nanoclusters with SiGe buffer interlayers on free-standing Si(001) island on SOI.

Thus, the Ge nanoheteroepitaxy at IHP aims on the experimental verification of the vision to realize Ge/Si nanostructures of high structural quality within this innovative growth approach. Fig. 1 shows on the left the selective growth of Ge nanostructures on free-standing Si(001) but due to i.e. the influence of the growth mask, no major compliance effect was observed and plastic relaxation occurred (see for further details G. Kozlowski et al. Applied Physics Letters 99, 141901 (2011)); in contrast, by the use of strain engineered Ge / SiGe / Si nanostructures on SOI - shown on the right of Fig.1 – compliant behavior was observed, resulting in fully coherent nanostructures without misfit dislocation networks at the interface (see for further details F. Montalenti et al. Phys. Rev. B. 89, 014101(2014)). Certainly, the successful establishment of such a complex growth scenario will also be of high importance for the integration of other lattice mismatched semiconductors (e.g. GaAs) on the Si technology platform.

 

The building and the infrastructure of the IHP were funded by the European Regional Development Fund of the European Union, funds of the Federal Government and also funds of the Federal State of Brandenburg.