Investigation on the Formation and Propagation of Defects in GeSn Thin Films

Silicon photonics is rapidly growing because of the recent introduction of GeSn alloys as a group IV direct bandgap material. Viability of GeSn alloys for complementary metal-oxide-semiconductor  technology, as demonstrated recently, is another step towards commercializing this material for integrated circuits applications [1].  However, ability to grow high quality materials is imperative for device applications.  In order to compensate the high lattice mismatch between the Si substrate and the GeSn film different strain relaxed buffer layers, such as Ge, are used. Relaxation of Ge buffer layer introduces a network of misfit dislocations. This can result in high threading dislocation density (TDD) in GeSn degrading the performance of the final device.  In this paper, we report on defect formation and propagation mechanism in Ge_1-xSn_x films.

Plastic relaxation of higher lattice constant materials (SiGe, Ge, GeSn) on top of Si forces the layers to form 60^o misfit dislocations.  These dislocations are caused by slip of (110) and (111) planes in the diamond lattice structure, which have a screw component that threads through the grown film.  Fig. 1 (a) schematically shows how defects form and propagate through the film. A network of misfit dislocations results in a cross-hatch pattern on the surface of the film. Fig. 1 (b) shows the plan view transmission electron microscopy (TEM) image of Ge_.93Sn_.07 on top of Ge buffer layer. The cross-hatch patterns formed at GeSn/Ge interface have overlapped with those of Ge/Si. The denser pattern is attributed to the Ge/Si interface due to higher lattice mismatch. Fig. 1 (c) delineates scanning electron microscopy image of decorated threading dislocations at the surface by using cold Schimmel solution (CrO₃+HF) for 10 minutes.  Density of pits (1.1×10⁷ cm^-2) is in good agreement with TDD measured from TEM micrographs (3×10⁷ cm²)[1]. It also confirms that the quality of GeSn material is higher than Ge buffer layer (TDD: 9.71×10⁸ cm^-2) by an order of magnitude.

Formation of Lomer misfit dislocaltion prevents propagation of threadig dislocation in the top layer.

The main reasons behind this phenomenon is that the Burgers vector in Lomer misfit dislocation, for a (001) substrate, lies parallel to the interface [2].  Therefore, the substrate and film lattices can match while no prependicular component exists.  In this case the film is grown relaxed without introducing structural imperfection such as threading dislocations, which degrades the material quality. 

Different models of Lomer misfit dislocation are proposed including a five-atom ring and seven-atom ring without dangling bonds [3].  Available models show that Lomer dislocations can combine two lattices through symmetric and asymmetric loops [4]. Fig. 1 (d-g) shows formation of the symmetric and asymmetric loops at Ge/GeSn interface  films using high resolution TEM images.

Our investigations show that formation of symmetric and asymmetric Lomer dislocation at the interface of Ge/GeSn film has reduced the TDD in GeSn film. Results from TEM images and etch pit density measurement verifies this conclusion by showing that 80% of the threading dislocations has not passed through the GeSn film. The current results show achievement of high quality GeSn film as well as indicating that GeSn is a unique candidate to be used as a universal compliant layer.

References:

[1] Mosleh et al. J. Electron. Mater., (2014). DOI: 10.1007/s11664-014-3089-2

[2] Mader et al. J Appl. Phys. 45, 4730 (1974).

[3] Hornstra, J. Phys. Chem. Solids 5, 129 (1958). 

[4] Bourret et al.  Phil Mag A 45, 1 (1982).