Transmission Electron Microscopy Studies of Bufferless Epitaxial GeSn on (0001) Sapphire

Epitaxial growth of GeSn films directly on (0001) sapphire substrates, has not been considered as a feasible task. Here, an ultra-thin and a 1 μm thick Ge_1-xSn_x (x≤0.1) film were deposited on (0001) sapphire substrates at 475 °C and 367 °C, respectively, through remote plasma-enhanced chemical vapor deposition (RPECVD). The ultra-thin Ge_1-xSn_x film (deposited at 475 °C) exhibits a distinct epitaxial/twin mushroom-like island morphology with a height ranging from 30-45 nm and a lateral width ranging from 40 - 200 nm. The Ge_1-xSn_x islands are covered by a ∼4 nm thick surface layer of Sn-rich amorphous material and present an atomically sharp and robust interface with the substrate. The epitaxial Ge_1-xSn_x lattices coherently join with the Al layer of the sapphire substrate. The 1 μm thick Ge_1-xSn_x film (deposited at 367 °C) consists of a thin epitaxial/twin layer below a nanocrystalline columnar layer. The nanocrystalline grains have varying Sn content that exceeds that in the epitaxial structure. The epitaxial/twin layer in this film has an ∼1 nm thick highly disrupted near amorphous layer at the interface. Quasiperiodic, two-dimensional hexagonal networks of misfit dislocations are formed at the interfaces of both films to accommodate the misfit strain. The dislocation periodic length was 13.3 Å and 13.1 Å for the films deposited at 475 °C and 367 °C, respectively. The epitaxial structures in both films have an identical orientation relationship of (111)_GeSn//(0001)_Sapphire, $[ {1\bar{1}0} ]$_GeSn//$[ {2\bar{1}\bar{1}0} ]$_Sapphire, $[ {21\bar{1}} ]$_GeSn//$[ {1\bar{1}00} ]$_Sapphire with the substrate, exhibiting lattice mismatches of ∼15% between the (220) GeSn and the $( {11\bar{2}0} )$ Al₂O₃ along the interface plane and -24% between the (111) GeSn and the (0003) Al₂O₃ planes along the film growth direction. The observed microstructures provide valuable feedback that can be used to optimize the RPECVD process for better quality epitaxial GeSn on (0001) sapphire substrates with no buffer layer required.