flash lamp annealing; GeSn; crystallization; heterogeneous integration; thin films
Menon Heera, Morgan Nicholas Paul, Hetherington Crispin, Athle Robin, Steer Matthew, Thayne Iain, Fontcuberta i Morral Anna, Borg Mattias (2021), Fabrication of Single Crystalline InSb‐on‐insulator by Rapid Melt Growth, in physica status solidi (a)
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Kúkoľová A., Dimitrievska M., Litvinchuk A. P., Ramanandan S. P., Tappy N., Menon H., Borg M., Grundler D., Fontcuberta i Morral A. (2021), Cubic, hexagonal and tetragonal FeGe x phases ( x = 1, 1.5, 2): Raman spectroscopy and magnetic properties, in CrystEngComm
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The crystalline quality of semiconductors relates directly to their functional properties such as charge carrier mobility and radiative recombination efficiency. Compared to bulk substrates, thin film technology provides the path of combining different materials for an increased functionality and a strong reduction in material utilization. Monocrystalline thin films may grow seamlessly on crystalline substrates, which provide the crystallographic orientation as well as mechanical support. Defects on the film appear when lattice constants between the two become too important. To circumvent this, a buffer layer may be employed to relax the strain and limit the defect formation close to the interface with the substrate. Alternatively, limiting growth at the nanoscale also results in single crystalline structures. In a hybrid strategy, growth can start in nanoscale regions that then overgrow laterally forming a thin film in the so-called lateral epitaxy method. Alternatively, amorphous thin films can be crystallized in a heating pulse or cycle by applying a laser pulse or an ultra-fast broad-band flash lamp. This strategy is used for example to re-crystallize amorphized regions of devices after ion implantation. These processes can also be applied to amorphous layers on glass, for example in transistors addressing pixels in flat panel displays. Critical to the final quality layer is limiting the nucleation to a low density so that the crystallized grains can be as large as possible. This reduces the amount of grain boundaries, resulting in improved functional properties. The goal of this project is to obtain crystalline thin films of metastable Ge1-xSnx semiconductor alloys on a silicon substrate. Ge1-xSnx with x>0.06 exhibits a direct bandgap in the mid-infrared, rendering this material interesting for optoelectronic and photo-detection applications in LiDAR, imaging and biosensing. Our approach combines the selective initiation of crystallization at nanoscale regions and the use of ultra-rapid annealing cycles. The amorphous Ge1-xSnx layer is first deposited on a patterned oxidized Si substrate, with nanoscale openings to the crystalline Si. Prior to the Ge1-xSnx deposition, the oxide is covered by a graphene layer. A few-ms flash annealing pulse is applied to trigger the crystallization. Crystallization should preferentially start in the nanoscale openings, adopting the crystalline orientation of the substrate. We will look for conditions in which crystallization then proceeds out of the nanoscale openings to laterally crystallize the whole film. The high speed of the thermal process (few ms) should suppress the diffusion of Sn out of the structure. The graphene should foster a higher selectivity of the crystallization process, compared to just using an oxide as a mask. The crystalline properties and functionality of the layers will be first scanned by Raman spectroscopy and X-Ray diffraction. The best samples will in addition be investigated in more detail by high-resolution transmission electron microscopy. The conditions leading to the best crystalline quality will be found by a machine-learning design of experiment approach. The routine will optimize the linewidth of the (004) diffraction peak. Crystalline Ge1-xSnx layers constitute a real alternative to much more costly and less sustainable materials absorbing in the mid-infrared such as InGaAs and lead-based compounds. In addition, we believe this process will be of general nature and will open new avenues for the defect-free thin film formation of other materials systems.