GeSn as next-generation material for short-wave infrared single-photon detection

In recent years, the automotive industry has aspired to bring self-driving vehicles to the generalpublic and light detection and ranging (LiDAR) sensors have emerged as the preferred solution forcar vision systems. At present, LiDAR technologies employ expensive Indium-based III-V materialsfor optimal performance. However, in view of future mass production of the technology, thisapproach is not sustainable due to the reliance on In, a scarce element already extensively used inthe semiconductor industry.In this context, this thesis explores the potential of GeSn as absorber material for single-photondetection in the short-wave infrared wavelengths to replace the current commercial III-V technologyemployed in LiDARs. Ge and Sn are more abundant elements compared to In, making thema more sustainable option for single-photon avalanche photodiodes (SPADs). Furthermore, thepossibility of monolithic integration of GeSn thin films on Si platforms allows for the utilization oflower amounts of these elements in contrast with III-V SPADs, where In constitutes the bulk ofthe device. Nevertheless, the use of the GeSn semiconductor comes with fundamental materialscience challenges related to the material metastability and electrically active defects arising fromthe thin film growth process on Si substrates.In this thesis, we propose to integrate a GeSn absorber on a Ge-buffered Si diode to achieve single photondetection targeting the wavelength of 1.55 &m. We aimed to demonstrate an all-group-IVSPAD device by epitaxially growing the Ge/GeSn absorber stack employing magnetron sputteringas the deposition method preferred for high-volume semiconductor production.The thesis starts with a review of the physics of SPAD devices, justifying the need of GeSn asabsorber material to access the wavelength of 1.55 &m in all-group-IV devices. Subsequently, Ipresent a detailed assessment of the understanding of the optoelectronic properties of Ge andGeSn thin films in the literature, reviewing additionally the works on sputtered epitaxial Ge andGeSn films. I then discuss the results of our scientific research in four chapters, each focused on adifferent layer composing the SPAD device.We first investigate the in situ p-type doping of GeSn by In, and show that In acts as a surfactantduring the epitaxial growth of GeSn, inducing phase separation via the formation of Sn-In liquiddroplets.Next, we move to the bulk of the research of the thesis, which involved extensive characterizationof epitaxial Ge and GeSn films grown by the magnetron sputtering method. We demonstratesuccessful epitaxy of both materials, evidencing the critical influence of the substrate latticemismatch in inducing defects in the film. We additionally provide characterization of the electricalproperties of GeSn, which showed to be promising but affected by high impurity levels in the films due to contamination in the employed sputtering tools.In the third section, we demonstrate the viability of flash-lamp annealing of Ge buffers as CMOS-compatibleannealing process, shedding light on the influence of Si-Ge intermixing in determinethe final defect density.Lastly, we present the design of a GeSn-on-Si SPAD structure and present results on their optoelectroniccharacteristics with sputtered GeSn, correlating them with the material's electricalproperties.