GeSn alloys are metastable semiconductors that have been proposed as building blocks for silicon-integrated short-wave and mid-wave infrared photonic and sensing platforms.Exploiting these semiconductors requires, however, the control of their epitaxy and their surface chemistry to reduce non-radiative recombination that hinders the efficiency of optoelectronic devices. Herein, we demonstrate that a combined sulfur-and iodine-based treatments yields effective passivation of Ge and Ge0.9Sn0.1 surfaces. X-ray photoemission spectroscopy and in situ spectroscopic ellipsometry measurements were used to investigate the dynamics of surface stability and track the reoxidation mechanisms. Our analysis shows that the largest reduction in oxide after HI treatment, while HF+(NH4)2S results in a lower reoxidation rate. A combined HI+(NH4)2S treatment preserves the lowest oxide ratio <10 % up to 1 hour of air exposure, while less than half of the initial oxide coverage is reached after 4 hours. These results highlight the potential of S-and I-based treatments in stabilizing the GeSn surface chemistry thus enabling a passivation method that is compatible with materials and device processing.

Germanium Tin is an emerging semiconductor, with high carrier mobility and tunable bandgap directness and energy, that has been attracting a great deal of interest for applications in silicon-compatible electronics and monolithic optoelectronics. In contrast to compound semiconductors which have in the limelight to address several challenges in these technologies, this new emerging family of silicon-compatible group IV semiconductors holds the promise to combine both cost effectiveness and performance in several devices such as tunnelling field effect transistors, infrared detectors and emitters. By tuning strain and composition of GeSn, the band structure can be modified, thus allowing to engineer a large variety of low-dimensional heterostructures relevant to these devices. In fact, even though Ge is an indirect bandgap material, the incorporation of Sn in its lattice allows a transition into a direct bandgap material. Thus, GeSn yields to higher rate of radiative transitions and band-to-band tunneling. However, there are still several outstanding challenges at the materials level that must be overcome before harnessing GeSn advantageous properties. For instance, effective processes to effectively passivate its surface at various Sn composition are yet to be established. It is known that the surface of a semiconductor contains electronically active states because of unsaturated surface bonds or dangling bonds states, which act as localized energy levels the band gap that can change the intrinsic electrical behavior of the material. Therefore, understanding and controlling the surface states are of compelling importance. Moreover, the native oxide layer that forms at the surface of GeSn contains defects resulting in trapping charge carriers thus decreasing their mobility. The poor quality of the surface can decrease the rate of radiative transitions and contribute to the dark current. An effective passivation is thus needed to enhance the optoelectronic performances of GeSn. Passivation layer allows improved charge-separation, reduces charge recombination at surface states, passivates the dangling bonds and decreases the reactivity of the surface. To address these issues, this work investigates several chemical passivation processes and evaluate their effects on the optoelectronic properties of GeSn. The GeSn samples investigated in this work were grown on silicon wafers using ~0.6-3 µm-thick Ge interlay – commonly known as virtual substrates (Ge-VS). The epitaxial growth was carried out by the chemical vapor deposition (CVD) using monogermane (GeH4) and tin-tetrachloride (SnCl4) precursors. Strain minimization and the reduced growth temperature below 350 °C are of paramount importance to enhance Sn incorporation in Ge lattice to reach compositions of 7 – 17 at.%, much larger than the equilibrium composition of 1at.%. Several passivation treatments were evaluated and their basic mechanisms were elucidated. We first evaluated the capacity to remove the native oxide, leave the surface clean and passivate the dangling bonds of GeSn. The kinetic of oxide regrowth was studied to assess the chemical stability of the passivation through X-rays photoelectron spectroscopy (XPS) analysis. For instance, for a surface treatment consisting of a dip in HF followed by a dip in (NH4)2S and then by a nitride drying. XPS measurements show that it is effective in cleaning and passivating the surface, it allows removing the major part of Ge and Sn oxides. Recorded spectra reveal that this treatment gives rise to a persistent sulfur bonds at 161.8 eV, which is the signature of sulphide compounds. Sulfur allows surface passivation and slows down the oxide regrowth. Moreover, alternative surface passivation processes will also be discussed, and their performances compared to the treatment above will be elucidated. Besides, to better understand the evolution of the surface state after the treatments, ellipsometric measurements combined with atomic force microscopy studies are conducted and will be presented. Also, complementary electrical measurements and photoluminescence emission studies will also be discussed to highlight the effectiveness of each treatment in improving the optoelectronic performances of GeSn.

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GeSn alloys are group IV semiconductors that have attracted remarkable interest owing to their ability of strain and bandgap engineering by controlling the Sn content, their compatibility with the Si CMOS platform and their tunable and direct band gap. These material properties make GeSn a promising candidate for many electronic and optoelectronic applications including among others tunnel field effect transistors, infrared (IR) photodectors, and IR emitters. However, in order to produce high quality GeSn devices to enable these applications, it is of paramount importance to develop ohmic metal contacts with very low specific contact resistivity on both n-type and p-type doped GeSn layers used in these devices. This could be achieved by realizing high doping levels and attaining low intrinsic barrier heights between the metal and the contacted GeSn layer. The growth of metastable GeSn semiconductors is typically performed at temperatures well below 400 ºC to avoid Sn segregation and phase separation, which would compromise the opto-electronic properties of the material. [1] Therefore, major care is required when developing post-growth processes that are commonly required for device fabrication. For instance, ohmic contact formation using the conventional process of rapid thermal annealing of nickel contacts cannot be made and the GeSn cannot be cured after dopant implantation because of the very limited thermal budget. We investigated both the epitaxial growth of highly doped layers, as well as the passivation of undoped GeSn samples in order to circumvent these limitations and achieve the desired barrier heights for both p-type and n-type GeSn while varying the Sn content. First, epitaxially grown GeSn layers with p- and n-type doping are demonstrated. For GeSn doping, the CVD growth of in-situ doped GeSn layers is done using B2H6 for p-type, and AsH3 for n-type. We show high active doping levels in the orders of 1019 and 1020 cm-3 for both p-GeSn and n-GeSn, respectively. These results were obtained from the capacitance voltage measurements (CV) using metal oxide semiconductor (MOS) back-to-back devices. In addition, secondary ion mass spectroscopy (SIMS) and atom probe tomography (APT) data support the high and uniform doping levels. Secondly, since the strong Fermi level pinning (FLP) of Ge is a major problem in the development of n-type contacts due to the loss of metal work function modulation of the barrier height [2], it is expected that the GeSn materials system with low Sn levels exhibits the same behavior, thereby degrading the properties of direct metal/n-GeSn contacts. Therefore, we develop processes aiming to release the Fermi level pinning at metal/GeSn interface, thus restoring the metal work function control of the barrier height. Samples used in this study are CVD grown intrinsic GeSn relaxed samples with increasing Sn concentrations up to 11 at. %. These samples are unintentionally p-type with defect doping levels in the order of 1017 cm-3 as obtained by CV measurements on MOS devices. Our process starts by chemical passivation of these samples, then the deposition and patterning of four metals with different work functions close to the band gap is realized to obtain transfer length method (TLM) structures used for current-voltage (IV) measurements. We then extract the contact properties of these devices and estimate the change of the barrier height induced by the chemical passivation due to the depinning of the FL. This demonstrates the potential use of our process in the fabrication of future metal/n-GeSn contacts. Acknowledgements O.M. acknowledges support from NSERC Canada (Discovery, SPG, and CRD Grants), Canada Research Chairs, Canada Foundation for Innovation, Mitacs, PRIMA Québec, and Defence Canada (Innovation for Defence Excellence and Security, IDEaS). References [1] S. Assali et al., Enhanced Sn incorporation in GeSn epitaxial layers via strain relaxation, Journal of Applied Physics Vol. 125, 025304 (2019). [2] T. Nishimura, K. Kita and A. Toriumi “Evidence for strong Fermi-level pinning due to metal-induced gap states at metal/germanium interface”, Appl. Phys. Lett. 91, 123123 (2007).