Integration of advanced materials on silicon: from classical to neuromorphic and quantum applications

Resume : Within the two last decades, group III-Nitrides on Silicon and especially GaN based structures have impressively developed, rising the hopes to rapidly integrate such materials in low cost CMOS compatible process lines. The story mainly started with high-frequency GaN high electron mobility transistors (HEMTs) to be developed on Silicon for reducing the cost of telecommunication systems such as for 5G, followed by light emitting diodes (LEDs) for solid state lighting. In-spite of serious difficulties to grow such materials on large diameter substrates, some succeeded and a new industry has emerged. But today, the GaN-on-Si industry experiences a new rise, much impressive than the original one, where in the one hand GaN HEMTs are developed to replace Silicon or SiC based electron devices in power switching systems and in the other hand, micro-LEDs are developed for displays. Even when there was some success in the development of alternative epitaxy strategies like nanowire-based LEDs, the great majority of III-Nitride materials grown today on Silicon relies on the use of planar growth on an Aluminum Nitride nucleation layer [1] able to protect the Silicon substrate from chemical reactions with metals like Gallium [2]. Furthermore, thanks to a lattice mismatch of 2.4% with GaN, the use of AlN induces a compressive strain in the GaN layers grown on it, which is the key to compensate the thermoelastic strain responsible for the generation of wafer bowing and cracks in the films. Such AlN nucleation layer can be used to integrate others materials like Graphene or cubic Silicon Carbide. Also, other devices can benefit the GaN-on-Si platform technology, such as micro-electro-mechanical systems on membranes easy to fabricate thanks to the selectivity of the etching of Silicon with regards to III-Nitrides, or surface or bulk acoustic wave devices for sensors or high-frequency filters. The preferred substrate orientations for III-Nitrides, Si(111) or Si(110) are not in the mainstream for the monolithic co-integration with Silicon CMOS (Si(100)). Even when such integration has been demonstrated via the use of substrates with a noticeable miscut angle or via hybrid SOI substrates (Si(111)/Si(100)) the noticeable progresses in the hetero-epitaxy of III-Nitrides on large diameter substrates and in the layer transfer on CMOS wafers make the approach interesting for heterogeneous co-integration. [1] A. Watanabe, T. Takeuchi, K. Hirosawa, H. Amano, K. Hiramatsu, I. Akasaki, The growth of single crystalline GaN on a Si substrate using AIN as an intermediate layer, J. Crystal Growth 128 (1993) 391. [2] A. Dadgar, A. Strittmatter, J. Bläsing, M. Poschenrieder, O. Contreras, P. Veit, T. Riemann, F. Bertram, A. Reiher, A. Krtschil, A. Diez, T. Hempel, T. Finger, A. Kasic, M. Schubert, D. Bimberg, F.A. Ponce, J. Christen, A. Krost, Metalorganic chemical vapor phase epitaxy of gallium-nitride on silicon, Phys. Status Solidi C 6 (2003) 1583.

Resume : Layered Metal-Dielectric-Metal (MDM) structures provide a highly versatile platform for light manipulation. We have demonstrated that this system can act as a photonic cavity in the visible sustaining ENZ resonances, and that the similarity of the Helmholtz to the Schroedinger equation enables to apply the formalisms of quantum mechanics, providing deep physical insights.[1-3] This analysis can be extended to multiple cavity layers that form ENZ bands, which can be described by the Kronig-Penney model.[4] The possibility to tune the splitting of the two low-energy cavity modes provides additional degrees of freedom for tuning the resonances to the emission and absorption bands of emitting dyes, which can be used to enhance their radiative rate. [5] Therefore, MDM cavities constitute ultrathin optical cavities with resonances in the visible range that can be implemented on large areas by cost-efficient thermal deposition methods. This platform allows to fabricate superabsorbers, optical cavities for light emission enhancement, modulation and photodetection, and can provide active elements in ultrafast signal processing.[6] Here, I will present our most recent progress on light-matter interaction [7,8] and coupled resonances in such cavities, and their implementation in devices for light emission enhancement. References [1] V. Caligiuri, M. Palei, G. Biffi, S. Artyukhin, R. Krahne, A Semi-Classical View on Epsilon-Near-Zero Resonant Tunneling Modes in Metal/Insulator/Metal Nanocavities, Nano Lett. 2019, 19, 3151-3160. [2] V. Caligiuri, M. Palei, G. Biffi, R. Krahne, Hybridization of epsilon-near-zero modes via resonant tunneling in layered metal-insulator double nanocavities, Nanophoton. 2019, 8, 1505. [3] B. Zappone, V. Caligiuri, A. Patra, R. Krahne, A. De Luca, Understanding and Controlling Mode Hybridization in Multicavity Optical Resonators Using Quantum Theory and the Surface Forces Apparatus, ACS Photonics 2021, 8, 3517-3525. [4] V. Caligiuri, G. Biffi, A. Patra, R. D. Pothuraju, A. De Luca, R. Krahne, One-Dimensional Epsilon-Near-Zero Crystals, Adv. Photon. Res. 2021, 2 , 2100053. [5] V. Caligiuri, M. Palei, M. Imran, L. Manna, R. Krahne, Planar Double-Epsilon-Near-Zero Cavities for Spontaneous Emission and Purcell Effect Enhancement, ACS Photonics 2018, 5, 2287-2294. [6] J. Kuttruff, D. Garoli, J. Allerbeck, R. Krahne, A. De Luca, D. Brida, V. Caligiuri, N. Maccaferri, Ultrafast all-optical switching enabled by epsilon-near-zero-tailored absorption in metal-insulator nanocavities, Commun. Phys. 2020, 3, 114. [7] V. Caligiuri, G. Biffi, M. Palei, B. Martín-García, R. D. Pothuraju, Y. Bretonnière, R. Krahne, Angle and Polarization Selective Spontaneous Emission in Dye-Doped Metal/Insulator/Metal Nanocavities, Adv. Opt. Mater. 2020, 8, 1901215. [8] A. Patra, V. Caligiuri, R. Krahne, A. De Luca, Strong Light–Matter Interaction and Spontaneous Emission Reshaping via Pseudo-Cavity Modes, Adv. Opt. Mater. 2021, 9, 2101076.

Resume : Recent improvements in THz sources and detectors have enabled novel technological platforms with promising new perspectives in several fields, such as medicine, pharmacy and security screening. Here, the use of the electromagnetic spectrum in the THz range (0.1-3 THz) allows label-free, reliable measurements of biomolecules and protein conformation changes [1, 2]. One of the major hurdles in this technology is that the interaction of bioanalytes with THz radiation is physically limited due to the huge size mismatch between the analytes (few nm) and the wavelengths of the electromagnetic field (several tens of µm) [1]. To overcome this problem, exploiting plasmonic effects has been suggested to enhance the spectroscopy signal with several order of magnitute thanks to the loal field enhancement [3]. In this work, we present the use of a highly n-doped Ge-based bow-tie-shaped plasmonic antennas operating in the THz range. The antennas have been fabricated using CMOS-compatible materials in an industrial-grade pilot line. According to finite element simulations, the antennas are able to induce sub-wavelength THz field enhancement within the hotspots, resulting in reliable semi-quantitative detection of adsorbed BSA proteins in the mg/mL range, featuring a layer thickness down to 30-50 nm. We futhermore study and discuss the effect of antenna design, size, and arrangement on its sensitivity. The results show CMOS-compatible Si/Ge-based antennas are a good choice due to their potential for mass fabrication of low-cost biosensors. We believe, the initial results could pave the way to a CMOS-integrated, low-cost, on-chip microfluidic biosensor platform. [1] T. Mancini, R. Mosetti, A. Marcelli, M. Petrarca, S. Lupi and A. D'Arco, "Terahertz Spectroscopic Analysis in Protein Dynamics: Current Status," Radiation, vol. 2, pp. 100-123, 7 February 2022. [2] N. L. Henry and D. F. Hayes, "Cancer biomarkers," Molecular Oncology, vol. 6, pp. 140-146, 2012. [3] S. Law, R. Liu and D. Wasserman, "Doped semiconductors with band-edge plasma frequencies," Journal of Vaccum Science & Technology B, vol. 32, 31 July 2014.

Resume : For future of electronics such as bioelectronics, 3D integrated electronics, and bendable electronics, needs for flexibility and stackability of electronic products have substantially grown up. However, conventional wafer-based single-crystalline semiconductors cannot catch up with such trends because they are bound to the thick rigid wafers such that they are neither flexible nor stackable. Although polymer- based organic electronic materials are more compatible as they are mechanically complaint and less costly than inorganic counterparts, their electronic/photonic performance is substantially inferior to that of single- crystalline inorganic materials. For the past half a decade, my research group at MIT has focused on mitigating such performance-mechanical compliance dilemma by developing methods to obtain cheap, flexible, stackable, single-crystalline inorganic systems. In today’s talk, I will discuss about our strategies to realize such a dream electronic system and how these strategies unlock new ways of manufacturing advanced electronic systems. I will highlight our remote epitaxy technique that can produce single-crystalline freestanding membranes from any compound materials with their excellent semiconducting performance. In addition, I will present unprecedented artificial heterostructures enabled by stacking of those freestanding 3D material membranes, e.g., world’s smallest vertically-stacked full color micro-LEDs, world’s best multiferroic devices, battery-less wireless e-skin, and reconfigurable hetero-integrated chips with AI accelerators.

Resume : Amorphous metal oxide semiconductors have been investigated for thin film transistor (TFT) applications because they offer higher mobility at low processing temperatures. While amorphous Indium Gallium Zinc Oxide (IGZO) and Indium Zinc Oxide (IZO) have been extensively studied as channel materials for TFTs, amorphous gallium oxide (a-GaOx) is an interesting transparent conducting oxide with wide bandgap (~4.9 eV) and ability to have n-type carriers. Additionally, it also has potential applications in UV detectors, solar cells, humidity sensors etc. This makes it suited for back-end-of-line (BEOL) integration of high voltage transistors and sensing devices. However, a detailed study of transistors based on an ALD grown a-GaOx channel has not yet been reported. In this work the amorphous GaOx films are deposited using plasma-enhanced atomic layer deposition (PE-ALD). The use of ALD allows for low deposition temperatures (250°C) and uniform films that are required for BEOL integration of TFTs. The plasma-enhanced ALD process developed recently by our group [1], allows tuning the conductivity of the a-GaOx layer through oxygen plasma exposure time during deposition. Utilizing this we show a-GaOx conductive channel back-gated transistors featuring 20 nm Al2O3 as the gate oxide. Three different channel thicknesses (22, 50 and 75 nm) were investigated. Detailed investigation of transistor characteristics such as subthreshold slope (SS), threshold voltage, ON current and their dependency on channel thickness and channel length is performed. Transistors with SS < 150 mV/dec and an ON/OFF ratio of 105 have been shown for a channel length of 6 µm. The devices show a hysteresis in the drain current with gate voltage sweep, which could be associated with charge traps in the channel surface exposed to atmosphere. Therefore, in order to minimize this effect, different encapsulations of the a-GaOx channel with in-situ ALD grown Al2O3 and ex-situ PECVD grown SiO2 were studied and the comparison of the two encapsulations will be presented. Based on these investigations, pathways to optimize the device characteristics (improvement in ON current, reduction in drain current hysteresis) will be discussed. [1] H. Kröncke et al., “Effect of O2 plasma exposure time during atomic layer deposition of amorphous gallium oxide.” Journal of Vacuum Science & Technology A 39, 052408 (2021)

Resume : The discovery of topological insulators (TIs) hold great promise for the field of spintronics thanks to the peculiar spin texture of their surface states. Indeed, the surface states of a TI possess spin-momentum locking which is expected to convert with high-efficiency a charge current into a spin current (Rashba-Edelstein effect) and viceversa (inverse Rashba-Edelstein effect). However, most of the experimental techniques employed in spintronics investigations require ferromagnetic films and TIs are known to chemically react when they are in contact with them [1]. This is detrimental for the spin-related properties of the TIs which are hosted by surface states. For this reason, a versatile platform allowing for the exploitation of the assets of TI is still lacking. Here, we circumvent the problem by exploiting germanium as a substrate for the growth of Bi2Se3, a prototypical TI. As probed by cross-sectional transmission electron microscopy, we obtain a very sharp Bi2Se3/Ge interface. Germanium adds two key features: first, it allows for efficient spin generation via optical orientation, thus avoiding any ferromagnetic material to generate the spin current; then, the 4% of lattice mismatch between Ge and Si allows for the integration of this system with the mainstream Si-based technology. To probe the spin properties of TIs we thus generate a spin population by optical spin orientation in Ge [2]. This photogenerated spins diffuse as a spin current toward the Bi2Se3, which acts as the spin detector. In this way, we probe the spin properties of the Bi2Se3/Ge pristine interface by investigating the spin-to-charge conversion taking place in the interface states. We compare the spin-to-charge conversion in Bi2Se3/Ge with the one taking place in Pt (a standard spin detector) in the same experimental conditions. Notably, the sign of the spin-to-charge conversion given by the TI detector is reversed compared to the Pt one, while the efficiency is comparable. By exploiting first-principles calculations, we ascribe the sign reversal to the hybridization of the topological surface states of Bi2Se3 with the Ge bands. [3] Hence, semiconductors not only preserve the elemental sharpness of TI and substrates at the interface, but the hybridization of electronics states allows engineering a device where, by gating the heterostructure, the spin-to-charge conversion could be tuned in magnitude and sign. These results pave the way for the implementation of highly efficient spin detection in TI-based architectures compatible with semiconductor-based platforms. [1] K. Ferfolja et al., J. Phys. Chem. C 122, 9980 (2018) [2] C. Zucchetti et al., Phys. Rev. B 96, 014403 (2017) [3] T. Guillet, C. Zucchetti et al., Phys. Rev. B 101, 184406 (2020)

Resume : A perfectly compliant substrate would allow the monolithic integration of high-quality semiconductor materials such as Ge and III-V on Silicon (Si) substrate, enabling novel functionalities on the well-established low-cost Si technology platform. Here, we demonstrate a compliant Si substrate allowing defect-free epitaxial growth of lattice mismatched materials. The method is based on the deep patterning of the Si substrate to form micrometer-scale pillars and subsequent electrochemical porosification. The investigation of the epitaxial Ge crystalline quality by X-ray diffraction, transmission electron microscopy and etch-pits counting demonstrates the full elastic relaxation of defect-free microcrystals. The achievement of dislocation free heteroepitaxy relies on the interplay between elastic deformation of the porous micropillars, set under stress by the lattice mismatch between Ge and Si, and on the diffusion of Ge into the mesoporous patterned substrate attenuating the mismatch strain at the Ge/Si interface.

Resume : In the last decade, mid-infrared integrated photonics has raised a great interest due to the envisioned applications in molecular sensing, security, environmental monitoring and free-space communications. The current technology, based on the mature SOI platform has already reached a significant technology readyness level, but it cannot be exploited at wavelegths > 3.2 μm. Extending the operational wavelengths toward the long-wave infrared region (LWIR) ( 8 μm – 12 μm) presents many challenges and many material platforms incuding III-V semiconductors, halides, chalchogenides are currently under investigation. In this framework, the SiGe-on-Si material platform is considered very promising. First of all, by taking advantage of the wide transparency range of Ge, low-loss waveguides operating up to λ =11 μm have been recently demonstrated, as well as many passive photonic components including Mach-Zehnder interferometers, resonators, spectrometers and modulators. Nevertheless, key functionalities such as wavelength conversion, photodetection and high speed optical modulation are still missing. In this framework, Ge/SiGe quantum wells could be exploited to fill this gap. Intersubband optical transitions in the valence band of such heterstructures can be used for light detection, for high-speed modulation through the quantum confined Stark effect, and for wavelength conversion through second harmonic generation. Ge/SiGe QWs can be easily grown on top of SiGe buffers, making them fully compatible with the existing SiGe-on-Si material platform. In this work, we show the experimental demonstration of second harmonic generation at mid-infrared frequencies and we propose a concept for waveguide integration of Ge quantum wells. We will also show recent experimental results on intersubband absorption and a feasibility study on high speed optical modulation through the quantum confined Stark effect.

Resume : The widening of quantum cascade laser (QCL) active materials to Group IV systems promises to create a breakthrough into semiconductor laser science and technology, enabling compact THz sources spanning the whole THz range and operating at room temperature. The recent observation of THz electroluminescence originating from intersubband transitions (ISBT) in the conduction band of n-type Ge/SiGe multi-quantum wells [1] suggests that the quality of this material system is approaching the standard required for QCL applications. Several criticalities still remain in the epitaxy of such complex structures, related to the high Ge/Si lattice mismatch and to the tricky control over the spatial localization of dopants in the Ge layers. We discuss here the advances achieved in the growth of n-type Ge/SiGe quantum cascade (QC) active regions by ultra-high-vacuum chemical vapor deposition (UHV-CVD). In particular, we report Fourier Transform Infrared absorption experiments performed at 10K to study the effect of the doping profile on the absorption spectra. Strain-compensated QC structures having 20 repetitions of the SW design used in Ref. 1 were grown on a Si(001) substrate. The central 3nm of different wells of the QC structure has a nominal 3x1017 cm-3 doping. The optical spectra were simulated using a multivalley self-consistent Schrödinger-Poisson solver. Since the expected spectra are strongly dependent on the dopant atom positions this was meant to explore the spatial control of dopants and the effect of field distortion produced by localized donors on the absorption spectra. When the dopant atoms are located in the second well of the structures as envisaged in the QCL design, a single absorption peak at 16 meV related to the lasing ISBT has been observed, in good agreement with the emission peak observed in [1]. In the samples doped in the fourth well a second more intense peak at 32 meV is instead present. The very good agreement between the measured and simulated spectra, confirms the good control achieved in the dopant position. Moreover, aiming at the realization of a Ge/SiGe QCL, we present the progress made in the deposition of very high-quality QC structures deposited on silicon on insulator (SOI) substrates, which facilitates the double metal waveguides fabrication process with respect to a standard Si substrate. A first series of samples, featuring 100 repetitions of the cascade structure (overall thickness of about 5 m), has been deposited. XRD and STEM data demonstrate the high quality of the material. Furthermore, a very low threading dislocation density of 1.5x106 cm -2 has been measured. This research has been supported by Regione Lazio, program POR FESR 2014-2020, project n. A0375-2020-36579 “Teralaser”. [1] D. Stark, et al. "THz intersubband electroluminescence from n-type Ge/SiGe quantum cascade structures", App. Phys. Lett. 118, 101101 (2021)

Resume : Detection in the mid-IR wavelength range is of great interest to enable gas sensing and biological spectroscopic detectors, which require an absorbing material in the 3-5 and 8-13 µm windows. Commercial detectors employ either MCT or InSb as sensing material, but they’re fragile, difficult to process and integrate on silicon, resulting in a very high cost. Cost effective MIR detectors may be fabricated by exploiting intersubband transitions (ISTs) in high-quality Ge multiple quantum wells nanostructures, thanks to the possibility of integration in standard CMOS processes. Moreover, the absorption of such quantum well infrared photodetectors (QWIP) can be tuned by varying the quantum well width and strain level, therefore shifting the transition energy to the desired value. n- and p-type SiGe QWIPs, which present a weaker absorption compared to pure Ge MQW structures, have already been demonstrated in the past, and germanium QWIPs have only been demonstrated by exploiting ISTs in the conduction band. Here p-type Ge MQWs structures are investigated as a platform for the fabrication of QWIPs which, thanks to the non-parabolicity and band-mixing effects in the valence band, present both TE and TM absorption and can therefore be employed in both vertical illuminated and waveguide geometries, while having a larger absorption coefficient compared to SiGe designs. To exploit this effect, three different Ge MQW designs, each with a different quantum well width, were grown by LEPECVD on high-resistivity Si wafers. Moreover, a second set of samples was grown with the same quantum well thickness to investigate the effect of different doping levels. A virtual substrate with a linearly graded concentration profile from pure Si to SiGe 80% was used as a buffer layer to achieve high quality QW superlattices with a low density of defects. High resolution x-ray diffraction (XRD) reciprocal space maps were then acquired and the relevant parameters (i.e. quantum well thickness, superlattice period, average Ge content of the MQW stack) and used as the starting point of the k·p simulations. Room-temperature absorption spectra of the MQW stacks were obtained by measuring the transmittance and reflectance spectra at normal incidence with a FTIR spectrometer and a LN2-cooled MCT detector, and the resulting spectra clearly show an absorption peak which depends on the QW thickness and can be attributed to the LH1-LH2 intersubband transition by 8-band k·p simulations. Moreover, the same measurement has been performed on samples with different doping levels, showing that in undoped samples the Fermi level is in the energy gap and no transitions can be observed, while at a large doping level free-carrier absorption is strongly increased and the absorption peak is less definite. Finally, transmission spectra have been measured in an optical cryostat as a function of temperature, clearly showing that the absorption peak does not shift with temperature, a strong signature of ISTs.

Resume : Germanium on silicon photodiodes have been studied for more than twenty years, mainly for integrated photonics in a waveguide configuration. [1] These devices tolerate fairly high dark current densities since the small volume and high optical power results in a sufficiently large signal to noise ratio. Vertically illuminated photodetectors are instead required for imaging applications of interest in the automotive and biomedical areas. In this case, a sufficiently low dark current density is required for the fabrication of multipixel devices. [2] [3] In recent years the reduction of threading dislocations [4] and the implementation of surface passivation [5] strategies have been investigated in order to reduce the dark current density. Instead, the effect of doping in the silicon-germanium heterostructure on the dark current has been not fully analyzed. In order to evaluate the impact of silicon substrate doping on dark current and photoresponse, a set of germanium photodiodes were grown by LEPECVD and microfabricated by optical lithography. All the investigated photodiodes feature a 1500 nm thick nominally intrinsic germanium layer and heavily doped germanium top contact layer with a thickness of 100 nm. The silicon substrates doping has been varied from 10^(14) cm^(-3) to 10^(19) cm^(-3). Current/voltage measurements have been performed on devices featuring different diameters to obtain the bulk and surface contribution to the total dark current density. Temperature dependent current/voltage and capacitance/voltage measurements have been performed to highlight the relative weight of the different physical mechanisms giving rise to the dark current such as diffusion, generation-recombination and trap-assisted tunnelling. To characterize the photoresponse of the photodetectors, the responsivity and the specific detectivity were measured for photodiodes at the interesting wavelengths (1000-1700 µm) and for different negative biases. We have observed a monotonic dependence of photodiodes performance as a function of substrate doping, highlighted by the relevant role of substrate resistivity in Ge on Si devices. [1] J. Michel, J. Liu, and L. C. Kimerling, Nat. Photonics 4, 527 (2010). [2] R. Kaufmann, G. Isella, A. Sanchez-Amores, S. Neukom, A. Neels, L. Neumann, A. Brenzikofer, A. Dommann, C. Urban, and H. von Känel, J. Appl. Phys. 110, 023107 (2011) [3] G. Xu et al., IEEE Photonics Technology Letters, vol. 34, no. 10, pp. 517-520, 15 May15, 2022, doi: 10.1109/LPT.2022.3168308. [4] H. Tetzner, I. A. Fischer, O. Skibitzki, M. M. Mirza, C. L. Magnanelli, G. Luongo, D. Spirito, D. J. Paul, M. De Seta, and G. Capellini, Appl. Phys. Lett. 119,153504 (2021) doi: 10.1063.5.0064477 [5] Joonas Isometsa, Tsun Hang Fung, toni P. Pasnen, Hanchen Liu, Marko Yli-koski, Ville Vahanissi, and Hele Savin, APL Materials 9, 1111113 (2021) doi: 10.1063/5.0071552

Resume : GeSn is a promising group IV material with a direct bandgap that could be most interesting for applications in photonics, microelectronics, or thermoelectrics. Furthermore, it could be potentially integrated into the complementary metal–oxide semiconductor (CMOS) technology. A key feature of this material system is that upon a suitable choice of strain and composition of the epitaxial layer, the band-gap structure of the GeSn can be largely engineered to obtain a direct band gap group IV semiconductor. To this aim, is of paramount importance to correctly evaluate the effect of composition, strain, and the deposition process on the crystal quality of GeSn layers. To this purpose, Raman spectroscopy is an effective experimental technique to determine these properties, as it is non-destructive, contactless, fast and locally resolve technique. In this work, we analyze the polarization dependence of Raman scattering from Ge1−xSnx (0.05 ≤ x ≤ 0.14) alloys grown by chemical vapor deposition (CVD) technique on Ge/Si virtual substrates. Polarized Raman scattering measurements were performed in backscattering geometry along the [001] and [110] directions with parallel and cross polarizations. The analysis of polarization dependence of spectra allowed us to separate unambiguously the different contributions to the Raman spectra, i.e. the main LO Ge-Ge, Ge-Sn, and Sn-Sn modes, as well as disorder-activated secondary multiphonons Ge-Ge modes. Our conclusions were supported by thorough structural characterization of the samples carried out by RBS, XRD, and TEM technics. Furthermore, Raman scattering measurements were performed on the same GeSn films at temperatures ranging from 83 to 325 K to investigate how the thermal-expansion and anharmonic phonon-coupling impact the temperature dependence of Raman modes. Our results help to understand the fundamental properties of GeSn alloys for their applications as light sources, optoelectronic or thermoelectric materials.

Resume : Formation of Sn-containing group-IV alloys, such as GeSn and SiGeSn which are the attractive candidates for thermoelectric generator, in the form of polycrystal with high Si and Sn contents is challenging due to low Sn solubility although high temperature is required for Si crystallization. Crystalline Sn nanodots were found to mediate the Ge or Si crystallization at low temperature as nuclei, besides, Sn atoms are introduced to the polycrystalline materials to be alloyed with them during the growth. Some results of phonon dispersion measurements at synchrotron will be also introduced if time allows.

Resume : Alternative to Si channel materials (SiGe or Ge) and new device concepts such as 3D transistor stacking, complementary FET (CFET) and gate-all-around (GAA) nanosheet devices, all considered for the upcoming technological nodes of 3 nm and below often require process temperatures < 500oC. Currently, the epitaxial growth of group IV semiconductor materials is used for the Source/Drain (S/D) engineering of both p- and n-MOS high performance transistors, allowing the deposition of SiGe:B and Si:P layers with active carrier concentrations ~ 1x1021 cm-3, ensuring low contact resistivities. Nevertheless, with reducing device dimensions, the growing importance of contact resistance in devices remains a major concern. To meet these temperature requirements, epitaxial processes based on the combination of high order Si and Ge precursors are explored. The strength of this approach has already been demonstrated by the successful implementation of Ge/SiGe stacks and Ge S/D deposited at low temperature for the production of pMOS Ge GAA devices exhibiting excellent electrostatic control at sub-30 nm gate lengths. In this contribution, we use advanced low temperature epitaxial processes to enable highly B-doped SiGe layers grown at temperatures down to 300°C with an active doping concentration exceeding 1x1021 cm-3. We also show that chemical concentration of B can be increased even higher up to few percents of B without deterioration of SiGe layers crystallinity. We will discuss the main properties of such layers and a possibility of their application as a S/D material in the advanced p-MOS transistors.

Resume : The growing effort to fabricate low-dimensional GeSn structures for (opto-)electronic applications increases the importance to understand the fundamental surface and interface properties of the material system. Accordingly, we investigate in this work the modification of the electronic structure of the Ge(001) surface after the adsorption and incorporation Sn. We extend a growth-model for the adsorption of Sn on Ge(001), previously established by scanning tunneling microscopy, by utilizing photoemission techniques. We show the k-space resolved valence band structure and reveal Sn induced changes to the Ge valence states, such as the removal of surface states, the modification of effective masses and the creation of a new Sn-related surface state. Post-deposition annealing leads to full incorporation of Sn and, consequently, to the disappearance of valence band state attributable to Sn ad-atoms. Independent of the adsorption and/or incorporation of Sn, we observe that Fermi-level remains pinned close to the Ge valence band maximum, indicating the initial stages of a Schottky barrier formation and allowing us to identify the origin of the strong Fermi-level pinning. Our results provide new fundamental insights into the electronic structure of the system, crucial for the development of SnGe electronics devices, and more generally of use for understanding the controlled alloying of isoelectronic layered materials.

Resume : Current research advances for classical [1] and quantum [2] Si(Ge)-based light emitters raise expectations to finally overcome the restrictions imposed by the indirect bandgap nature of group-IV semiconductors. However, emitter integration in efficient electrically-pumped devices is limited by the absence of lattice-matched compounds that allow confining injected carriers to the emitter-site via type-I double-heterostructures (DHS), as it is state-of-the-art for the III-V (Al)GaAs system. Common knowledge suggests that the type-II band alignment and the limited critical thickness of strained, pseudomorphic, and Ge-rich Si1 xGex on Si(001) layers impede DHS-formation. We present an extensive study of Si1-xGex epilayers with variable Ge contents of 0.35

Resume : Plasmonic excitations in metallic nanostructures have potential applications ranging from enhancing the functionality of optoelectronic devices to integrated biosensors. The optical properties of structures such as plasmonic nanohole arrays are highly sensitive to changes in the refractive index in the vicinity of the structures. The combination of plasmonic nanohole arrays with group-IV optoelectronic devices can be a strategy for the large scale fabrication of miniaturized and cost-effective biosensors on the Si platform. However, complementary metal-oxide-semiconductor (CMOS) fabrication processes place restrictions in particular on the metals that can be used for the fabrication of the structures. Here, we investigate the optical properties of Titanium Nitride (TiN) nanohole arrays fabricated using high-precision industrial fabrication processes for possible applications in integrated, plasmonic biosensors. Reflectance measurements show pronounced Fano-shaped resonances that can be attributed to extraordinary optical transmission through the nanohole arrays. Using the measured material permittivity as an input, the measured spectra are reproduced by simulations to a very large degree of accuracy. Our results show that the material, despite featuring higher losses compared to metals such as Ag or Au, is very promising for applications in on-chip plasmonic refractive index sensors.

Resume : Forksheet transistors are lateral nanosheet devices with a forked gate structure. The physical separation of N- and PFETs by a dielectric wall enables N-P space scaling and consequently sheet width maximization within the limited footprint of low-track-height standard cells. Bottom dielectric isolation has been proposed to circumvent the junction isolation trade-off between punch-through suppression on the one hand and junction leakage and capacitance on the other hand. A typical fabrication scheme includes the epitaxial growth of Si/Si1-yGey/multi-{Si1-xGex/Si} epi stacks (y>x) where the bottom Si1-yGey layer is later replaced by a SiN/SiCO isolation. A similar approach is followed for complementary FET (CFET) devices where pFET and nFET are stacked on top of each other with a dielectric isolation in between. Both fabrication schemes rely on the selective etching of the sacrificial Si1-yGey layer with respect to the {Si1-xGex/Si} multi-stack. Owing to the very small dimensions (e.g. sub-10 nm nanowire channel diameter), high etch selectivity towards both Si1-xGex and Si, and excellent process controls are mandatory. This sets stringent requirements on the epitaxial stacks (thicknesses and composition control, sharpness of interfaces, and absence of strain relaxation) as well as on the etch process itself (high selectivity, limited Si1-xGex and Si consumption). The selective Si1-yGey removal requires a great precision in its adjustment, with the risk of experiencing process variabilities and yield issues. Selective SiGe etching is typically done in advanced wet or dry chemistries and is sensitive to both strain in and oxidation of individual layers. Also, HCl-based vapor etching has been reported for SiGe removal, with high selectivity towards Si. The HCl vapor etching requires a relative high process temperature (typically ≥ 600C). This makes it less attractive for fabrication schemes with bottom or middle isolations. The presence of the Ge-rich Si1-yGey layer in the as-grown epi stack results in an enhanced risk for unwanted layer relaxation. A low temperature Br2-based vapor etching process is proposed as an alternative for the selective Si1-yGey removal in the isolation fabrication. Using Br2 as etching gas for Si1-xGex etching, in-stead of HCl or HBr, allows to reduce the processing temperature by ~300C. The difference in etching rate as a function of layer composition is the highest for Br2. After initial process screening on blanket epi layers to compare etching behavior for different process gases as function of material composition and crystallinity, it is demonstrated on fin patterned test structures that Br2 etching enables high etch selectivity of 10 nm Si0.5Ge0.5 towards 3x {9 nm Si1-xGex / 9 nm Si} multi-layers (x=0.2, 0.23, and 0.3).

Resume : A neuromorphic computing system employs a great number of artificial synapses which transfer information between neurons. In this paper we present CMOS compatible artificial synapses based on a ferroelectric Schottky barrier MOSFET on silicon. The ferroelectric polarization switching dynamics gradually modulate the Schottky barriers, thus programming the device conductance by applying positive and negative pulses to imitate the excitation and inhibition behaviors of biological synapse. The three and four terminal configurations of the device enable both homo- and hetero-synaptic plasticity with multi-functionalities, high endurance, low power consumption and high speed. Logic in-memory units, like AND, NAND, XOR, NOR, which are highly demanded in neuromorphic computing, are also demonstrated with the device.

Resume : The last decades have been marked by a shift of paradigm in computing. From generic hardware and specialized algorithms, current approaches are relying more and more on specialized hardware with much more generic algorithms. This is particularly true in the context of Artificial Intelligence where implementing generic algorithms of Artificial Neural Networks requires a specialized hardware in order to keep the energy consumption low. Nevertheless, standard fabrication approaches are largely top-down and rely on deterministic hardware where all components and their organization needs to be known a priori. At the opposite, biology is relying on a bottom-up strategy where elementary components assemble and evolve toward the objective function during development resulting on optimal resources utilization. In this talk, we will present a strategy that can reproduce the bottom-up approach of biology for implementing meaningful computing functions. We propose to use bipolar AC electropolymerization to engineer dendritic-like fibers of PEDOT:PSS. We will show how such unconventional structures can implement various neuromorphic concept such as structural plasticity and synaptic plasticity of biological neural networks. Finally, we will show how this approach can be used to find optimal topologies of neural networks for tasks such as classification and signal reconstruction.

Resume : Neuromorphic computing is being seen as a solution to address the memory bottleneck persistent with the present computing paradigm. Artificial synapses and neurons need to be built to realize such an architecture. One way to emulate a bio-synapse requires a material with a metal-insulator phase transition (MIT). VO2 undergoes a structural phase transformation (SPT) from a monoclinic structure at room temperature to tetragonal at approximately 70°C. The SPT is accompanied by an MIT leading to a large variation in its electrical (about 4 orders of magnitude of its resistivity) and optical properties, in particular, in its complex refractive index in the mid-IR frequency range. To keep with the current trends of the microelectronic industry, it is imperative to integrate VO2 on silicon. However, the higher lattice mismatch and formation of oxides and silicates at the interface between VO2 and crystalline Si degrade the quality and functionality of VO2 film. Additionally, VO2(M1) is a challenging material to integrate into patterned heterostructures because it can exist not only as multiple polymorphs (A, B, M1) but the high-temperature depositions can lead to the formation of various oxidation states phases that are present in the V-O system (VnO2n-1, VnO2n 1). This work was conducted to study the growth of VO2 on silicon with oxide buffer layers using RF magnetron sputtering of a V2O5 ceramic target in an argon atmosphere. We studied the structure-property relationships, specifically electrical and optical properties as a function of temperature across the Tc. Structural and compositional characterization are carried out using x-ray diffraction, atomic force microscopy, and x-ray photoemission spectroscopy respectively, optical responses are studied under spectroscopic ellipsometry and electrical characterizations are performed using the four-point probe method. With the use of a very thin metal oxide buffer layer between the silicon substrate and VO2 film, we demonstrate a high resistivity ratio (of the order 3 between the two phases) and investigate the scope of improvement. The results show the influence of substrate temperature, VO2 grain size, and strain on it as well as the crystal structure of the buffer layer on the structural and physical properties of interfaces and film morphology which subsequently affect the electrical bistability of VO2. The preliminary findings mentioned here are being utilized to improve the electrical bistability, thus allowing us to improve the reproducibility in operational modes (switching, memory, logical operations, etc.) of neuromorphic devices.

Resume : A promising route to neuromorphic systems is using resistance switching in oxide-based, intrinsic ReRAM devices. During operation, under an applied field, oxygen is driven across the metal-oxide-metal (MIM) stacks modulating the resistance between the electrodes. The electrode is thought to act as an oxygen reservoir enabling the reversible exchange of oxygen with the high resistance oxide. How effective the electrodes are as a reservoir is a significant factor in determining the cycling endurance and stability of devices. Here we report the measurement of oxygen movement in silicon oxide-based ReRAM devices using a novel Secondary Ion Mass Spectrometry (SIMS) normalisation technique that allows the measurement of ionic movement with unparalleled sensitivity. Our technique allows us for the first time to observe the movement of 16O across electrically biased silicon oxide (SiOx) ReRAM stacks, measuring bulk concentration changes in a continuous profile with unprecedented sensitivity. Applying this to three devices, using different electrode materials, we systematically examine the exchange of oxygen across the oxide-metal interface. A clear link is revealed between the thermodynamics and microstructure of the electrode material with the scale and nature of this diffusion across the oxide-metal interface. Where the electrode is unable to accept oxygen, delamination and breakdown of the device is observed. Where the electrode acts as a reservoir we see a stable and reversible

Resume : Due to the rapid development of artificial intelligence and internet of things, neuromorphic computing and brain-inspired architectures that mimic the cognitive functions of the brain in hardware have become a primary challenge. In this scenario, it is crucial to find suitable elementary units for both performing neuromorphic functions and fabricating devices in large scale. The volatile resistive switching memories (RRAMs), which feature spontaneous change of device conductance, own the distinct combination of high similarity to biological neurons and unique physical mechanisms. They rely on the formation and disruption of a metallic conductive filament (CF) with a lifetime ranging from few microseconds to several seconds, thus by controlling and predicting the CF lifetime, devices can be engineered for a wide range of applications, such as non-volatile memory for data storage, tunable short/long term memory for synaptic neuromorphic computing, and online-learning for edge-computing. Here we present Ag/HfO2/C RRAMs with 1-transistor and 1-resistor (1T1R) structure and 10 nm of oxide layer. The devices start from a high resistive state (HRS) higher than 1 TΩ and switch to a low resistive state in the range of 1 KΩ. The retention time of the CF can be easily tuned by changing the transistor gate voltage and thus the maximum current that can flow inside, showing that the retention time increases with the maximum current, going from few milliseconds to seconds. The set event is dependent on the voltage applied across the devices, revealing that the higher the voltage the bigger the switching probability. The combination of the stochastic behavior of these devices and the tunability of such processes make them perfect candidates to implement short-term memory functions. Here we present a simple full-memristive architecture able to store and distinguish ten patterns, associated to different items, with an overall retention capability in the order of several hundred of milliseconds. Different experimental conditions have been tested, in terms of stimulation voltage (linked to the switching probability) and spike rate (related to the retention time stochasticity of the RRAMs), to deeply analyze the network response and compare it with biological systems. By decreasing the spike rate from 50 to 10 Hz the system starts forgetting the stored item, since the retention time is comparable or smaller than the spike distance. On the other hand, a high switching probability (20% probability to have a set with a certain voltage) makes the system saturated and unable to store and distinguish any more the items. The best condition was reached working with small switching probability (5%) and fast spike rate (50Hz), in perfect agreement with the biological mechanisms that happen in the brain.

Resume : Computational problems of combinatorial nature including non-deterministic polynomial-time (NP) complete problems currently require exponential time to explore the solution space as the problem size increases, making conventional serial computation intractable, and parallel computation a necessity. Network-based Biocomputation (NBC) was demonstrated to solve the subset sum problem (SSP) [1], by encoding it into a graphical network of channels in a nanofabricated device, which is then explored by cytoskeletal filaments propelled by molecular motors to find all possible solutions. This approach of parallel computation promises to require orders of magnitude less energy than conventional computers due to high energy efficiency of the molecular motors. The current study focusses on using NBC to solve another problem, namely Exact Cover (ExCov). For an ExCov problem with a collection X of subsets with elements from a set Y, an exact cover is a subcollection X* such that each element in Y is contained in exactly one subset in X*. We demonstrate an algorithm [2] to translate an ExCov problem with 32 potential solutions into an SSP network which is then solved by using a molecular motor system (actin-myosin II). This work demonstrates that the NBC approach can be used for directly encoding more than one combinatorial problem, and upscaling of the solution space that can be solved by NBC by a factor of four compared to previous work [3]. We have recently detailed the challenges and requirements for further upscaling of NBC [4]. In future, our approach could be applicable in solving real world ExCov problems like airplane fleeting, designing electric circuits and solving puzzles like Sudoku and Rubik’s cubes. References [1] Nicolau, Dan V., et al. "Parallel computation with molecular-motor-propelled agents in nanofabricated networks." PNAS 113.10 (2016): 2591-2596. [2] Korten, Till, et al. "Design of network-based biocomputation circuits for the exact cover problem." New Journal of Physics 23.8 (2021): 085004. [3] Surendiran, Pradheebha, et al. “Solving Exact Cover Instances with Molecular-Motor-Powered Network-Based Biocomputation.” ACS Nanoscience Au (2022) (Accepted manuscript). [4] Zhu, Jingyuan, et al. "Physical requirements for scaling up network-based biocomputation." New Journal of Physics 23.10 (2021): 105004. Keywords: parallel computation, molecular motors, biocomputation, NP-complete Funding: This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 732482 (Bio4Comp).

Resume : Strongly Correlated Materials such as NbO2, Vanadium Dioxide(VO2) exhibit insulator-to-metal transition (IMT) with increasing temperature. This transition can be electrically achieved via Joule heating, leading to threshold switching and Negative Differential Resistance (NDR) regions in the I-V characteristics of nano-and micro-scale devices. Devices operating within the NDR region can self-oscillate. Since VO2 has an IMT of 340K which is just above room temperature, the power expended in generating the self-oscillations is relatively low. The nature of these oscillations were shown to be periodic, stochastic, or chaotic by various authors. Here we demonstrate a transition between periodic oscillations to clustered oscillations in defective VO2 (100 nm) thin-films grown epitaxially on sapphire. The voltage-controlled I-Vs show a volatile hysteresis, but current-controlled I-V profiles exhibit a rugged and complex NDR profile (unlike the smooth S-shape NDRs observed by others), that can be correlated to the defects in the VO2 thin-film. With a load resistance in series, at low pulse voltages, the devices showed a simple capacitive charging and discharging. Between a threshold voltage (Vth) and a transition voltage (Vt), we observe periodic current oscillations (~MHz), whose frequency increases with increasing voltage. These properties of oscillations are similar to the simple integrate-and-fire model in neurons. At voltages beyond Vt, the device characteristics change, showing clusters of oscillations. These clustered oscillations closely resemble tonic bursting in neurons, and generating such oscillations from low-power devices is significant for neuromorphic computation. High amplitude oscillations disappeared on further increasing voltage, and the device settled to a particular state. Along with the electrical oscillations, we simultaneously observed mechanical oscillations. Interestingly, the frequency of electrical and mechanical oscillations is precisely the same, which predicts the phase transition is the cause of mechanical and electrical oscillations. This work also provides material design guidelines for generating complex NDR regions and thus clustered oscillations.