Browsing by Author "Clavel, Michael B."

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    Atomic Layer Deposited Tantalum Silicate on Crystallographically-Oriented Epitaxial Germanium: Interface Chemistry and Band Alignment
    Clavel, Michael B.; Bhattacharya, Shuvodip; Hudait, Mantu K. (Royal Society of Chemistry, 2022-05-13)
    The interface chemistry and energy band alignment properties of atomic layer deposited (ALD) tantalum silicate (TaSiOx) dielectrics on crystallographically-oriented, epitaxial (001)Ge, (110)Ge, and (111)Ge thin-films, grown on GaAs substrates by molecular beam epitaxy, were investigated. The ALD process, consisting of a 6 : 1 Ta : Si precursor super-cycle, was analyzed via sputter depth-dependent elemental analysis utilizing X-ray photoelectron spectroscopy (XPS). The XPS investigations revealed uniform Si incorporation throughout the TaSiOx dielectric, and a measurable amount of cross-diffusion between Ge and Ta atomic species in the vicinity of the oxide/semiconductor heterointerface. The formation of a thin SiO2 interfacial oxide, through the intentional pre-pulsing of the Si precursor prior to the Si : Ta super-cycle process, was observed via cross-sectional transmission electron microscopy analysis. Moreover, the bandgap of Ta-rich Ta0.8Si0.2Ox dielectrics, analyzed using the photoelectron energy loss technique centered on the O 1s binding energy spectra, was determined to be in the range of 4.62 eV-4.66 eV (±0.06 eV). Similarly, the XPS-derived valence band and conduction band offsets (ΔEV and ΔEC, respectively) were found to be ΔEV > 3.0 ± 0.1 eV and ΔEC > 0.6 ± 0.1 eV for the (001)Ge, (110)Ge, and (111)Ge orientations, promoting the increased carrier confinement necessary for reducing operational and off-state leakage current in metal-oxide-semiconductor devices. Thus, the empirical TaSiOx/Ge interfacial energy band offsets, coupled with the uniform dielectric deposition observed herein, provides key guidance for the integration of TaSiOx dielectrics with Ge-based field-effect transistors targeting ultra-low power logic applications.
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    Design, Theoretical, and Experimental Investigation of Tensile-Strained Germanium Quantum-Well Laser Structure
    Hudait, Mantu K.; Murphy-Armando, Felipe; Saladukha, Dzianis; Clavel, Michael B.; Goley, Patrick S.; Maurya, Deepam; Bhattacharya, Shuvodip; Ochalski, Tomasz J. (American Chemical Society, 2021-10-14)
    Strain and band gap engineered epitaxial germanium (ϵ-Ge) quantum-well (QW) laser structures were investigated on GaAs substrates theoretically and experimentally for the first time. In this design, we exploit the ability of an InGaAs layer to simultaneously provide tensile strain in Ge (0.7-1.96%) and sufficient optical and carrier confinement. The direct band-to-band gain, threshold current density (Jth), and loss mechanisms that dominate in the ϵ-Ge QW laser structure were calculated using first-principles-based 30-band k·p electronic structure theory, at injected carrier concentrations from 3 × 1018 to 9 × 1019 cm-3. The higher strain in the ϵ-Ge QW increases the gain at higher wavelengths; however, a decreasing thickness is required by higher strain due to critical layer thickness for avoiding strain relaxation. In addition, we predict that a Jth of 300 A/cm2 can be reduced to <10 A/cm2 by increasing strain from 0.2% to 1.96% in ϵ-Ge lasing media. The measured room-temperature photoluminescence spectroscopy demonstrated direct band gap optical emission, from the conduction band at the Γ-valley to heavy-hole (0.6609 eV) from 1.6% tensile-strained Ge/In0.24Ga0.76As heterostructure grown by molecular beam epitaxy, is in agreement with the value calculated using 30-band k·p theory. The detailed plan-view transmission electron microscopic (TEM) analysis of 0.7% and 1.2% tensile-strained ϵ-Ge/InGaAs structures exhibited well-controlled dislocations within each ϵ-Ge layer. The measured dislocation density is below 4 × 106 cm-2 for the 1.2% ϵ-Ge layer, which is an upper bound, suggesting the superior ϵ-Ge material quality. Structural analysis of the experimentally realistic 1.95% biaxially strained In0.28Ga0.72As/13 nm ϵ-Ge/In0.28Ga0.72As QW structure demonstrated a strained Ge/In0.28Ga0.72As heterointerface with minimal relaxation using X-ray and cross-sectional TEM analysis. Therefore, our monolithic integration of a strained Ge QW laser structure on GaAs and ultimately the transfer of the process to the Si substrate via an InGa(Al)As/III-V buffer architecture would provide a significant step toward photonic technology based on strained Ge on a Si platform.
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    Enhanced Erbium-Doped Ceria Nanostructure Coating to Improve Solar Cell Performance
    Shehata, Nader; Clavel, Michael B.; Meehan, Kathleen; Samir, Effat; Gaballah, Soha; Salah, Mohamed (MDPI, 2015-11-12)
    This paper discusses the effect of adding reduced erbium-doped ceria nanoparticles (REDC NPs) as a coating on silicon solar cells. Reduced ceria nanoparticles doped with erbium have the advantages of both improving conductivity and optical conversion of solar cells. Oxygen vacancies in ceria nanoparticles reduce Ce4+ to Ce3+ which follow the rule of improving conductivity of solar cells through the hopping mechanism. The existence of Ce3+ helps in the down-conversion from 430 nm excitation to 530 nm emission. The erbium dopant forms energy levels inside the low-phonon ceria host to up-convert the 780 nm excitations into green and red emissions. When coating reduced erbium-doped ceria nanoparticles on the back side of a solar cell, a promising improvement in the solar cell efficiency has been observed from 15% to 16.5% due to the mutual impact of improved electric conductivity and multi-optical conversions. Finally, the impact of the added coater on the electric field distribution inside the solar cell has been studied.
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    Growth, structural, and electrical properties of germanium-on-silicon heterostructure by molecular beam epitaxy
    Ghosh, Aheli; Clavel, Michael B.; Nguyen, Peter D.; Meeker, Michael A.; Khodaparast, Giti A.; Bodnar, Robert J.; Hudait, Mantu K. (American Institute of Physics, 2017)
    The growth, morphological, and electrical properties of thin-film Ge grown by molecular beam epitaxy on Si using a two-step growth process were investigated. High-resolution x-ray diffraction analysis demonstrated 0.10% tensile-strained Ge epilayer, owing to the thermal expansion coefficient mismatch between Ge and Si, and negligible epilayer lattice tilt. Micro-Raman spectroscopic analysis corroborated the strain-state of the Ge thin-film. Cross-sectional transmission electron microscopy revealed the formation of 90°Lomer dislocation network at Ge/Si heterointerface, suggesting the rapid and complete relaxation of Ge epilayer during growth. Atomic force micrographs exhibited smooth surface morphology with surface roughness < 2 nm. Temperature dependent Hall mobility measurements and the modelling thereof indicated that ionized impurity scattering limited carrier mobility in Ge layer. Capacitanceand conductance-voltage measurements were performed to determine the effect of epilayer dislocation density on interfacial defect states (Dit ) and their energy distribution. Finally, extractedDit values were benchmarked against publishedDit data for GeMOS devices, as a function of threading dislocation density within the Ge layer. The results obtained were comparable with GeMOSdevices integrated on Si via alternative buffer schemes. This comprehensive study of directly-grown epitaxial Ge-on-Si provides a pathway for the development of Ge-based electronic devices on Si.
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    Heteroepitaxial Ge MOS Devices on Si Using Composite AlAs/GaAs Buffer
    Nguyen, Peter D.; Clavel, Michael B.; Goley, Patrick S.; Liu, Jheng-Sin; Allen, Noah P.; Guido, Louis J.; Hudait, Mantu K. (IEEE, 2015-07-01)
    Structural and electrical characteristics of epitaxial germanium (Ge) heterogeneously integrated on silicon (Si) via a composite, large bandgap AlAs/GaAs buffer are investigated. Electrical characteristics of N-type metal-oxide-semiconductor (MOS) capacitors, fabricated from the aforementioned material stack are then presented. Simulated and experimental X-ray rocking curves show distinct Ge, AlAs, and GaAs epilayer peaks. Moreover, secondary ion mass spectrometry, energy dispersive X-ray spectroscopy (EDS) profile, and EDS line profile suggest limited interdiffusion of the underlying buffer into the Ge layer, which is further indicative of the successful growth of device-quality epitaxial Ge layer. The Ge MOS capacitor devices demonstrated low frequency dispersion of 1.80% per decade, low frequency-dependent flat-band voltage, VFB , shift of 153 mV, efficient Fermi level movement, and limited C-V stretch out. Low interface state density (Dit) from 8.55 × 1011 to 1.09 × 1012 cm-2 eV-1 is indicative of a high-quality oxide/Ge heterointerface, an effective electrical passivation of the Ge surface, and a Ge epitaxy with minimal defects. These superior electrical and material characteristics suggest the feasibility of utilizing large bandgap III-V buffers in the heterointegration of high-mobility channel materials on Si for future high-speed complementary metal-oxide semiconductor logic applications.
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    Heterogeneous Integration of Epitaxial Ge on Si using AlAs/GaAs Buffer Architecture: Suitability for Low-power Fin Field-Effect Transistors
    Hudait, Mantu K.; Clavel, Michael B.; Goley, Patrick S.; Jain, Nikhil; Zhu, Yan (Nature Publishing Group, 2014-11-07)
    Germanium-based materials and device architectures have recently appeared as exciting material systems for future low-power nanoscale transistors and photonic devices. Heterogeneous integration of germanium (Ge)-based materials on silicon (Si) using large bandgap buffer architectures could enable the monolithic integration of electronics and photonics. In this paper, we report on the heterogeneous integration of device-quality epitaxial Ge on Si using composite AlAs/GaAs large bandgap buffer, grown by molecular beam epitaxy that is suitable for fabricating low-power fin field-effect transistors required for continuing transistor miniaturization. The superior structural quality of the integrated Ge on Si using AlAs/GaAs was demonstrated using high-resolution x-ray diffraction analysis. High-resolution transmission electron microscopy confirmed relaxed Ge with high crystalline quality and a sharp Ge/AlAs heterointerface. X-ray photoelectron spectroscopy demonstrated a large valence band offset at the Ge/AlAs interface, as compared to Ge/GaAs heterostructure, which is a prerequisite for superior carrier confinement. The temperature-dependent electrical transport properties of the n-type Ge layer demonstrated a Hall mobility of 370 cm2/Vs at 290 K and 457 cm2/Vs at 90 K, which suggests epitaxial Ge grown on Si using an AlAs/ GaAs buffer architecture would be a promising candidate for next-generation high-performance and energy-efficient fin field-effect transistor applications.
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    Heterogeneous integration of InAs/GaSb tunnel diode structure on silicon using 200 nm GaAsSb dislocation filtering buffer
    Liu, J. S.; Clavel, Michael B.; Pandey, R.; Datta, Suman; Xie, Y.; Heremans, Jean J.; Hudait, Mantu K. (AIP Publishing, 2018-10-08)
    An InAs/GaSb tunnel diode structure was heterogeneously integrated on silicon by solid source molecular beam epitaxy using a 200 nm strained GaAs1-ySby dislocation filtering buffer. X-ray analysis demonstrated near complete strain relaxation of the metamorphic buffer and a quasi-lattice-matched InAs/GaSb heterostructure, while high-resolution transmission electron microscopy revealed sharp, atomically abrupt heterointerfaces between the GaSb and InAs epilayers. In-plane magnetotransport analysis revealed Shubnikov-de Haas oscillations, indicating the presence of a dominant high mobility carrier, thereby testifying to the quality of the heterostructure and interfaces. Temperature-dependent current-voltage characteristics of fabricated InAs/GaSb tunnel diodes demonstrated Shockley-Read-Hall generation-recombination at low bias and band-to-band tunneling transport at high bias. The extracted conductance slope from the fabricated tunnel diodes increased with increasing temperature due to thermal emission (Ea ~ 0.48 eV) and trap-assisted tunneling. Thus, this work illustrates the significance of defect control in the heterointegration of metamorphic InAs/GaSb tunnel diode heterostructures on silicon when using GaAs1-ySby dislocation filtering buffers.
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    High carrier lifetimes in epitaxial germanium-tin/Al(In)As heterostructures with variable tin composition
    Hudait, Mantu K.; Johnston, Steven W.; Clavel, Michael B.; Bhattacharya, Shuvodip; Karthikeyan, Sengunthar; Joshi, Rutwik (Royal Society of Chemistry, 2022-06-23)
    Group IV-based germanium-tin (Ge1−ySny) compositional materials have recently shown great promise for infrared detection, light emission and ultra-low power transistors. High carrier lifetimes are desirable for enhancing the detection limit and efficiency of photodetectors, low threshold current density in lasers, and low tunneling barrier height by lowering defects and dislocations at the heterointerface of a source and a channel. Here, carrier lifetimes in epitaxial germanium (Ge) and variable tin (Sn) compositional Ge1−ySny materials were experimentally determined on GaAs substrates using the contactless microwave photoconductive decay (μ-PCD) technique at an excitation wavelength of 1500 nm. Sharp (2 × 2) reflection high energy electron diffraction patterns and low surface roughness were observed from the surface of the Ge0.97Sn0.03 epilayer. X-ray rocking curves from Ge0.97Sn0.03 and Ge0.94Sn0.06 layers demonstrated the pseudomorphic and lattice-matched growth on AlAs and In0.12Al0.88As buffers, respectively, further substantiated by reciprocal space maps and abrupt heterointerfaces evident from the presence of Pendellösung oscillations. High effective carrier lifetimes of 150 ns to 450 ns were measured for Ge1−ySny epilayers as a function of Sn composition, surface roughness, growth temperature, and layer thickness. The observed increase in the carrier lifetime with an increasing Ge layer thickness and a reducing surface roughness, by incorporating Sn, were explained. The enhancement of the carrier lifetime with an increasing Sn concentration was achieved by controlling the defects with lattice-matched Ge0.94Sn0.06/In0.12Al0.88As heterointerfaces or the pseudomorphic growth of Ge0.94Sn0.06 on GaAs. Therefore, our monolithic integration of variable Sn alloy compositional Ge1−ySny materials with high carrier lifetimes opens avenues to realize electronic and optoelectronic devices.
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    Integration of lead-free ferroelectric on HfO2/Si (100) for high performance non-volatile memory applications
    Kundu, Souvik; Maurya, Deepam; Clavel, Michael B.; Zhou, Yuan; Halder, Nripendra N.; Hudait, Mantu K.; Banerji, Pallab; Priya, Shashank (Nature Publishing Group, 2015-02-16)
    We introduce a novel lead-free ferroelectric thin film (1-x)BaTiO3-xBa(Cu1/3Nb2/3)O3 (x 5 0.025) (BT-BCN) integrated on to HfO2 buffered Si for non-volatile memory (NVM) applications. Piezoelectric force microscopy (PFM), x-ray diffraction, and high resolution transmission electron microscopy were employed to establish the ferroelectricity in BT-BCN thin films. PFMstudy reveals that the domains reversal occurs with 1806 phase change by applying external voltage, demonstrating its effectiveness forNVMdevice applications. X-ray photoelectron microscopy was used to investigate the band alignments between atomic layer deposited HfO2 and pulsed laser deposited BT-BCN films. Programming and erasing operations were explained on the basis of band-alignments. The structure offers large memory window, low leakage current, and high and low capacitance values that were easily distinguishable even after ,106 s, indicating strong charge storage potential. This study explains a new approach towards the realization of ferroelectric based memory devices integrated on Si platform and also opens up a new possibility to embed the system within current complementary metal-oxide-semiconductor processing technology.
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    Lead-free epitaxial ferroelectric material integration on semiconducting (100) Nb-doped SrTiO3 for low-power non-volatile memory and efficient ultraviolet ray detection
    Kundu, Souvik; Clavel, Michael B.; Biswas, Pranab; Chen, Bo; Song, Hyun-Cheol; Kumar, Prashant; Halder, Nripendra N.; Hudait, Mantu K.; Banerji, Pallab; Sanghadasa, Mohan; Priya, Shashank (Springer Nature, 2015-07-23)
    We report lead-free ferroelectric based resistive switching non-volatile memory (NVM) devices with epitaxial (1-x)BaTiO3-xBiFeO(3) (x = 0.725) (BT-BFO) film integrated on semiconducting (100) Nb (0.7%) doped SrTiO3 (Nb: STO) substrates. The piezoelectric force microscopy (PFM) measurement at room temperature demonstrated ferroelectricity in the BT-BFO thin film. PFM results also reveal the repeatable polarization inversion by poling, manifesting its potential for read-write operation in NVM devices. The electroforming-free and ferroelectric polarization coupled electrical behaviour demonstrated excellent resistive switching with high retention time, cyclic endurance, and low set/reset voltages. X-ray photoelectron spectroscopy was utilized to determine the band alignment at the BT-BFO and Nb: STO heterojunction, and it exhibited staggered band alignment. This heterojunction is found to behave as an efficient ultraviolet photo-detector with low rise and fall time. The architecture also demonstrates half-wave rectification under low and high input signal frequencies, where the output distortion is minimal. The results provide avenue for an electrical switch that can regulate the pixels in low or high frequency images. Combined this work paves the pathway towards designing future generation low-power ferroelectric based microelectronic devices by merging both electrical and photovoltaic properties of BT-BFO materials.
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    Mapping the Interfacial Electronic Structure of Strain-Engineered Epitaxial Germanium Grown on InxAl1–xAs Stressors
    Clavel, Michael B.; Liu, Jheng-Sin; Bodnar, Robert J.; Hudait, Mantu K. (American Chemical Society, 2022-02-08)
    The indirect nature of silicon (Si) emission currently limits the monolithic integration of photonic circuitry with Si electronics. Approaches to circumvent the optical shortcomings of Si include band structure engineering via alloying (e.g., SixGe1–x–ySny) and/or strain engineering of group IV materials (e.g., Ge). Although these methods enhance emission, many are incapable of realizing practical lasing structures because of poor optical and electrical confinement. Here, we report on strong optoelectronic confinement in a highly tensile-strained (ε) Ge/In0.26Al0.74As heterostructure as determined by X-ray photoemission spectroscopy (XPS). To this end, an ultrathin (∼10 nm) ε-Ge epilayer was directly integrated onto the In0.26Al0.74As stressor using an in situ, dual-chamber molecular beam epitaxy approach. Combining high-resolution X-ray diffraction and Raman spectroscopy, a strain state as high as ε ∼ 1.75% was demonstrated. Moreover, high-resolution transmission electron microscopy confirmed the highly ordered, pseudomorphic nature of the as-grown ε-Ge/In0.26Al0.74As heterostructure. The heterointerfacial electronic structure was likewise probed via XPS, revealing conduction- and valence band offsets (ΔEC and ΔEV) of 1.25 ± 0.1 and 0.56 ± 0.1 eV, respectively. Finally, we compare our empirical results with previously published first-principles calculations investigating the impact of heterointerfacial stoichiometry on the ε-Ge/InxAl1–xAs energy band offset, demonstrating excellent agreement between experimental and theoretical results under an As0.5Ge0.5 interface stoichiometry exhibiting up to two monolayers of heterointerfacial As–Ge diffusion. Taken together, these findings reveal a new route toward the realization of on-Si photonics.
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    Multivalley Electron Conduction at the Indirect-Direct Crossover Point in Highly Tensile-Strained Germanium
    Clavel, Michael B.; Murphy-Armando, F.; Xie, Y.; Henry, K.; Kuhn, M.; Bodnar, Robert J.; Khodaparast, Giti; Smirnov, D.; Heremans, Jean; Hudait, Mantu K. (American Physical Society, 2022-12-01)
    As forward-looking electron devices increasingly adopt high-mobility low-band-gap materials, such as germanium (Ge), questions remain regarding the feasibility of strain engineering in low-band-gap systems. Particularly, the Ge L-Γ valley separation (∼150 meV) can be overcome by introducing a high degree of tensile strain (ε ≥ 1.5%). It is therefore essential to understand the nature of highly strained Ge transport, wherein multivalley electron conduction becomes a possibility. Here, we report on the competitiveness between L- and Γ-valley transport in highly tensile-strained (ε ∼ 1.6%) Ge/In0.24Ga0.76As heterostructures. Temperature-dependent magnetotransport analysis reveals two contributing carrier populations, identified as lower- and higher-mobility L- and Γ-valley electrons (in Ge), using temperature-dependent Boltzmann transport modeling. Coupling this interpretation with electron-cyclotron-resonance studies, the effective mass (m*) of the higher-mobility Γ-valley electrons is probed, revealing m* = (0.049 ± 0.007)me. Moreover, a comparison of empirical and theoretical m* indicates that these electrons reside primarily in the first-two quantum sublevels of the Ge Γ valley. Consequently, our results provide an insight into the strain-dependent carrier dynamics of Ge, offering alternative pathways toward efficacious strain engineering.
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    Reduced erbium-doped ceria nanoparticles: one nano-host applicable for simultaneous optical down- and up-conversions
    Shehata, Nader; Meehan, Kathleen; Hassounah, Ibrahim; Hudait, Mantu K.; Jain, Nikhil; Clavel, Michael B.; Elhelw, Sarah; Madi, Nabil (Springer, 2014-05-13)
    This paper introduces a new synthesis procedure to form erbium-doped ceria nanoparticles (EDC NPs) that can act as an optical medium for both up-conversion and down-conversion in the same time. This synthesis process results qualitatively in a high concentration of Ce3+ ions required to obtain high fluorescence efficiency in the down-conversion process. Simultaneously, the synthesized nanoparticles contain the molecular energy levels of erbium that are required for up-conversion. Therefore, the synthesized EDC NPs can emit visible light when excited with either UV or IR photons. This opens new opportunities for applications where emission of light via both up- and down-conversions from a single nanomaterial is desired such as solar cells and bio-imaging.
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    Structural and optical properties of sulfur passivated epitaxial step-graded GaAs₁₋ySby materials
    Hudait, Mantu K.; Clavel, Michael B.; Saluru, Sarat K.; Liu, Jheng-Sin; Meeker, Michael A.; Khodaparast, Giti A.; Bodnar, Robert J. (American Institute of Physics, 2018-11-15)
    The impact of bulk and surface defect states on the vibrational and optical properties of step-graded epitaxial GaAs₁₋ySby (0 ≤ y ≤ 1) materials with and without chemical surface treatment by (NH₄)₂S was investigated. Tunable antimony (Sb) composition GaAs₁₋ySby epitaxial layers, grown by solid source molecular beam epitaxy (MBE), were realized on GaAs and Si substrates by varying key growth parameters (e.g., Sb/Ga flux ratio, growth temperature). Raman and photoluminescence (PL) spectroscopic analysis of (NH₄)₂S-treated GaAs₁₋ySby epitaxial layers revealed composition-independent Raman spectral widths and enhanced PL intensity (1.3x) following (NH₄)₂S surface treatment, indicating bulk defect-minimal epitaxy and a reduction in the surface recombination velocity corresponding to reduced surface defect sites, respectively. Moreover, quantification of the luminescence recombination mechanisms across a range of measurement temperatures and excitation intensities (i.e., varying laser power) indicate the presence of free-electron to neutral acceptor pair or Sb-defect-related recombination pathways, with detectable bulk defect recombination discernible only in binary GaSb PL spectra. In addition, PL analysis of the short- and long-term thermodynamic stability of sulfur-treated GaAs₁₋ySby/Al₂O₃ heterointerfaces revealed an absence of quantifiable atomic interdiffusion or native oxide formation. Leveraging the combined Raman and PL analysis herein, the quality of the heteroepitaxial step-graded epitaxial GaAs₁₋ySby materials can be optimized for optical devices.
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    TBAL: Tunnel FET-Based Adiabatic Logic for Energy-Efficient, Ultra-Low Voltage IoT Applications
    Liu, Jheng-Sin; Clavel, Michael B.; Hudait, Mantu K. (IEEE, 2019-01-07)
    A novel, tunnel field-effect transistor (TFET)-based adiabatic logic (TBAL) circuit topology has been proposed, evaluated and benchmarked with several device architectures (planar MOSFET, FinFET, and TFET) and AL implementations (efficient charge recovery logic, 2N-2N2P, positive feedback adiabatic logic) operating in the ultra-low voltage (0.3 V ≥ VDD ≤ 0.6 V) regime. By incorporating adiabatic logic functionality into standard combinational logic, an 80% reduction in energy/cycle was achieved. A further 80% reduction in energy/cycle was demonstrated by utilizing near broken-gap TFET devices and simultaneous scaling of supply voltage to 0.3 V, resulting in a 96% reduction in energy/cycle as compared to conventional Si CMOS. Extension of operating frequency beyond 10 MHz, coupled with sub-threshold circuit operation, shows the feasibility of TBAL for energy-efficient Internet of Things applications.