Browsing by Author "Liu, Jheng-Sin"

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    Advanced Energy-Efficient Devices for Ultra-Low Voltage System: Materials-to-Circuits
    Liu, Jheng-Sin (Virginia Tech, 2018-01-18)
    The overall energy consumption of portable devices has been projected to triple over the next decade, growing to match the total power generated by the European Union and Canada by 2025. The rise of the internet-of-things (IoT) and ubiquitous and embedded computing has resulted in an exponential increase in such devices, wherein projections estimate that 50 billion smart devices will be connected and online by 2020. In order to alleviate the associated stresses placed on power generation and distribution networks, a holistic approach must be taken to conserve energy usage in electronic devices from the component to the circuit level. An effective approach to reduce power dissipation has been a continual reduction in operating voltage, thereby quadratically down-scaling active power dissipation. However, as state-of-the-art silicon (Si) complimentary metal-oxide-semiconductor (CMOS) field-effect transistors (FETs) enter sub-threshold operation in the ultra-low supply voltage regime, their drive current is noticeable degraded. Therefore, new energy-efficient MOSFETs and circuit architectures must be introduced. In this work, tunnel FETs (TFETs), which operate leveraging quantum mechanical tunneling, are investigated. A comprehensive investigation detailing electronic materials, to novel TFET device designs, to memory and logic digital circuits based upon those TFETs is provided in this work. Combined, these advances offer a computing platform that could save considerable energy and reduce power consumption in next-generation, ultra-low voltage applications.
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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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    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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    Performance Analysis of TaSiOx Inspired Sub-10 nm Energy Efficient In₀.₅₃Ga₀.₄₇As Quantum Well Tri-Gate Technology
    Saluru, Sarat K.; Liu, Jheng-Sin; Hudait, Mantu K. (IEEE, 2017-10-24)
    In this paper, for the first time, the performance analysis of short channel In₀.₅₃Ga₀.₄₇As quantum well (QW) 3-D tri-gate technology with advanced high-κ gate dielectric, TaSiOx is presented. We benchmark the projected performance of sub-10 nm In₀.₅₃Ga₀.₄₇As transistor technology as a function of fin width, fin aspect ratio, and gate length scaling based on present-day lithographic advancement aiding InGaAs QW tri-gate technology as a replacement to Si for sub-10 nm transistor technology. The highly scaled oxide (EOT ∼ 12Å) while retaining superior interfacial properties (Dit ∼ 4 × 10¹¹ cm⁻²eV⁻¹) provides higher ON current for given idle performance. Furthermore, the simulated In₀.₅₃Ga₀.₄₇As tri-gate transistor exhibits superior gate electrostatic control with low OFF-state current (IOFF) ∼ 24.5 nA/μm, peak transconductance (gm) ∼ 2 mS/ μm and high ION/IOFF ratio ∼ 2.3 × 10³, aiding the case of alternate channel transistors for high-speed and low-power CMOS logic.
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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.
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    Temperature and Doping-Dependent Interplay between Direct and Indirect Optical Response in Buffer-Mediated Epitaxial Germanium
    Hudait, Mantu K.; Meeker, Michael; Liu, Jheng-Sin; Clavel, Michael; Bhattacharya, Shuvodip; Khodaparast, Giti (Elsevier, 2022-09-01)
    The structural and optical properties of buffer mediated epitaxial germanium (Ge) layer were investigated and compared with bulk n-type and p-type Ge substrates. An interconnected dual-chamber molecular beam epitaxy (MBE) system was used to grow a 280 nm thin Ge epilayer on (100)GaAs substrate with an intermediate AlAs buffer layer. The lattice-matched, abrupt Ge/AlAs heterointerface was analyzed using cross-sectional transmission electron microscopic analysis, and no elemental interdiffusion was detected via secondary ion mass spectrometry. A strong direct gap transition, compared to the indirect gap transition, and a series of phonon-assisted transitions was observed by photoluminescence (PL) spectroscopy. In addition, the intensity of the direct gap recombination decreases with decreasing PL measurement temperatures, which was ascribed to the reduced density of Γ-valley electrons available for recombination at lower temperature. Furthermore, the intensity ratio between the direct and indirect optical transition drastically decreases with decreasing temperature in both n-type epitaxial and p-type bulk Ge. An empirical relation in both direct and indirect peak position with temperature was established. The observed strong luminescence in 280 nm thick epitaxial Ge at room temperature is vital for Ge-based photonic devices. In addition, the quality of the epitaxial Ge layer grown via MBE is on par with bulk Ge substrates.