Department of Electrical and Computer Engineering Faculty Publications
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https://docs.lib.purdue.edu/ecepubs
Recent documents in Department of Electrical and Computer Engineering Faculty Publications
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Thu, 28 Apr 2022 02:34:25 PDT
3600

Human Body–Electrode Interfaces for WideFrequency Sensing and Communication: A Review
https://docs.lib.purdue.edu/ecepubs/159
https://docs.lib.purdue.edu/ecepubs/159
Tue, 26 Apr 2022 13:06:57 PDT
Several onbody sensing and communication applications use electrodes in contact with the human body. Body–electrode interfaces in these cases act as a transducer, converting ionic current in the body to electronic current in the sensing and communication circuits and vice versa. An ideal body–electrode interface should have the characteristics of an electrical short, i.e., the transfer of ionic currents and electronic currents across the interface should happen without any hindrance. However, practical body–electrode interfaces often have definite impedances and potentials that hinder the free flow of currents, affecting the application’s performance. Minimizing the impact of body–electrode interfaces on the application’s performance requires one to understand the physics of such interfaces, how it distorts the signals passing through it, and how the interfaceinduced signal degradations affect the applications. Our work deals with reviewing these elements in the context of biopotential sensing and human body communication.
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Kurian Polachan et al.

Limitations of zT as a Figure of Merit for Nanostructured Thermoelectric Materials
https://docs.lib.purdue.edu/ecepubs/158
https://docs.lib.purdue.edu/ecepubs/158
Wed, 04 Dec 2019 12:37:12 PST
Thermoelectric properties of nanocomposites are numerically studied as a function of average grain size or nanoparticle density by simulating the measurements as they would be done experimentally. In accordance with previous theoretical and experimental results, we find that the Seebeck coefficient, power factor and figure of merit, zT, can be increased by nanostructuring when energy barriers exist around the grain boundaries or embedded nanoparticles. When we simulate the performance of a thermoelectric cooler with the same material, however, we find that the maximum temperature difference is much less than expected from the given zT. This occurs because the measurements are done in a way that minimizes Joule heating, but the Joule heating that occurs in operating devices has a large effect for these kinds of materials. The same nanocomposite but without energy barriers at the grain boundaries has a lower measured zT but a higher maximum temperature difference. The physical reason for these results is explained. The results illustrate the limitations of zT as a figure of merit for nanocomposites with electrically active grain boundaries.
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Xufeng Wang et al.

50GhzSpaced Comb of HighDimensional FrequencyBin Entangled Photons from an OnChip Silicon Nitride Microresonator
https://docs.lib.purdue.edu/ecepubs/157
https://docs.lib.purdue.edu/ecepubs/157
Wed, 02 Oct 2019 14:27:47 PDT
Quantum frequency combs from chipscale integrated sources are promising candidates for scalable and robust quantum information processing (QIP). However, to use these quantum combs for frequency domain QIP, demonstration of entanglement in the frequency basis, showing that the entangled photons are in a coherent superposition of multiple frequency bins, is required. We present a verification of qubit and qutrit frequencybin entanglement using an onchip quantum frequency comb with 40 mode pairs, through a twophoton interference measurement that is based on electrooptic phase modulation. Our demonstrations provide an important contribution in establishing integrated optical microresonators as a source for highdimensional frequencybin encoded quantum computing, as well as dense quantum key distribution.
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Poolad Imany et al.

Characterization of Coherent Quantum Frequency Combs Using ElectroOptic Phase Modulation
https://docs.lib.purdue.edu/ecepubs/156
https://docs.lib.purdue.edu/ecepubs/156
Wed, 02 Oct 2019 14:27:35 PDT
We demonstrate a twophoton interference experiment for phase coherent biphoton frequency combs (BFCs), created through spectral amplitude filtering of biphotons with a continuous broadband spectrum. By using an electrooptic phase modulator, we project the BFC lines into sidebands that overlap in frequency. The resulting highvisibility interference patterns provide an approach to verify frequencybin entanglement even with slow singlephoton detectors; we show interference patterns with visibilities that surpass the classical threshold for qubit and qutrit states. Additionally, we show that with entangled qutrits, twophoton interference occurs even with projections onto different final frequency states. Finally,we showthe versatility of this scheme for weaklight measurements by performing a series of twodimensional experiments at different signalidler frequency offsets to measure the dispersion of a singlemode fiber.
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Poolad Imany et al.

HighOrder Coherent Communications Using Modelocked DarkPulse Kerr Combs from Microresonators
https://docs.lib.purdue.edu/ecepubs/155
https://docs.lib.purdue.edu/ecepubs/155
Fri, 27 Sep 2019 12:59:34 PDT
Microresonator frequency combs harness the nonlinear Kerr effect in an integrated optical cavity to generate a multitude of phaselocked frequency lines. The line spacing can reach values in the order of 100 GHz, making it an attractive multiwavelength light source for applications in fiberoptic communications. Depending on the dispersion of the microresonator, different physical dynamics have been observed. A recently discovered comb state corresponds to the formation of modelocked dark pulses in a normaldispersion microcavity. Such darkpulse combs are particularly compelling for advanced coherent communications since they display unusually high powerconversion efficiency. Here, we report the first coherenttransmission experiments using 64quadrature amplitude modulation encoded onto the frequency lines of a darkpulse comb. The high conversion efficiency of the comb enables transmitted optical signaltonoise ratios above 33 dB, while maintaining a laser pump power level compatible with stateoftheart hybrid silicon lasers.
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Attila Fülöp et al.

Measurement of the Lifetime of the 7s2s1/2 State in Atomic Cesium Using Asynchronous Gated Detection
https://docs.lib.purdue.edu/ecepubs/154
https://docs.lib.purdue.edu/ecepubs/154
Fri, 27 Sep 2019 12:59:18 PDT
We report a measurement of the lifetime of the cesium 7s 2S1/2 state using timecorrelated singlephoton counting spectroscopy in a vapor cell. We excite the atoms using a Dopplerfree twophoton transition from the 6s 2S1/2 ground state, and detect the 1.47μm photons from the spontaneous decay of the 7s 2S1/2 to the 6p 2P3/2 state. We use a gated singlephoton detector in an asynchronous mode, allowing us to capture the fluorescence profile for a window much larger than the detector gate length. Analysis of the exponential decay of the photon count yields a 7s 2S1/2 lifetime of 48.28 ± 0.07 ns, an uncertainty of 0.14%. These measurements provide sensitive tests of theoretical models of the Cs atom, which play a central role in parity violation measurements.
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George Toh et al.

Observation of Breathing Dark Pulses in Normal Dispersion Optical Microresonators
https://docs.lib.purdue.edu/ecepubs/153
https://docs.lib.purdue.edu/ecepubs/153
Fri, 27 Sep 2019 12:59:01 PDT
Breathers are localized waves in nonlinear systems that undergo a periodic variation in time or space. The concept of breathers is useful for describing many nonlinear physical systems including granular lattices, BoseEinstein condensates, hydrodynamics, plasmas, and optics. In optics, breathers can exist in either the anomalous or the normal dispersion regimes, but they have only been characterized in the former, to our knowledge. Here, externally pumped optical microresonators are used to characterize the breathing dynamics of localized waves in the normal dispersion regime. HighQ optical microresonators featuring normal dispersion can yield modelocked Kerr combs whose timedomain waveform corresponds to circulating dark pulses in the cavity. We show that with relatively high pump power these Kerr combs can enter a breathing regime, in which the timedomain waveform remains a dark pulse but experiences a periodic modulation on a time scale much slower than the microresonator round trip time. The breathing is observed in the optical frequency domain as a significant difference in the phase and amplitude of the modulation experienced by different spectral lines. In the highly pumped regime, a transition to a chaotic breathing state where the waveform remains darkpulselike is also observed, for the first time to our knowledge; such a transition is reversible by reducing the pump power.
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Chengying Bao et al.

On the Carrier Mobility in ForwardBiased Semiconductor Barriers
https://docs.lib.purdue.edu/ecepubs/152
https://docs.lib.purdue.edu/ecepubs/152
Sat, 16 Feb 2019 06:09:30 PST
A simple onespeed solution to the Boltzmann equation is used to evaluate the mobility and diffusion coefficient for carriers in forwardbiased semiconductor barriers. The analysis shows that although the average kinetic energy of carriers remains near thermal equilibrium, the mobility and diffusion coefficient are strongly reduced by the builtin field. Conventional macroscopic transport equations, which treat the carrier mobility and diffusion coefficient as single valued functions of the kinetic energy will improperly treat transport in forwardbiased barriers. The results are important for the careful analysis of metalsemiconductor and heterojunction diodes.© 1995 American Institute of Physics.
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Mark S. Lundstrom et al.

Computational Electronics for the 21st Century: Reflections on the Past, Present, and Future
https://docs.lib.purdue.edu/ecepubs/151
https://docs.lib.purdue.edu/ecepubs/151
Sat, 16 Feb 2019 06:09:12 PST
The author’s career has coincided with the development of numerical simulation into an essential component of semiconductor device technology research and development. We now have a sophisticated suite of simulation capabilities along with new challenges for 21st Century electronics. This talk presents a short history of the field and a description of the current state of the art, but it concentrates on lessons learned and thoughts about how computational electronics can continue to contribute effectively to the development of new electronic device technologies. The author will argue that electronics is changing, and that computational electronics can play a key role in this evolution. In addition to supporting the continuing development of a small suite of physically detailed / first principles tools, he will argue for more emphasis on analytically compact, strongly physical, conceptual models. Such models help guide the development of physically detailed models, connect to circuit and application designers, and advance device science itself.
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Mark S. Lundstrom

Ballisticity of nanotube fieldeffect transistors: Role of phonon energy and gate bias
https://docs.lib.purdue.edu/ecepubs/150
https://docs.lib.purdue.edu/ecepubs/150
Sat, 16 Feb 2019 06:08:59 PST
We investigate the role of electronphonon scattering and gate bias in degrading the drive current of nanotube fieldeffect transistors FETs. Optical phonon scattering significantly decreases the drive current only when gate voltage is higher than a welldefined threshold. For comparable electronphonon coupling, a lower phonon energy leads to a larger degradation of drive current. Thus in semiconductor nanowire FETs, the drive current will be more sensitive than in carbon nanotube FETs because of the smaller phonon energies in semiconductors. Acoustic phonons and other elastic scattering mechanisms are most detrimental to nanotube FETs irrespective of biasing conditions.
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Siyuranga O. Koswatta et al.

A Simple Boltzmann Transport Equation for Ballistic to Diffusive Transient Heat Transport
https://docs.lib.purdue.edu/ecepubs/149
https://docs.lib.purdue.edu/ecepubs/149
Sat, 16 Feb 2019 06:08:45 PST
Developing simplified, but accurate, theoretical approaches to treat heat transport on all length and time scales is needed to further enable scientific insight and technology innovation. Using a simplified form of the Boltzmann transport equation (BTE), originally developed for electron transport, we demonstrate how ballistic phonon effects and finitevelocity propagation are easily and naturally captured. We show how this approach compares well to the phonon BTE, and readily handles a full phonon dispersion and energydependent meanfreepath. This study of transient heat transport shows (i) how fundamental temperature jumps at the contacts depend simply on the ballistic thermal resistance, (ii) that phonon transport at early times approach the ballistic limit in samples of any length, and (iii) perceived reductions in heat conduction, when ballistic effects are present, originate from reductions in temperature gradient. Importantly, this framework can be recast exactly as the Cattaneo and hyperbolic heat equations, and we discuss how the key to capturing ballistic heat effects is to use the correct physical boundary conditions.
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Mark S. Lundstrom et al.

Steadystate heat transport: Ballistictodiffusive with Fourier's law
https://docs.lib.purdue.edu/ecepubs/148
https://docs.lib.purdue.edu/ecepubs/148
Sat, 16 Feb 2019 06:08:34 PST
It is generally understood that Fourier's law does not describe ballistic phonon transport, which is important when the length of a material is similar to the phonon meanfreepath. Using an approach adapted from electron transport, we demonstrate that Fourier's law and the heat equation do capture ballistic effects, including temperature jumps at ideal contacts, and are thus applicable on all length scales. Local thermal equilibrium is not assumed, because allowing the phonon distribution to be outofequilibrium is important for ballistic and quasiballistic transport. The key to including the nonequilibrium nature of the phonon population is to apply the proper boundary conditions to the heat equation. Simple analytical solutions are derived, showing that (i) the magnitude of the temperature jumps is simply related to the material properties and (ii) the observation of reduced apparent thermal conductivity physically stems from a reduction in the temperature gradient and not from a reduction in actual thermal conductivity.We demonstrate how our approach, equivalent to Fourier's law, easily reproduces results of the Boltzmann transport equation, in all transport regimes, even when using a full phonon dispersion and meanfreepath distribution.
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Jesse Maassen et al.

A computational study of the thermoelectric performance of ultrathin Bi2Te3 films
https://docs.lib.purdue.edu/ecepubs/147
https://docs.lib.purdue.edu/ecepubs/147
Sat, 16 Feb 2019 06:08:23 PST
The ballistic thermoelectric performance of ultrathin films of Bi2Te3, ranging in thickness from 1 to 6 quintuple layers, is analyzed using density functional theory combined with the Landauer approach. Our results show that the thinnest film, corresponding to a single quintuple layer, has an intrinsic advantage originating from the particular shape of its valence band, leading to a large power factor and figureofmerit exceeding bulk Bi2Te3. The interaction between the top and bottom topological surface states is key. The thinnest film yields a sixfold increase in power factor compared to bulk.
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Jesse Maassen et al.

Thermoelectric properties of epitaxial ScN films deposited by reactive magnetron sputtering onto MgO(001) substrates
https://docs.lib.purdue.edu/ecepubs/146
https://docs.lib.purdue.edu/ecepubs/146
Sat, 16 Feb 2019 06:08:11 PST
Epitaxial ScN(001) thin films were grown on MgO(001) substrates by dc reactive magnetron sputtering.The deposition was performed in an Ar/N2 atmosphere at 2 x 10−3 Torr at a substrate temperature of 850 °C in a high vacuum chamber with a base pressure of 10−8 Torr. In spite of oxygen contamination of 1.6 +/ 1 at. %, the electrical resistivity, electron mobility, and carrier concentration obtained from a typical filmgrown under these conditions by room temperature Hall measurements are 0.22 mΩ cm, 106 cm2 V−1 s−1, and 2.5 x 1020 cm−3, respectively. These films exhibit remarkable thermoelectric power factors of 3.3–3.5 × 10−3 W/mK2 in the temperature range of 600 K to 840 K. The crossplane thermal conductivity is 8.3 W/mK at 800 K yielding an estimated ZT of 0.3. Theoretical modeling of the thermoelectric properties of ScN calculated using a meanfreepath of 23 nm at 300 K is in very good agreement with the experiment. These results also demonstrate that further optimization of the power factor of ScN is possible. Firstprinciples density functional theory combined with the site occupancy disorder technique was used to investigate the effect of oxygen contamination on the electronic structure and thermoelectric properties of ScN. The computational results suggest that oxygen atoms in ScN mix uniformly on the N site forming a homogeneous solid solution alloy. Behaving as an ntype donor, oxygen causes a shift of the Fermi levelin ScN into the conduction band without altering the band structure and the density of states.
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Polina V. Burmistrova et al.

Analysis of thermal conductance of ballistic point contacts
https://docs.lib.purdue.edu/ecepubs/145
https://docs.lib.purdue.edu/ecepubs/145
Sat, 16 Feb 2019 06:08:01 PST
Substantial reduction of thermal conductance (Kph) was recently reported for air gap heterostructures (AGHs) in which two bulk layers were connected by lowdensity nanopillars. We analyze Kph using a full phonon dispersion and including important phonon scattering. We find a transition from ballistic at low temperatures to quasiballistic transport near room temperature and explain the slow rolloff in Kph that occurs near room temperature. We show that the density of nanopillars deduced from the analysis depends strongly on the phonon dispersion assumed. Our model provides a good agreement with experiment that will be necessary to design AGHs for thermoelectric applications
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Changwook Jeong et al.

On the best bandstructure for thermoelectric performance: A Landauer Perspective
https://docs.lib.purdue.edu/ecepubs/144
https://docs.lib.purdue.edu/ecepubs/144
Sat, 16 Feb 2019 06:07:52 PST
The question of what bandstructure produces the best thermoelectric device performance is revisited from a Landauer perspective. We find that a deltafunction transport distribution function (TDF) results in operation at the MahanSofo upper limit for the thermoelectric figureofmerit, ZT. We show, however, the MahanSofo upper limit itself depends on the bandwidth (BW) of the dispersion, and therefore, a finite BW dispersion produces a higher ZT when the lattice thermal conductivity is finite. Including a realistic model for scattering profoundly changes the results. Instead of a narrow band, we find that a broad BW is best. The prospects of increasing ZT through high valley degeneracy or by distorting the densityofstates are discussed from a Landauer perspective. We conclude that while there is no simple answer to the question of what bandstructure produces the best thermoelectric performance, the important considerations can be expressed in terms of three parameters derived from the bandstructure  the densityofstates, D(E), the number of channels, M(E), and the meanfreepath, k(E)
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Changwook Jeong et al.

Thermal conductivity of bulk and thinfilm silicon: A Landauer approach
https://docs.lib.purdue.edu/ecepubs/143
https://docs.lib.purdue.edu/ecepubs/143
Sat, 16 Feb 2019 06:07:42 PST
The question of what fraction of the total heat flow is transported by phonons with different meanfreepaths is addressed using a Landauer approach with a full dispersion description of phonons to evaluate the thermal conductivities of bulk and thin film silicon. For bulk Si, the results reproduce those of a recent molecular dynamic treatment showing that about 50% of the heat conduction is carried by phonons with a meanfreepath greater than about 1 μm. For the inplane thermal conductivity of thin Si films, we find that about 50% of the heat is carried by phonons with meanfreepaths shorter than in the bulk. When the film thickness is smaller than ∼0.2 μm, 50% of the heat is carried by phonons with meanfreepaths longer than the film thickness. The crossplane thermal conductivity of thinfilms, where quasiballistic phonontransport becomes important, is also examined. For ballistic transport, the results reduce to the wellknown Casimir limit [H. B. G. Casimir, Physica 5, 495–500 (1938)]. These results shed light on phonontransport in bulk and thinfilm silicon and demonstrate that the Landauer approach provides a relatively simple but accurate technique to treat phonontransport from the ballistic to diffusive regimes.
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Changwook Jeong et al.

Can morphology tailoring improve the open circuit voltage of organic solar cells?
https://docs.lib.purdue.edu/ecepubs/142
https://docs.lib.purdue.edu/ecepubs/142
Sat, 16 Feb 2019 06:07:32 PST
While the effect of interfacial morphology on the short circuit current (ISC) of organic photovoltaic devices (OPVs) is well known, its impact on open circuit voltage (VOC) and fillfactor (FF) are less clear. Since the output power of a solar cell Pout 1/4 ISCVOCFF, such understanding is critical for designing highperformance, morphologyengineered OPVs. In this letter, we provide an explicit analytical proof that any effort to radically improve VOC by tailoring bulk heterojunction morphology is futile, because any increase in ISC due to larger interface area is counterbalanced by corresponding increase in recombination current, so that the upper limit of VBHJ OC cannot exceed that of the corresponding planar heterojunction devices, i.e... We discuss the implication of this VOCconstraint on the efficiency optimization of organic solar cells.
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Biswajit Ray et al.

Computational study of energy filtering effects in onedimensional composite nanostructures
https://docs.lib.purdue.edu/ecepubs/141
https://docs.lib.purdue.edu/ecepubs/141
Sat, 16 Feb 2019 06:07:22 PST
Possibilities to improve the Seebeck coefficient S versus electrical conductance G tradeoff of diffusive composite nanostructures are explored using an electrothermal simulation framework based on the nonequilibrium Green’s function method for quantum electron transport and the lattice heat diffusion equation. We examine the role of the grain size d, potential barrier height UB, grain doping, and the lattice thermal conductivity jL using a onedimensional model structure. For a uniform jL, simulation results show that the power factor of a composite structure may be improved over bulk with the optimum UB being about kBT, where kB and T are the Boltzmann constant and the temperature, respectively. An optimum UB occurs because the current flow near the Fermi level is not obstructed too much while S still improves due to barriers. The optimum grain size dopt is significantly longer than the momentum relaxation length kp so that G is not seriously degraded due to the barriers, and dopt is comparable to or somewhat larger than the energy relaxation length kE so that the carrier energy is not fully relaxed within the grain and jSj remains high. Simulation results also show that if jL in the barrier region is smaller than in the grain, S and power factor are further improved. In such cases, the optimum UB and dopt increase, and the power factor may improve even for UB (d) significantly higher (longer) than kBT (kE). We find that the results from this quantum mechanical approach are readily understood using a simple, semiclassical model.
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Raseong Kim et al.

Computational study of the Seebeck coefficient of onedimensional composite nanostructures
https://docs.lib.purdue.edu/ecepubs/140
https://docs.lib.purdue.edu/ecepubs/140
Sat, 16 Feb 2019 06:07:12 PST
The Seebeck coefficient (S) of composite nanostructures is theoretically explored within a selfconsistent electrothermal transport simulation framework using the nonequilibrium Green’s function method and a heat diffusion equation. Seebeck coefficients are determined using numerical techniques that mimic experimental measurements. Simulation results show that, without energy relaxing scattering, the overall S of a composite structure is determined by the highest barrier within the device. For a diffusive, composite structure with energy relaxation due to electronphonon scattering, however, the measured Sis an average of the positiondependent values with the weighting factor being the lattice temperature gradient. The results stress the importance of selfconsistent solutions of phonon heat transport and the resulting lattice temperature distribution in understanding the thermoelectric properties of a compositestructure. It is also clarified that the measured S of a composite structure reflects its power generation performance rather than its cooling performance. The results suggest that the lattice thermal conductivity within the composite structure might be engineered to improve the power factor over the bulk by avoiding the conventional tradeoff between S and the electrical conductivity.
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Raseong Kim et al.