Publications

My full list of publications can be also found in my Google Scholar Profile.

2020

1. N-polar GaN/AlN Resonant Tunneling Diodes Cho^★, YongJin, Encomendero^★, Jimy, Ho, Shao-Ting, Xing, Huili Grace, and Jena, Debdeep Applied Physics Letters 117, 143501 (2020) [Abstract] [DOI] [PDF] ★ Equal contributions

   N-polar GaN/AlN resonant tunneling diodes are realized on a single-crystal N-polar GaN bulk substrate by plasma-assisted molecular beam epitaxy growth. The room-temperature current–voltage characteristics reveal a negative differential conductance (NDC) region with a peak tunneling current of 6.8±0.8 kA/cm² at a forward bias of ∼8 V. Under reverse bias, the polarization-induced threshold voltage is measured at ∼−4 V. These resonant and threshold voltages are well explained with the polarization field, which is opposite to that of the metal-polar counterpart, confirming the N-polarity of the resonant tunneling diodes (RTDs). When the device is biased in the NDC-region, electronic oscillations are generated in the external circuit, attesting to the robustness of the resonant tunneling phenomenon. In contrast to metal-polar RTDs, N-polar structures have the emitter on the top of the resonant tunneling cavity. As a consequence, this device architecture opens up the possibility of seamlessly interfacing—via resonant tunneling injection—a wide range of exotic materials with III-nitride semiconductors, providing a route towards unexplored device physics.

2. Fighting Broken Symmetry with Doping: Toward Polar Resonant Tunneling Diodes with Symmetric Characteristics Encomendero, Jimy, Protasenko, Vladimir, Sensale-Rodriguez, Berardi, Fay, Patrick, Rana, Farhan, Jena, Debdeep, and Xing, Huili Grace Physical Review Applied 13, 034048 (2020) [Abstract] [DOI] [PDF]

   The recent demonstration of resonant tunneling transport in nitride semiconductors has led to an invigorated effort to harness this quantum transport regime for practical applications. In polar semiconductors, however, the interplay between fixed polarization charges and mobile free carriers leads to asymmetric transport characteristics. Here, we investigate the possibility of using degenerately doped contact layers to screen the built-in polarization fields and recover symmetric resonant injection. Thanks to a high doping density, negative differential conductance is observed under both bias polarities of GaN/AlN resonant tunneling diodes (RTDs). Moreover, our analytical model reveals a lower bound for the minimum resonant-tunneling voltage achieved via uniform doping, owing to the dopant solubility limit. Charge storage dynamics is also studied by impedance measurements, showing that at close-to-equilibrium conditions, polar RTDs behave effectively as parallel-plate capacitors. These mechanisms are completely reproduced by our analytical model, providing a theoretical framework useful in the design and analysis of polar resonant-tunneling devices.

3. Gallium Nitride Tunneling Field-effect Transistors Exploiting Polarization Fields Chaney, Alexander, Turski, Henryk, Nomoto, Kazuki, Hu, Zongyang, Encomendero, Jimy, Rouvimov, Sergei, Orlova, Tatyana, Fay, Patrick, Seabaugh, Alan, Xing, Huili Grace, and Jena, Debdeep Applied Physics Letters 116, 073502 (2020) [Abstract] [DOI] [PDF]

   This report showcases a vertical tunnel field effect transistor (TFET) fabricated from a GaN/InGaN heterostructure and compares it to a gated vertical GaN p-n diode. By including a thin InGaN layer, the interband tunneling in the TFET is increased compared to the gated homojunction diode. This leads to an increased drain current of 57 μA/μm and a reduced subthreshold swing of 102 mV/dec, from 240 mV/dec. However, trap assisted tunneling prevents devices from realizing subthreshold slopes below the Boltzmann limit of 60 mV/dec. Nevertheless, this work shows the capability of tunnel field effect transistors to be realized in GaN by taking advantage of the spontaneous and piezoelectric polarization in the III-N material system.

4. Resonant Tunneling Transport in Polar III-Nitride Heterostructures Encomendero, Jimy, Jena, Debdeep, and Xing, Huili Grace In High Frequency GaN Electronic Devices. Springer, Cham (2020) [Abstract] [DOI]

   During the last two decades, a considerable effort has been dedicated to engineer resonant tunneling transport within the family of III-Nitride semiconductors. At the heart of this initiative, lies the outstanding material properties that this revolutionary family of semiconductors offers for manufacturing high-power ultra-fast oscillators and terahertz quantum cascade lasers that operate at room temperature. Despite these efforts, it is only recently that room temperature resonant tunneling transport has been demonstrated within III-Nitride materials. In this chapter we discuss various aspects of heterostructure design, epitaxial growth and device fabrication, which have led to the first unequivocal demonstration of robust resonant tunneling transport, and reliable room temperature negative differential conductance resulting in the generation of GHz oscillations in III-Nitride semiconductors. These advances allowed us to shed light into the physics of resonant tunneling transport in polar semiconductors which had remained hidden until now. This insight was obtained using a combined experimental and theoretical approach, leading to the discovery of new tunneling features, unique in polar RTDs. After identifying the intimate connection between the polarization fields and the resonant tunneling current, we harness this relationship by introducing a completely new approach to measure the magnitude of these internal polarization fields. Moreover, precise epitaxial growth at the single-monolayer level is employed to engineer resonant tunneling currents >200 kA/cm² and demonstrate the first microwave oscillator driven by a GaN/AlN resonant tunneling diode.The findings presented in this chapter pave the way for the realization of III-Nitride-based high-speed oscillators and quantum cascade lasers that operate at wavelengths that remain unreachable by other semiconductor materials.

2019

1. Broken Symmetry Effects due to Polarization on Resonant Tunneling Transport in Double-Barrier Nitride Heterostructures Encomendero, Jimy, Protasenko, Vladimir, Sensale-Rodriguez, Berardi, Fay, Patrick, Rana, Farhan, Jena, Debdeep, and Xing, Huili Grace Physical Review Applied 11, 034032 (2019) [Abstract] [DOI] [PDF] ★ Editor’s Suggestion

   The phenomenon of resonant tunneling transport through polar double-barrier heterostructures is systematically investigated by a combined experimental and theoretical approach. On the experimental side, GaN/AlN resonant tunneling diodes (RTDs) are grown by molecular beam epitaxy. In situ electron diffraction is used to monitor the number of monolayers incorporated into each tunneling barrier of the RTD active region. Using this precise epitaxial control at the monolayer level, we demonstrate exponential modulation of the resonant tunneling current density as a function of barrier thickness. At the same time, both the peak voltage and the characteristic threshold bias exhibit a dependence on barrier thickness as a result of the intense electric fields present within the polar heterostructures. To get further insight into the asymmetric tunneling injection originating from the polar active region, we present an analytical theory for tunneling transport across polar heterostructures. A general expression for the resonant tunneling current that includes contributions from coherent and sequential tunneling processes is introduced. After the application of this theory to the case of GaN/AlN RTDs, their experimental current-voltage characteristics are reproduced over both bias polarities, with tunneling currents spanning several orders of magnitude. This agreement allows us to elucidate the effect of the internal polarization fields on the magnitude of the tunneling current and broadening of the resonant tunneling line shape. Under reverse bias, we identify new tunneling features originating from highly attenuated resonant tunneling phenomena, which are completely captured by our model. The analytical form of our quantum transport model provides a simple expression that reveals the connection between the design parameters of a general polar RTD and its current-voltage characteristics. This new theory paves the way for the design of polar resonant tunneling devices exhibiting efficient resonant current injection and enhanced tunneling dynamics as required in various practical applications.

2. New physics in GaN resonant tunneling diodes Xing, Huili Grace, Encomendero, Jimy, and Jena, Debdeep In Proceedings SPIE 10918, Gallium Nitride Materials and Devices XIV, 109180Z (2019) [Abstract] [DOI] [PDF]

   The outstanding material properties of III-Nitride semiconductors, has prompted intense research efforts in order to engineer resonant tunneling transport within this revolutionary family of wide-bandgap semiconductors. From resonant tunneling diode (RTD) oscillators to quantum cascade lasers (QCLs), III-Nitride heterostructures hold the promise for the realization of high-power ultra-fast sources of terahertz (THz) radiation. However, despite the considerable efforts over last two decades, it is only during the last three years that room temperature resonant tunneling transport has been demonstrated within the III-Nitride family of semiconductors. In this paper we present an overview of our current understanding of resonant tunneling transport in polar heterostructures. In particular we focus on double-barrier III-Nitride RTDs which represents the simplest device in which the dramatic effects of the internal polarization fields can be studied. Tunneling transport within III-heterostructures is strongly influenced by the presence of the intense spontaneous and piezoelectric polarization fields which result from the non-centrosymmetric crystal structure of III-Nitride semiconductors. Advances in heterostructure design, epitaxial growth and device fabrication have led to the first unequivocal demonstration of robust and reliable negative differential conductance. which has been employed for the generation of microwave power from III-Nitride RTD oscillator. These significant advances allowed us to shed light into the physics of resonant tunneling transport in polar semiconductors which had remained hidden until now.

2018

1. Synchronized Plasma Wave Resonances in Ultrathin-Membrane GaN Heterostructures Condori Quispe, H. O., Chanana, A., Nahata, A., Sensale-Rodriguez, Berardi, Encomendero, Jimy, Zhu, Mingda, Jena, Debdeep, Xing, Huili Grace, and Trometer, N. In 43rd International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz) (2018) [Abstract] [DOI] [PDF]

   In this work we report on synchronized plasma wave resonances in ultrathin-membrane GaN heterostructures. In contrast to commonly employed grating-gate configurations, the analyzed structure contains periodically-patterned ohmic contacts to the two-dimensional electron gas (2DEG), which are laid-out parallel to the gate fingers. Our work demonstrates that the proposed approach allows: more efficient excitation of high order plasmon modes, and superior overall coupling, even in configurations having less number of devices per unit area.

2. Comparison of Unit Cell Coupling for Grating‐gate and High electron Mobility Transistor Array THz Resonant Absorbers Condori Quispe, Hugo O., Chanana, Ashish, Encomendero, Jimy, Zhu, Mingda, Trometer, Nicole, Nahata, Ajay, Jena, Debdeep, Xing, Huili Grace, and Sensale-Rodriguez, Berardi Journal of Applied Physics 124, 093101 (2018) [Abstract] [DOI] [PDF] ★ Featured as the Journal Cover

   We report experimental studies on the excitation of synchronized plasmon resonances in AlGaN/GaN High Electron Mobility Transistor (HEMT) arrays. In contrast to the commonly employed grating-gate configurations, the analyzed structure contains periodically patterned ohmic contacts to the two-dimensional electron gas, which are laid-out parallel to the gate fingers. In this structure, the terahertz to plasmon coupling mechanism is fundamentally different from that in grating-gate configurations. Whereas the grating-gate configuration constitutes a coupled resonant system in which the resonance frequency depends on the grating periodicity, when periodical ohmic contacts are incorporated, the system behaves as a synchronized resonant system in which each unit cell is effectively independent. As a result, in a HEMT-array, the resonance is no longer set by the periodicity but rather by the gate and the ungated region length. Experimental results of fabricated samples compare well with numerical simulations and theoretical expectations. Our work demonstrates that the proposed approach allows: (i) more efficient excitation of high order plasmon modes and (ii) superior overall terahertz to plasmon coupling, even in configurations having less number of devices per unit area. From this perspective, our results reveal a simple way to enhance the terahertz to plasmon coupling and thus improve the performance of electron plasma wave-based devices; this effect can be exploited, for example, to improve the response of HEMT-based terahertz detectors.

3. Room temperature microwave oscillations in GaN/AlN resonant tunneling diodes with peak current densities up to 220 kA/cm² Encomendero, Jimy, Yan, Rusen, Verma, Amit, Islam, S. M., Protasenko, Vladimir, Rouvimov, Sergei, Fay, Patrick, Jena, Debdeep, and Xing, Huili Grace Applied Physics Letters 112, 103101 (2018) [Abstract] [DOI] [PDF] ★ Featured as the Journal Cover

   We report the generation of room temperature microwave oscillations from GaN/AlN resonant tunneling diodes, which exhibit record-high peak current densities. The tunneling heterostructure grown by molecular beam epitaxy on freestanding GaN substrates comprises a thin GaN quantum well embedded between two AlN tunneling barriers. The room temperature current-voltage characteristics exhibit a record-high maximum peak current density of ∼220 kA/cm². When biased within the negative differential conductance region, microwave oscillations are measured with a fundamental frequency of ∼0.94 GHz, generating an output power of ∼3.0 μW. Both the fundamental frequency and the output power of the oscillator are limited by the external biasing circuit. Using a small-signal equivalent circuit model, the maximum intrinsic frequency of oscillation for these diodes is predicted to be ∼200 GHz. This work represents a significant step towards microwave power generation enabled by resonant tunneling transport, an ultra-fast process that goes beyond the limitations of current III-Nitride high electron mobility transistors.

2017

1. New Tunneling Features in Polar III-Nitride Resonant Tunneling Diodes Encomendero, Jimy, Faria, Faiza Afroz, Islam, S. M., Protasenko, Vladimir, Rouvimov, Sergei, Sensale-Rodriguez, Berardi, Fay, Patrick, Jena, Debdeep, and Xing, Huili Grace Physical Review X 7, 041017 (2017) [Abstract] [DOI] [PDF] ★ Featured by Semiconductor Today Magazine

   For the past two decades, repeatable resonant tunneling transport of electrons in III-nitride double barrier heterostructures has remained elusive at room temperature. In this work we theoretically and experimentally study III-nitride double-barrier resonant tunneling diodes (RTDs), the quantum transport characteristics of which exhibit new features that are unexplainable using existing semiconductor theory. The repeatable and robust resonant transport in our devices enables us to track the origin of these features to the broken inversion symmetry in the uniaxial crystal structure, which generates built-in spontaneous and piezoelectric polarization fields. Resonant tunneling transport enabled by the ground state as well as by the first excited state is demonstrated for the first time over a wide temperature window in planar III-nitride RTDs. An analytical transport model for polar resonant tunneling heterostructures is introduced for the first time, showing a good quantitative agreement with experimental data. From this model we realize that tunneling transport is an extremely sensitive measure of the built-in polarization fields. Since such electric fields play a crucial role in the design of electronic and photonic devices, but are difficult to measure, our work provides a completely new method to accurately determine their magnitude for the entire class of polar heterostructures.

2. Experimental demonstration of enhanced terahertz coupling to plasmon in ultra-thin membrane AlGaN/GaN HEMT arrays Quispe, H. O. C., Chanana, A., Encomendero, Jimy, Zhu, M., Nahata, A., Jena, D., Xing, H. G., and Sensale-Rodriguez, B. In 75th Annual Device Research Conference (DRC) (2017) [Abstract] [DOI] [PDF]

   Here we present the first experimental demonstration of enhanced THz coupling to electron plasma wave or plasmon in ultra-thin membrane HEMT arrays via plasmon synchronization. A thin-membrane configuration enables us to remove substrate effects and further enhance the coupling. The proposed approach allows: (i) more efficient excitation of high order plasmonic modes, and (ii) superior overall coupling -even in configurations having less number of devices per unit area-. Our results reveal a simple way to enhance the THz to plasmon coupling and thus improve the performance of electron plasma wave based devices; this effect can be exploited, for example, to improve the response of HEMT THz detectors.

2016

1. Terahertz plasmon amplification in RTD-gated HEMTs with a grating-gate Quispe, Hugo O. Condori, Encomendero, Jimy, Xing, Huili Grace, and Rodriguez, Berardi Sensale In Proceedings SPIE 9920, Active Photonic Materials VIII, 992027 (2016) [Abstract] [DOI] [PDF]

   We analyze amplification of terahertz plasmons in a grating-gate semiconductor hetero-structure. The device consists of a resonant-tunneling-diode gated high-electron-mobility transistor (RTD-gated HEMT), i.e. a HEMT structure with a double-barrier gate stack enabling resonant tunneling from gate to channel. In these devices, the key element enabling substantial power gain is the coupling of terahertz waves into and out of plasmons in the RTD-gated HEMT channel, i.e. the gain medium, via the grating-gate itself, part of the active device, rather than by an external antenna structure as in previous works, enabling amplification with associated power gain >> 30 dB at room temperature.

2. Terahertz amplification in RTD-gated HEMTs with a grating-gate wave coupling topology Condori Quispe, Hugo O., Encomendero-Risco, Jimy J., Xing, Huili Grace, and Sensale-Rodriguez, Berardi Applied Physics Letters 109, 063111 (2016) [Abstract] [DOI] [PDF]

   We theoretically analyze the operation of a terahertz amplifier consisting of a resonant-tunneling-diode gated high-electron-mobility transistor (RTD-gated HEMT) in a grating-gate topology. In these devices, the key element enabling substantial power gain is the efficient coupling of terahertz waves into and out of plasmons in the RTD-gated HEMT channel, i.e., the gain medium, via the grating-gate itself, part of the active device, rather than by an external antenna structure as discussed in previous works, therefore potentially enabling terahertz amplification with associated power gains >40 dB.

2015

1. THz devices based on 2D electron systems Xing, Huili Grace, Yan, Rusen, Song, Bo, Encomendero, Jimy, and Jena, Debdeep In Proceedings SPIE 9476, Automatic Target Recognition XXV, 94760Q (2015) [Abstract] [DOI] [PDF]

   In two-dimensional electron systems with mobility on the order of 1,000—10,000 cm²/Vs, the electron scattering time is about 1 ps. For the THz window of 0.3 – 3 THz, the THz photon energy is in the neighborhood of 1 meV, substantially smaller than the optical phonon energy of solids where these 2D electron systems resides. These properties make the 2D electron systems interesting as a platform to realize THz devices. In this paper, I will review 3 approaches investigated in the past few years in my group toward THz devices. The first approach is the conventional high electron mobility transistor based on GaN toward THz amplifiers. The second approach is to employ the tunable intraband absorption in 2D electron systems to realize THz modulators, where I will use graphene as a model material system. The third approach is to exploit plasma wave in these 2D electron systems that can be coupled with a negative differential conductance element for THz amplifiers/sources/detectors.

2014

1. Direct electrical observation of plasma wave-related effects in GaN-based two-dimensional electron gases Zhao, Y., Chen, W., Li, W., Zhu, M., Yue, Y., Song, B., Encomendero, J., Sensale-Rodriguez, B., Xing, H., and Fay, P. Applied Physics Letters 105, 173508 (2014) [Abstract] [DOI] [PDF]

   In this work, signatures of plasma waves in GaN-based high electron mobility transistors were observed by direct electrical measurement at room temperature. Periodic grating-gate device structures were fabricated and characterized by on-wafer G-band (140–220 GHz) s-parameter measurements as a function of gate bias voltage and device geometry. A physics-based equivalent circuit model was used to assist in interpreting the measured s-parameters. The kinetic inductance extracted from the measurement data matches well with theoretical predictions, consistent with direct observation of plasma wave-related effects in GaN-channel devices at room temperature. This observation of electrically significant room-temperature plasma-wave effects in GaN-channel devices may have implications for future millimeter-wave and THz device concepts and designs.

2. Metal-Ion Effects on the Polarization of Metal-Bound Water and Infrared Vibrational Modes of the Coordinated Metal Center of Mycobacterium tuberculosis Pyrazinamidase via Quantum Mechanical Calculations Salazar-Salinas, Karim, Baldera-Aguayo, Pedro A., Encomendero-Risco, Jimy J., Orihuela, Melvin, Sheen, Patricia, Seminario, Jorge M., and Zimic, Mirko The Journal of Physical Chemistry B, 118, 34, 10065-10075 (2014) [Abstract] [DOI] [PDF]

   Mycobacterium tuberculosis pyrazinamidase (PZAse) is a key enzyme to activate the pro-drug pyrazinamide (PZA). PZAse is a metalloenzyme that coordinates in vitro different divalent metal cofactors in the metal coordination site (MCS). Several metals including Co2+, Mn2+, and Zn2+ are able to reactivate the metal-depleted PZAse in vitro. We use quantum mechanical calculations to investigate the Zn2+, Fe2+, and Mn2+ metal cofactor effects on the local MCS structure, metal–ligand or metal–residue binding energy, and charge distribution. Results suggest that the major metal-dependent changes occur in the metal–ligand binding energy and charge distribution. Zn2+ shows the highest binding energy to the ligands (residues). In addition, Zn2+ and Mn2+ within the PZAse MCS highly polarize the O–H bond of coordinated water molecules in comparison with Fe2+. This suggests that the coordination of Zn2+ or Mn2+ to the PZAse protein facilitates the deprotonation of coordinated water to generate a nucleophile for catalysis as in carboxypeptidase A. Because metal ion binding is relevant to enzymatic reaction, identification of the metal binding event is important. The infrared vibrational mode shift of the C═Nε (His) bond from the M. tuberculosis MCS is the best IR probe to metal complexation.

2013

1. Noise performance of RTD-gated plasma-wave HEMT THz detectors Encomendero-Risco, J. J., Sensale-Rodriguez, B., and Xing, H. G. In 38th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz) (2013) [Abstract] [DOI] [PDF]

   In this paper, we study the noise performance of RTD-gated plasma-wave HEMT THz detectors. It is shown that noise in these devices is dominated by gate tunneling shot noise, and that a smaller effective electron mass promises much improved noise performance by boosting the responsivity while slightly decreasing the noise spectral density (NSD). This implies that it is desirable to realize RTD-gated plasma-wave HEMT THz detectors in material systems with low effective mass.