Department of Electrical Engineering

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To understand the different mechanism occurring at the MoS2-silicon interface, we have fabricated a MoS2/Si heterojunction by exfoliating MoS2 on top of the silicon substrate. Raman spectroscopy and atomic force microscopy (AFM) measurement expose the signature of few-layers in the deposited MoS2 flake. Herein, the temperature dependence of the energy barrier and carrier density at the MoS2/Si heterojunction has been extensively investigated. Furthermore, to study band alignment at the MoS2/Si interface, we have calculated a valence band offset of 0.66 ± 0.17 eV and a conduction band offset of 0.42 ± 0.17 eV using X-ray and Ultraviolet photoelectron spectroscopy. We determined a type-II band alignment at the interface which is very conducive for the transport of photoexcited carriers. As a proof-of-concept application, we extend our analysis of the photovoltaic behavior of the MoS2/Si heterojunction. This work provides not only a comparative study between MoS2/p-Si and MoS2/n-Si heterojunctions but also paves the way to engineer the properties of the interface for the future integration of MoS2 with silicon. © 2018 Author(s).

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Journal of Applied Physics

Determination of band alignment at two-dimensional MoS2/Si van der Waals heterojunction

We report on a high-performance NO2 sensor based on a one dimensional MoS2 nanowire (NW) network. The MoS2 NW network was synthesized using chemical transport reaction through controlled turbulent vapor flow. The crystal structure and surface morphology of MoS2 NWs were confirmed by X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy. Further, the sensing behavior of the nanowires was investigated at different temperatures for various concentrations of NO2 and the sensor exhibited about 2-fold enhanced sensitivity with a low detection limit of 4.6 ppb for NO2 at 60 °C compared to sensitivity at room temperature. Moreover, it showed a fast response (16 s) with complete recovery (172 s) at 60 °C, while sensitivity of the device was decreased at 120 °C. The efficient sensing with reliable selectivity toward NO2 of the nanowires is attributed to a combination of abundant active edge sites along with a large surface area and tuning of the potential barrier at the intersections of nanowires during adsorption/desorption of gas molecules. © 2018 Author(s).

Applied Physics Letters

High performance NO2 sensor using MoS2 nanowires network

Molybdenum disulfide (MoS2), a just three atoms thick semiconductor has opened up new avenues in gas sensing because of its high surface-to-volume ratio. However, the low sensitivity, incomplete recovery and poor selectivity at room temperature restricts its uses for practical gas sensors. Here, we report a facile strategy to develop highly sensitive, extremely selective and fully recoverable gas sensor at room temperature through photo excitation. Although, pristine MoS2 gas sensor showed a considerable response and incomplete recovery to NO2 at room temperature. Though after photo excitation, the MoS2 sensor exhibited an improved response along with the complete recovery upon NO2 exposure. © 2018 Author(s).

AIP Conference Proceedings

Enhanced sensing response with complete recovery of MoS2 sensor under photoexcitation

Layered transition metal dichalcogenide materials grown over a conventional 3D semiconductor substrate have ignited a spark of interest in the electronics industry. The integration of these 2D layered materials extensively addresses the formidable challenges faced by a new generation of opto-electronic and photovoltaic devices. Herein, we have demonstrated a 2D/3D heterojunction type photodetector by depositing MoS2 on a GaN substrate with a mass-scalable sputtering method. Spectroscopic and microscopic characterizations expose the signature of the highly crystalline, homogeneous and controlled growth of a deposited few-layer MoS2 film. The greater light absorption of few-layer MoS2 results in the high performance of the MoS2/GaN photodetector. Our device shows high external spectral responsivity (∼103 A W-1) and detectivity (∼1011 Jones) with a very fast response time (∼5 ms). Our obtained results are significantly better than previous MoS2- and GaN-based photodetectors. This work unveils a new perspective in MoS2/GaN heterojunctions for high-performance optoelectronic applications. © 2018 IOP Publishing Ltd.

Journal of Physics D: Applied Physics

Wafer-scale synthesis of a uniform film of few-layer MoS2 on GaN for 2D heterojunction ultraviolet photodetector

Bifurcation in a resonant inductive power transfer (RIPT) system causes hard switching of the primary side inverter, decrease in efficiency and loss of control stability. Bifurcation can be avoided by either selecting complicated control strategies or by calculating the parameters of RIPT system in such a way that the system has only one resonant frequency for the entire expected range of load and coupling variations. Many control methods to tackle the bifurcation issue have been covered in the literature. This paper aims at handling the bifurcation by proposing an analytical design procedure. A fabricated system, based on the parameters calculated using presented design steps, avoids the bifurcation phenomenon for the entire coupling and load variations. A 3.3 kW wireless charger setup using series-series compensated RIPT (SS-RIPT) system and 2-phase interleaved boost power factor correction (PFC) as a front end converter has been designed using the proposed method as an example. Simulation results using MATLAB are presented to verify the proposed design methodology. © 2017 IEEE.

2017 IEEE Electrical Power and Energy Conference, EPEC 2017

Design of 3.3 kW wireless battery charger for electric vehicle application considering bifurcation

Near atmospheric N2 ambient sputtered Au thin films exhibit significant spectral absorptivity over medium to long wave infrared radiations. Thin films were found adequately robust for micropatterning using conventional photolithography and metal lift off processes. Since long term spectral absorptivity is major practical concern for Au blacks, this paper reports on aging behavior of near atmospheric Ar and Ar + N2 (1:1) ambient sputtered infrared absorber Au thin films. Comparative analysis on electrical, morphological and spectral absorption behavior of twenty-five weeks room temperature/vacuum aged Au infrared absorber thin films is performed. The Ar and Ar + N2 ambient sputtered Au thing films have shown anticipated consistency in their physical, electrical and spectral properties regardless the long term aging in this work. © 2018 Elsevier B.V.

Infrared Physics and Technology

Aging behavior of near atmospheric N2 ambient sputtered/patterned Au IR absorber thin films

In this paper, we present a gas sensor platform technology which is non-MEMS and doesn't require any complex packaging for example hanging the chips in air while packing to reduce the power consumption. Our work is based on changing the substrate material to borofloat glass, which has much lower thermal conductivity in comparison to silicon and alumina, hence reducing the thermal losses making it possible to operate the device even with lower power consumption. The presented process has lesser complexities and the process cost is certainly reduced, as it does not require thermal oxidation and bulk micromachining. Different substrates (silicon, alumina and glass) have been simulated using COMSOL to depict the benefit of lower thermal conductivity. Fabricated gas sensor platform is rugged and reliable; the same has been verified by maximum temperature test, long term ON test and ON-OFF pulse test. © 2018 IEEE.

Proceedings of IEEE Sensors

Gas Sensor Platform for Rugged Applications

A regeneration based BGR circuit is proposed in this paper. The regeneration using positive feedback results in the positive temperature coeffecient of 2.34 mV/° C from the single stage at room temperature. This results in 60 % saving in power and over 80 % saving in area, when compared with the state-of-the-art trimming less BGR circuits. The obtained temperature coefficient is 33 ppm/° C for the temperature range 0° C-90° C. The circuit is designed and simulated in UMC 180nm CMOS process. The circuit consumes power of 40 nW and occupies an area of 0.003 mm2. The reference voltage of 819 mV is achieved at 1.2 V power supply. The power supply rejection ratio at 40 kHz is -47 dB. © 2017 IEEE.

Proceedings - International SoC Design Conference 2017, ISOCC 2017

A 1.2 V, 33 ppm/°C, 40 nW, regeneration based BGR circuit for nanowatt CMOS LSIs

In this paper, a nonlinear distortion analysis of power delivery network and the impact of nonlinear characteristics of DC-DC converter on the PDN output have been analyzed. The harmonic components for an equivalent PDN model have been estimated using Volterra series for 50 Hz input ripples. The mean relative error between analytical and simulated results for the fundamental (H1(jω1)), the second (H2 (jω1, jω2)) and the third harmonic (H2(jω1, jω2, jω3)) responses are 3%, 1% and 7%, respectively. For input intercept points (IIP2 and IIP3), the difference between analytical and simulation results are 0.5 dB and 2.8 dB, respectively. © 2017 IEEE.

2017 IEEE Electrical Design of Advanced Packaging and Systems Symposium, EDAPS 2017

The harmonics impact study of a DC-DC buck converter through a power delivery network

Demand of minimal hardware requirement and low energy consumption for internet of things (IoT) has gained the importance of techniques enhancing the physical layer security. Looking at this, we propose a channel-based mapping diversity (CBMD) scheme for a source (S), destination (D) pair to increase the average bit error rate (ABER) at an eavesdropper (E). CBMD leads to an ABER of 0.5 at E when the channels between S - D and S - E are independent. In the case where S - D and S - E are correlated (due to the proximity of E at D), we derive an optimal strategy at E which leads to the best performance at E. We also derive an optimal strategy at S, assuming E uses its optimal strategy, thereby, considering the worst case scenario. A sub-optimal strategy is also proposed at S which does not require any statistical information about the link S - E and hence, has low implementation complexity. We analyze the variation of ABER at E with the signal to noise ratio (SNR) and the channel correlation coefficient. Our simulation results demonstrate that CBMD indeed leads to significant performance degradation at E even at moderate channel correlation for a properly chosen strategy. © 2017 IEEE.