arXiv subjects

Krishna Murali

Publications and source records attributed to Krishna Murali.

10 recordsLinked to original sources

arXiv2024

Ultrathin BIC metasurfaces based on ultralow-loss Sb2Se3 phase-change material

Phase-change materials (PCMs) are increasingly recognised as promising platforms for tunable photonic devices due to their ability to modulate optical properties through solid-state phase transitions. Ultrathin and low-loss PCMs are highly valued for their fast and more effective phase transitions and applications in reconfigurable photonic chips, metasurfaces, optical modulators, sensors, photonic memories, and neuromorphic computing. However, conventional PCMs such as GST, GSST, VO2, and In3SbTe2, despite optimisation for tunable meta-optics, suffer from high intrinsic losses in the near-infrared (NIR) region, limiting their potential for high quality factor (Q-factor) resonant metasurfaces. Here we present the design and fabrication of tunable bound states in the continuum (BIC) metasurfaces using the ultralow-loss PCM Sb2Se3. Our BIC metasurfaces, only 25 nm thick, achieve high modulation depth and broad resonance tuning in the NIR with high Q-factors up to 130, without the need for additional materials. Experimentally, we employ these BIC metasurfaces to modulate photoluminescence in rare earth-doped upconversion nanoparticles, reducing the excitation power for multiphoton photoluminescence and enabling emission polarisation manipulation. This work offers a promising platform for developing active resonant metasurfaces in the NIR region, with broad applications including super resolution imaging, optical modulation, ultrafast switches, harmonic generation, colour filtering, and optical sensing.

physics.optics↗

arXiv2021

Accurate Extraction of Schottky Barrier Height and Universality of Fermi Level De-pinning of van der Waals Contacts

Due to Fermi level pinning (FLP), metal-semiconductor contact interfaces result in a Schottky barrier height (SBH), which is usually difficult to tune. This makes it challenging to efficiently inject both electrons and holes using the same metal - an essential requirement for several applications, including light-emitting devices and complementary logic. Interestingly, modulating the SBH in the Schottky-Mott limit of de-pinned van der Waals (vdW) contacts becomes possible. However, accurate extraction of the SBH is essential to exploit such contacts to their full potential. In this work, we propose a simple technique to accurately estimate the SBH at the vdW contact interfaces by circumventing several ambiguities associated with SBH extraction. Using this technique on several vdW contacts, including metallic 2H-TaSe$_2$, semi-metallic graphene, and degenerately doped semiconducting SnSe$_2$, we demonstrate that vdW contacts exhibit a universal de-pinned nature. Superior ambipolar carrier injection properties of vdW contacts are demonstrated (with Au contact as a reference) in two applications, namely, (a) pulsed electroluminescence from monolayer WS$_2$ using few-layer graphene (FLG) contact, and (b) efficient carrier injection to WS$_2$ and WSe$_2$ channels in both nFET and pFET modes using 2H-TaSe$_2$ contact.

cond-mat.mes-hall↗

arXiv2021

Contact-Barrier Free, High Mobility, Dual-Gated Junctionless Transistor Using Tellurium Nanowire

Gate-all-around nanowire transistor, due to its extremely tight electrostatic control and vertical integration capability, is a highly promising candidate for sub-5 nm technology node. In particular, the junctionless nanowire transistors are highly scalable with reduced variability due to avoidance of steep source/drain junction formation by ion implantation. Here we demonstrate a dual-gated junctionless nanowire \emph{p}-type field effect transistor using tellurium nanowire as the channel. The dangling-bond-free surface due to the unique helical crystal structure of the nanowire, coupled with an integration of dangling-bond-free, high quality hBN gate dielectric, allows us to achieve a phonon-limited field effect hole mobility of $570\,\mathrm{cm^{2}/V\cdot s}$ at 270 K, which is well above state-of-the-art strained Si hole mobility. By lowering the temperature, the mobility increases to $1390\,\mathrm{cm^{2}/V\cdot s}$ and becomes primarily limited by Coulomb scattering. \txc{The combination of an electron affinity of $\sim$4 eV and a small bandgap of tellurium provides zero Schottky barrier height for hole injection at the metal-contact interface}, which is remarkable for reduction of contact resistance in a highly scaled transistor. Exploiting these properties, coupled with the dual-gated operation, we achieve a high drive current of $216\,\mathrm{μA/μm}$ while maintaining an on-off ratio in excess of $2\times10^4$. The findings have intriguing prospects for alternate channel material based next-generation electronics.

cond-mat.mes-hall↗

arXiv2020

Highly tunable layered exciton in bilayer WS$_2$: linear quantum confined Stark effect versus electrostatic doping

In 1H monolayer transition metal dichalcogenide, the inversion symmetry is broken, while the reflection symmetry is maintained. On the contrary, in the bilayer, the inversion symmetry is restored, but the reflection symmetry is broken. As a consequence of these contrasting symmetries, here we show that bilayer WS$_2$ exhibits a quantum confined Stark effect (QCSE) that is linear with the applied out-of-plane electric field, in contrary to a quadratic one for a monolayer. The interplay between the unique layer degree of freedom in the bilayer and the field driven partial inter-conversion between intra-layer and inter-layer excitons generates a giant tunability of the exciton oscillator strength. This makes bilayer WS$_2$ a promising candidate for an atomically thin, tunable electro-absorption modulator at the exciton resonance, particularly when stacked on top of a graphene layer that provides an ultra-fast non-radiative relaxation channel. By tweaking the biasing configuration, we further show that the excitonic response can be largely tuned through electrostatic doping, by efficiently transferring the oscillator strength from neutral to charged exciton. The findings are prospective towards highly tunable, atomically thin, compact and light, on chip, reconfigurable components for next generation optoelectronics.

cond-mat.mes-hall↗

arXiv2020

Astability versus Bistability in van der Waals Tunnel Diode for Voltage Controlled Oscillator and Memory Applications

Van der Waals (vdW) tunnel junctions are attractive due to their atomically sharp interface, gate tunablity, and robustness against lattice mismatch between the successive layers. However, the negative differential resistance (NDR) demonstrated in this class of tunnel diodes often exhibits noisy behaviour with low peak current density, and lacks robustness and repeatability, limiting their practical circuit applications. Here we propose a strategy of using a 1L-WS$_2$ as an optimum tunnel barrier sandwiched in a broken gap tunnel junction of highly doped black phosphorus (BP) and SnSe$_2$. We achieve high yield tunnel diodes exhibiting highly repeatable, ultra-clean, and gate tunable NDR characteristics with a signature of intrinsic oscillation, and a large peak-to-valley current ratio (PVCR) of 3.6 at 300 K (4.6 at 7 K), making them suitable for practical applications. We show that the thermodynamic stability of the vdW tunnel diode circuit can be tuned from astability to bistability by altering the constraint through choosing a voltage or a current bias, respectively. In the astable mode under voltage bias, we demonstrate a compact, voltage controlled oscillator without the need for an external tank circuit. In the bistable mode under current bias, we demonstrate a highly scalable, single element one-bit memory cell that is promising for dense random access memory applications in memory intensive computation architectures.

cond-mat.mes-hall↗

arXiv2019

Highly Sensitive, Fast Graphene Photodetector with Responsivity $>10^6$ A/W Using Floating Quantum Well Gate

Graphene, owing to its zero bandgap electronic structure, is promising as an absorption material for ultra-wideband photodetection applications. However, graphene-absorption based detectors inherently suffer from poor responsivity due to weak absorption and fast photocarrier recombination, limiting their viability for low intensity light detection. Here we use a graphene/WS$_2$/MoS$_2$ vertical heterojunction to demonstrate a highly sensitive photodetector, where the graphene layer serves dual purpose, namely as the light absorption layer, and also as the carrier conduction channel, thus maintaining the broadband nature of the photodetector. A fraction of the photoelectrons in graphene encounter ultra-fast inter-layer transfer to a floating monolayer MoS$_2$ quantum well providing strong quantum confined photogating effect. The photodetector shows a responsivity of $4.4\times 10^6$ A/W at 30 fW incident power, outperforming photodetectors reported till date where graphene is used as light absorption material by several orders. In addition, the proposed photodetector exhibits an extremely low noise equivalent power ($N\!E\!P$) of $<4$ fW/$\sqrt{Hz}$ and a fast response ($\sim$ milliseconds) with zero reminiscent photocurrent. The findings are attractive towards the demonstration of graphene-based highly sensitive, fast, broadband photodetection technology.

physics.app-ph↗

arXiv2019

Gate controlled large resistance switching driven by charge density wave in 1T-TaS2/2H-MoS2 heterojunction

1T-TaS2 is a layered material that exhibits charge density wave (CDW) induced distinct electrical resistivity phases and has attracted a lot of attention for interesting device applications. However, such resistivity switching effects are often weak, and cannot be modulated by an external gate voltage - limiting their widespread usage. Using a back-gated 1T-TaS2/2H-MoS2 heterojunction, here we show that the usual resistivity switching in TaS2 due to different phase transitions is accompanied with a surprisingly strong modulation in the Schottky barrier height (SBH) at the TaS2/MoS2 interface - providing an additional knob to control the degree of the phase-transition-driven resistivity switching by an external gate voltage. In particular, the commensurate (C) to triclinic (T) phase transition results in an increase in the SBH owing to a collapse of the Mott gap in TaS2. The change in SBH allows us to estimate an electrical Mott gap opening of ~71 +/- 7 meV in the C phase of TaS2. On the other hand, the nearly-commensurate (NC) to incommensurate (IC) phase transition results in a suppression in the SBH, and the heterojunction shows a gate-controlled resistivity switching up to 17.3, which is ~14.5 times higher than that of standalone TaS2. The findings mark an important step forward showing a promising pathway to externally control as well as amplify the CDW induced resistivity switching. This will boost device applications that exploit these phase transitions, such as ultra-broadband photodetection, negative differential conductance, fast oscillator and threshold switching in neuromorphic circuits.

cond-mat.mes-hall↗

arXiv2018

Self-Powered, Highly Sensitive, High Speed Photodetection Using ITO/WSe2/SnSe2 Vertical Heterojunction

Two dimensional transition metal di-chalcogenides (TMDCs) are promising candidates for ultra-low intensity photodetection. However, the performance of these photodetectors is usually limited by ambience induced rapid performance degradation and long lived charge trapping induced slow response with a large persistent photocurrent when the light source is switched off. Here we demonstrate an indium tin oxide (ITO)/WSe$_2$/SnSe$_2$ based vertical double heterojunction photoconductive device where the photo-excited hole is confined in the double barrier quantum well, whereas the photo-excited electron can be transferred to either the ITO or the SnSe$_2$ layer in a controlled manner. The intrinsically short transit time of the photoelectrons in the vertical double heterojunction helps us to achieve high responsivity in excess of $1100$ A/W and fast transient response time on the order of $10$ $μ$s. A large built-in field in the WSe$_2$ sandwich layer results in photodetection at zero external bias allowing a self-powered operation mode. The encapsulation from top and bottom protects the photo-active WSe$_2$ layer from ambience induced detrimental effects and substrate induced trapping effects helping us to achieve repeatable characteristics over many cycles.

cond-mat.mes-hall↗

arXiv2018

Gate tunable WSe2/SnSe2 backward diode with ultrahigh reverse rectification ratio

Backward diodes conduct more efficiently in the reverse bias than in the forward bias, providing superior high frequency response, temperature stability, radiation hardness, and 1/f noise performance than a conventional diode conducting in the forward direction. Here we demonstrate a van der Waals material based backward diode by exploiting the giant staggered band offsets of WSe2/SnSe2 vertical heterojunction. The diode exhibits an ultra-high reverse rectification ratio (R) of ~2.1x10^4 and the same is maintained up to an unusually large bias of 1.5 V - outperforming existing backward diode reports using conventional bulk semiconductors as well as one- and two-dimensional materials by more than an order of magnitude, while maintaining an impressive curvature coefficient (γ) of ~37 per V. The transport mechanism in the diode is shown to be efficiently tunable by external gate and drain bias, as well as by the thickness of the WSe2 layer and the type of metal contacts used. These results pave the way for practical electronic circuit applications using two-dimensional materials and their heterojunctions.

physics.app-ph↗

arXiv2017

Substrate effects in high gain, low operating voltage SnSe2 photoconductor

High gain photoconductive devices find wide spread applications in low intensity light detection. Ultra-thin layered materials have recently attracted a lot of attention from researchers in this regard. However, in general, a large operating voltage is required to obtain large responsivity in these devices. In addition, the characteristics are often confounded by substrate induced trap effects. Here we report multi-layer SnSe2 based photoconductive devices using two different structures: (1) SiO2 substrate supported interdigitated electrode (IDE), and (2) suspended channel. The IDE device exhibits a responsivity of ~ 10^3 A/W and 8.66x10^4 A/W at operating voltages of 1 mV and 100 mV, respectively - a superior low voltage performance over existing literature on planar 2D structures. However, the responsivity reduces by more than two orders of magnitude, while the transient response improves for the suspended device - providing insights into the critical role played by the channel-substrate interface in the gain mechanism. The results, on one hand, are promising for highly sensitive photoconductive applications consuming ultra-low power, and on the other hand, show a generic methodology that could be applied to other layered material based photoconductive devices as well for extracting the intrinsic behavior.

cond-mat.mes-hall↗