Rajendra Singh’s research while affiliated with Indian Institute of Technology Delhi and other places

Achieving low contact resistance in advanced electronic devices remains a significant challenge. As the demand for faster and more energy-efficient devices grows, 2D contact engineering emerges as a promising solution for next-generation electronics. Beyond graphene, 1T-WTe2 has gained attention due to its outstanding electrical transport properties, quantum phenomena, and Weyl semimetallic characteristics. We demonstrate the direct wafer-scale growth of 1T-WTe2 via molecular beam epitaxy (MBE) and use it as a 2D contact for layered materials like InSe, which exhibits broad photoresponsivity. The performance of this 2D electrode in InSe-based photodetectors is compared with conventional metal electrodes. Under near-infrared (NIR) to deep ultraviolet (DUV) illumination, the InSe/1T-WTe2 configuration shows a broad photoresponsivity range from 0.14 to 217.58 A/W, with fast rise/fall times of 42/126 ms in the visible region. In contrast, the InSe/Ti-Au configuration exhibits a peak photoresponsivity of 3.64 A/W in the DUV range, with an overall lower responsivity spanning from 0.000865 A/W to 3.64 A/W under NIR and DUV illumination, respectively. Additionally, in the visible regime, it exhibits slower rise and fall times of 150 ms and 144 ms, respectively, compared to InSe/1T-WTe2. These findings indicate that MBE-grown 1T-WTe2 serves as an effective 2D electrode, delivering higher photoresponsivity and faster photodetection compared to traditional metal contacts. This approach offers a simplified, high-performance alternative for layered material-based devices, eliminating the need for complex heterostructure configurations.

Vanadium dioxide (VO2) (M1) exhibits a unique metal–insulator transition (MIT) near room temperature, garnering considerable attention for its applications in bolometer, terahertz/infrared detectors, and microelectronic devices. Here, we explore the potential of epitaxially grown VO2 (M1) thin films for near-infrared (IR) detection by optimizing the growth conditions, followed by structural characterization and device fabrication. Alongside the VO2 (M1) phase, two other oxides from the vanadium oxide family, VO2 (A) and V2O5, were also grown on a c-cut sapphire substrate using a pulsed laser deposition (PLD) system. In-depth analysis using temperature-dependent XRD and Raman spectroscopy confirmed the crystalline structure and the quality of epitaxial thin film formation of VO2 (M1), while also unveiling structural phase transition (SPT) behavior and the critical temperature of transition. At elevated temperatures during electrical measurement, the VO2 (M1) epilayer exhibits a first-order phase transition from the metallic to the insulating state, accompanied by a significant change in resistance exceeding three orders of magnitude unveiling its potential in thermal switches, memory-based devices etc. In depth, electrical analysis on all the grown oxides shows that VO2 (M1) and V2O5 exhibit a higher temperature coefficient of resistance (TCR) (3%/K and 2%/K) and a lower 1/f noise (in the order pA / Hz at 0.1 Hz) as compared to VO2 (A), paving scope for further analysis of these two oxides toward important applications in the domain of thermal sensors. Additionally, VO2 (M1) exhibited good bolometric response (in the order of ms) to IR radiation, proving its candidature for the application in IR detectors as well.

Despite advancements in terahertz (THz) modulators, achieving a balance between large modulation depth (MD) and fast modulation speed in scalable devices remains a significant challenge. Optically pumped THz modulators with high MD, broad bandwidth, and fast response times are essential for progress in THz technology. Here, scalable MoTe2/Si van der Waals (vdW) heterostructures are grown via molecular beam epitaxy (MBE) as an optically pumped THz modulators, leveraging its favorable band alignment and seamless integration with silicon complementary metal‐oxide semiconductor (CMOS) technology. High‐quality bilayer, 5‐layer, and 7‐layer MoTe2 films are grown on high‐resistivity silicon, with the bilayer modulator achieving a maximum 74.6% modulation depth under 355 nm illumination of 4.8 W mm⁻² at 0.25 THz. The bilayer device exhibited fast rise and fall times of 1.8 and 0.6 µs, respectively, and provided broadband modulation across the 0.1 to 1.0 THz range. Furthermore, this work demonstrates that higher modulation depth significantly enhances THz imaging quality with promising applications in detecting pharmaceutical forgeries. Overall, these ultrafast, broadband THz modulators provide a promising route for advancing next‐generation THz technology, enhancing applications in wireless communication, precision detection, and high‐resolution THz imaging, fostering future innovation.

This study investigates the electrical DC and Low-frequency noise (LFN) characteristics of GaN-on-Si power MIS-HEMTs across an extensive temperature spectrum from 4 K to 420 K to assess their viability for high to cryogenic temperatures. As the temperature drops, improvements are observed in the input DC characteristics, including enhanced transconductance and an increased ON-to-OFF current ratio. Correspondingly, the output DC characteristics affirmed the significant enhancements in drain current and reduced ON-resistance across the same temperature span. These improvements in DC performance are achieved without any evidence of carrier-freeze out throughout the temperature expanse. Furthermore, the LFN characteristics from 10 Hz to 100 KHz reveal a consistent 1/f noise behavior within the measured temperature domain. The trap density inferred from the LFN characteristics increases substantially by two orders of magnitude at deep cryogenic temperatures. Collectively, these experimental outcomes demonstrate that GaN-on-Si power MIS-HEMTs maintain a robust electrical performance over a wide temperature range, providing valuable insights for enhancing their deployment in quantum computing and space-based applications.

A detailed investigation of deep traps in halide vapor-phase epitaxy (HVPE)-grown β-Ga2O3 epilayers has been done by performing deep-level transient spectroscopy (DLTS) from 200 K to 500 K on Pt/β-Ga2O3 and Ni/β-Ga2O3 Schottky diodes. Similar results were obtained with a fill pulse width of 100 ms irrespective of the different Schottky metal contacts and epilayers. Two electron traps at E2 (EC–ET = 0.65 eV) and E3 (EC–ET = 0.68–0.70 eV) with effective capture cross-sections of 4.10 × 10⁻¹⁴ cm² and 5.75 × 10⁻¹⁵ cm² above 300 K were observed. Below 300 K, a deep trap with a negative DLTS signal peak was also observed at E1 (EC–ET = 0.34–0.35 eV) with a very low capture cross-section of 3.28 × 10⁻¹⁷ cm². For a short pulse width of 100 μs, only two electron traps, E2 and E3, at energies of 0.72 eV and 0.73 eV were observed, and one order of higher corresponding effective capture cross-sections. All traps were found to be unaffected by the electric field during the field-dependent DLTS study. From the filling pulse width dependence DLTS study, a decrease in the capacitance transient amplitude with the increasing pulse width was observed opposite to the capture barrier kinetics of the traps and attributed to the emission of carriers during the capture process. Trap concentrations were found to be high at the interface using depth profiling DLTS. Based on the available literature, it is suggested that these traps are related to FeGa, Fe-related centers, and complexes with hydrogen or shallow donors, and might be affected or generated during metallization by the electron beam evaporator and chemical mechanical polishing.