2025 Theses Doctoral

Towards achieving high-performance electronic devices with air-sensitive two-dimensional materials

Rajendran, Anjaly Thekkevilayil

Air-sensitive two-dimensional (2D) materials are a subclass of atomically thin 2D materials that degrade upon exposure to ambient conditions like oxygen and moisture. Examples include transition metal dichalcogenides (TMDCs) like MoTe₂, NbSe₂, and TaS₂, three materials this thesis is based on. The air-sensitive TMDCs exhibit unique optoelectronic and quantum properties, but their instability in ambient conditions leads to degradation of intrinsic properties. To mitigate this,these materials require encapsulation which hermetically seals them from ambient conditions. The device fabrication requires an inert atmosphere like a glovebox and transportation requires vacuum suitcases. The major obstacle to applications of these materials is that the doping and contact techniques developed for regular 2D materials need further modifications to be applicable to the air-sensitive 2D materials. This thesis demonstrates solutions to tackle these pressing problems which enable fabrication of high-performance quantum and optoelectronic devices with air-sensitive 2D
materials.

The first part of the thesis describes the potential of van der Waals materials to realize a scaled, low-loss parallel plate capacitors (PPC) for qubits. Breakthroughs in materials science like growth of high-purity semiconductors, better qubit and resonator designs, and advanced fabrication techniques like precision etching have enhanced the lifetime of qubits and reproducibility significantly. Superconducting qubits is one of the multiple technologies the community is working on. Capacitors, a key component of a transmon (a design variation of superconducting qubit) are conventionally fabricated as large-footprint coplanar capacitors. However, their extensive geometric size limits qubit scalability and introduces unwanted crosstalk. This thesis exploits the unique properties of a superconducting air-sensitive 2D material, NbSe₂ and develop a technique for fabricating ultra-low loss contacts with a resistance of 192 𝜇Ω to them. The microwave losses of 2D dielectric hexagonal boron nitride (ℎBN) is calculated to be 2.4x10⁻⁵, making it an excellent dielectric at microwave frequencies. Finally, transmon qubit employing an all vdW PPC is demonstrated with a 1000× reduction in geometric footprint compared to conventional coplanar designs and a coherence time T₁ of 1 𝜇𝑠.

In the second part, a device to study the rich charge density wave (CDW) phase transitions in few-layer TaS₂ using near-field imaging is demonstrated. Encapsulation, coupled with the requirements for near-field imaging makes the device architecture design and fabrication quite challenging. Near-field imaging is a high-resolution technique that overcomes the diffraction limit by utilizing evanescent waves on the surface to capture fine details beyond conventional optical microscopy. An abrupt metal to insulator transition in few-layer limit is observed, in contrast to previous studies. The difference in temperature evolution and the bimodality of the insulating and metallic states highlights the need to consider the role of sample encapsulation and sample thickness. The CDW transitions in few-layer TaS₂ allows for resistive switching making it an ideal candidate for neuromorphic computing.

The third part of this thesis shows fabrication and characterization of a high-performance ptype transistor based on 1L-MoTe₂. As transistor sizes approach the atomic scale, issues related to scattering, heat dissipation, and quantum effects have become increasingly difficult to overcome. Limitations of traditional silicon-based technologies have prompted the exploration of alternative materials and architectures. Enter air-sensitive semiconducting monolayer MoTe₂ (1L-MoTe₂) - 0.7 nm thick and with a direct bandgap of 1.1 eV, making it a suitable choice for CMOS circuits and near infrared (NIR) photonic applications. Robust p-type modulation doping using oxidized WSe₂ and a low-loss tunneling contact with a resistance of 2 kΩ · 𝜇𝑠 is presented. The doping and contact engineering also result in hole field-effect mobility of 102 cm²/Vs, the highest reported till date for 1L-MoTe₂-based FETs. Here, 1L-MoTe₂ is hermetically sealed and has long-term stability.