Engineering Nano-Electronic Devices using 2-D Materials: CMOS Logic to Biosensing

Abstract

Technology scaling has driven the development of semiconductor technology that forms a ubiquitous part of daily utilities such as smartphones, computers, and wearables. However, efforts to continue scaling have met numerous challenges, both from engineering limitations and the fundamental limits of silicon. Two-dimensional (2-D) materials are a potential candidate for highly scaled nodes due to their atomically thin nature and excellent electrostatics. The IRDS roadmap (2021) has forecasted 2-D channels as possible contenders for 1.5 nm nodes and beyond. In this regard, there is a need to explore the potential of 2-D materials for complementary metal oxide semiconductor (CMOS) compatible logic platforms and address the challenges that limit their adoption. The large surface-to-volume ratio and the sensitivity to external surroundings also make 2-D materials excellent for bio-sensing applications. The field effect transistor (FET) technology can be leveraged for ion and biomolecule sensing by appropriate functionalization. In this thesis, we use 2-D materials like MoS2, WSe2, etc., along with high-k dielectric materials (HfO2, Al2O3) to develop scalable and CMOS-compatible FETs and ion-sensitive FETs (ISFETs) for logic and biosensing applications, respectively. The first part of this thesis deals with process engineering and optimization for contact engineering and short-channel devices. A modified surface treatment process using ammonium sulfide in an alcohol medium is introduced for better Ni-MoS2 contacts with a low Schottky barrier height of 130 m eV, resulting in lower contact resistance, low variability, and better yield. The process is less aggressive and compatible with the back end of line processing on pre-patterned substrates with metal interconnects. Then, we optimize electron beam lithography for ultra-short channel device patterning for the smallest feature lengths of 80 nm and 30 nm, using manual dose correction and algorithmic proximity error correction, respectively. Back-gated FETs with Ni-MoS2 contacts and a short channel of 80 nm show contact resistance as low as 1.3 kΩ μm with alcohol-based sulfur treatment. Secondly, we explore alloying to tune the electrochemical characteristics of 2-D materials. Ternary alloys of form MoS2(1-x)Se2x show composition-dependent bandgap, strain, and carrier concentrations. Using these alloys, we tune the threshold voltage, subthreshold slope, mobility, and drain currents in back-gated FETs. Integrating them with SiO2 and HfO2 dielectrics provided exciting insights into their interfaces and highlighted the benefit of high-k dielectrics for high-performance FETs with enhancement mode operation. Low-power logic circuits require steep switching (<60 mV/dec) FETs, whereas ISFETs with high sensitivity (> 59 mV/pH) are desirable for biosensing applications. Third, we engineer new device architectures to surpass the conventional limits of these devices. A steep switching MoS2 FET is developed using gate connected nickel ferrite (NF) threshold switching (TS) device. By integrating the TS device to the top-gate stack of a MoS2 FET, we achieve steep subthreshold slopes as low as 8.5 mV/dec, much lower than the Boltzmann limit of 60 mV/dec. A super-Nernstian ISFET is developed using the vdW heterostructure of WSe2 and MoS2. The double-gated WSe2/MoS2 ISFET achieved a sensitivity of 362 mV/pH, well above the Nernst limit of 59 mV/pH, by exploiting the charge screening effects of the hetero-interface. Further sensitivity enhancement can be achieved using an experiment calibrated TCAD model and numerical solutions for the ferroelectric negative capacitance effect. The NC-WSe2/MoS2 ISFET shows a massive enhancement in sensitivity to 4.38 V/pH with a resolution of 0.002 units of pH. Biomolecule detection (total cholesterol) is also demonstrated using a functionalized MoS2 channel. The bio-FET device uses a copper (II) oxide nanoparticle-linked cholesterol oxidase and esterase as the primary sensing platform. The device shows a proportional increase in current with increasing cholesterol concentrations and achieved a normalized peak sensitivity of 1.95 μA/(μm2 mg/mL). This work opens the paths for scalable and CMOS-compatible 2-D material platforms - for low-power computing applications; for point-of-care diagnostic devices with high sensitivity and throughput.