SURFACE AND INTERFACE EFFECTS IN TRANSITION-METAL-DICHALCOGENIDE-BASED FUNCTIONAL OPTOELECTRONIC DEVICES

TWO-DIMENSIONAL (2D) TRANSITION METAL DICHALCOGENIDES (TMDS) HAVE ATTRACTED SIGNIFICANT INTEREST AS BUILDING BLOCKS FOR NEXT-GENERATION ELECTRONIC AND OPTOELECTRONIC TECHNOLOGIES. THEIR ATOMIC-SCALE THICKNESS, LARGE SURFACE-TO-VOLUME RATIO, TUNABLE BAND STRUCTURES, AND STRONG LIGHT–MATTER INTERACTIONS ENABLE DIVERSE TRANSPORT AND PHOTORESPONSE MECHANISMS, WHILE MAKING THEIR BEHAVIOUR HIGHLY SENSITIVE TO ENVIRONMENTAL EXPOSURE, STRUCTURAL DEFECTS, AND INTERFACE QUALITY. A KEY PREREQUISITE FOR RELIABLE 2D TECHNOLOGIES IS SURFACE STABILITY. USING A SYNERGISTIC COMBINATION OF AFM, SEM, STM, STEM-EDX, XPS, RAMAN SPECTROSCOPY, AND DFT SIMULATIONS, THE OXIDATION OF ZRSE₂, AN AIR-SENSITIVE GROUP-IV TMD, WAS INVESTIGATED. OXIDATION INITIATES AT SURFACE DEFECTS AND EDGES, FORMING SE-RICH PROTRUSIONS AND NANOWIRES OVER TIME. ENCAPSULATION IS THUS REQUIRED TO LIMIT DEGRADATION, HIGHLIGHTING THE CRITICAL ROLE OF SURFACE PASSIVATION. IN TMD SEMICONDUCTORS WITH REVERSIBLE SURFACE INTERACTIONS, SURFACE-MEDIATED PROCESSES ACT BOTH AS A SOURCE OF TUNABILITY AND A KEY PERFORMANCE BOTTLENECK. THIS INTERPLAY WAS STUDIED VIA PRESSURE- AND TEMPERATURE-DEPENDENT MEASUREMENTS OF FEW-LAYER RES₂ FIELD-EFFECT TRANSISTORS. AT AMBIENT PRESSURE, AIR-INDUCED ELECTRON TRAPPING REDUCES MOBILITY, BROADENS HYSTERESIS, AND CAUSES PERSISTENT PHOTORESPONSE, WHILE VACUUM DESORPTION INCREASES CARRIER DENSITY AND RESPONSE SPEED. TEMPERATURE-DEPENDENT STUDIES (80–350 K) REVEAL TRAP-MEDIATED AND CONTACT-LIMITED TRANSPORT AT METAL- AND OXIDE-SEMICONDUCTOR INTERFACES. PHOTORESPONSE TRANSITIONS FROM FAST, LINEAR BEHAVIOUR AT LOW TEMPERATURE TO SLOWER, THERMALLY ACTIVATED DYNAMICS AT HIGHER TEMPERATURES. TO ACHIEVE ROBUST AND EFFICIENT ARCHITECTURES, MIXED-DIMENSIONAL RES₂/SI VAN DER WAALS HETEROJUNCTIONS WERE FABRICATED, WHERE A VERTICAL BUILT-IN ELECTRIC FIELD ENABLES RAPID SEPARATION OF PHOTOGENERATED CARRIERS. THESE DEVICES COMBINE THE STRONG LIGHT–MATTER INTERACTION AND DIRECT BANDGAP OF RES₂ WITH MATURE SILICON PROCESSING TECHNOLOGY. THE RES₂/N-SI DEVICE EXHIBITS DIODE-LIKE BEHAVIOUR IN THE DARK, CONSISTENT WITH FIRST-PRINCIPLES CALCULATIONS CONFIRMING TYPE-II BAND ALIGNMENT. UNDER ILLUMINATION, A LINEAR, STABLE, MULTIMODE PHOTODETECTION RESPONSE IS OBSERVED, WITH MICROSECOND-SCALE SWITCHING IN SELF-POWERED MODE. OPTIMIZING DEVICE GEOMETRY AND FABRICATION YIELDS IMPROVED RESPONSIVITY (UP TO 10^3 A/W) AND ENHANCED WEAK-SIGNAL DETECTIVITY ACROSS THE VISIBLE–NIR RANGE, MAINTAINING FAST AND RELIABLE OPERATION EVEN AT LOW TEMPERATURE. THIS WORK ALSO DEMONSTRATES HOW THE INTRINSIC RESPONSIVENESS OF 2D MATERIALS CAN ENABLE ADAPTIVE, MULTIFUNCTIONAL ELECTRONIC ARCHITECTURES. AMBIPOLAR SEMICONDUCTORS WERE CHOSEN FOR THEIR ABILITY TO SUPPORT BOTH N- AND P-TYPE CONDUCTION, PROVIDING A BROADER SET OF ELECTRO-OPTICAL RESPONSES THAN UNIPOLAR 2D MATERIALS. IN THIS FRAMEWORK, A MOTE₂-BASED FET EXHIBITS GATE-TUNABLE BIDIRECTIONAL PHOTOCONDUCTIVITY, WITH POSITIVE AND NEGATIVE PHOTOCURRENTS DEPENDING ON ELECTROSTATIC CONFIGURATION AND TRAP-STATE DYNAMICS, PROVIDING A PROOF-OF-CONCEPT ANALOGY TO RETINAL ON/OFF-LIKE RESPONSE. WSE₂-BASED TRANSISTORS SHOW A LIGHT-INDUCED TRANSITION FROM AMBIPOLAR TO ANTI-AMBIPOLAR BEHAVIOUR, PRODUCING A Λ-SHAPED TRANSFER CURVE AND THREE DISTINCT CURRENT LEVELS, PROMISING FOR TERNARY OR MULTI-BIT LOGIC ARCHITECTURES BEYOND CONVENTIONAL BINARY OPERATION. BY SYSTEMATICALLY EXAMINING HOW STRUCTURAL PROPERTIES, EXTERNAL STIMULI, AND DEVICE ARCHITECTURE INFLUENCE CHARGE TRANSPORT AND PHOTORESPONSE IN 2D SEMICONDUCTORS, THIS THESIS HIGHLIGHTS THE VERSATILITY AND TECHNOLOGICAL POTENTIAL OF TMD SYSTEMS, PROVIDING A PATHWAY FOR THE RELIABLE INTEGRATION OF 2D MATERIALS INTO FUNCTIONAL OPTOELECTRONIC DEVICES.