van der Waals Metallization to Probe Defect States of Semiconductors

Abstract

The semiconductor defect states have been recognized as a critical factor determining the characteristics of final semiconductor devices. This study observed that the electrical and optical properties of various semiconductors are distinctly influenced by semiconductor’s own defect states when employing pristine van der Waals (vdW) metallization method. Leveraging this observation, the study has confirmed the substantial improvement in key issues of current semiconductors, such as interfacial resistance and luminescent quantum yield, by effectively addressing semiconductor defect states. The vdW technique employed in this study has gained attention as an innovative process for non-destructive tool for target semiconductors. However, the prevalent use of wet processes in most vdW integration procedures leads to the introduction of contaminants at the target substrates and the final integrated interfaces. As a response to this challenge, our research group endeavors to bridge this gap by showcasing the performance of vdW- integrated devices, achieved through an entirely dry and uncontaminated process. Our investigations demonstrate that the substantial improvements in electronic and optical performance are closely related with their intrinsic defect states of semiconductors, achieved through our pristine vdW integration approach. In Chapter 2, the study introduces the pristine vdW integrated metal/graphene/silicon junction achieved through the dry transfer technique implemented in this study and present a comprehensive analysis of its electrical characteristics. The introduction of metal (M) contacts to semiconductors (S) gives rise to metal-induced gap states (MIGS), which exert a strong influence on the Schottky barrier (ΦB) and result in Fermi-level pinning. In the case of metal/graphene (M/Gr) bilayers, wherein graphene serves as the metallized layer, a contact to silicon (Si) can be established while minimizing MIGS through weak interactions. However, creating a pristine M/Gr/Si junction is a formidable challenge due to the conventional fabrication process involving the wet-transfer of graphene. This process exposes the graphene surface to various wet-chemicals, leading to the introduction of contaminant-related interfacial states. Here, the study introduces successfully achieved pristine M/Gr/Si junctions utilizing an entirely dry fabrication process, and the study have conducted a comprehensive investigation of their electrical properties. Metal/graphene contacts on silicon exhibit distinct current-voltage (J-V) characteristics in comparison to metal/silicon (M/Si) junctions, showcasing a significantly increased reverse bias current by several orders of magnitude (approximately 105 times for M=Au) at room temperature. These observations are attributed to the effective reduction of MIGS by the graphene interlayer, with Fermi-level pinning being induced by carefully controlled surface states on the silicon substrate. Our findings not only offer valuable insights into understanding the role of a 2D barrier in metal/semiconductor interfaces but also suggest versatile opportunities for precise interface engineering through the utilization of pristine 2D interlayers. In Chapter 3, the study demonstrates that, when applying vdW metal deposition to halide perovskites without altering other factors, it is possible to solely manipulate the hole concentration, thereby confirming a fivefold enhancement in luminescent quantum yield by slowing down the electron-hole recombination rate through variations in defect states. In the realm of light-emitting diodes (LEDs), low-dimensional halide perovskites have emerged as the predominant approach for achieving cutting-edge performance. Nonetheless, despite the recognized importance of free-carrier doping in quantum-confined systems concerning their light-emitting characteristics, there has been limited exploration of its influence on these materials. Here, the study presents evidence of electronic doping achieved through van der Waals contact between an aluminum (Al) layer and a single-layer hexagonal boron nitride (1L-h-BN) film onto a two-dimensional tin halide perovskite, phenethylammonium tin iodide (PEA2SnI4), resulting in a substantial 8-fold enhancement in photoluminescence (PL) intensity. Notably, this intensity enhancement exhibited strong metal-type dependence and reverted to the original level upon contact release. Our investigation employing ultraviolet photoemission spectroscopy (UPS) revealed a Fermi- level upshift of 0.6 eV, while maintaining the electronic structure of PEA2SnI4 without distortion. The study proposed that electronic doping minimizes Shockley-Read-Hall recombination, a phenomenon that provides insights into our observations in terms of time- resolved PL and temperature-dependent PL intensity variations. Furthermore, I measured a photoluminescence quantum yield (PLQY) of 10%, marking a record-high value among reported PEA2SnI4 materials. These findings underscore the pivotal role played by free- carrier doping in shaping the luminescent properties of low-dimensional halide perovskites due to the dominance of p-type deep-trap states induced by VSn vacancy. Separately, in Chapter 4, the study introduces our new technique for the fabrication of organic single crystals utilizing a rapidly developed spin-coating method, known for its exceptionally fast production speed. The spin-coating technique has emerged as a notably rapid and uncomplicated process for material solidification. While conventionally employed for the fabrication of polycrystalline thin films, recent investigations have ventured into its potential for epitaxial growth, with a primary focus on inorganic materials. Here, the study introduces a novel approach: the solvent-moisturized spin-coating method, which facilitates the rapid growth of large organic single crystals (OSCs). This method encompasses both p-type semiconductor (tetracyanoquinodimethane (TCNQ)) and n-type semiconductor (tetrathiafulvalene (TTF)) OSCs. In a mere 2-hour time frame, I successfully produced OSCs with controllable dimensions of up to 2,000 μm, a feat that conventionally necessitates several weeks using slower solvent evaporation techniques. The accuracy of OSC replication was verified through Raman mapping and UV-vis absorption spectroscopy. To elucidate this innovative OSC fabrication process, the study proposed a model based on the supersaturated dynamic fluid phenomenon. Furthermore, the study demonstrates the integration of these OSCs into electronic devices designed for charge-transfer complex channels. During gate sweep measurements, these OSC-integrated devices exhibit ambipolar behavior. This pioneering approach to OSC production holds substantial potential for advancing various fields of science and electronics, traditionally hindered by the limited availability of suitably sized OSC samples. In conclusion, this thesis highlights the significance of the van der Waals integration strategy, employing a dry-transfer process for a wide range of material types, including silicon, light-emitting layers, and small-molecule organic semiconductors. The improvements observed in electrical and optical properties were strongly related with their intrinsic defect states of target semiconductors. The uses of the dry processes apparently underscore the importance of pristine interfaces, a strategy that has yet to become universally adopted in mainstream research. This thesis serves as a valuable contribution to advancing the development of such strategies, with a specific focus on the pristine process, which holds great promise for meeting various researches and industrial demands.