Electronics and Optoelectronics Devices from Low-Dimensional Systems
The notion of low dimensionality becomes more than a pure mathematical ideal, as scaling of modern electronics reaches a state of the art in which electrons are strongly confined at the interface between silicon and the gate oxides in MOSFETs. Moreover, the improvement of material synthesis in the last two decades gives rise to many fascinating materials, in which the electronic states are confined at least for one dimension, like quantum dots (QD) for 0 dimension (0D), carbon nanotubes (CNT) for 1 dimension (1D), graphene and transitional metal dichalcogenides (TMD) as 2 dimensions (2D). In these materials, quantum confinement due to the reduced dimensionality can significantly change their electronic and optical properties as compared to their bulk counterparts. Those materials offer great potential and opportunity for electronics and optoelectronic devices beyond traditional bulk materials. As novel and unique as the low dimensional materials, they require a change of understanding of charge transport from the macroscopic to the mesoscopic level, and in many cases from classical Ohm’s law to quantum tunneling. This thesis focuses on understanding charge transport in low dimensional material systems, and on building prototype devices for electronic and optoelectronic applications. I studied Two different material systems in detail: 1) 2D transition metal dichalcogenides (TMDs); 2) heterojunction between 0D CdSe QDs and 1D CNTs. TMD materials have unique van der Waal interactions in the out-of-plane direction. Understanding charge transport is limited due to a lack of periodicity in the out-of-plane direction. Thus, both experiments and simulations are conducted to extract the effective mass in the out-of-plane direction for the first time. Based on this understanding, I have also explored vertical-TMD based memory devices for RRAM and selector. The CNT-QDs heterostructure is designed to combine the excellent light absorption of QDs and exceptional mobility of CNTs for light harvesting. The key, to achieve a high-efficiency solar cell, lies in the charge transport through QDs and CNTs interface. After initially gaining insight into the impact of functionalization of QDs, the transport properties of FET based on CNT-QDs heterojunctions are studied with different QD sizes and different laser wavelengths. The electron transfer from QDs to CNT is confirmed, and the tunneling nature of this electron transport is revealed. In the last chapter, the current drive capability is compared between 1D and 2D devices to show the benefit of low dimensional devices.
Appenzeller, Purdue University.
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