Self-Aligned Heterojunction Diodes and Transistors

Beck, Megan

doi:https://doi.org/10.21985/n2-7q9v-vk76

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Self-Aligned Heterojunction Diodes and Transistors

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Low-dimensional materials have emerged as a promising platform for ultrathin electronic and optoelectronic devices. The span of electronic properties covers the spectrum from metallic through small and medium bandgap semiconductors to large bandgap insulators, providing all the necessary components to fabricate a variety of devices. Compared to bulk-semiconductor based devices, devices made from low-dimensional materials should have much lower leakage currents because charge carriers are confined to an atomically channel, be fully modulated by an electric field because of reduced dielectric screening, and suffer from fewer interfacial traps from the lack of dangling bonds at the surface. The weak van der Waals interactions and lack of dangling bonds also allow for stacking of heterostructures from arbitrary and disparate material systems. These strain-free and abrupt interfaces result in interesting charge transport that can be tuned electrostatically, leading to novel device phenomenon that is not accessible in bulk semiconductor devices. While the realization of function devices using these atomically thin materials is significant, current demonstrations do not yet present definitive advantages over existing silicon-based technology. Additionally, most demonstrations are lateral devices with large footprints or vertical devices with very thick flakes. Neither of which takes advantage of the unique properties of these materials. This motivates investigation into fabrication methods and heterostructure geometries that exploit the novel properties associated with the atomically thin nature of these materials to (1) understand intrinsic charge-transport in van der Waals devices by reducing extrinsic effects using a self-aligned scheme, (2) develop scalable fabrication techniques to reduce lithographic steps and enable device geometries optimized for low-dimensional materials and (3) explore the characteristics and functionalities of more complicated device structures. This thesis presents a novel approach for manipulating existing fabrication techniques to enable device geometries that overcome some of the challenges associated with current methods. Controlling the undercut profile of either electron-beam lithography or photolithography resists, enables the encapsulation of metal electrodes with a dielectric extension in a single lithography step in a self-aligned way. The resulting dielectric extension, as determined by the undercut, controls the smallest channel dimension of the devices, and the encapsulation of the electrode enables semi-vertical geometries. This method reduces the number of lithography steps for fabricating complicated geometries and relies on the dimensions of the resist undercut rather than the limitations of lithography equipment and transfer methods. For local-gated MoS2 transistors, the self-aligned structure enables multiple biasing geometries which, due to the difference in electrostatics, can mitigate short-channel effects that are commonly seen in lateral short-channel devices. These transistors exhibit superior current saturation and improved ON/OFF ratios under source-gated biasing. Using the self-aligned process to fabricate MoS2-BP p-n heterojunctions results in significant reduction of the constituent semiconductor series resistances, and the semi-vertical geometry allows for the overlap junction region to be modulated by both a top and bottom gate. The devices show dual-gate tunability of rectifying behavior as well as additional control of the antiambipolar shape and peak position that is not seen in other lateral p-n junctions from low-dimensional materials. The self-aligned heterojunction fabrication process can be extended to large-area using CVD-grown MoS2 and solution-processed CNTs. The similar gate stacks in the dual-gate geometry enable multiple biasing methods resulting in dual-gate control of the rectification behavior and unprecedented control of the peak height, peak position, and full-width-half-maximum of the Gaussian antiambipolar response. This Gaussian behavior is exploited in a spiking neuron circuit based on the Hodgkin-Huxley model to demonstrate a variety of biologically relevant spiking neuron responses for neuromorphic computing.

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