Developing and Understanding Materials Processing-Property Relationships for Carbon Nanotube Electronics

Gaviria Rojas, William A

doi:https://doi.org/10.21985/n2-4dav-qf09

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Developing and Understanding Materials Processing-Property Relationships for Carbon Nanotube Electronics

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The commercial success of personal computing has led to the rapid creation and proliferation of diverse electronic systems including desktops, laptops, tablets, mobile devices, and embedded systems. For the past five decades, silicon has served as the base material for computing electronics. However, with increasing demand for unconventional electronics (e.g., ultrathin flexible wearables), new computing platforms are required that meet increasingly diverse mechanical, electronic, and functional requirements. Conventional silicon integrated circuit technology faces significant challenges in meeting these demands due to its limited mechanical flexibility, high temperature processing, and scaling limitations. Emerging alternative computing platforms based on other crystalline semiconductors suffer from similar limitations. Consequently, next-generation electronics necessitates the exploration of radically different electronic materials. Single-walled carbon nanotubes (SWCNTs) are among the most promising and highly studied nanoelectronic materials. Due to their small size, solution-processability, chemical stability, and chirality-dependent optoelectronic properties, SWCNTs offer a number of unique advantages and are compatible with the complex requirements of future electronic devices. Recent advances in post-processing methods have allowed SWCNTs to be used as semiconducting channels in diverse settings including charge transport devices, optical emitters and detectors, and chemical sensors. With this tunable functionality, a wide range of SWCNT-based electronic applications have been realized, such as printed digitial logic and complementary metal-oxide-semiconductor (CMOS) field-effect transistors (FETs). Despite this progress, most applications realized to date have largely focused on digital logic, and demonstrations of other key logic and analog functions remain largely unexplored. Moreover, silicon electronics are often used to perform these functions, further limiting the impact of SWCNT-based devices in practical electronic systems. Therefore, SWCNT-based electronics capable of performing these key functions must be realized to fully exploit the potential of solution-processed SWCNTs in emerging electronics. To this end, work presented in this dissertation focuses on the development of materials processing methods and their impact on SWCNT device properties. With this understanding of processing-property relationships, this work demonstrates SWCNT devices with novel function and performance in two key applications, overcoming fundamental challenges in the development SWCNT-based security and sensing technologies. Innovative materials processing and device operation are at the core of this thesis and have led to the development of the first true random number generator based on a solution-processed semiconductor. Processing optimization of charge transport through tunable doping, encapsulation and transistor design enabled the realization of low-power, complementary SWCNT static random access memory (SRAM) cells. Further characterization of device operation under dynamic biasing conditions allowed for the development of biasing strategies to operate these SWCNT SRAM cells as random bit generators through digitization of thermal noise. This work shows that this approach requires minimal computational overhead to produce highly random bit streams, as is confirmed through a series of rigorous tests including the National Institute of Standards and Technology (NIST) randomness statistical test suite (STS) and the TestU01 battery tests. This thesis work thus overcomes a key application-specific challenge to low cost, flexible security electronics by demonstrating a ubiquitous security primitive using a solution-processed semiconductor. In turn, this provides a path for improving security in the rapidly growing global network of interconnected electronic and sensing devices. Further innovative transistor design and understanding of material processing-property relationships led to the development of a novel ohmic-contact-gated transistor (OCGT) using solution-processed SWCNTs. Through the development of a novel self-aligned photolithography technique and processing optimization of components critical in charge transport (i.e., contacts and channel), a key advantage of the OCGT device geometry is realized through gating of the semiconducting channel with both the bottom gate and top contact electrode without the need of additional terminals beyond the conventional gate-source-drain configuration. In turn, this novel transistor design enables unprecedented levels of output current saturation in short channel limits (i.e., channel lengths < 300 nm) using atomically thin semiconductors without compromising the output current drive. Solution-processed SWCNT random networks are used to implement OCGTs that mitigate short channel effects to achieve low output conductance with high output current levels, overcoming the tradeoff relationship that is typically observed in conventional field-effect transistors FETs. These SWCNT OCGTs are then used in common-source amplifiers to attain the highest output current density and length-scaled signal gain to date for amplifiers based on solution-processed semiconductors, overcoming a key challenge in the development of practical sensing technologies. The utility and robustness of these amplifiers is further demonstrated by amplifying a number of analog biological signals from sensors commonly found in Internet of Things (IoT) and medical devices. Because the facile OCGT fabrication design can be generalized to other semiconducting nanomaterials, this thesis work has wide-ranging implications for solution-processed analog electronics.

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