Abstract

The expanding consumer electronics industry has spurred significant advancements in display technology. Thin-Film Transistors (TFTs) serve as active matrix switching components in LCD and OLED panels. Amorphous metal-oxide semiconductors (AOS) support large-area deposition at low temperatures and boasts an electron mobility many times greater than that of amorphous silicon. This work presents a comprehensive study on AOS materials and devices for their introduction to display, monolithic integration, and heterogeneous integration applications. Key studies have focused on advancing the state of (Indium Gallium Zinc Oxide) IGZO TFTs by addressing challenges in device uniformity, reliability, and modeling. Device uniformity was improved by modifying process parameters to allow for higher degree of film uniformity during deposition. Devices fabricated with this modified process demonstrate exceptional resistance to the application of traditional bias stress. Application of intensive bias and illumination-bias stress treatments led to distinctive transfer characteristics, differing in shift magnitude, distortion, and hysteresis behavior. Silvaco TCAD and a previously defined mobility model were used to simulate this behavior and explore the defect states created during intensive bias stress. Utilizing ion implantation for self-aligned source/drain regions present a path towards sub-micron device scaling. Past reports have demonstrated boron implanted self-aligned TFTs with excellent on-state and off-state performance. However, when subjected to thermal stresses above 175ºC the device transfer characteristics gradually shift over time. From this work it is hypothesized that this instability is related to the implanted boron dose, with higher doses presenting more shifting. Interpretation of electrical results suggests that boron can exist in two states: an active form that bonds with interstitial oxygen, increasing oxygen vacancies and enhancing conductivity, and an inactive form as an isolated interstitial atom. At low boron doses, the active state is dominant, improving conductivity, whereas at higher doses, the inactive state prevails, leading to reduced current and, in extreme cases, charge injection degradation. A thermally-activated diffusive mobility and percolation theory are two contending processes that have been proposed to govern electron transport in IGZO. This work builds upon previous investigations on the temperature dependence of channel mobility in IGZO TFTs, where transport behavior from 170 K to room temperature (RT) is clearly described by a thermally-activated diffusive mobility. The isolation of thermally dependent mechanisms via TCAD enabled the separation of the intrinsic and extrinsic components of the observed field-effect mobility. The methods used resulted in a quantitative assessment of the thermally-activated diffusive mobility and the free/total charge ratio. These advancements allowed for the development of a platform to realize the heterogeneous integration of µLEDs on an IGZO TFT backplane. µLEDs were successfully transferred onto an IGZO TFT backplane using a micro transfer printing technique employing a PDMS stamp. Metal deposition techniques were investigated for post passivation anneal interconnects between printed µLEDs and the completed IGZO backplane. Emission of single pixel and RGB pixel circuits was confirmed and a new RGB pixel was designed to minimize pixel cross-talk and sub-pixel leakage. While IGZO is the only AOS technology that has matured enough for commercialization, the electron channel mobility (≈10 cm2/Vs) presents a limitation in its application in back-end-of-line (BEOL) monolithic and heterogeneous integration. As such, alternate candidate AOS materials exist that exhibit channel mobilities 2-3x higher than that of IGZO. Studies in alternate AOS materials such as Indium Tungsten Oxide (IWO), Indium Tin Gallium Oxide (ITGO), and Indium Gallium Zinc Tin Oxide (IGZTO) have been conducted. The investigation on IWO TFTs revealed an unusual metastable device behavior dependent upon annealing temperature. This is believed to be the first report of such behavior, as published works adhere to either a low-temperature or high-temperature regime. The investigations into ITGO and IGZTO serve as preliminary studies; device characteristics support the claims of high channel mobility; however, the influence of defect states clearly indicates the need for further process development. The advancements realized in IGZO TFTs in this work will serve as a foundation for these alternative AOS materials.

Library of Congress Subject Headings

Amorphous semiconductors--Materials; Metal oxide semiconductors; Indium gallium zinc oxide; Light emitting diodes; Thin film transistors

Publication Date

8-8-2025

Document Type

Dissertation

Student Type

Graduate

Degree Name

Microsystems Engineering (Ph.D.)

Department, Program, or Center

Microsystems Engineering

College

Kate Gleason College of Engineering

Advisor

Karl D. Hirschman

Advisor/Committee Member

Parsian Mohseni

Advisor/Committee Member

Jing Zhang

Campus

RIT – Main Campus

Plan Codes

MCSE-PHD

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