NSF-BSF: High-mobility amorphous-iodide-based channel materials for p-type thin-film transistors and complementary TFT circuitry

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Nontechnical description: Over the past decade, displays in electronic devices like laptops have been revolutionized in part by using transparent conducting amorphous oxides due to their processing versatility and higher carrier mobility combined with low-temperature, large-area deposition conditions. A team of investigators from Brown University and Technion (Israel) is investigating a new class of materials, amorphous metal iodides like CuSnI and CuPbI, that would add to the arsenal of transparent conducting materials for use in displays and other transparent circuit technologies. Compared to other amorphous materials, the amorphous metal iodides feature a different physical mechanism: under an applied voltage, the currents are carried by positively charged holes. This offers new low-power dissipation circuit possibilities, as long as the materials properties of these metal iodides, including impurity content, phase, and temperature behavior can be tuned using composition and processing parameters. The research program covers a number of topics, ranging from fundamental experimental materials science of material deposition and characterization to theoretical modeling of phase transformation and impurity incorporation to prototype transistor device fabrication, brings together transparent electronic material and device expertise at Brown University with cutting-edge materials characterization and modeling at the Technion. The research project has an educational impact in and out of the classroom, including graduate student support, Brown-Technion graduate student interaction and exchange visits, as well as outreach to underprivileged middle-school students in the Providence area.Technical description: Two Brown experimentalists with complementary expertise in amorphous electronic materials and device physics, in collaboration with a Technion team experienced in atomic/nano/micro characterization and physical modeling, are focusing on the need for a high-mobility wide-bandgap low-temperature p-type material for thin film transistors (TFTs). The PIs have identified amorphous iodide-based materials as the most promising: they have a wide bandgap (3 eV), high hole mobility (up to 40 cm2/V.s), native vacancy doping, and are compatible with arbitrary substrates. They are also compatible with low-temperature-deposited amorphous n-type zinc oxide-based materials, opening the way for complementary TFT circuitry. The Brown PIs are leveraging their recent demonstration of high-performance indium-zinc-oxide materials by reconfiguring an oxide sputtering tool for in-situ iodide deposition. The Brown team is developing and optimizing the synthesis of a-Cu1-xMxI thin films (M = Sn, Pb, In, and others), studying the doping mechanism via Brouwer analysis, investigating phase stability, and fabricating prototype TFT demonstrator circuits. The experimental work is complemented by the detailed materials characterization and physical modeling performed by the Technion team, that has extensive experience in nanoscale amorphous and crystalline films. The final experimental goal is to develop the deposition of both n- and p-type transparent conducting materials in the same sputter-deposition process at low temperature on arbitrary substrates. The project provides training opportunities to the participating graduate and undergraduate students in cross-cutting electronic materials and devices fields, to the Brown-Technion visitor and student exchanges, as well as to the local underprivileged middle-school students.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.