(Invited) Flexible Multifunctional III-N Semiconductor Materials and Devices for Energy Applications

Abstract

We present recent development in flexible wide-bandgap semiconductor materials and devices. Especially, we focus on mechanically bendable group-III-nitride (III-N) thin-film heterostructures and their photonic, electronic, and energy-harvesting devices in energy applications. The presentation will cover various topics including (1) multi-functionality of flexible III-N devices, (2) direct growth of high-quality single-crystal-like GaN semiconductor thin films on low-cost flexible metal tapes, and (3) piezoelectric generators and sensors for self-powered wearable systems by harvesting and sensing ambient biomechanical energy. Flexible III-N thin-film heterostructures have an implication of more than just mechanically flexible materials. Bendable devices based on III-N heterostructures can be equipped with new functionalities and even further improved performance characteristics using a new concept of active polarization engineering by controlled external strains. We propose to develop multi-functional and/or further-improved-performance devices by utilizing the interactions between electronic and optical properties and mechanical forces in the flexible III-N heterostructures. The concept will enable new mechano-electro-photonic (MEP) devices. We will show by device modeling that new device concept in flexible transistors based on AlGaInN/GaN heterostructure is possible, including modulation of 2-dimensional electron gas (2DEG) density by bending, strain-effect transistors (Figure 1: I-V characteristics of strain-effect transistors), and high-hole-mobility transistors. Furthermore, photon emitters based on flexible III-N heterostructure can result in higher internal quantum efficiency (IQE), higher wall-plug efficiency (WPE), and color modulation by optimum bending conditions (Figure 2: Concept of color-tunable white light-emitting diodes). We study and develop nearly-single-crystalline GaN thin film on flexible metal tapes by a direct deposition technique for the demonstration of a new wide-bandgap semiconductor platform, targeting high-performance yet economical, flexible, and versatile device technology. Data and analysis are presented for the single-crystal-like film, as representatively shown in Figure 3 (Flexible hybrid tape substrate consisting of GaN thin film and Cu tape with crystallinity-transformational buffer layers). Energy harvesters that scavenge biomechanical energy are promising power supply candidates for wearable and implantable electronics. Of the most widely used energy harvesters, piezoelectric generators can generate more electric charge than their triboelectric counterparts with similar device size, thus are more suitable to make compact wearable devices. We develop a flexible piezoelectric generator (F-PEG) with chemically stable and biocompatible III-N thin film. Data and analysis are presented for the flexible III-N F-PEG. We also demonstrate that the F-PEG can directly power electronics such as light-emitting diodes and electric watches, and charge commercial capacitors and batteries to operate an optical pulse sensor, as representatively shown in Figure 4 (Pulse sensor powered by F-PEG and heart rate measurement on a fingertip). Figure 1