InGaN/GaN MQW crystal array as multi-color micro-LED pixels on a flexible substrate

Abstract

The increasing demands for three-dimensional (3D) electronic and optoelectronic devices have triggered interest in epitaxial growth of 3D semiconductor materials. Appropriately designed optical metamaterials consisting of nano- and micro-structured elements have been proven to yield superior physical characteristics. However, most of the epitaxially-grown nano- and micro-structures available so far are limited to certain forms of crystal arrays, and the level of control is still very low. To further develop the capability of 3D epitaxy, we demonstrate here the heteroepitaxial growth of GaN architectures on a lattice-matched ZnO template grown hydrothermally in solution. Advancements in controlling the precise shape, geometry, size and orientation enable three-dimensionally architectured templates, which can be used to produce complex 3D structured GaN crystals. By introducing multi quantum well (MQW) structures on these 3D GaN light emitting crystals, we demonstrate the potential of InGaN/GaN-based micro-crystals (µ-crystals) as an individually-addressable, self-contained, and color-tunable emitting pixel of light-emitting diodes (LEDs) that are amenable to integration on flexible substrates. Our study reveals the multicolor emission mechanism of InGaN/GaN QW µ-crystal LEDs through in-depth optical and structural measurements and correlated bias-voltage and diameter-dependent wavelength change with variation in composition and thickness of multiple QWs on the polyhedral facets. This further provides important design and growth considerations for realizing multi-color displays with ultra-small pixels on non-planar and/or non-rigid surfaces. Furthermore, by applying our unique 3D GaN crystals to the pressure sensor, we propose a new approach to enable a large wavelength response to mechanical stimuli in the pixel arrays of a µ-crystal LED. Importantly, this dissertation has exploited the pressure-dependent variation of the dominantly emitting InGaN/GaN MQWs so that the LEDs could produce an unprecedented large change in the electroluminescence spectra (wavelength shift of >50 nm at 8 MPa). This unique wavelength tunability is fully investigated through finite element analysis (FEA) simulation combined with piezo-potential induced band-gap deformation at MQW junction. This study suggests a new capability for dynamic color mapping of the pressure distribution with a high spatial resolution and provides new opportunities.