(Invited) InGaN/AlGaN Red-Emitting Nanowire LEDs for Monolithic Micro-LED Displays

Abstract

High efficiency, high color rendition and low-cost light-emitting diodes (LEDs) with low power consumption, long lifetime and high reliability are highly expected for general lighting illumination, smartphones, smartwatches, virtual reality (VR), augmented reality (AR) headsets, and micro-display applications. Nonetheless, the achievement of deep green to red-emitting LEDs using conventional III-nitride quantum well heterostructures has been difficult, due to the presence of large densities of dislocations, strong polarization fields, poor hole transport, and carrier delocalization [1]. The external quantum efficiency (EQE) of the blue and green InGaN LEDs surpassed 80% and 53%, respectively. However, due to the aforementioned issues, the current InGaN red LEDs with high Indium composition exhibit extremely low EQE which is less than 3% [2]. In this regard, LEDs using nanowire structures offer dramatically reduced strain-induced polarization fields and dislocation densities, providing ideal material structure for high efficiency full-color and even white light emission without using phosphor-converters. In this study, we have successfully designed, and fabricated high efficiency red-emitting nanowire heterostructures and demonstrated micro-LEDs with stable and strong emission at ~645 nm. Moreover, the micro-LEDs have high internal quantum efficiency of >40%.The InGaN/AlGaN nanowire micro-LED structures are grown by RF plasma-assisted molecular beam epitaxy (MBE) under nitrogen-rich condition. The LED structure consists of a 250 nm n-GaN nanowire template, 10 couples of 3 nm AlGaN quantum barrier (QB)/ 3 nm InGaN quantum well (QW) served as the active region, and a 200 nm p-GaN. During the epitaxial growth of AlGaN barriers, an AlGaN shell spontaneously forms, enabling a unique InGaN/AlGaN core-shell structure [3]. The emission color of the micro-LEDs can be defined by controlling the ratio of Ga/In flux and the substrate temperature during the MBE growth process. Detailed growth conditions and the device fabrication can be found elsewhere [3-5]. The nanowires are uniformly arranged on Si substrates, as illustrated in Figure 1(a). Figure 1(b) shows the schematic structure of the fabricated micro-LEDs. Strong red emissions were measured from the InGaN/AlGaN core-shell LEDs, as shown in Figure 1(c). At injection current of 400mA, the peak emission wavelength is at ~645nm. The red-emitting micro-LEDs exhibit stable emissions with a blue-shift of only ~ 4nm under injection current from 50 mA to 400 mA, attributed to the significantly reduced quantum-confined Stark effect (QCSE) in the nanowire structures. Moreover, full color micro-LEDs with device size from 10x10 µm2 to 100x100 µm2 have been fabricated using similar approach. Such high efficiency, high color rendering properties, and low power consumption micro-LEDs are promising candidates for emerging AR/VR devices and micro-LED display applications.