A Simple Approach to Achieving Ultrasmall III-Nitride Microlight-Emitting Diodes with Red Emission

Abstract

The microdisplays for augmented reality and virtual reality require ultrasmall micro light-emitting-diodes (μLEDs) with a dimension of ≤5 μm. Furthermore, the microdisplays also need three kinds of such μLEDs each emitting red, green, and blue emission. Currently, in addition to a great challenge for achieving ultrasmall μLEDs mainly based on III-nitride semiconductors, another fundamental barrier is due to an extreme difficulty in growing III-nitride-based red LEDs. So far, there has not been any effective approach to obtain high indium content InGaN as an active region required for a red LED while maintaining high optical performance. In this paper, we have demonstrated a selective epitaxy growth approach using a template featuring microhole arrays. This allows us to not only obtain the natural formation of ultrasmall μLEDs but also achieve InGaN with enhanced indium content at an elevated growth temperature, at which it is impossible to obtain InGaN-based red LEDs on a standard planar surface. By means of this approach, we have demonstrated red μLEDs (at an emission wavelength of 642 nm) with a dimension of 2 μm, exhibiting a high luminance of 3.5 × 10 7 cd/m 2 and a peak external quantum efficiency of 1.75% measured in a wafer form (i.e., without any packaging to enhance an extraction efficiency). In contrast, an LED grown under identical growth conditions but on a standard planar surface shows green emission at 538 nm. This highlights that our approach provides a simple solution that can address the two major challenges mentioned above.

Nanomaterial Integration in Micro LED Technology: Enhancing Efficiency and Applications

“…Multi-color pixels and micron-scale platelets, pyramids, nanowires were reported by Kishino et al's studies [302]. Recent research by Wang et al [303], [37,304]however, demonstrated green LED matrices with a peak EQE of ~9% and red-emitting micro-LEDs with a peak EQE of 1.75%, both grown utilising selective area epitaxy [37,304]. For instance, an integration between a micro-LED and a high-electron-mobility-transistor (HEMT) resulted in a modulation bandwidth of 1.2 GHz, which is the highest ever recorded in this context [305].…”

“…Reduced quantum efficiency due to crystal defects like dislocations in III-nitride nanostructures underscores the need for defect mitigation to improve crystal quality. Improved growth techniques are required to reduce defects while enhancing the structural integrity of III-nitride nanostructures [304,337]. In spite of these hurdles, III-nitride nanostructures, especially gallium nitride (GaN), hold great promise across various applications [338].…”

The micro-LED roadmap: status quo and prospects

“…Based on the CSE approach, ultrasmall µLEDs with a record EQE in the green and red spectral regions have been achieved [98,99]. The CSE approach has also led to the demonstration of epitaxially integrating a microcavity and ultrasmall µLEDs achieving a very stable emission wavelength and the demonstration of monolithically integrating µLEDs with HEMTs for VLC and microdisplay applications [100][101][102][103].…”

Review of a direct epitaxial approach to achieving micro–LEDs

“…Based on our CSE approach, the Sheffield team has achieved ultra-small µLEDs without involving dry-etching processes, leading to the demonstration of ultra-small and ultra-compact µLEDs with a record EQE and small reverse leakage current in the green and red spectral regions. [33,34,36,37] Based on this CSE approach, by epitaxially integrating a microcavity and ultra-small µLEDs, ultrasmall and ultra-compact µLEDs with a stabilized emission wavelength have been achieved, [35] which is extremely important for practical applications and has addressed the longstanding issue in the field of III-nitride optoelectronics. In addition, the CSE approach has also been employed to monolithically integrate µLEDs with high electron mobility transistors (HEMTs) for visible light communication (VLC) and display applications.…”

“…In our di-rect epitaxial approach, µLEDs are grown in the regularly arrayed micro-holes, naturally forming relaxation in the lateral direction and thus enhancing indium incorporation into GaN. Using regularly arrayed micro-hole masks (namely, 2 µm diameter and 1.5 µm-spacing), red µLEDs with an emission wavelength at 642 nm and a peak EQE of 1.75% have been achieved, [36] as shown in Fig. 4.…”