Abstract
Visible light communications (VLC) require III-nitride visible micro-light-emitting diodes (μLEDs) with a high-modulation bandwidth. Such μLEDs need to be driven at a high injection current density on a kA/cm2 scale, which is about 2 orders of magnitude higher than those for normal visible LED operation. μLEDs are traditionally fabricated by dry-etching techniques where dry-etching-induced damages are unavoidable, leading to both a substantial reduction in performance and a great challenge to viability at a high injection current density. Furthermore, conventional biasing (which is simply applied across a p–n junction) is good enough for normal LED operation but generates a great challenge for a single μLED, which needs to be modulated at a high injection current density and at a high frequency. In this work, we have proposed a concept for an epitaxial integration and then demonstrated a completely different method that allows us to achieve an epitaxial integration of a single μLED with a diameter of 20 μm and an AlGaN/GaN high-electron-mobility transistor (HEMT), where the emission from a single μLED is modulated by tuning the gate voltage of its HEMT. Furthermore, such a direct epitaxial approach has entirely eliminated any dry-etching-induced damages. As a result, we have demonstrated an epitaxial integration of monolithic on-chip μLED-HEMT with a record modulation bandwidth of 1.2 GHz on industry-compatible c-plane substrates.
Highlights
The patterned high-electron-mobility transistor (HEMT) template is reloaded into a metalorganic vapor-phase epitaxy (MOVPE) system for further μLED growth
It is essential that the overgrown n-GaN of μLEDs within the microhole areas directly contacts the interface between the AlGaN barrier and the GaN buffer of the HEMT so that each single μLED is electrically connected with the HEMT through the two-dimensional electron gases
We have employed our direct epitaxial approach to achieving small μLEDs on predefined microhole arrays formed by SiO2 masks on an AlGaN/GaN HEMT template, demonstrating an epitaxially monolithic on-chip integration of HEMT-μLED, where a single μLED with 20 μm in diameter is modulated and stably by the gate bias of its HEMT instead of conventional biasing methods
Summary
One of the greatest challenges the current VLC technology is facing is due to its limited modulation bandwidth, which is far from satisfactory and poses an insurmountable barrier for promoting VLC applications. A modulation bandwidth is determined by the larger of the RC time constant of the junction capacitance and the carrier recombination lifetime of III-nitride visible LEDs used as a transmitter. RF emissions span a limited range from 3 kHz to 300 GHz, while the wavelengths of visible light are much shorter than those of RF emissions leading to a huge range from 430 to 750 THz, which is more than 3 orders of magnitude larger than RF. VLC presents another unique advantage which Wi-Fi and 5G lack, namely, security-
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