Paving the Way for High‐Performance UVB‐LEDs Through Substrate‐Dominated Strain‐Modulation

Abstract

AlGaN‐based ultraviolet‐B light‐emitting diodes (UVB‐LEDs) exhibit great potential in phototherapy, vitamin D3 synthesis promotion, plant growth regulation, and so on. However, subjected to the excess compressive strain induced by the large lattice mismatch between multiple quantum wells (MQWs) and AlN, UVB‐LEDs that simultaneously satisfy the requirements of high light output power (LOP), low working voltage, and excellent stability are rarely reported. Here, a substrate‐dominated strain‐modulation strategy is proposed. By precisely manipulating the strain in AlN grown on nano‐patterned sapphire substrate (NPSS) to a slightly tensile one, the compressive strain in the following Al0.55Ga0.45N underlayer and Al0.28Ga0.72N/Al0.45Ga0.55N MQWs is successfully suppressed. As a result, an outstanding UVB‐LED with a peak wavelength at 303.6 nm is achieved. The 20 × 20 mil2 UVB‐LED chip shows a wall‐plug efficiency (WPE) of 3.27% under a forward current of 20 mA and a high LOP of 57.2 mW with an extremely low voltage of 5.87 V under a forward current of 800 mA. It is more exciting that the LOP degradation is as low as 17% after 1000 h operation under a forward current density of 75 A cm−2, showing excellent stability. The here‐developed UVB‐LED, with a high LOP and excellent reliability, will definitely promote the applications of AlGaN‐based UVB‐LEDs.

Monolithic Dual-Wavelength AlGaN-Based UV-LEDs with Controllable Spectral Profile

“…The corresponding WPE (WPE = LOP I × V ) decreases from 3.86 to 3.07% for DW-LED and from 3.73 to 2.74% for REF-LED, respectively, as shown in Figure c. The decreasing trend of WPE with increasing injection current is common in DUV-LEDs, which may be related to electron leakage under high current injection. , It is worth mentioning that both DW-LED and REF-LED exhibit high maximum WPE values of 3.86 and 3.73% under 10 mA, respectively, which are comparable to the values reported for single-peak UVB-LEDs and UVC-LEDs. ,, …”

Progress in efficient doping of Al-rich AlGaN

“…Moreover, doping in the cladding layers indirectly influences the LEE in UV-LEDs. Specifically, owing to the lack of a thorough solution for p-type doping in Al-rich AlGaN, a thick p-GaN layer is widely adopted as a substitute [25,27,33,37,39,41,43] . Hence, the vast majority of the UV light emitted towards the p-region is absorbed, restricting the increase of LEE as well as WPE.…”

Toward High-Performance AlGaN-Based UV-B LEDs: Engineering of the Strain Relaxation Process

“…5 The reported maximum light output power (LOP) of 20 × 20 mil 2 UV-B chips at 304 nm was 57.2 mW under an operating current of 800 mA, with 17% degradation after 1000 h of operation. 7 The major bottlenecks lie in proper strain management and light extraction. 4−11 For AlGaN grown on AlN bulk/templates, the huge misfit strain will trigger the strain relaxation inducing: (1) the generation of misfit dislocations (MDs) which enhance nonradiative recombination; 12 (2) severe phase separation and surface roughness causing local current leakage; 13−16 (3) increasing point defects contributing to the nonradiative recombination and current leakage.…”

“…An on-wafer EQE of 9.6% was realized by using a highly reflective Ni/Al electrode but still suffered from an operating voltage exceeding 40 V under 250 mA, leading to a relatively low wall-plug efficiency (WPE) of <2% . The reported maximum light output power (LOP) of 20 × 20 mil 2 UV-B chips at 304 nm was 57.2 mW under an operating current of 800 mA, with 17% degradation after 1000 h of operation . The major bottlenecks lie in proper strain management and light extraction. − For AlGaN grown on AlN bulk/templates, the huge misfit strain will trigger the strain relaxation inducing: (1) the generation of misfit dislocations (MDs) which enhance nonradiative recombination; (2) severe phase separation and surface roughness causing local current leakage; − (3) increasing point defects contributing to the nonradiative recombination and current leakage. ,, Although the strain relaxation of AlGaN-based LEDs has been reported widely, − the relaxation mechanisms including the critical condition and triggering sequence for high Al-content AlGaN have been less than decisive.…”

“…The reported maximum light output power (LOP) of 20 × 20 mil 2 UV-B chips at 304 nm was 57.2 mW under an operating current of 800 mA, with 17% degradation after 1000 h of operation . The major bottlenecks lie in proper strain management and light extraction. − For AlGaN grown on AlN bulk/templates, the huge misfit strain will trigger the strain relaxation inducing: (1) the generation of misfit dislocations (MDs) which enhance nonradiative recombination; (2) severe phase separation and surface roughness causing local current leakage; − (3) increasing point defects contributing to the nonradiative recombination and current leakage. ,, Although the strain relaxation of AlGaN-based LEDs has been reported widely, − the relaxation mechanisms including the critical condition and triggering sequence for high Al-content AlGaN have been less than decisive. Clarifying the relaxation mechanisms and engineering the misfit strain during AlGaN growth is crucial to defining the final structural quality, which is a vital step toward performance improvement of AlGaN-based UV-B LEDs and potential for practical applications.…”