Nearly size-independent micro-LED UV-A performance

New: LED diodes

June 27, 2024

The University of Wisconsin-Madison in the USA reports atomic layer deposition (ALD) passivation as a way to overcome the effects of sidewall damage on light-emitting diodes (microLEDs) at the micron scale, resulting in almost size-independent performance in the 8 μm and 100 μm range (Guangying Wang and in., physica status solidi (RRL) published online on May 23, 2024).

The researchers comment on the smaller devices: “Micro LEDs can exhibit greater stability at high current density while providing higher light extraction efficiency and power output than regular-sized devices due to improved heat dissipation and lower stress with decreasing device sizes. “

Smaller devices may experience higher leakage currents, which reduce efficiency and indicate non-radiative recombination near the sidewalls due to plasma etching damage and dangling chemical bonds.

The researchers emphasize that while microLEDs of other wavelengths—visible (>420 nm) and UV-C (<280 nm)—have been the subject of much research, relatively little has been done on devices operating in the near-ultraviolet range (UV-A, 315–400 nm).

“UV-A LEDs can be used in a variety of applications such as curing, disinfection, lithography, counterfeit detection and 3D printing,” the team comments. Scientists also see potential for use in displays that use UV-A devices to power appropriate color conversion materials for pixels of different colors.

The researchers fabricated the microLEDs using metal-organic chemical vapor deposition (MOCVD) of UV-A wafers (Figure 1). The active 13x multi-quantum well (MQW) light-emitting region consisted of indium gallium nitride (InGaN) wells and aluminum gallium nitride (AlGaN) barriers. The p-type contact region included an electron-blocking layer (EBL) to avoid electron overshoot, which would lead to increased nonradiative recombination in the p-contact layers.

Figure 1: Cross-sectional diagram of a UV-A LED with an optical microscope image.

The p-electrode consisted of 100 nm thick nickel (Ni) first deposited prior to MES insulation by plasma etching up to the n contact layers. The sidewall defect was passivated: first using ALD of aluminum oxide (Al2ABOUT2), and then using plasma-assisted CVD of silicon nitride (SiNX). The nip electrodes were made by etching and deposition of titanium (Ti) and Ni.

The manufactured devices had a square shape with sides ranging from 8 to 100 μm. All devices exhibited a reverse leakage current density of less than ~10-5A/cm2. The smaller devices had a higher reverse current density than the larger LEDs.

The injection current density for a given forward bias was also higher for the smaller devices. The researchers comment: “This behavior may be due to reduced current density and improved thermal dissipation in the smaller microLEDs.”

The electroluminescence spectrum showed a peak at 368.5 nm with a full width at half maximum (FWHM) of 14.6 nm. The current-dependent spectral peak shifts were on the order of 0.028 nm, close to the detection limit of the researchers’ equipment.

The team comments: “These UV-A LEDs have been designed with minimal indium concentration (<1.5%) in the quantum well. This low indium concentration results in less indium segregation, which increases the uniformity of the wafer composition and thus leads to a low FWHM and stable emission wavelength. The low indium concentration also reduces the quantum-limited Stark effect caused by the InGaN/AlGaN interface, resulting in low blueshift at higher injection current density.”

The quantum-confined Stark effect (QCSE) refers to shifts in electron energy levels caused by internal electric fields created by different charge polarizations of chemical bonds between different alloys.

The external quantum efficiency (EQE) behavior of the LEDs was similar to 200 A/cm2 current density (Figure 2). The team reports: “The devices showed a high on-board EQE of approximately 5.5%, with a difference of less than 0.5% between all sizes at 100 A/cm2. This result indicates that we have successfully suppressed the leakage current of the smaller-sized micro-LED and achieved similar performance of the smaller-sized micro-LED as that of the regular-sized micro-LED.”

Figure 2: On-board EQE measurements of various LEDs.

EQE results on the wafer are typically lower than those for fully packaged LEDs measured in an integrating sphere. The team claims that >5% EQE of an 8μm LED is the highest EQE reported on a UV-A microdiode wafer smaller than 10μmx10μm in size. The 15μm device showed the highest EQE value of 5.78% at 200 A/cm.2.

EQE slope with increasing current in the range of 100–200 A/cm2 was positive, indicating that the precipitation area was shifted to >200A/cm2 region. The device had the greatest slope of 15 μm. The researchers suggest that the smaller 15-μm device benefited from improved heat dissipation, and the optimized passivation adequately eliminated the effects of sidewall etching damage, leading to compensation between the two factors.

Tags: Micro LEDs UV-A UV-C InGaN ALD MOCVD

Visit: https://doi.org/10.1002/pssr.202400119

Author Mike Cooke is a freelance technology journalist who has been working in the semiconductor and high-tech sectors since 1997.