Reducing Micro-LED Sidewall Impact
The team from University of Science and Technology of China, Suzhou Institute of Nano-tech and Nano-bionics, and Jiangsu Institute of Advanced Semiconductors, reports: “Treatment of neutral reactive N/H species effectively mitigates the surface Fermi-level pinning and reduces the defect-related surface states. This step is directly followed by vacuum-interconnected atomic layer deposition (ALD) passivation, preventing air exposure and secondary impurity adsorption.”
The use of neutral N/H radicals also has the advantage of avoiding surface charging, generating strong local electric fields, and consequent anomalous band bending at the sidewalls.
Micro-LEDs are particularly sought for next-generation high-resolution displays and for communication technologies due to high brightness, high efficiency, fast response, and long operational lifetime. However, micro-LEDs suffer from increased sidewall negative impacts due to the perimeter/area ratio scaling inversely with size.
Conventional methods to reduce sidewall defects and efficiency losses often involve etching the surface back. This can smooth the surface, making it more liable to reflect light back into the device, reducing the light extraction efficiency.
Device fabrication (Figure 1) began with commercial blue LED epitaxial material consisting of silicon-doped n-GaN, 20 cycle strain release layer (SRL), six pairs of multiple quantum well (MQW), and magnesium-doped p-GaN. Indium tin oxide (ITO) was added by magnetron sputtering to encourage current-spreading.
The device mesa was etched by inductively coupled plasma (ICP) with a silicon dioxide (SiO2) layer used as a mask. The mask was removed by buffered oxide etching (BOE).
The sidewall was repaired by exposure to radio frequency (RF) sourced nitrogen/hydrogen (N/H) radicals. The material was then transferred to an ALD chamber in vacuum to avoid oxidation and other secondary contamination of the sidewall. The ALD process was used to apply 30nm aluminium oxide (Al2O3) as sidewall passivation.
The titanium/aluminium/nickel/gold metal electrodes were deposited through ICP-etched contact vias.
The impact of the sidewall treatment was most significant on the smallest 5μm LED: the peak external quantum efficiency (EQE) increased to 18.52% from 13.76%, an increase of 34.6%. The larger devices benefited to a lesser extent: from 26.92% to 29.71%, and from 24.63% to 28.13%, for the 20μm and 10μm LEDs, respectively.
The reverse current leakage at −5V was also significantly reduced by up to five orders of magnitude: from 7.53×10−3A/cm2 to 6.37×10−7A/cm2, from 8.44×10−2A/cm2 to 2.26×10−6A/cm2 and from 1.30A/cm2 to 9.64×10−6A/cm2 for the 20μm, 10μm and 5μm devices, respectively. Also, the treatment eliminated a low-forward-bias hump in the current–voltage characteristics. This enhanced current–voltage performance is credited to suppression of etching-induced leakage by the N/H radical treatment.
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