These authors have contributed equally to this work
This article was submitted to Physical Chemistry and Chemical Physics, a section of the journal Frontiers in Chemistry
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Strain-reduced micro-LEDs in 50 μm × 50 μm, 100 μm × 100 μm, 200 μm × 200 μm, 500 μm × 500 μm, and 1,000 μm × 1,000 μm sizes were grown on a patterned c-plane sapphire substrate using partitioned growth with the metal-organic chemical-vapor deposition (MOCVD) technique. The size effect on the optical properties and the indium concentration for the quantum wells were studied experimentally. Here, we revealed that the optical properties can be improved by decreasing the chip size (from 1,000 to 100 µm), which can correspondingly reduce the in-plane compressive stress. However, when the chip size is further reduced to 50 μm × 50 μm, the benefit of strain release is overridden by additional defects induced by the higher indium incorporation in the quantum wells and the efficiency of the device decreases. The underlying mechanisms of the changing output power are uncovered based on different methods of characterization. This work shows the rules of thumb to achieve optimal power performance for strain-reduced micro-LEDs through the proposed partitioned growth process.
During the past few decades, InGaN/GaN based light-emitting diodes (LEDs) have been extensively studied to improve the external quantum efficiency (EQE) and optical output power (
In this work, strain-reduced micro-LEDs with various sizes of 50 μm × 50 μm, 100 μm × 100 μm, 200 μm × 200 μm, 500 μm × 500 μm, and 1,000 μm × 1,000 µm are demonstrated to be directly grown on patterned c-plane sapphire substrates by using our MOCVD system. To protect these PG micro-LEDs during the dicing process, a margin (10 µm) was included between two individual chip partitions. Here we systematically investigated the size effect on the strain and studied the electrical and optical properties for the proposed PG micro-LEDs which are critical to optimize the size for PG micro-LEDs.
The LED epitaxial wafers were grown on c-plane single polished sapphire substrates by the MOCVD system. The sapphire substrate was first deposited with SiO2 (100 nm) by plasma enhanced chemical vapour deposition (PECVD), and the SiO2 layer was then patterned and etched with reactive-ion etching (RIE) to obtain square opening regions with different sizes. The margin width between two opening regions was set to 10 µm. Trimethylaluminum (TMAl), trimethylindium (TMIn), trimethylgallium (TMGa), and ammonia (NH3) were used as Al, In, Ga, and N precursors, respectively. The growth was initiated with a 3 µm thick unintentionally doped GaN, followed by a 5.5 µm thick Si-doped N-GaN (doping concentration ≈ 5 × 1018/cm3). Then, six pairs of In0.15Ga0.85N/GaN multiple quantum wells (MQWs) (thickness of 3 nm/12 nm) were grown. A Mg-doped Al0.15Ga0.85N electron blocking layer (EBL), with a thickness of 20 nm, was grown to reduce the electron overflow. Finally, a 200 nm thick Mg-doped GaN (with a free hole concentration of 3 × 1017/cm3) was grown as the hole source layer. In both of the EBL and the hole source layers, bis(cyclopentadienyl)magnesium (Cp2Mg) was used as the Mg precursor.
The images of different PG micro-LEDs were taken by a scanning electron microscopy (SEM) (JEOL JSM-5600LV) system. Micro-Raman spectra were also recorded using a spectrometer (Horiba JY-T64000) equipped with an excitation laser of 532 nm wavelength to reveal the strain level. Electroluminescence (EL) spectra and the optical output power were acquired by an Ocean Optics spectrometer (QE65000) attached to an integrating sphere. The micro-LEDs were fabricated into the same size (1 mm × 1 mm) using micro-fabrication technique. A LED tester (M2442S-9A Quatek Group) was used to measure the current-voltage characteristics of the resulting LED chips.
SEM images of the proposed PG micro-LEDs grown in different sizes: top-view images for
The optical output power and the external quantum efficiency (EQE) for these PG micro-LEDs of different sizes are presented in
Raman scattering is a widely used method to study the strain for III-nitrides (
As the in-plane stress reduces, the blueshift for the electroluminescence wavelength caused by the reduced QCSE in the quantum wells is expected. The EL spectra and the peak EL wavelength for the studied PG micro-LEDs at 20 mA are depicted in
EL spectra of the PG micro-LEDs of different sizes at 20 mA:
We further carried out the current voltage (I–V) characteristics for the PG micro-LEDs of different sizes in
I–V characteristics for PG micro-LEDs of different sizes.
In this work, to study the size effect on strain release, strain-reduced micro-LEDs in 50 μm × 50 μm, 100 μm × 100 μm, 200 μm × 200 μm, 500 μm × 500 μm, and 1,000 μm × 1,000 µm sizes were grown on patterned c-plane sapphire substrates by the MOCVD technique. Various characterization methods were performed to study the strain for different sizes. The output power and EQE characteristics show that the optical performance can be enhanced when the PG micro-LED size is reduced from 1,000 μm × 1,000 µm–100 μm × 100 µm. The improved EQE is due to the reduced QCSE in the InGaN/GaN quantum wells for those PG micro-LEDs. However, for the 50 μm × 50 µm size, the optical performance is limited by the smallest effective lighting area and additional defects induced by higher indium incorporation. Therefore, in our case, the 100 μm × 100 µm size delivers the highest output power and EQE. In summary, our experimental results indicate that to obtain high-quality epitaxy based on partitioned growth, a small size is needed provided that the defects density and margin ratio are carefully controlled. Since the partitioned growth method is easy and low-cost to apply to the sapphire substrate LEDs growth process, such directly grown micro-LEDs hold great promise for being adopted in commercial product lines.
The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding authors.
All authors listed have made a substantial, direct, and intellectual contribution to the work and approved it for publication.
This work is supported by the Singapore National Research Foundation under Grant No., NRF-CRP-6-2010-2 and the Singapore Agency for Science, Technology and Research (A*STAR) SERC Pharos Program under Grant No. 1527300025. HD gratefully acknowledges TUBA.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.