Progress of InGaN-Based Red Micro-Light Emitting Diodes

: InGaN-based red micro-size light-emitting diodes ( µ LEDs) have become very attractive. Compared to common AlInGaP-based red µ LEDs, the external quantum efﬁciency (EQE) of InGaN red µ LEDs has less inﬂuence from the size effect. Moreover, the InGaN red µ LEDs exhibit a much more robust device performance even operating at a high temperature of up to 400 K. We review the progress of InGaN red µ LEDs. Novel growth methods to relax the strain and increase the growth temperature of InGaN red quantum wells are discussed.


Introduction
Micro-size light-emitting diodes (µLEDs) have attracted huge attention as the nextgeneration display technology for wide applications, such as wearable watches, virtual/augmented reality, micro-projectors, and ultra-large televisions [1][2][3][4]. InGaN-based red µLEDs have become the research focus now [5][6][7][8]. To achieve red emission, the indium composition in the InGaN quantum wells (QWs) should be greater than 35%, which causes a lot of challenges. First, the large lattice mismatch leads to a high defect density. Second, a much lower growth temperature is required to increase the indium incorporation efficiency in the InGaN red QWs, resulting in poor materials crystal quality. Moreover, the high indium composition InGaN red QWs generate a large piezoelectric field and severe quantum-confinement Stark effect (QCSE), leading to a poor internal quantum efficiency (IQE) [8]. The common AlInGaP-based red light-emitting diodes (LEDs) show high efficiency in regular size devices (0.1 mm 2 ) but the µLEDs devices suffer from a severe size-effect in external quantum efficiency (EQE) and poor thermal stability at a high temperature [9][10][11][12]. Our group has demonstrated that InGaN µLEDs can overcome EQE size effect by proper sidewalls passivation/treatment. InGaN µLEDs also exhibit a robust thermal stability in commercial InGaN blue and green LEDs [6,13,14]. Therefore, InGaN red µLEDs are promising candidates for µLEDs display application.
Our group has reported high efficiency InGaN green LEDs using AlGaN cap layer grown on top of InGaN green QWs [15]. More recently, we reported InGaN red µLEDs with an EQE over 3% using similar active region design [5][6][7]. Other novel growth strategies have been proposed to achieve InGaN red µLEDs, such as InGaNOS pseudo-substrate [16][17][18][19], nano-porous GaN [20], strain relaxed InGaN buffer layer using in-situ InGaN decomposition layer [21,22]. These technologies aim to relax the strain of InGaN buffer layer and increase the indium intake efficiency in the red InGaN QWs. Thus, the red InGaN QWs can be grown at a higher temperature, which promises a high crystal quality. Moreover, a new model of 3-dimension "V" pits injections was proposed and high-efficiency InGaN red LEDs were reported [23,24].
In this review, we discuss the advantages of InGaN red µLEDs. We review the recent progress of InGaN red LEDs/µLEDs grown on all kinds of substrate/templates, such as patterned sapphire substrate (PSS), Si (111), InGaNOS, nano-porous GaN template, and strain relaxed InGaN layer.  Figure 1a shows the uniform emission (~3 A/cm 2 ) by optical microscopy images for the InGaN red µLEDs with size reduces from 100 × 100 to 20 × 20 µm 2 [6]. These µLEDs show a packaged EQE varied from 2.4% to 2.6% as the size reduces from 100 × 100 to 20 × 20 µm 2 ( Figure 1). The emission wavelength is centered at 607 nm and the full-width half maximum (FWHM) is 50 nm. A comparison of normalized EQE vs. µLEDs size between InGaN and AlInGaP red µLEDs points out that the EQE of AlInGaP red µLEDs reduces dramatically as the size decreases to 20 × 20 µm 2 (only 43% EQE left) [10]. This is caused by Shockley-Read-Hall (SRH) nonradiative recombination due to sidewalls damage and a higher surface recombination velocity. It is very difficult to overcome this challenge since it is related to the intrinsic AlInGaP materials property. In contrast, the EQE of InGaN red µLEDs has less influence from the size effect down to 20 × 20 µm 2 due to a lower surface recombination velocity. For the size down to 20 × 20 µm 2 , the electroluminescence (EL) is very uniform in InGaN red µLEDs. It is promising to achieve ultra-small (<10 × 10 µm 2 ) high-efficiency red µLEDs using InGaN materials system instead of the common AlInGaP materials. (c) comparison of normalized peak EQE for different sizes InGaN and AlInGaP red µLEDs. These figures are reproduced from [6], with the permission of AIP Publishing.

Robust Temperature Property of InGaN Red µLEDs
The output power and EQE of AlInGaP red µLEDs decrease dramatically when the operation temperature gets higher [12]. This is caused by carrier leakage and injection efficiency reduction. In contrast, InGaN red µLEDs show a robust thermal property. P. Li et al. from UCSB investigated the high-temperature EL properties from 300 K to 400 K of InGaN red µLEDs (40 × 40 µm 2 ) [7,14]. The 600 nm InGaN red µLEDs show a high peak EQE of 3.2%. From Figure 2, the peak EQE decreases as the temperature increases, especially at a low current density (J). The hot/cold factor (HC factor) was defined by the ratio of high-temperature EQE to room-temperature EQE to quantify the thermal droop. At 50 A/cm 2 , the EQE HC factor is as high as 0.85 at 360 K and 0.72 at 400 K. The high HC in InGaN red µLEDs agrees with commercial InGaN blue LEDs, which is a general property in the InGaN materials system. The thermal droop of output power in AlInGaP red µLEDs is much more severe. Lee et al. reported that the output power of AlInGaP red µLEDs was reduced by more than 70% as the operating temperature increased to 120 • C (~400 K) [12]. Therefore, InGaN red µLEDs is much more robust at a high temperature, making them more promising for AR and VR application.  In 2020, Iida et al. from KAUST reported InGaN red LEDs growing on PSS using 8-µm-thick n-GaN under layer with a lower residual in-plane stress. The red LEDs show a peak emission wavelength of 633 nm, a light output power of 0.64 mW, and external quantum efficiency of 1.6% at 20 mA [25]. Meanwhile, they demonstrated 10 × 10 arrays of InGaN 17 × 17 µm 2 µLEDs [26]. The peak wavelength was blue-shifted from 662 to 630 nm as the J increased from 10 to 50 A/cm 2 . The on-wafer EQE was 0.18% at 50 A/cm 2 and the output power density was 1.76 mW/mm 2 .
In 2021, Li et al. from UCSB demonstrated significant progress of InGaN red µLEDs on PSS [5][6][7]. As discussed in Section 2.1, the packaged 607 nm InGaN red µLEDs show a peak EQE of 2.4% to 2.6% as the size reduces from 100 × 100 µm 2 to 20 × 20 µm 2 . The same team also demonstrated InGaN red µLEDs with a peak EQE of 3.2% and a much more robust thermal stability as compared to the common AlInGaP red µLEDs. Moreover, Li et al. from UCSB reported ultra-small 5 × 5 µm 2 InGaN amber µLEDs grown on PSS with a peak EQE greater than 2% (Figure 4) [5]. The leakage current was only 10 −9 A at −5 V. Since the AR and VR display demand µLEDs with a size smaller than 10 × 10 µm 2 , the excellent optical and electrical performance of the 5 × 5 µm 2 InGaN amber µLEDs suggests the promising application for AR and VR micro-display using InGaN materials. The Light Tool simulation reveals that the light extraction efficiency (LEE) is 76% in the 5 × 5 µm 2 InGaN red µLEDs, which is higher than 64% in the 100 × 100 µm 2 µLEDs due to an enhanced light scatting from the sidewalls. However, the EQE of 5 × 5 µm 2 InGaN µLEDs is lower than that of 100 × 100 µm 2 µLEDs (3.2%). Therefore, the impact of nonradiative recombination from the sidewalls damage can't be neglected in such small µLEDs. Fewer damage sidewalls etching process is a good approach to improve the efficiency of ultra-small InGaN red µLEDs. More recently, Li et al. from UCSB reported 623 nm red InGaN µLEDs (60 × 60 µm 2 ) with a peak EQE of 4.5% using epitaxial tunnel junction (TJ) contact [27]. GaN TJ offers a better current spreading and less optical loss, which can improve efficiency. These EQE values are one order higher than the previous reports about InGaN red µLEDs and represent the start-of-the-are results for InGaN red µLEDs [17,[19][20][21][22].

High-Efficiency InGaN Red LEDs Grown on Silicon Substrate Using 3-Dimension "V" Pits Injection
In 2019, Jiang et al. from Nanchang University proposed a "3D pn junction" from V-pits injection [23]. The V-pits can screen the dislocations and increase the hole injection into the active region. In 2020, the same team reported efficient InGaN-based red LEDs grown on Si (111) substrates [24]. The voids of V-pits reduce compressive strain and benefit the growth of high In composition InGaN QWs. The hybrid MQWs design with yellow QWs and orange QWs enables a high peak wall-plug efficiency (WPE) of 24% at 0.8 A/cm 2 with an emission wavelength of 608 nm. 621 nm InGaN red LEDs with a peak WPE of 16.8% were achieved with a very low forward voltage of 1.96 V at 0.8 A/cm 2 . Theoretical simulation of "3D V-pits injection" has been investigated [28].

InGaN Red µLEDs Using Semi-Relaxed InGaNOS Pseudo-Substrate
In 2017, Even et al. from LETI and Soitec observed a significant enhancement of indium incorporation in full InGaN heterostructure grown on semi-relaxed InGaN pseudosubstrate (InGaNOS) due to the composition pulling effect [16]. The InGaNOS was developed by Soitec based on its Smart CutTM technology and offers a thin relaxed InGaN seed layer. The photoluminescent (PL) was red-shifted by~50 nm by growing InGaN QWs on InGaNOS compared to the common GaN template ( Figure 5). In 2021, Dussaigne et al. from LETI and Soitec reported InGaN red µLEDs grown on InGaNOS with a lattice parameter of 3.210 Å [17]. The 10 µm diameter circular 625 nm InGaN red µLEDs show an EQE of 0.14% at 8 A/cm 2 , despite the LEE estimated below 4%. Moreover, White et al. from UCSB and Soitec have greatly improved the EQE of 80 × 80 µm 2 609 nm InGaN µLEDs on InGaNOS to 0.83% by reducing the defect density in the InGaN/GaN buffer layer and optimizing the p-type structure [18,19]. Noted that the growth temperature for red QWs on InGaNOS is much higher than the typical red QWs grown on GaN template (close to green InGaN QWs growth temperature).

InGaN Red µLEDs Grown on Strain Relaxed Nano-Porous InGaN Template
In 2021, Pasayat et al. from UCSB developed compliant GaN on porous GaN pseudosubstrates (PSs) for strain relaxed InGaN layer [20]. A 10 × 10 µm 2 pattern with a 100 nm compliant GaN cap layer was grown on top of the porous GaN layer. Doping selective electrochemical (EC) etching was used to form the nano-porous structure. The 440 nm thick InGaN layer with 3-4% indium composition grown on the PSs shows a 56% strain relaxation from the X-ray diffraction reciprocal space map (RSM) (Figure 6). The peak wavelength was red-shifted by 56 nm. The 632 nm InGaN red µLEDs (6 × 6 µm 2 ) show a peak on-wafer EQE of 0.2% ( Figure 6). The output power density was 2.1 mW/mm 2 at 100 A/cm 2 . However, such technology involves a complicated fabrication process and materials overgrowth. The thermal conductivity and reliability remain poor for the nano-porous template.

InGaN Red µLEDs Grown on Strained Relaxed Layer Using In-Situ Decomposition Layer
In 2021, Chan et al. from UCSB developed a novel technology using in-situ InGaN decomposition layer (DL) to achieve highly strain relaxed InGaN PSs [21]. A 3 nm thick very high indium composition InGaN DL and a 100 nm deposition stop layer were grown, and then thermal annealing at 1000 • C was carried out. Voids were formed in the DL by TEM (Figure 7). The 200 nm In 0.04 Ga 0.96 N grown on top were highly relaxed with an 85% biaxial relaxation confirmed by XRD RSM. The PL wavelength was red-shifted by 75 nm from 440 nm to 515 nm. Moreover, the same team reported 100% fully strain relaxed In 0.04 Ga 0.96 N layer. Red emission was obtained by growing the QWs at a very high temperature of 870 • C [22]. The 633 nm red LEDs show a peak on-wafer EQE of 0.05% (Figure 8). The operation forward voltage is as low as 2.25 V at 25 A/cm 2 . This technology is much more practical without introducing any extra complicated process or overgrowth as compared to InGaNOS and nano-porous templates. The surface of the relaxed InGaN buffer layer is very rough compared to the atomic force micrograph (AFM) with a lot of pits. Further epitaxy optimization is needed to minimize the surface roughness and reduce the defect density.

Other Novel Methods to Achieve InGaN Red Emissions
There are also some novel techniques to achieve InGaN red emissions, such as europium (Eu) doping, InGaN platelets, and N-polar GaN templates. The Eu-doped GaN layers performed by ion implantation with post-thermal annealing can be used to realize red emission using p-type GaN/Eu-doped GaN/n-type GaN structure. Ichikawa et al. reported a red emission with a narrow linewidth at 621 nm due to the intra-4f shell transitions of 5 D 0 -7 F 2 in Eu 3+ ions [29,30]. Bi et al. reported InGaN platelets made by in situ annealing of InGaN pyramid and selective area growth for the growth of high indium composition InGaN QWs and observed a weak red emission from electroluminescence (EL) [31]. More recently, Pandey. et al. showed InGaN red LEDs from EL measurement using n-polar InGaN/GaN nanowires structure grown by plasma-assisted molecular beam epitaxial (PA-MBE) [32]. Pantzas et al. proposed a semi-bulk InGaN/GaN structure, which is similar to that of bulk InGaN intended for the improvement of indium composition and the GaN insertion layer can effectively reduce the dislocation density [33]. However, the reported efficiency in these InGaN red LEDs/µLEDs remains low, which requires a significant improvement for the practical application of the µLEDs display technology.

Conclusions
In conclusion, we review the progress of InGaN red LEDs/µLEDs. Our group has demonstrated 623 nm InGaN red µLEDs with a peak EQE up to 4.5% grown on common PSS. New growth strategies to relax the strain and increase the growth temperature of InGaN red QWs are discussed, although the devices still show a relatively low EQE. Generally, InGaN-based red µLEDs hold the promise of high efficiency and thermal robustness in ultra-small size devices, which is very important for the full colors AR/VR display.