Progress of InGaN-Based Red Micro-Light Emitting Diodes

Abstract

InGaN-based red micro-size light-emitting diodes (μLEDs) have become very attractive. Compared to common AlInGaP-based red µLEDs, the external quantum efficiency (EQE) of InGaN red µLEDs has less influence from the size effect. Moreover, the InGaN red µLEDs 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 μLEDs. Novel growth methods to relax the strain and increase the growth temperature of InGaN red quantum wells are discussed.

1. Introduction

Micro-size light-emitting diodes (μLEDs) have attracted huge attention as the next-generation 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 mm²) 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.

2. Advantages of InGaN Red μLEDs

2.1. Size-Independent Peak EQE of InGaN Red μLEDs

The EQE of AlInGaP red µLEDs reduces dramatically by decreasing the device size. Ultra-small size AlInGaP red µLEDs show a much lower efficiency, which cannot meet the requirement of AR and VR applications. P. Li et al. from UCSB firstly demonstrated size-independent EQE in the InGaN red µLEDs. Figure 1a shows the uniform emission (~3 A/cm²) by optical microscopy images for the InGaN red µLEDs with size reduces from 100 × 100 to 20 × 20 μm² [6]. These µLEDs show a packaged EQE varied from 2.4% to 2.6% as the size reduces from 100 × 100 to 20 × 20 μm² (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 μm² (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 μm² due to a lower surface recombination velocity. For the size down to 20 × 20 μm², the electroluminescence (EL) is very uniform in InGaN red µLEDs. It is promising to achieve ultra-small (<10 × 10 μm²) high-efficiency red µLEDs using InGaN materials system instead of the common AlInGaP materials.

2.2. 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 µm²) [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/cm², 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.

3. Progress of InGaN Red LEDs and μLEDs

3.1. InGaN Red LEDs/μLEDs Grown on PSS

In 2014, Hwang et al. from Toshiba reported regular size (0001) InGaN red LEDs grown on PSS [8]. A high Al composition Al_0.9Ga_0.1N layer was inserted on top of InGaN red QWs, resulting in the uniform red emission from fluorescence images. From Figure 3, the 460 × 460 μm² size InGaN red LEDs show an output power of 11 mW, an EQE of 2.9% and an emission wavelength of 629 nm at 20 mA. Another weak peak located at 430 nm was observed in the spectra, which is related to the phase-separated component in the InGaN red QWs.

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 μm² μLEDs [26]. The peak wavelength was blue-shifted from 662 to 630 nm as the J increased from 10 to 50 A/cm². The on-wafer EQE was 0.18% at 50 A/cm² and the output power density was 1.76 mW/mm².

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 μm² to 20 × 20 μm². 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 μm² 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 μm², the excellent optical and electrical performance of the 5 × 5 μm² 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 μm² InGaN red μLEDs, which is higher than 64% in the 100 × 100 μm² μLEDs due to an enhanced light scatting from the sidewalls. However, the EQE of 5 × 5 μm² InGaN μLEDs is lower than that of 100 × 100 μm² 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 μm²) 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].

3.2. 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/cm² 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/cm². Theoretical simulation of “3D V-pits injection” has been investigated [28].

3.3. 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 pseudo-substrate (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/cm², despite the LEE estimated below 4%. Moreover, White et al. from UCSB and Soitec have greatly improved the EQE of 80 × 80 μm² 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).

3.4. InGaN Red μLEDs Grown on Strain Relaxed Nano-Porous InGaN Template

In 2021, Pasayat et al. from UCSB developed compliant GaN on porous GaN pseudo-substrates (PSs) for strain relaxed InGaN layer [20]. A 10 × 10 μm² 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 μm²) show a peak on-wafer EQE of 0.2% (Figure 6). The output power density was 2.1 mW/mm² at 100 A/cm². However, such technology involves a complicated fabrication process and materials overgrowth. The thermal conductivity and reliability remain poor for the nano-porous template.

3.5. 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_0.04Ga_0.96N 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_0.04Ga_0.96N 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/cm². 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.

3.6. 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 ⁵D₀–⁷F₂ in Eu³⁺ 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.

4. 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.