Recent Advancements in GaN LED Technology

Abstract

Gallium nitride (GaN)-based solid state lighting technology has revolutionized the semiconductor industry. The GaN technology has played a crucial role in reducing world energy demand as well as reducing the carbon footprint. As per the reports, the global demand for lighting has reduced around 13% of total energy consumption in 2018. The Department of Energy (USA) has estimated that bright white LED source could reduce their energy consumption for lighting by 29% by 2025. Most of the GaN LEDs are grown in c-direction, and this direction gives high growth rate and good crystal integrity. On the other hand, the c-plane growth induces piezoelectric polarization, which reduces the overall efficiency of LEDs since the last decade researchers round the globe working on III-N material to improve the existing technology and to push the limit of III-V domain. Now, the non-polar and semi-polar grown LEDs are under investigation for improved efficiency. With the recent development, the GaN is not only limited to lighting, but latest innovations also led the development of micro-LEDs, lasers projection and point source. These developments have pushed GaN into the realm of display technology. The miniaturization of the GaN-based micro-LED and integration of GaN on silicon driving the application into fast response photonic integrated circuits (ICs). Most of the recent advancements in GaN LED field would be discussed in detail.

1. Introduction

Group III-nitrides (GaN, AlN, and InN) and their alloys have been considered the most promising semiconductor materials for various optoelectronic applications due to their excellent physical properties and stability in harsh environmental conditions [1, 2, 3]. Today, III-nitrides-based light-emitting diodes (LEDs) are widely used for solid-state lighting (SSL) applications all over the world because of their high efficiency, low power consumption, and longer lifetime than fluorescent and incandescent bulbs [4, 5]. Specifically, white light LEDs are a more promising low-power light resource to replace conventional fluorescent, as shown in Figure 1. Along with LEDs, III-nitride-based laser diodes (LDs), high-power electronics, photodetectors, etc., are other extended optoelectronic applications that are also demonstrated [7, 8]. However, developing III-nitride-based devices is not straightforward due to various difficulties and challenges. Among these, the preparation of high-quality and large-area single-crystalline bulk III-nitride semiconductors which has been one of the significant challenges over the last few decades because of the poor solubility of nitrogen gas in III-metals (In, Ga and Al) [9, 10, 11]. For example, GaN single crystal growth requires a high growth temperature (2220°C) and high growth pressure (6 GPa) [9]. The bulk single-crystal GaN growth cannot be employed in the standard Czochralski or Bridgman techniques, generally used to prepare bulk Si and GaAs substrates [11]. However, to take advantage of III-nitrides material physical properties and their chemical stability, III-nitrides material is hetero-epitaxially grown on various substrates [1, 3].

Figure 1.

(a) Structure of white LED, (b) YAG-ce phosphor used to generate white LED by blue emission; and (c) emission spectra of first white LED (covers blue – Yellow- red) [4], where the typical distance between +ve and -ve lead wire is 0.3 mm for 5 mm LED [6].

In semiconductor epitaxy, the most favorable approach is homoepitaxy in which same substrate is used for the growth of targeted semiconductor material. But this approach is not possible for III-nitride based material due to the unavailability of bulk GaN substrates. Hence, hetroepitaxy route is chosen for the growth of GaN based material. Hetroepitaxy means growth of material on foreign substrate. For example, most of the GaN devices are grown on sapphire which is highly lattice mismatched with GaN. Due to heteroepitaxial growth process the grown epilayers suffer from high level of in-plane strain and structural defects like dislocation, stacking faults, etc. Generally, nitridation of sapphire is performed before GaN growth as pre-nitridation step. This pre-nitridation helps in formation of an intermediate AlN layer which compensate for plane strain to some extent [12, 13, 14, 15]. Because of the said reason, large area singe- crystalline bulk III-nitride semiconductor fabrication is challenging. Over the last few decades, research has been done to improve the structural, optical, and electronic properties of GaN devices.

1.1 Breakthrough: P-type doping

Today, scientific advancement brings tremendous progress in improving the crystalline quality of III-nitrides semiconductors, which enable the development of highly efficient and durable III-nitride-based optoelectronic devices for commercial applications [4, 5, 7, 8]. The GaN material was first time discovered by Johnson et al. in the year of 1932. This work obtained polycrystalline GaN by passing ammonia (NH₃) gas over hot gallium metal at 900–1000°C temperature conditions [16]. However, the epitaxial growth of GaN film on sapphire (0001) was presented in 1969s by Maruska and Tietjen using the hydride vapor phase epitaxy (HVPE) growth technique [17]. Thereafter, Manasevit et al. reported the growth of epitaxial GaN film on a sapphire substrate using metalorganic vapor phase epitaxy (MOVPE) [18]. However, these hetero-epitaxial grown GaN films on sapphire substrates have poor crystal quality and high-density n-type carriers (>10¹⁸ cm⁻³), which makes it challenging to achieve p-type doping into the GaN films [19, 20]. Yoshida et al. addressed that the molecular beam epitaxy (MBE) growth of GaN film on AlN-coated sapphire has shown better electrical and optical properties than the directly grown GaN on the sapphire substrate [21]. Amano et al. reported that the thin AlN buffer layer on the sapphire substrate improves the epitaxial GaN film crystalline quality [22]. Nakamura et al. also noted the growth of enhanced crystalline quality GaN on the low-temperature GaN buffer layer on the sapphire substrate before the primary GaN film growth [23].

The major development of III-nitride optoelectronic devices was initiated by Amano et al. after successfully p-type (Mg-acceptor) doping into the GaN film by using a low-energy electron beam irradiation method, which has opened a way to develop the p-n junction-based LEDs [24]. However, it was noticed that the Mg acceptor doped GaN film has a high electrical resistivity due to the formation of acceptor-H complexes [3, 20]. Nakamura et al. have shown that the p-GaN film hole carrier activation was achieved by a simple thermal annealing process in the N₂ gas environment [25]. The high temperature thermally annealed p-GaN film has shown a high hole carrier concentration of ~3 × 10¹⁷ cm⁻³ with mobility of 10 cm²/Vs and low contact electrical resistivity of 2 Ω.cm as shown in Figure 2 [25]. Furthermore, the growth of high crystalline InGaN film on GaN film by using metalorganic chemical vapor deposition (MOCVD) has a strong band-edge emission peak at room temperature photoluminescence (PL), which is another breakthrough for developing LEDs devices [27]. In 1993, Nakamura et al. successfully reported the first GaN-based blue LED with a light output power of 125 μW and external quantum efficiency (EQE) of 0.22% [26]. Later, the significant development progress in III-nitride semiconductor research enabled highly efficient SSL LED devices (Figure 3).

Figure 2.

Resistivity of Mg-doped GaN films for different annealing temperatures. (b) Output power comparison between commercially available SiC LEDs and p-n junction GaN LEDs [26].

Figure 3.

The SSL LEDs efficiency improvement at laboratory demonstration and comparison with incandescent and fluorescent bulb efficiencies [4].

1.2 III-N alloy for wide spectral emission

III-N-based materials are the leading contender to fill the wavelength from 500 to 600 nm. Alloys of AlGaInN/GaN cover a broad spectrum of deep ultraviolet to near-infrared (AlN-6.2 eV, GaN-3.4 eV, and InN-0.65 eV) by easing the severe bottleneck of low-output efficiency by other semiconductor emitters such as ZnSe as shown in Figure 1.

Around the 1990s, Nakamura et al. successfully developed high-quality GaN epi-film and made efficient p-type GaN film using high-temperature annealing to realize high-efficiency blue-light LED. Taking advantage of the momentum, researchers push blue GaN LEDs EQE to exceed 80%, while emissions wavelengths move to the red spectra range. In 1996, Nakamura et al. demonstrated the first short wavelength (417 nm) GaN LD. Since then, the 405 nm violet InGaN laser source has been commercially successful for high-definition video and multi-layer data storage. Using GaN-based blue LDs increases the data storage capabilities per disc in the blue-ray systems [28]. For their significant contribution in developing III-nitride-based LEDs, Shuji Nakamura, Hiroshi Amano, and Isamu Akasaki received the prestigious Noble Prize in physics in 2014 [29].

Today, the III-nitride-based LEDs have been proven to be highly efficient, have a long lifetime, and are environment friendly [4, 30]. The III-nitride-based white LEDs have high efficiency of 200 lm/W, and are commercially available for light applications [4]. The III-nitride semiconductors-based LEDs are used in almost every traffic signal, medical application, cell phone display, monitor, house/street, car, and so on [2, 30, 31]. Haitz et al. addressed that SSL LED lighting decreases electrical power consumption by more than 50% and is directly related to reducing CO₂ gas emissions by approximately 200 Megatons per year [32].

2. Polar-InGaN/GaN for blue and green LEDs

Acceleration of blue and green LEDs development from InGaN/GaN quantum well (QW) started after the successful invention of p-type GaN by low-energy electron irradiation in late 1989 by Amano et al. [24]. This invention led to high quantum efficiency and output power. However, InGaN/GaN growth along c-plane sapphire [0001] induces a polarization effect which causes quantum confined Stark effect (QCSE) and carrier loss (droop effect). The QCSE reduces the probability of radiation recombination by separating the hole and electron wave function on the QW. Especially, QCSE and polarization effects are highly pronounced with high indium (In) content incorporation in the InGaN epilayer [29].

The InGaN has the advantage of covering a wide spectral range from blue, green, and ultraviolet by tuning energy from 0.7 to 3.4 eV. Theoretically, when the indium composition is between 15 and 20% and 25–30%, the emission wavelengths belong to blue and green LEDs, respectively. However, to cover ultraviolet emission, the indium mole fractions should increase to a large extent, increasing threading dislocation density in the active layer. High threading dislocation density reduces internal quantum efficiency; therefore, InGaN material is not suitable for ultraviolet spectral emission.

Growth of InGaN/GaN with more than a 30% indium composition is difficult due to the different MOCVD growth temperature window [2, 33]. High-quality GaN requires a relatively high growth temperature (1050°C), whereas InGaN needs 800°C. Therefore, MOCVD needs high control on temperature fluctuations for better stability growth and to avoid inhomogeneous indium composition.

2.1 Polarization effect: Spontaneous polarization

Asymmetry of the wurtzite structure in (0001) sapphire direction (lattice constant c/a < 1.63 A, the electronegativity between Ga and N is vastly different) causes spontaneous polarization in the III-N material system [34].

AlN material has the largest spontaneous polarization compared to GaN and InN, as shown in Figure 4. Theoretical calculation of spontaneous polarization of ternary material can be expressed by [35]:

Figure 4.

Spontaneous polarization vs. lattice constant of III-N material system.

$$P_{\mathit{InxGa}1-xN}^{Sp}=-0.042x-0.034\left(1-x\right)+0.037x\left(1-x\right)$$E1

$$P_{\mathit{AlxGa}1-xN}^{Sp}=0.09x-0.034\left(1-x\right)+0.019x\left(1-x\right)$$E2

2.2 Polarization effect: Piezoelectric polarization

Piezoelectric polarization effects are generated in the III-N materials system when the heterostructure undergoes huge lattice distortion from biaxial tensile/compressive stress. The generated piezoelectric effect will be in [000–1] (tensile) / [0001] (compressive) direction [29, 36, 37, 38, 39].

The piezoelectric effect from III-N ternary material can be calculated from biaxial stress and lattice mismatch. Therefore, piezoelectric polarization can be given as:

$$P_{PZ}=2(e_{33}{\epsilon }_z+e_{31}\left({\epsilon }_x+{\epsilon }_y\right)$$E3

where e₃₃ and e₃₁ are piezoelectric constants and ε_z is in-plane strain.

ε_z can be calculated from:

$${\epsilon }_z=\frac{\left(c-c_0\right)}{c_0}$$E4

where $c,c_0$ are in-plane lattice constant and relaxed lattice constant, respectively.

2.3 Quantum-confined stark effect

The polarization effect generates an electric field that eventually tilts the conduction and valance bands of InGaN/GaN multi-quantum wells (MQWs). Also, the electric field reduces spatial overlap between holes and electrons waves, thereby reducing the probability of radiative recombination, as shown in Figure 5 [40, 41]. This effect is called QCSE, which highly depends on QW width. When the QW width increases, the QW luminance is red-shifted, whereas it is blue-shifted when the QW width decreases [42, 43, 44, 45, 46]. So, the effect of QCSE by polarization should consider two facts on efficient emission: the transition level and luminous intensity of InGaN QW.

Figure 5.

(a) Wurtzite crystal structure of GaN (c-plane), and (b) electron and holes wavefunction separation due to polarization effect.

First, the impact of transition level is a function of several parameters, such as QW width, biaxial stress, growth temperature, sub-band energy levels, etc. Since the spontaneous polarization constant difference is more negligible for GaN and InN, and lattice mismatch is significant, the contribution of piezoelectric polarization is dominant in InGaN ternary alloy. As the lattice mismatch increases, the electron and hole transition level decreases. Secondly, the electroluminescence intensity of InGaN/GaN MQW highly depends on the QW width. Electrons and holes are spatially separated when the QW width increases, reducing the wave function overlap. So, the peak wavelengths have blue-shift and eventually reduce luminescence intensity.

Due to lattice mismatch, huge screw and threading dislocations and point defects will be present in the QW layer. These defects can cause fluctuations that confine carriers in the minimum value of potential energy. Three significant facts potentially vary the localization of carriers and luminescence property of InGaN/GaN LEDs: indium clustering, well width fluctuations, and ternary alloy variations [46, 47, 48].

2.4 Methods to improve green LEDs efficiency using polar surface

Epitaxial growth of InGaN/GaN MQW structures on the polar substrate can reach more than 80% internal quantum efficiency (IQE). However, as the wavelength increases, InGaN/GaN-based LEDs’ efficiency decreases by around 30%. Also, the efficiency of green LEDs decreases faster as the current injection increases. This problem leads to improper light mixing efficiency for white LEDs. Therefore, increasing the efficiency of green LEDs is the primary concern of RGB-LEDs. However, using the existing epitaxial growth technique, few methods can improve the green-based LED IQE, Figure 6 continuous improvisation of epitaxial materials quality with less threading dislocation in the QW region, inserting pre-strain layer to accommodate lattice strain, and sharp QW interface by growth interruption method [29]. On the other hand, growing green LEDs epilayers on non-polar and semipolar substrates reduce the piezoelectric effect at the QW and avoid blue-shifted wave emission. Moreover, it increases the probability of radiative recombination to attain maximum IQE [49, 50].

Figure 6.

Optical output power and EQE vs. current densities.

3. Recent advancements in non-polar and Semipolar LEDs

Many researchers are developing the technology to reduce the piezoelectric polarization electric field effect in the active region of MQW devices. However, few methods have been proposed to increase the quantum efficiency of green LEDs to 163% by introducing a gradient electron blocking layer.

3.1 Non-polar LEDs

Several non-polar crystal planes occur in a wurtzite structure orthogonal to the basal planes. The non-polar m-planes $\left\{10\overline{1}0\right\}$ and a-planes $\left\{11\overline{2}0\right\}$ have zero inherent spontaneous polarization effect. InGaN QW grown on non-polar GaN orientation exhibits flat energy bands due to zero strain-induced piezoelectric effect, as shown in Figure 7. The device grown on an m-plane displays high brightness and one hundred times less decay [49, 50].

Figure 7.

The m-plane and a-plane of GaN wurtzite crystal structure. Wave function overlaps due to a lack of polarization for better radiative recombination.

Researchers at University of California Santa Barbara (UCSB) established violet LEDs based on m-plane, which has low defect density, and the device performance is comparable with c-plane devices [51]. In addition, more uniform current injection, low current droop, and small blue-shift were observed for m-plane-oriented devices [52]. On the other hand, due to the lack of QCSE of non-polar InGaN/GaN MQW, the QW width can be increased without a significant loss in radiative recombination [52, 53]. However, the growth window for non-polar GaN is narrow and indium incorporation is limited with non-polar InGaN material [49].

Experimentally, it is well proven that growth of m-plane and a-plane GaN on non-polar direction reduces the spontaneous polarization effect. These planes are widely grown on sapphire or silicon substrates which normally shows five times higher (0.5 ~ 0.6 which is equivalent to 10¹⁰/cm²) full width half maximum (FWHM) compared to that of grown on c-sapphire substrate. This high FWHM indicates a very high treading dislocation and stacking fault in GaN layer which reduces the crystalline and optical quality of GaN epilayer. Generally, the growth rate in a- or m- direction is lower when compared to c-plane direction. Due to this uneven growth rate the striation is seen in GaN epilayer. During the growth of non-polar GaN, it is important to supress the growth rate in vertical c-plane direction to have smooth morphology by avoiding island formation. The separation of c-plane and enhance of a-plane growth could be achieved in narrow growth temperature window [54, 55, 56].

3.2 Semipolar LEDs

The semipolar plane has a growth plane between the c-plane and non-polar plane, as shown in Figure 8. The semipolar plane shows a less polarization electric field effect than the c-plane [39, 57]. Semipolar directional growth has the advantage of high indium incorporation in the InGaN epi layer, which can be used for longer wavelengths. The total polarization and polarization discontinuity between InGaN and GaN will be dependent on the angle between the c-plane and the InGaN epilayer.

Figure 8.

Two different Semipolar wurtzite structures of GaN [49].

The motivation of using semipolar plane with different arbitrary angles is to reduce the polarization electric field to suppress the QCSE effect, as shown in Figure 9. Among these semipolar planes, $\left\{\overline{2}021\right\}$ orientation showed better performance due to high compositional homogeneity across QW [58]. The semipolar directional growth yields a low polarization electric field; however, stacking fault formation is inevitable due to the material’s anisotropy property. The first semipolar LED was demonstrated on $\left\{30\overline{3}8\right\}$ while the first semipolar green LED was demonstrated on GaN along $\left\{10\overline{1}3\right\}$ direction [59, 60]. Currently, the IQE of c-plane-based yellow-green LEDs is surpassed by semipolar long wavelengths of yellow-green LEDs. In 2011, Mg-doped quantum barriers homoepitaxially grown on GaN increased radiative recombination probability to improve green LEDs’ performance [61]. Researchers at UCSB have reported output power of 31.1 mW and 9.9 mW at 20 mA, with EQE of 54% and 20.4% of blue and green LEDs, respectively. These blue and green LEDs were grown on a semipolar growth plane of $\left\{10\overline{1}1\right\}$ and $\left\{11\overline{2}2\right\}$, respectively [62, 63]. The significant differences between non-polar and semi-polar based GaN LEDs devices are given in Table 1.

Figure 9.

Polarization discontinuity of GaN for various arbitrary angles with respect to the c-plane [50].

Non-polar	Semi-polar
Only two different plane orientations	Vast variants of planes
Gives maximum wave function overlap	Substantial wave function overlap (comparatively high than c-plane)
Absence of QCSE	QCSE effect presents except few planes (e.g $20\overline{2}\overline{1}$)
Indium is difficult to incorporate which reduces the growth window	Indium incorporation is high which enhances the growth window
Not suitable for long wavelength emission due to defects in QWs	Promising for longer wavelength emission
Larger degree of polarized emission	Degree of polarization is comparatively less than non-polar

Table 1.

The important properties of non-polar and semi-polar GaN LEDs devices.

4. Micro LED

Display-based LED technology is considered the most promising for next-generation display devices. Every display screen consists of several small fractions known as a pixel, and the size of these pixels determines the resolution of the display screen. The resolution of the display screen is measured in the term of PPI, which means pixel per square inch. We could estimate this PPI term by comparing the resolution of conventional Organic Light Emitting Diode (OLED) displays and modern smartphones; commercial OLED televisions currently have a pixel density of about 100 to 200 PPI, while the resolution of new smartphones is around 400 to 500 PPI. This pixel density is proportional to the size of a single light emitting unit, which in this case is in the sub-millimeter range. The resolution of the micro-LED display can be increased by fabricating sub-micron LEDs. The individual LEDs would be one pixel for the display, and the size of the LEDs could be reduced according to the desired resolution. In addition, these micro-LEDs have fast response, and are energy efficient than previously developed display technology. The OLED technology, which was developed in the early 90s, was ahead of its time, and during the later decade, it was commercially available in some consumer displays. Its significant advantages over the LCD were self-luminous, wide viewing angle, high contrast, and fast response.

On the other hand, these displays were not used widely in consumer electronics [64, 65, 66]. This event proved to be a fracture in display technology; major market players and researchers started looking for other materials and technology. However, the main objective remains the same: high contrast, efficiency, resolution, and the process should be well established. Currently, the research is focused on developing micro-LEDs of size <100 μm for high-resolution. Furthermore, according to market research, it is estimated that the micro-LED display market will grow by around 20.5 billion USD in 2025 with an annual growth rate of about 80%.

Jiang’s group did the first endeavor for the growth of micro-LED. During their quest, they could fabricate a blue GaN micro-LED chip with an individual pixel diameter of 12 μm [67]. The display made from these pixels measured 0.5 X 0.5 mm² and had 10 × 10 pixels. Soon after, micro-LED became a hot topic among researchers, leading to the further development of the fabrication technology. Thenceforth, the researchers have extensively understood the nature and epitaxy of III-N material. This has resulted in efficient and complex structures of GaN-based LEDs. Modern LED structures may consist of many layers up to 100 and a lifetime of more than fifty thousand hours. In 2014, Christian et al. fabricated a 50 X 50 μm² LED and successfully transferred it to a flexible substrate. The fabricated LED emits 60 μW optical power at 1 mA current injection [68]. This research paves the path towards a flexible screen, which overcomes the limitation that III-N micro-LED faced while competing with OLED technology.

The color contrast of the micro-LED display is based on three primary colors (red, green, and blue, also called RGB) that can be combined in different ratios to give all colors in nature. Thus, combining three micro-LED (RGB) on a single pixel solves the problem. Figure 10 shows the schematic of such pixel consisting of RGB micro-LEDs. In this arrangement, the different currents would be applied to control the brightness of each LED to realize the combination of RGB primary color to achieve the full-color display. However, integrating three different material LEDs on the same pixel is difficult to mass produce. For example, if there is a need to fabricate a 4 K resolution display, nearly 25 million micro-LEDs are needed to assemble or fabricate. This hurdle could be overcome by fabricating all LEDs of the same material. The solution is fabricating the GaN/InGaN base LEDs where the emission wavelength could be tuned by varying the molar ration of In in InGaN alloy. The said approach was developed by A. Even et al. used a very innovative substrate to grow InGaN base LEDs. The unique substrate is InGaNOS (InGaN on pseudo-substrate), which overcomes the lattice mismatch and reduces the stress in as-grown films. The epitaxy results show that the grown LED structures could cover blue (482 nm) to red (617 nm) [69]. The schematic is shown in Figure 11. Similarly, Pasayat et al. fabricated an LED structure on GaN porous pseudo substrate. The fabricated LEDs range from 8 x 8 μm² to 20 x 20 μm². It was observed that the reduction in size leads to an EL redshift from 525 to 561 nm [70]. As per the study, the color-tunable monolithic integration nitride-based RGB technique can fabricate micro-LED arrays.

Figure 10.

Schematic of RGB micro-LED full-color pixel.

Figure 11.

Schematic of InGaN LED structures grown on InGaNOS modified substrate.

Another efficient approach was developed by Prof. Kuo, in which the full-color display pixel could be achieved by employing UV LEDs and a down-conversion QDs (Quantum Dots) solution. In this approach, all three pixels are fabricated as AlGaN/GaN-based UV LED, and QDs are placed above them. The targeted QDs down convert UV emission to respective wavelength, i.e., RGB. The schematic of this approach is shown in Figure 12 [71, 72].

Figure 12.

Schematic of UV micro-LED and QDs integration approach to realize the full-color pixel.

5. Conclusion

We reviewed the history of GaN epitaxy, III-N material improvement strategies, and the advancement of LEDs. It is necessary to reduce the MQW threading dislocation density, QCSE effect, and carrier localization effect to enhance the IQE. To get a high hole concentration by manipulating the Fermi level, we must design strain compensated InGaN/AlInGaN and doped InGaN/GaN barriers. For c-plane growth, ultra-sharp (thickness ~ 3 nm) QW structure with low defect density can yield high IQE. However, for good wavefunction matching and low blue and redshift on the spectral emission, the non-polar and semipolar substrate can be a potential candidate for the reported output power of 31.1 mW and 9.9 mW at 20 mA, with EQE of 54% and 20.4% of blue and green LEDs, respectively. For non-polar GaN epitaxy of $\left\{11\overline{2}0\right\}$a-plane and $\left\{10\overline{1}0\right\}$m-planes can be grown on r-plane Al₂O₃, a-plane SiC, and m-plane SiC, (100), LiAlO₂ are the promising epitaxial substrates. For Semi-polar GaN epitaxy, $\left\{\overline{2}021\right\}$ orientation on patterned r-plane Al₂O₃ substrates showed better performance due to their high compositional homogeneity across QW. With the recent developments, GaN is not limited to lighting, but the latest innovation also led to the development of micro-LEDs, laser projection, and point sources. These developments have pushed GaN into the realm of display technology. The miniaturization of the GaN-based micro-LEDs and integration of GaN on silicon will help drive the application into photonic integrated circuits (ICs).