Full-Color III-Nitride Nanowire Light-Emitting Diodes

III-nitride nanowire based lightemitting diodes (LEDs) have been intensively studied as promising candidates for future lighting technologies. Compared to conventional GaN-based planar LEDs, III-nitride nanowire LEDs exhibit numerous advantages including greatly reduced dislocation densities, polarization elds, and quantum-con ned Stark e ect due to the e ective lateral stress relaxation, promising high e ciency full-color LEDs. Beside these advantages, however, several issues have been identi ed as the limiting factors for further enhancing the nanowire LED quantum e ciency and light output power. Some of the most probable causes have been identi ed as due to the lack of carrier con nement in the active region, non-uniform carrier distribution, electron over ow, and the nonradiative recombination along the nanowire lateral surfaces. Moreover, the presence of large surface states and defects contribute signi cantly to the carrier loss in nanowire LEDs. Consequently, reported nanowire LEDs show relatively low output power. Recently, III-nitride core-shell nanowire LED structures have been reported as the most e cient nanowire white LEDs with a record high output power which is more than 500 times stronger than that of nanowire white LEDs without using core-shell structure. In this context, we will review the current status, challenges and approaches for the high performance IIInitride nanowire LEDs. More speci cally, we will describe the current methods for the fabrication of nanowire structures including top-down and bottom-up approaches, followed by characteristics of III-nitride nanowire LEDs. We will then discuss the carrier dynamics and loss mechanism in nanowire LEDs. The typical designs for the enhanced performance of III-nitride nanowire LEDs will be presented next. The color tunable nanowire LEDs with emission wavelengths in the visible spectrum, and phosphorfree nanowire white LEDs will be nally discussed.

III-nitride nanowire based lightemitting diodes (LEDs) have been intensively studied as promising candidates for future lighting technologies. Compared to conventional GaN-based planar LEDs, III-nitride nanowire LEDs exhibit numerous advantages including greatly reduced dislocation densities, polarization elds, and quantum-conned Stark eect due to the eective lateral stress relaxation, promising high eciency full-color LEDs. Beside these advantages, however, several issues have been identied as the limiting factors for further enhancing the nanowire LED quantum eciency and light output power. Some of the most probable causes have been identied as due to the lack of carrier connement in the active region, non-uniform carrier distribution, electron overow, and the nonradiative recombination along the nanowire lateral surfaces. Moreover, the presence of large surface states and defects contribute signicantly to the carrier loss in nanowire LEDs. Consequently, reported nanowire LEDs show relatively low output power. Recently, III-nitride core-shell nanowire LED structures have been reported as the most ecient nanowire white LEDs with a record high output power which is more than 500 times stronger than that of nanowire white LEDs without using core-shell structure. In this context, we will review the current status, challenges and approaches for the high performance IIInitride nanowire LEDs. More specically, we will describe the current methods for the fabrication of nanowire structures including top-down and bottom-up approaches, followed by characteristics of III-nitride nanowire LEDs. We will then discuss the carrier dynamics and loss mechanism in nanowire LEDs. The typical designs for the enhanced performance of III-nitride nanowire LEDs will be presented next. The color tunable nanowire LEDs with emission wavelengths in the visible spectrum, and phosphorfree nanowire white LEDs will be nally discussed. etching followed by an anisotropic wet etch [4].
Consequently, the nanowires exhibit straight, smooth and well-dened facets. In their report, they 2 fields. Bae et al. also demonstrated the size-controlled fabrication of InG arrays by combining a top-down method with a wet treatment process [5]. T photoluminescence (PL) intensity of the as-etched nanowires was the lowe induced damage, while the wet-treated nanowires displayed a significa intensity. Chiu et al. reported that the wet-treated nanowires displayed enha due to the reduced surface damage [2]. For instance, the PL intensity of n using ICP and photo-enhanced chemical (PEC) wet oxidation process gained about 6 times compared to that of the as-etch nanowire samples. and Fe as catalyst to assist the nucleation and growth of nanowires [6]. The metal particle sizes and densities are carefully controlled to achieve the proper position and the diameter of the nanowires. In addition, growth parameters such as temperature and pressure are very important for the proper growth of nanowires. The VLS growth of III-nitride nanowire structures was performed by using a quartz tube furnace [7], low pressure metal-organic vapor phase epitaxy (MOVPE) [8], thermal chemical vapor deposition (CVD) [9] and low-temperature halide chemical vapor deposition [10]. Generally, in metal catalyst assisted nanowire growth, metal catalyst remains on the tip of nanowire and it may diuse into semiconductor nanowires during the epitaxial growth, which may introduce impurity in the nanowires. The optical and electrical properties of the related nanostructured LEDs are strongly deteriorated resulted from such unintentionally introduced impurities [11,12].
III-nitride nanowires can also be grown spontaneously without using any foreign metal catalysts [13,14]. Molecular beam epitaxy (MBE) is commonly used for the spontaneous growth of III-nitride nanowires. The growth of GaN nanowires has been demonstrated in this approach on Si (111) [2] and Si (001) [17], on AlN [18] and SiO 2 [19] buer layers on Si, and low-temperature GaN [18] buer layers on c-  , epin to the n-type layer to form the nanowire array structure using an ICP OG was then spin-coated on the nanowire sample followed by a curing igure 1.3(c). An appropriate etch-back process needs to be performed to r of the nanowires for later deposition of p-type metal contacts. Figure  coated nanowire LEDs after the etch-back process. In the next step, the r procedures were conducted following standard fabrication of planar ased LEDs. Apart from SOG, similar gap-filling materials with properties nd high transparency, such as polyimide and SU8, were also employed. nother method to process the GaN nanowire LED by flip-chip packaging g the GaN nanowire LED with silver nanoparticles or metal foils [37]. ated the fabrication of nanowire LEDs by using chemical mechanical nanowire contact and using SiO 2 layer for sidewall passivation [38]. chnique of nanowire LEDs is based on coalesced p-GaN growth, firstly al. [39]. By adjusting the growth parameters during p-GaN growth, the plete layer allowing conventional planar device to be processed.    Philip et al. used polyimide as filling material in their nanowire LED fabrication sinc material has low absorption of visible light and can be heated up to relatively high tempe [40]. The fabrication process starts with the epitaxial growth of nanowire LED structure, in  SOG was used to ll gaps between nanowires; (d) after the etching back process to expose the top p-type layer [3].
were also employed. Waag [44]. An enhancement factor of more than 4.3 times was measured for the light output power of such well-in-a-wire LEDs compared to conventional LEDs at 20 mA, illustrated in Although compare with the conventional planar LEDs, GaN nanowire LEDs oer many potential advantages such as signicantly reduced dislocation densities and polarization elds [36,[45][46][47], as well as enhanced light extraction eciency (LEE) [48], however, III-nitride nanowire based LEDs generally exhibit slow rise of EQEs with increased injection current and extremely low output power. For example, the peak quantum eciency of such devices can only be measured under relatively large injection conditions (>100 Acm −2 ) [6,8,9], compared to that (∼1020 Acm    6 EDs, respectively [3]. Their experiment was performed at room temperature and at current jection of 20 mA, shown in Figure 2.1. More significant enhancement in the light output power or the green nanowire LED manifested that the emission enhancement is mainly attributed to the rge strain relaxation occurring during the fabrication of nanowire and thus a significant nhancement in the internal quantum efficiency (IQE). Using the method of silica nanoparticle thography, Wang [41]. They found that the shallower nanowires possess the fewer surface defects and thus ive a higher light output power, while longer nanowires show smaller efficiency droop due to ore relaxed strain. Chen  In case of bottom-up nanowire LEDs, significantly progress was reported. As shown in igure 2.2 (a), Kikuchi et al. reported that strong EL emissions at green (530 nm), yellow (580 m), and red (645 nm) were achieved at room temperature [39]. This was mainly attributed to the igh electron-hole recombination efficiency, achieved from the quantum disks in nanowires. eported by Kim et al., strong EL emission at ~460 nm is observed for a well-in-a-wire LED ith negligible peak wavelength shift with the increase of injection current as shown in Figure  .2 (b) [44]. An enhancement factor of more than 4.3 times was measured for the light output ower of such well-in-a-wire LEDs compared to conventional LEDs at 20 mA, illustrated in  of the nanowire LEDs were then measured by comparing the EL intensity at 300 K to that measured at 5 K. The devices exhibit an IQE of ~ 36.7% at room temperature under electrical injection and efficiency droop was not observed up to injection current density of 200 A/cm 2 [50], shown in the   [39]. (b) Room temperature EL of well-in-a-wire LED with varying injection currents [44]. (c) Light output power of well-in-a-wire LEDs and thin film LEDs versus injection currents [44].
Although compare with the conventional planar LEDs, GaN nanowire LEDs offer many potential advantages such as significantly reduced dislocation densities and polarization fields [36,[45][46][47], as well as enhanced light extraction efficiency [48], however, III-nitride nanowire based LEDs generally exhibit slow rise of EQEs with increased injection current and extremely low output power. For example, the peak quantum efficiency of such devices can only be measured under relatively large injection conditions (>100 A cm −2 ) [6,8,9], compared to that (~10-20 A cm −2 ) of InGaN/GaN planar LEDs [10][11][12], indicating the presence of a significant level of nonradiative carrier recombination processes [13]. The surface states/defects are identified as one of the main factors that strongly affect the performance of nanowire LEDs [49]. The performance of such nanoscale LEDs is more susceptible to electron leakage out of the device active region due to the presence of large densities of states/defects along the wire surface and the one-dimensional carrier transport process. The resulting carrier loss and nonradiative (b) Room temperature EL of well-in-a-wire LED with varying injection currents [44]. (c) Light output power of well-in-a-wire LEDs and thin lm LEDs versus injection currents [44].
carrier recombination on nanowire surfaces severely limit the device performance. The next session will present the maina few underlying causes of the poor carrier injection efficiency and strategies to minimize the carrier loss processes for the enhanced performance of nanowire LEDs.

Carrier dynamics and loss mechanism of axial nanowire LEDs
In this section, plausible causes of carrier losses in axial nanowire LEDs will be discussed. The probable carrier loss processes can be listed as Auger recombination, electron overflow/leakage, and nonradiative surface recombination. To analyze the IQE of nitride-based LEDs, the ABF model [36,51] is utilized which is described below: where ɳ i is presented for the device IQE, and N, A, and B are the carrier density in the active region, the SRH nonradiative recombination coefficient, and the radiative recombination ( ) represents the third or other higher order effects. coefficient, respectively. The performance of nanowire LEDs is defined by their external quantum efficiency (EQE), which is a product of carrier injection efficiency, IQE and light extraction efficiency (LEE). The performance of such devices has been severely limited by their low IQE and low LEE. The where η i is presented for the device IQE, and N, A, and B are the carrier density in the active region, the SRH nonradiative recombination coecient, and the radiative recombination coecient, respectively. f (N ) represents the third or other higher order eects.
The performance of nanowire LEDs is dened by their external quantum eciency (EQE), which is a product of carrier injection eciency, IQE and LEE. The performance of such devices has been severely limited by their low IQE and low LEE. The Auger recombination, poor hole transport, electron leakage and overow [52,53], and defects/dislocations are among the loss mechanism which can reduce the IQE in the electrically injected white-color nanowire LEDs.

Auger recombination
Auger recombination, which refers to a threecarrier nonradiative recombination process, is c 2019 Journal of Advanced Engineering and Computation (JAEC) 557 VOLUME: 3 | ISSUE: 4 | 2019 | December found to be a reason for the performance degradation in GaN based LEDs at higher carrier concentrations [54][55][56]. In this process, a third carrier gets the excess energy released by the recombination of an electron-hole pair. The third carrier, which can be an electron or a hole with high kinetic energy, is then excited to a higher energy state, which leads to a large eciency droop observed in GaN LEDs. Shen  Huang-Rhys factor of nanowires is much smaller than that of as-grown multi-quantum well sample due to a reduced coupling between LOphonon and exciton [59]. It is suggested that

Electron leakage
In addition to the previously described carrier loss mechanism, the electron leakage and overow is another major process which can highly aect the performance of nanowire LEDs at high current levels [70][71][72][73][74][75]. As mentioned be- erformance is highly affected by the heavy effective mass and low mobility of se highly inefficient hole transport in device active regions [64,65]. Compared to tion of electrons, the holes are generally distributed in the quantum wells near N layer, and are significantly degraded toward the n-GaN region [36,66,67]. e Auger recombination significantly and leads to increase the electron overflow  presence of large densities of states/defects along the wire surface and th carrier transport process. Figure 3.2 shows the electron overflow mechanism in the LED structure holes injected into the device can recombine in the active region. Howev having sufficient energy can surmount the barrier and flow out of the act recombining with holes. The electron overflow has been reduced by implem Blocking Layer (EBL) in the p-side of the LED. A wide bandgap energy A typically inserted between the active region and the p-side in the GaN LE electrons from overflowing out of the active region. However, the EBL cou injection efficiency depending on the band offset.  energy than that of the active region. Shown in Figure 3.3(a), only one peak emission at ~ 550 nm which is from the quantum dot active region can be observed at low injection current levels (< 100 mA). However, due to the enhanced electron overflow, emission from the test well (~ 430 nm) becomes progressively stronger with increasing current. Therefore, at high injection current levels, electron overflow in the nanowire LED devices is more pronounced.  decreasing temperature. They suggested that the strong nonradiative recombination on the nanowire surface is the main reason causing efficiency droop. Using different passivation techniques, the nonradiative surface recombination can be effectively reduced and consequently the performance of nanowire LEDs can be significantly improved [84][85][86][87]. Several materials have been reported for the surface passivation of III-V semiconductors including SiO 2 , SiN x and sulf ide [88 -90]. The sulf ur surf ace pas siva tion tec hni que was effectively used to remove the surface states and recovering the band-edge emission of InAs nanowires [15].     [85]. Consequently, the carrier lifetime for nanowire measured very small (~ 0.2 ns, or less) [91][92][93]. The carrier injection efficiency is by Zhang et al. for different surface recombination velocities and nanowire diameters in Figure 3.5 [94]. The carrier injection efficiency decreases significantly with g nanowire diameter, and it is well below 10% for nanowire diameters in the range of uch an extremely small carrier injection efficiency results in a very low output power imilar to conventional nanowire LEDs.
: Variations of the carrier injection efficiency of the nanowire LEDs vs. surface recombination (from 5 ×~ 10 3 cm/s to 1 ×~ 10 5 cm/s) for different nanowire diameters (from 50 nm to 1000 rrier injection efficiency for relatively small nanowires is much significantly smaller nanowires. Compared to other III-V materials, although the surface recombination s smaller for GaN material [95], the surface states defects can significantly reduce the LEDs performances with the large surface-to-volume ratios. The large surface-toatio is necessary to have effective lateral stress relaxation of nanowire LEDs and he large light extraction efficiency. Reducing the surface recombination velocity is the ctive way for the carrier injection efficiency enhancement, which was proposed by  method [98]. Shown in    [98]. Shown in Figure 4.1(a), LED devices are formed on p-GaN grown on sapphire substra The emission wavelength of devices can be tuned by controlling the In composition in nanowir as shown optical images and emission spectra in Figures 4.1(b) and (c), respectively.    16 as illustrated in the inset of Figure 4.2(a). Moreover, the peak emission wavelength can be controlled in the entire visible wavelength range by changing the In composition in the LED active region, illustrated in Figure 4.2(b). Additionally, white light emission was also obtained using this approach as shown in the inset of Figure 4.2(b).

Disk/Well-in-a-wire LEDs
To increase the carrier confinement in the LED active region, well/disk/dot-in-a-wire LED structures were developed in which multiple InGaN/GaN wells, disks or dots were embedded in GaN wires. Kikuchi et al. [39] demonstrated an efficient approach to enhance the carrier confinement by the use of InGaN/GaN multiple quantum disks (MQD) embedded in a single nanowire by MBE on Si (111).

Dot-in-a-wire LEDs
The performance of nanowire LED devices can be significantly improved by achieving superior carrier confined in three dimensions. Nguyen et al. recently reported an InGaN/GaN dot-in-awire heterostructure grown by MBE [36]. In their LED structures, multiple InGaN/GaN dots are

p-Type modulation doping for the enhanced hole transport
Recent studies have shown that the poor hole transport severely limit the performance of GaN based LEDs. The holes injected to the GaN nanowire LEDs are mostly resided close to the p-GaN region because of large effective mass and low mobility. This nonuniform carrier distribution mainly in the device active region leads to enhance Auger recombination and increase electron overflow [101]. Nguyen et al. showed the improvement on hole transport and injection process using the p-doping in the device active region [36]. LEDs, the IQE of p-doped nanowire LEDs was significantly enhanced. The p-doped dot-in-awire LEDs exhibit a maximum IQE of ∼50.2% under optical pumping, shown in the inset. The reported IQEs are drastically higher than any nanowire LEDs which is previously proposed. The higher IQEs are mainly attributed to the enhanced hole transport in the p-doped dot-in-a-wire LEDs. The enhanced hole transport also leads to electron overflow reduction, and less Auger recombination [36].   ainly in the device active region leads to enhance Auger recombination and tron overflow [101]. Nguyen et al. showed the improvement on hole transport and cess using the p-doping in the device active region [36]. The improved hole lts in significantly enhanced internal quantum efficiencyIQE. The p-doped dot-inproposed fabricated by Nguyen et al. shows high IQE under electrical injection at ature ~ 56.8%, presented in Figure 4.7. Compared to the undoped dot-in-a-wire E of p-doped nanowire LEDs was significantly enhanced. The p-doped dot-in-axhibit a maximum IQE of ∼50.2% under optical pumping, shown in the inset. The s are drastically higher than any nanowire LEDs which is previously proposed. The are mainly attributed to the enhanced hole transport in the p-doped dot-in-a-wire nhanced hole transport also leads to electron overflow reduction, and less Auger n [36].

Core-shell nanowire LED structure for the reduced nonradia recombination and enhanced carrier injection efficiency
Addition to the high bandgap AlGaN EBLs, core shell nanowires are alternativ reduce the electron overflow. The strong carrier confinement is the most importa the other and is provided in Figure 4.11 (c). The greatly enhanced output power is attributed to e increased electrical injection efficiency, due to the drastically reduced nonradiative surface ecombination resulted from the usage of efficient core-shell structure.      Nguyen et al. reported a unique InGaN/AlGaN core-shell nanowire LED with significantly reduced electron overflow, presented in the high resolution TEM image in Figure 4.12 (a) [97]. They suggested that multiple AlGaN shells are spontaneously form during the growth of the AlGaN barriers in the active region. The AlGaN shell formed on the sidewall leads to significantly reduce the nonradiative surface recombination. The carrier injection efficiency is also remarkably increased due to the strong carrier confinement offered by the large bandgap AlGaN shell. The derived carrier lifetime is significantly enhanced from 0.3 ns to 4.5 ns, due to the reduced surface recombination, shown in Figure 4.12 (b). They demonstrated that the AlGaN barrier layers between quantum dots act as distributed EBLs effectively reduce the electron overflow. They obtained the core-shell LEDs which emit strong white light emission with output power of more than 5 mW, shown in Figure 4.12 (c).

Tunnel junction-induced nanowire LEDs
I-nitride material-based LEDs have resistive p-type layers which leads to have poor ohm ntact. In addition, due to high activation energies of Mg dopants, it is difficult make heav ped p-GaN layer. These create serious hole transportation problem in LEDs. This problem c reduced with the incorporation of tunnel junction (TJ) in the LED. When a TJ is integrat ith single active region LED, LED can be forward biased while the TJ is operating in reve ased condition. In this case, TJ works as carrier conversion center which supplies holes to t tive region, due to this p-GaN can be replaced by n-GaN. Sharif et al. reported that t onolithically integrated metal/semiconductor TJ based nanowire LED whe + GaN/Al/p ++ GaN considered as TJ, schematic diagram is provided in Figure 4.15 (a) [10 ue to very high Mg-doping, defects are present at the Al/p++-GaN interface, which c amatically improve the carrier transport from p ++ GaN to Al in a similar manner nventional trap-assisted tunneling. As a result, effective tunneling barrier width reduces whi     wer nanowire LEDs on metal substrates could be beneficial for multi inary applications thanks to the chemical stability of nitride materials, high ratio of nanowires, flexibility, high-temperature operation.

ible light emitters
ith color tunability are essential for various ranges of applications such as sma and displays [117]. The white LEDs utilizing phosphor converters hav ance because of their low CRI and Stokes fluorescence loss [118]. Full-colo perating in the visible wavelength region and phosphor-free white-color nanow sented below.
lor tunable nanowire LEDs with emission wavelengths in the red, green, s ithically integrated RGB LEDs enable the spectral tuning to achieve the green, y ich can lead to full-color LEDs with low power consumption, extremely smal olor rendition. The lattice mismatched between the substrates and the high m wells results in very low efficiency GaN-based LEDs in the green a processes [112,113] and shown in

Visible light emitters
LEDs with color tunability are essential for various ranges of applications such as smart-lighting [116], and displays [117]. The white LEDs utilizing phosphor converters have limited performance because of their low CRI and Stokes uorescence loss [118]. erature, the In/Ga flux ratio as well as the growth duration are among the factors to emission wavelengths. Moreover, they demonstrated the phosphor-free InGaN/GaN e white LEDs mixing different colors emitted from large inhomogeneous broadening N/GaN quantum dots. They showed that the IQE of GaN nanowires LEDs could 22% at room temperature [50].
he electroluminescence spectra of GaN nanowire LEDs at room temperature which shows and orange light emissions and their optical microscopy images (insets) [50].
al. demonstrated the tunable full color GaN-LEDs utilizing integration of blue, , and orange/red light emissions as shown in Figure 5.2 [119]. In their experiment, nowire LEDs were grown directly on Si substrate by three step selective area MBE g SiO 2 mask. The EL spectra of nanowire LED subpixels grown at different steps are gure 5.3 (a). The emission properties of each nanowire LED could be controlled by e dimension and/or compositions of the dots. Such LED arrays on the same chip r-tunable characteristic in a wide range CCT which is from 3800 K to 6500 K by dependent injection currents to each LED subpixel. Moreover, these integrated color LED arrays can also exhibit very high color rendering index (>90), shown in b). These small size RGB nanowire LED arrays offer highly promising applications art lighting, and displays.

28
applying independent injection currents to each LED subpixel. Moreover, tunable full-color LED arrays can also exhibit very high color rendering index Figure 5.3 (b). These small size RGB nanowire LED arrays offer highly prom for future smart lighting, and displays. Recent studies have presented that the emission color of nitride-based nanowire LEDs can be varied by controlling the composition of the InGaN active region through the changing nanowire diameters as shown in Figure 5.4 [120,121]. Sekiguchi et al. demonstrated GaN nanowire LEDs utilizing Ti-mask selective area MBE growth [120]. They reported the GaN nanowire LEDs which exhibit different light emission ranges from 480 nm to 632 nm with different diameters from 143 nm to 270 nm. The peak emission wavelengths changed from blue (480 nm) to red (632 nm) with increasing nanowire diameter and can be seen from Figure 5 Albert et al. showed nanowire LEDs with different emission wavelengths by varying the In/Ga, III/V ratios, and the growth temperature [122]. They further reported white light emission achieved by single nanowire mixing the red-green-blue colors from different InGaN portions of the nanowire [122]. Recently, Wang et al. developed the AlInGaN quaternary core shell nanowire LEDs with tunable emission wavelengths from ~430 nm to ~630 nm. They can significantly suppress the nonradiative surface recombination utilizing an Al-rich shell which enhance the output power ~30 mW [104].
Recent studies have further shown that amorphous SiO x can be used as substrate for superior quality nanowire LED heterostructures [112,123,124], which pave the way for realizing high efficiency, flexible, and multi-color nanowire LEDs. Nguyen   creasing nanowire diameter and can be seen from  osphor-free nanowire white LEDs hough significant progresses have been made for the conventional III-nitr m well white LEDs, these devices still suffers from several major performance ng severe efficiency droop at high injection current and significant reduced cy in the green and even longer emission wavelength region, both of which ar ted with strong polarization fields [82,125], Auger recombination [3], inhom distribution [67], defects/dislocation [126], and/or electron leakage [75]. T ED lighting utilize phosphors to down-conversion of blue light of an InGaN  Lin et al. proposed phosphor-free white LEDs using InGaN/GaN nanowire heterostruc mixing multiple colors emitted from a single nanowire [139]. They reported that changin wth temperature and In/Ga beam fluxes can tune the color emission. Figure 5.5(a) pre PL spectra of InGaN nanodisk with different growth temperature. In Figure 5.5(b) aN nanodisk LED emissions cover the entire visible light range producing natural w t.

31
ribed in previous sections. These devices have already shown relatively high IQE, t sion across the entire visible spectral range, and droop-free operation [36, 99, ntly, InGaN nanowire LEDs has been demonstrated which show efficient light em ss nearly the entire visible spectral range [99,138,139]. in et al. proposed phosphor-free white LEDs using InGaN/GaN nanowire heterostru ixing multiple colors emitted from a single nanowire [139]. They reported that chang th temperature and In/Ga beam fluxes can tune the color emission. Figure 5.5(a) p PL spectra of InGaN nanodisk with different growth temperature. In Figure 5.5(b N nanodisk LED emissions cover the entire visible light range producing natural .

Conclusion
In conclusion, we reviewed the current progress of research on full-color III-nitride nanowire  [140,141], water splitting [142,143], food processing/horticulture [144,145], photo-therapy/medical diagnostics [146], and many more. Especially, device minia- 588 "This is an Open Access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium provided the original work is properly cited (CC BY 4.0)."