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MQW PL of (a) nanorod LEDs and (b) planar LED. (c) MQW PL peak position plotted as a function of pump power for planar LEDs and nanorod LEDs.
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Vertically aligned InGaN/GaN nanorod light emitting diode (LED) arrays were created from planar LED structures using a new top-down fabrication technique consisting of a plasma etch followed by an anisotropic wet etch. The wet etch results in straight, smooth, well-faceted nanorods with controllable diameters and removes the plasma etch damage. 94%...
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... higher IQE, and higher lifespans. This result also shows that nanorods formed by top-down etching can be nearly dislocation free even though they are not grown strain-relaxed like bottom-up nanowires. The room temperature PL spectra of the nanorod LED InGaN MQWs excited by a 413.1 nm Krypton ion laser are compared to the planar LED in Figs. 4(a) and 4(b). The PL spectra peak positions were determined using Gaussian fit and plotted as a function of pumping power in Fig. 4(c). For both structures there is a blue shift in the emission wavelength with pump power (carrier injection) caused by free carrier screening of the QCSE. The QCSE results from spontaneous polarization field caused by ...
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... free even though they are not grown strain-relaxed like bottom-up nanowires. The room temperature PL spectra of the nanorod LED InGaN MQWs excited by a 413.1 nm Krypton ion laser are compared to the planar LED in Figs. 4(a) and 4(b). The PL spectra peak positions were determined using Gaussian fit and plotted as a function of pumping power in Fig. 4(c). For both structures there is a blue shift in the emission wavelength with pump power (carrier injection) caused by free carrier screening of the QCSE. The QCSE results from spontaneous polarization field caused by low symmetry of c-plane III-nitride crystal structure and piezoelectric polarization field caused by lattice mismatch ...
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... that the change in strain linearly alters the piezoelectrically dominated Stark shift, we estimate a ~3.0 nm blue shift (0.16 x 19 nm) for the nanorod QW emission relative to that of the planar sample. In Fig. 4(c), we see a measured blue shift of ~4 nm (from about 445 to 441 nm for the planar vs. nanorod sample) at low power, in approximate agreement with our estimates and in support of partial QW strain relaxation due to wire formation. A smaller maximum blue shift of ~2 nm in the nanorod InGaN peak position as the pumping power is increased from 0 to 100 mW compared to the 6 nm shift observed in the planar LED is also evidence for reduced QCSE in the nanorod LEDs, consistent with a previous report [7]. ...
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... of the nanowires did not compromise the IQE in an overly detrimental way. Additionally, it is possible that the decrease in IQE in the nanorod LEDs at relatively low pump powers may be caused by heating from the laser, due to the poor thermal transport properties of nanowires. This is supported by the red shift at higher pump powers observed in Fig. 4, consistent with a previous report where laser-induced produced redshifts in GaN nanowires during PL experiments [24]. Despite the reduced piezoelectric polarization and their mostly dislocation-free nature, the measured InGaN MQW IQEs of the nanorod and planar LEDs are not significantly different. It is possible that the benefit of ...
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Citations
... In the next step, the sample is annealed and the Pt film breaks up into an ensemble of nanoislands. These islands serve as a mask for the nanopatterning of the GaN following previously reported etching steps [21,22]. First, the buffer layer that is not protected by the nanoislands is removed by reactive ion etching (RIE). ...
... The sidewalls are smooth, and the diameter is homogeneous along the nanowires. This beneficial morphology is a consequence of the final KOH wet etch, that is known to remove the inclined sidewalls and defects induced by dry etching that are characteristic for the ICP etching [21,28,29]. ...
The dewetting of thin Pt films on different surfaces is investigated as a means to provide the patterning for the top-down fabrication of GaN nanowire ensembles. The transformation from a thin film to an ensemble of nanoislands upon annealing proceeds in good agreement with the void growth model. With increasing annealing duration, the size and shape uniformity of the nanoislands improves. This improvement speeds up for higher annealing temperature. After an optimum annealing duration, the size uniformity deteriorates due to the coalescence of neighboring islands. By changing the Pt film thickness, the nanoisland diameter and density can be quantitatively controlled in a way predicted by a simple thermodynamic model. We demonstrate the uniformity of the nanoisland ensembles for an area larger than 1 cm². GaN nanowires are fabricated by a sequence of dry and wet etching steps, and these nanowires inherit the diameters and density of the Pt nanoisland ensemble used as a mask. Our study achieves advancements in size uniformity and range of obtainable diameters compared to previous works. This simple, economical, and scalable approach to the top-down fabrication of nanowires is useful for applications requiring large and uniform nanowire ensembles with controllable dimensions.
... Additionally, due to changes in the internal polarization field of the QWs, the emission wavelength of the QWs can be blueshifted. Precise control of the nanostructure size allows for accurate modulation of the emission wavelength range [29,[72][73][74][75][76][77][78]. Based on this concept, in 2017, the Kuo research group fabricated nanoring arrays on LED epitaxial wafers by depositing nickel on the surface and etching out nanorings with an outer diameter of 800 nm and an inner diameter of approximately 700 nm [35], the fabrication process is illustrated in Figure 6a-f. ...
... Additionally, due to changes in the internal polarization field of the QWs, the emission wavelength of the QWs can be blueshifted. Precise control of the nanostructure size allows for accurate modulation of the emission wavelength range [72,73,29,[74][75][76][77][78]. Based on this concept, in 2017, the Kuo research group fabricated nanoring arrays on LED epitaxial wafers by depositing nickel on the surface and etching out nanorings with an outer diameter of 800 nm and an inner diameter of approximately 700 nm [35], the fabrication process is illustrated in Figure 6a-f. ...
Micro-light-emitting diodes (μLEDs), with their advantages of high response speed, long lifespan, high brightness, and reliability, are widely regarded as the core of next-generation display technology. However, due to issues such as high manufacturing costs and low external quantum efficiency (EQE), μLEDs have not yet been truly commercialized. Additionally, the color conversion efficiency (CCE) of quantum dot (QD)-μLEDs is also a major obstacle to its practical application in the display industry. In this review, we systematically summarize the recent applications of nanomaterials and nanostructures in μLEDs and discuss the practical effects of these methods on enhancing the luminous efficiency of μLEDs and the color conversion efficiency of QD-μLEDs. Finally, the challenges and future prospects for the commercialization of μLEDs are proposed.
... With respect to singularly addressable light emitter sources, Orenstein and co-workers reached an important milestone in their pioneering work where matrix driven arrays with surface emitting laser diodes were developed [21][22][23][24] albeit for signal processing and optical communication. The further developments, which represent a significant step forward, were reported on the arrangement of LEDs in arrays [25][26][27][28][29][30][31] and the possibility to drive them preferably singularly (among others for displays) [32][33][34][35][36][37][38][39][40][41][42][43]. Hence, if all of these "ingredients/building blocks" are available within a lithography system, in principle, next generation lithography can be carried out mask-free, allowing time and resources savings, flexible structure patterning on demand. ...
LED devices are increasingly gaining importance in lithography approaches due to the fact that they can be used flexibly for mask-less patterning. In this study, we briefly report on developments in mask-free lithography approaches based on nano-LED devices and summarize our current achievements in the different building blocks needed for its application. Individually addressable nano-LED structures can form the basis for an unprecedented fast and flexible patterning, on demand, in photo-chemically sensitive films. We introduce a driving scheme for nano-LEDs in arrays serving for a singularly addressable approach. Furthermore, we discuss the challenges facing nano-LED fabrication and possibilities to improve their performance. Additionally, we introduce LED structures based on a hybrid nanocrystal/nano-LED approach. Lastly, we provide an outlook how this approach could further develop for next generation lithography systems. This technique has a huge potential to revolutionize the field and to contribute significantly to energy and resources saving device nanomanufacturing.
... The top-down fabrication process refers to the direct preparation of desired nanostructures on the substrate (or bulk material) through a series of thin film deposition, photolithography, and etching techniques. By employing top-down patterning and inductively coupled plasma techniques, highly uniform and well-oriented NWs can be fabricated on a high-quality epitaxial single crystal film [48][49][50][51]. The top-down method has also been widely applied to the preparation of devices such as 2D materials-based transistors. ...
Highlights
The operating mechanism of piezotronic neuromorphic devices and related manufacturing techniques are presented.
Recent research advances in piezotronic neuromorphic devices including multifunctional applications are summarized.
Challenges and prospects for modulating novel neuromorphic devices with piezotronic effects are discussed.
... Slanted sidewalls can complicate subsequent deposition steps, and surface damage due to the dry etch process contributes to nonradiative recombination which significantly reduces the optical output and degrades device efficiency. 8,9,21,22 To optimize device performance, the dry etch damaged III-Nitride micropillar sidewall surface must be fully removed. Therefore, here, we propose an analytical study on the use of KOH-based wet etch that can effectively passivate vertical III-N nanowires and micropillars. ...
... Passivation through atomic layer deposition (ALD) of dielectrics or treatment with sulfide solutions have been shown to minimize surface recombination, improving device performance and efficiency. [22][23][24] Specifically, ALD passivation layers typically provide benefits, such as improved thermal or chemical stability, or can act as optical coating. However, this may require additional time consuming and expensive processing, and will add material to the device surface that can affect the optical properties of the device in the case of optoelectronics such as mLEDs and may not be desirable for power transistors if direct contacts are needed. ...
III-Nitride micropillar structures show great promise for applications in micro light-emitting diodes and vertical power transistors due to their excellent scalability and outstanding electrical properties. Typically, III-Nitride micropillars are fabricated through a top-down approach using reactive ion etch which leads to roughened, nonvertical sidewalls that results in significant performance degradation. Thus, it is essential to remove this plasma etch induced surface damage. Here, we show that potassium hydroxide (KOH) acts as a crystallographic etchant for III-Nitride micropillars, preferentially exposing the vertical <11¯00> m-plane, and effectively removing dry etch damage and reducing the structure diameter at up to 36.6 nm/min. Both KOH solution temperature and concentration have a dramatic effect on this wet etch progression. We found that a solution of 20% AZ400K (2% KOH) at 90°C is effective at producing smooth, highly vertical sidewalls with RMS surface roughness as low as 2.59 nm, ideal for high-performance electronic and optoelectronic devices.
... However, wet etching has been proven to be an excellent tool to eliminate plasma induced damage. It has been already demonstrated the impact of plasma induced damage on the PL emission intensity along with positive effect of wet etching [34]. Additionally, wet etching can be utilized to control the morphology of the NRs. ...
Gallium nitride (GaN) based low-dimensional optoelectronic devices offer distinct advantages over bulk counterparts owing to enhanced surface-to-volume ratio and better strain management. Among these devices, InGaN/ GaN multi-quantum wells (MQWs) based on nanostructured GaN have garnered attention due to tunable bandgap along with reduced size for its possible integration into display technology. However, to mitigate the effects of polarization induced by polar planes of GaN, such as quantum confined stark effect, researchers are exploring the growth of MQWs on non-polar planes as a practical approach to improve emission characteristics and device performance. In this work, we present the fabrication of GaN nanorods (NRs) using a two-step etching method, followed by the growth of core-shell InGaN/GaN MQWs on the exposed non-polar {1010} sidewalls of GaN NRs. Our metrological studies confirmed the successful growth of InGaN/GaN MQWs on the non-polar sidewalls of strain-relaxed core GaN NRs. Furthermore, power-dependent luminescence study showed emission characteristics with absence of polarization-induced effects. Cathodoluminescence study validated the emission characteristics of InGaN/GaN MQWs grown on the exposed sidewalls of the GaN NRs.
... The extent of this interface is one of the main causes of the early onset of nanoLED heating, i.e., temperature droop and very low efficiency. 24 So far, using the nanopillar-based LED architectures, measurements show a rollover in the EQE, and efficiency droop is observed. 25 ZnO fin LEDs with a nanoscale dimension of about 160 nm have shown some advantages relative to nanopillars in suppression of non-radiative Auger recombination at high current densities. ...
Previously, we showed within a sub-micron fin shape heterojunction, as current density increases, the non-radiative Auger recombination saturates mediated by the extension of the depletion region into the fin, resulting in a droop-free behavior. Here, we investigate the dependence of the fin aspect ratio (height to width ratio) on external quantum efficiency (EQE) of single n-AlGaN fin/p-GaN heterojunctions. Fins are arranged in an array format varying in width from 3000 to 200 nm. In this architecture, an n-metal contact is interfaced with the non-polar side facet of the fin. At a fixed current density, as the aspect ratio increases from 0.2 to 3 (the fin width reduces), we systematically observe an increase in the ultraviolet (UV) excitonic emission of the AlGaN fin and a 7× enhancement in the EQE. We explain this phenomenon by conserving the volume of the carrier depletion region within a fin. As the fin gets thinner, the base area of the depletion volume shrinks, whereas its height increases within the fin. This geometrical advantage allows a 200 nm wide fin to operate at 1/3rd the current density compared to a 3000 nm wide fin while generating a UV emission with a comparable power of 1 μW. These findings show additional parameters that can be used for developing brighter light sources, including the shape and aspect ratio of a heterojunction at the micro- or nano-scale.
... ,81 ...
Third-generation semiconductors have richer and better properties than traditional semiconductors, and show promising application prospects in high-power, high-temperature, high-frequency, and optoelectronic devices. Therefore, they have gained increasing interest and received extensive research attention in recent years. Electrochemical etching plays an important role in exploring the properties of third-generation semiconductors and related device fabrication. This paper systematically reviews the electrochemical etching process of silicon carbide and gallium nitride which are the typical representative of the third-generation semiconductors. Through subdividing the electrochemical etching approach into anodic oxidation etching, photoelectrochemical etching, and electroless photoelectrochemical etching, the mechanism of each electrochemical etching method is expounded, the influences of various etching parameters on the etching results are discussed, and the related applications of electrochemical etching in characterizing crystal defects, processing micro-nano structures, and fabricating microelectronic devices are summarized. Finally, future development in achieving more efficient electrochemical etching is briefly discussed. In general, this paper provides a systematic review of the electrochemical etching of third-generation semiconductors, which is helpful for researchers to supplement the content in this field, and even non-researchers in this field will be able to familiarize themselves with the relevant content quickly.
... To produce the initial GaN NW array with high uniformity, we follow a conventional top-down fabrication scheme comprising mask patterning, plasma etching and wet etching in concentrated KOH [36]. This approach schematically depicted in figure 1(a) was shown effective for synthesizing micrometer long GaN NWs with a minimum diameter of ≈100 nm. ...
... Similar conclusions are drawn concerning the NW diameter. Interestingly, straight NW sidewalls are already obtained after 130 s, whereas etching times of several hours were required in [36,46]. We associate the faster etching observed in this work (20 nm min −1 ) to the smaller volume of material to be etched and to the higher temperature of the KOH bath. ...
... This morphology is also consistent with previous studies [40,41,59]. Straight NW sidewalls are eventually obtained during KOH etching ( figure 3(b)), in agreement with previous reports [36,45]. However, the roughness of the sidewalls is larger than in the presence of SiN x . ...
We introduce a facile route for the top-down fabrication of ordered arrays of GaN nanowires with aspect ratios exceeding 10 and diameters below 20 nm. Highly uniform thin GaN nanowires are first obtained by lithographic patterning a bilayer Ni/SiNxhard mask, followed by a combination of dry and wet etching in KOH. The SiNxis found to work as an etch stop during wet etching, which eases reproducibility. Arrays with nanowire diameters down to (33 ± 5) nm can be achieved with a uniformity suitable for photonic applications. Next, a scheme for digital etching is demonstrated to further reduce the nanowire diameter down to 5 nm. However, nanowire breaking or bundling is observed for diameters below ≈ 20 nm, an effect that is associated to capillary forces acting on the nanowires during sample drying in air. Explicit calculations of the nanowire buckling states under capillary forces indicate that nanowire breaking is favored by the incomplete wetting of water on the substrate surface during drying. The observation of intense nanowire photoluminescence at room-temperature indicates good compatibility of the fabrication route with optoelectronic applications. The process can be principally applied to any GaN/SiNxnanostructures and allows regrowth after removal of the SiNxmask.
... Whilst nanopillars of 150 nm diameters for the blue-pixel regions are chosen due to process constraints relating to etch selectivity, the strain relaxation induced at this dimension does not provide sufficient spectral blue-shift for blue emission. As such, the 150 nm diameter nanopillars undergo shrinkage by means of wet etching in a 95°C AZ400K solution for an hour, targeted to reduce the diameter by 100 nm [33], as illustrated by figure 1(g), with the ITO current spreading layer on top of the GaN surface acting as a self-aligned protective layer. An average diameter of 45 nm is achieved after wet etch, as estimated from SEM images illustrated in figure 2(c). ...
Chip-scale red, green and blue (RGB) light emission on an InGaN/GaN multi-quantum well (MQW) wafer adopting a top-down fabrication approach is demonstrated in this study, facilitated by shadow-masked nanosphere lithography for precise site-controlled nano-patterning. Exploiting the strain relaxation mechanism by fabricating arrays of nanosphere-defined nanopillars of two different dimensions utilizing a sequential shadow-masked nanosphere coating approach into the blue and green light-emitting pixel regions on a red-light emitting InGaN/GaN wafer, RGB light emission from a monolithic chip is demonstrated. The red, green and blue light-emitting pixels emit at 645 nm – 680 nm, 510 nm – 521 nm and 475 nm – 498 nm respectively, achieving a maximum color gamut of 60% NTSC and 72% sRGB. Dimensional fluctuations of the nanopillars of 73% and 71% for the green and blue light-emitting pixels, respectively, are estimated from scanning electron microscope (SEM) images of the fabricated device, corresponding to fluctuations in spectral blue-shifts of 5.4 nm and 21.2 nm as estimated by strain-coupled k . p Schrödinger calculations, consistent with observations from -PL mapping which shows deviations of emission wavelengths for the red, green and blue light-emitting pixels to be 8.9 nm, 14.9 nm and 23.7 nm, respectively. The RGB pixels are also configured in a matrixaddressable configuration to form an RGB microdisplay, demonstrating the feasibility of the approach towards chip-scale color displays.
