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Diagrams of fabrication processes for VLEDs (a) wafer bonding, (b) LLO, and (c) chemical wet etching and electrode deposition.
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In this letter, the nitride-based near-ultraviolet (NUV) vertical-injection light-emitting diodes (VLEDs) with roughened mesh-surface are proposed and demonstrated by a combination of pattern sapphire substrate, wafer bonding, laser lift-off, and chemical wet etching processes. With the help of adopting a roughened mesh-surface, the light-output po...
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... superlattice struc- ture. Fig. 1(b) shows a cross-sectional SEM micrograph of a GaN-based LED grown on a PSS. According to Fig. 1(b), the PSS can be buried completely by a GaN epitaxial layer without appearance of void. By performing a detail comparison, both types of NUV-LED wafers with and without PSS were subjected to the VLEDs processes. Fig. 2 shows the diagrams of fabrication process for VLEDs. The fabrication process of VLEDs on Si began with the deposition of the highly reflec- tive ohmic contact layer Ni-Ag-Pt and Cr-Au bonding layer on p-GaN. Both types of samples were then bonded onto a Cr-Au-coated p-type conducting Si substrate at 350 C for 1 h to form the structure ...
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... The fabrication process of VLEDs on Si began with the deposition of the highly reflec- tive ohmic contact layer Ni-Ag-Pt and Cr-Au bonding layer on p-GaN. Both types of samples were then bonded onto a Cr-Au-coated p-type conducting Si substrate at 350 C for 1 h to form the structure of sapphire (with and without PSS)/GaN LED/NiAgPt-CrAu-AuCr-Si [ Fig. 2(a)]. The wafer bonded samples were then subjected to the LLO process to form the u-GaN (with and without mesh-surface)/n-GaN/MQW/p-GaN structure on Si [ Fig. 2(b)]. A KrF excimer laser at a wavelength of 248 nm with a pulsewidth of 25 ns was used to remove the sapphire substrate. The incident laser with a beam size of 1.0 mm 1.0 mm was ...
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... types of samples were then bonded onto a Cr-Au-coated p-type conducting Si substrate at 350 C for 1 h to form the structure of sapphire (with and without PSS)/GaN LED/NiAgPt-CrAu-AuCr-Si [ Fig. 2(a)]. The wafer bonded samples were then subjected to the LLO process to form the u-GaN (with and without mesh-surface)/n-GaN/MQW/p-GaN structure on Si [ Fig. 2(b)]. A KrF excimer laser at a wavelength of 248 nm with a pulsewidth of 25 ns was used to remove the sapphire substrate. The incident laser with a beam size of 1.0 mm 1.0 mm was incident from the polished backside of the sapphire substrate onto the sapphire-GaN interface to decompose GaN into Ga and N. After removing the sapphire ...
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... In order to further increase the light extraction efficiency of VLEDs, the top n-GaN sur- faces treated through a chemical etching using 40% KOH by weight dissolved in ethylene-glycol solution at 120 C for 120 s [10]. Finally, a Cr-Pt-Au electrode was deposited as the n-type contact and the VLEDs with and without mesh-surface was obtained [ Fig. 2(c)]. The surface morphology of VLEDs was examined by SEM as shown in Fig. 3. It is obviously observed that the mesh-surface was naturally formed due to the epi-growth on PSS. According to Fig. 3(b), the hole-array mesh-surface of 3-m diameter, 3-m spacing, and 1-m depth shows a complementary structure from the PSS and was nearly ...
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Wet etching of InAl(Ga)N/GaN structures has been studied in detail by means of Rutherford backscattering spectroscopy, x-ray diffraction, atomic force microscopy and capacitance–voltage profiling (C–V). The samples used for the study were grown on three different substrates (sapphire, silicon carbide and silicon(111)). Nearly lattice-matched compos...
Citations
... Furthermore, the inferior performance of the sapphire substrate in terms of conducting the heat generated in the GaN-based vertical LEDs (VLEDs) hinders high-power LED performance [6]. To resolve these issues, different methods are being pursued to improve the output power by enhancing the light-extraction efficiency (LEE) through random scattering from a roughened surface, these methods include nano-imprint lithography [7], photonic crystal structures [8], nanostructure arrays [9], patterned sapphire substrates [10,11], surface texturing by laser irradiation [12,13], and wet chemical etching [14]. In addition, although implementing a distinctive laser lift-off (LLO) procedure has been suggested to resulting in high output power for high-performance LEDs, this technique has not yet been fully investigated [4]. ...
A controllable, mask-free, and wafer-level surface texturing method is proposed, which is applied to n-GaN protrusion array using laser irradiation treatment targeting to achieve high-performance vertical light-emitting diodes. The size and density of the n-GaN protrusions could be modified by controlling the energy density and pulse number of laser irradiation. Measurement results of current-voltage and light output power (LOP) reveal that the textured surface structure increases the LOP up to 47.89% at an injection current of 350 mA without affecting the current spread, such an enhancement is attributed to a higher probability that light could escape from the textured n-GaN surface.
... 6,9 In this connection, various dry-etching methods with etching masks formed using electron-beam lithography, imprint, nanosphere coating, and Ag nanoparticles have been employed. [10][11][12][13][14][15] Specifically, the use of Ag nanoparticles, produced by annealing Ag layers at 400°C-500°C, 15 is considered a facile route for roughening GaN surfaces. 12,15 However, this relatively high annealing temperature can damage the contact layer between the hetero-substrate and the wafer-bonded thin-GaN LED layer. ...
We investigated the effects of etching conditions on the performance of light-emitting diodes (LEDs) of various sizes aimed at vehicle headlamp applications. Photoluminescence (PL) images showed that after wet etching, the percentage of bad LED arrays significantly increased from 75% to 94%, and the leakage current at −5 V significantly increased from 1.14 × 10⁻⁹ A to 5.02 × 10⁻⁶ A. It was shown that plasma etching turned an Ag layer into Ag particles, the size and density of which depended on the treatment time and Ag layer thickness. These Ag particles served as micro-masks during dry etching. Plasma etching produced relatively uniform hillocks of diameters 0.9–1.43 μm and heights 0.85–2.5 μm. Moreover, the PL images showed that dry etching did not degrade the LED arrays. Furthermore, the light output power of the dry-etched LEDs was higher than that of the wet-etched LEDs. For example, the output power levels of the dry-etched LEDs (chip sizes: 240 × 290 μm², 240 × 490 μm², and 490 × 1190 μm²) at 100 mA were 20.4%, 15.0%, and 11.2% higher than those of the corresponding wet-etched LEDs, respectively. Moreover, we demonstrated a vehicle headlamp unit consisting of Ag-particle-based plasma-etched LEDs.
... Surface roughening through the chemical wet etching process [1,2], nanoimprint lithography [3], and nanostructures [4,5] has been applied to roughen the top surface of optoelectronic devices. Surface roughening has attracted considerable interest for applications such as solar cells (SCs), light-emitting diodes (LEDs), ultraviolet photodetectors (UV-PDs), and gas sensors [6][7][8][9]. ...
The fabrication of silicon oxynitride (SiON)/ZnO nanotube (NT) arrays and their application in improving the energy conversion efficiency (η) of crystalline Si-based solar cells (SCs) are reported. The SiON/ZnO NT arrays have a graded-refractive-index that varies from 3.5 (Si) to 1.9 2.0 (Siand ZnO) to 1.72 1.75 (SiON) to 1 (air). Experimental results show that the use of 0.4 m long ZnO NT arrays coated with a 150 nm thick SiON film increases Δ η / η by 39.2% under AM 1.5 G (100 mW/cm2) illumination as compared to that of regular SCs with a Simicropyramid surface. This enhancement can be attributed to SiON/ZnO NT arrays effectively releasing surface reflection and minimizing Fresnel loss.
... 5,6,9 It is obvious that a combination of the advantages of PSS-LED with the thin-GaN LED process will improve the LED performance. [10][11][12][13][14][15] However, compared with conventional flat thin-GaN LED (FT-LED), the critical laser energy of PT-LED was high, 14 the yield rate was very low, and the leakage current was high. 15 In this study, the requirement of high laser energy and the root causes of low yield rate and high leakage current were investigated. ...
Wafer bonding and laser lift-off (LLO) processes were employed to fabricate pattern sapphire thin-GaN light-emitting diodes LEDs (PT-LED). During the LLO process, the required laser energy for PT-LED was much higher than that for flat thin-GaN LED (FTLED). The yield rate of PT-LED was low, and the leakage current was high. In this study, the laser lift-off mechanisms of PT-LEDs were investigated.
... Various surface textures, such as surface roughening by alkaline-solution-based wet etching [6]- [8], photonic crystals [9]- [12], and nanorods [13], [14] have recently yielded enhanced light extraction efficiencies in VLEDs. Owing to the large thickness ð2 $ 3 mÞ and heavy doping concentration of the n-GaN top layer of the VLED, texturing methods that require dry or wet etching of the n-GaN can be applied to VLEDs without degrading their electrical properties [15], [16]. Additionally, the light extraction efficiency of the VLED can be effectively enhanced by combining highly reflective mirrors and light-extracting structures in the n-GaN layer [17], [18]. ...
A high-brightness GaN-based vertical light-emitting diode (VLED) was demonstrated by introducing a large-area low-cost direct printing process. A hexagonally close-packed micrometer convex array was fabricated on the n-GaN top layer of the VLED by direct printing using a hydrogen silsesquioxane solution and subsequent inductively coupled plasma etching. To confirm that the enhancement of light extraction by this structure, a conventional wet-chemical-etched structure was also fabricated on the n-GaN top layer of the VLED, yielding randomly oriented pyramid structures on the layer. Both VLEDs showed much stronger electroluminescence emission than an unpatterned VLED. However, the micrometer convex array improved the light extraction significantly more than the random pyramid structure owing to its greater ability to enlarge the light escape cone, attributed to its 50°-tapered profile and large extraction area. After chip packaging with silicone encapsulation, the light output power of the micropatterned VLED was 11.4% and 106% greater than those of the wet-etched and unpatterned VLEDs, respectively, under a 350-mA drive current.
... Recently, a new LED structure (vertical-GaN LED) was developed for the high-power LED applications1234 . The two key processes of vertical-GaN LEDs are (1) wafer bonding and (2) laser-lift off (LLO) techniques. ...
Vertical thin-GaN LED was successfully fabricated on the GaN LED epi-layers grown on the patterned-sapphire substrate with the pyramidal pattern by low-temperature Cu/Sn/Ag wafer bonding at 150°C. An inverted pyramidal pattern formed on the n-GaN surface after the GaN epi-layer was transferred onto Si wafer, which resulted from the pyramidal pattern on the patterned-sapphire substrate. The inverted pyramidal pattern has an equivalent function with roughening the n-GaN surface. With higher inverted pyramidal pattern coverage, the light extraction efficiency can be greatly enhanced. In addition, we found that the 4-fold increase (from 13.6% to 53.8%) in the pyramidal pattern coverage on patterned-sapphire substrate only gives the GaN LED epi-layer about 5.7% enhancement in the internal quantum efficiency.
... 4,5) However, with the existence of a large refractive index difference between GaN (n ¼ 2:45) and air (n ¼ 1), a low critical angle of total internal reflection ($23:6 ) and a poor light extraction ratio (4%) 6) impede the light output power of VLEDs. Over the past decade, numerous surface roughening technologies, such as the use of a patterned sapphire substrate with a laser lift-off (LLO) process, 7) surface roughening using inductively coupled plasma (ICP) dry etching, 8) photoelectrochemical (PEC) etching, 9) the use of a micro-photolithographic system with dry and wet etching, 10) PEC for the roughening of oxide passivation mesa sidewalls, 11) and the use of a Ni nanomask with laser etching, 12) have been demonstrated to increase light extraction efficiency through random scattering from the roughened surface for high-power GaN-based LEDs. ...
GaN-based thin-film vertical-structured light-emitting diodes (VLEDs) with a GaOx film atop an n-GaN layer roughened via KrF laser irradiation and a TiO2/SiO2 distributed Bragg reflector (DBR) are proposed and investigated. As compared with regular VLEDs with an Al reflector and without a roughened GaOx film, the proposed VLEDs with a chip size of 1 mm2 show a typical increase in light output power by 68% at 350 mA and by 51% at 750 mA, which is attributed to the enhanced reflectivity and current blocking capability of the DBR layer, the surface roughening with circular GaN protrusions, and the formation of a surface GaOx film by KrF laser irradiation.
... However, the mirror reflectivity could be damaged easily after the bonding process. The p-side up thin GaN devices have also been reported previously, where a transferred epilayer to the sapphire substrate is employed [23], [25]. Obviously, the use of sapphire substrate will hamper the thermal dissipation of the device under high driving currents. ...
A p-side-up GaN-based light-emitting diode (LED) on a silicon substrate was designed and fabricated using a combination of omnidirectional reflector (ODR) and double-side textured surface (both p-GaN and undoped-GaN) structures via surface-roughening, laser lift-off (LLO) and wafer-bonding technologies. The reflectivity of the designed ODR can reach 99.1% at a wavelength of 460 nm. The textured surface of top p-GaN was achieved under low temperature (LT) conditions using metalorganic chemical vapor deposition. It was found that the GaN LED with an extra 200-nm-thick LT p-GaN layer exhibits a 50% enhancement in luminance intensity. The luminance efficiency of double-side roughened silicon-ODR-GaN LED with a small chip size of 250 mum times 500 mum can be improved from 23.2% to 28.2% at an injection current of 20 mA. For the case of 1 mm times 1 mm in chip size, the saturation behavior of the light output power is not observed when an injection current increased from 20 to 350 mA, where the luminance efficiency at 20 mA can reach 28.9%, demonstrating an enhancement by 46%, as compared with that of the conventional GaN-sapphire LEDs. These enhanced results can be attributed to higher reflectivity from the ODR and multiple chances of light emitted from the active region to escape, as well as a centralizing effect of light along the vertical direction.
GaN-based thin-film vertical-structured light-emitting diodes (VLEDs) with a GaOx film atop an n-GaN layer roughened via KrF laser irradiation and a TiO2/SiO2 distributed Bragg reflector (DBR) are proposed and investigated. As compared with regular VLEDs with an Al reflector and without a roughened GaOx film, the proposed VLEDs with a chip size of 1 mm² show a typical increase in light output power by 68% at 350 mA and by 51% at 750 mA, which is attributed to the enhanced reflectivity and current blocking capability of the DBR layer, the surface roughening with circular GaN protrusions, and the formation of a surface GaOx film by KrF laser irradiation.
To further enhance the light extraction efficiency of GaN-based thin-film flip-chip light-emitting diodes (TFFC-LEDs), a surface roughening technique using KrF excimer laser ablation and chemical wet etching is demonstrated. Both optical ray-tracing simulations and experimental results of the light emission characteristics are presented and discussed. With the proposed twofold surface texturing scheme with circular protrusions superimposed by hexagonal cones, the angular randomization of photons at the emission surface was maximized, enhancements in light output power of 13.08% (12.81%) and wall-plug efficiency of 2.87% (2.25%) at 350 mA (700 mA) compared to those of a TFFC-LED without surface texturing were obtained.
