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High-oriented Li–Al layered double hydroxide (LDH) films were grown on an InGaN light-emitting diode (LED) structures by immersing in an aqueous alkaline Al3+- and Li+-containing solution. The stand upward and adjacent Li-Al LDH platelet structure was formed on the LED structure as a textured film to increase the light extraction efficiency. The light output power of the LED structure with the Li-Al LDH platelet structure had a 31% enhancement compared with a conventional LED structure at 20 mA. The reverse leakage currents, at −5V, were measured at −2.3 × 10−8A and −1.0 × 10−10A for the LED structures without and with the LDH film that indicated the Li-Al LDH film had the insulated property acted a passivation layer that had potential to replace the conventional SiO2 and Si3N4 passivation layers. The Li-Al LDH layer had the textured platelet structure and the insulated property covering whole the LED surface that has potential for high efficiency InGaN LED applications.
©2012 Optical Society of America
1. Introduction
InGaN/GaN light-emitting diodes (LEDs) have been utilized in traffic lights, the back lights of liquid crystal displays, indoor lighting and in other applications. However, the internal quantum efficiency [1] and the extraction efficiencies of commercial LEDs continue to need improvements. The low light extraction efficiency is due to a larger refractive index difference between the GaN (2.5) layer and the surrounding air (1.0). Bottom patterned Al2O3 substrates [2], photonic crystal structures formation [3,4], periodic deflector embedded structures [5], pattern-nanoporous GaN:Mg surface [6], nano-cone array on a p-GaN surface [7], polydimethylsiloxane concave microstructures [8], inverted micropyramid structures [9], ZnO nanotips prepared by the aqueous solution deposition [10], double-embedded photonic crystals [11], self-assembled microlens arrays [12–14], and self-organized patterning method [15] have all been used to increase light-extraction efficiency in InGaN-based LEDs on Al2O3 substrates. The threading dislocation densities in the GaN layers were reduced and the internal quantum efficiency (IQE) in InGaN-based LED was increased by using the nanoscale patterned sapphire substrates [16, 17]. The high light extraction efficiency in the GaN-based LED was increased when grown on the micron-sized dimensions patterned sapphire substrates [18]. InGaN quantum wells ΠQWs) design with large electron-hole wave functions overlap [19, 20] and the surface plasmon coupled InGaN QWs [21, 22] were used to increase the IQE value and the radiative recombination rates in InGaN LED structures. InAlN electron-blocking layers [23] and InGaN QW with 15-Å AlGaN barriers surrounding the QW [24] have been reported to suppress the carrier leakages and improve the efficiency-droop in the InGaN based LED structures. The effective anti-reflection (AR) coating with the indium tin oxide (ITO) nanocolumns [25], the ITO nanorods [26], the textured ITO layer [27], and the CsCl nano-islands [28] were used to increase the light extraction efficiency. The conventional passivation layers were SiO2 [29] and Si3N4 layers for optical devices to prevent the surface leakage current which were deposited through the electron beam evaporation system and the plasma-enhanced chemical vapor deposition system.
In this paper, a Li-Al layered double hydroxide (LDH) film was grown and covered the whole LED structure by immersing in an aqueous alkaline Al3+ and Li+ containing solution. Our previous study [30] had shown the method to prepare the solution. The Li-Al LDH layer acted an insulated passivation layer to prevent from the surface leakage current occurring on the mesa sidewall surface. The Li-Al LDH platelets stand upward on the GaN surface provided a high light scattering structure to increase the light extraction efficiency. The surface morphology and the optical properties of the InGaN LED structures with the Li-Al LDH film are analyzed in detail.
2. Experiments
InGaN-based LED structures were grown on c-face (0001) 2”-diameter sapphire substrates with the truncated triangle-shaped patterns through a metalorganic chemical vapor deposition (MOCVD) system. The detail dimensions of the patterned sapphire substrate consisted of a 2.2µm-width of the top truncated plane, a 4.8µm-width of the bottom plane, and a 2.0µm-height for the truncated triangle-shaped patterns. Trimethylgallium (TMGa), trimethylaluminum (TMAl), trimethylindium (TMIn), and ammonia (NH3) were used as the Ga, Al, In, and N sources, respectively. The doping source gas for the Si, as the donor, was monosilane (SiH4) and for the Mg, as the acceptor, was bis-cyclopentadienylmagnesium (Cp2Mg). These LED structures consisted of a low-temperature grown 30nm-thick GaN buffer layer, a 6.2μm-thick n-type GaN:Si layer, 10 pairs of the InGaN/GaN multiple quantum wells (MQWs) active layers, and a 0.2μm-thick magnesium-doped p-type GaN:Mg layer. The active layers consisted of a 35Å-thick InGaN-well layer and a 120Å-thick GaN-barrier layer. The dimension of the chip was 570 × 240 µm2 in size and the mesa region of 540 × 210 µm2 with a 1.0µm-depth was defined through the inductively coupled plasma (ICP) etcher. A 240nm-thick indium tin oxide layer (ITO) was deposited on the mesa region as a transparent contact layer (TCL). The Cr/Au metal layers were deposited as n-type and p-type contact pads. Al-Li intermetallic compound (IMC) was used as a raw material to prepare ionic solution for the deposition of the Li-Al layered-double-hydroxide (LDH) layer. Because of the brittle nature of the IMC, grinding the IMC in a mortar was carried out to prepare IMC powder. 0.2 g of the powder was then added into 500 ml deionized water (DI water) at 5°C under continuous stirring for 5 min in the ambient atmosphere. Vigorous hydrolysis reaction of the IMC powder in the 5°C water would proceed. The reaction solution was filtered through a filter paper (No.5A; Advantec). The filtered solution was the Al3+− and Li+− containing solution for forming the Li-Al LDH film on GaN surface. The LDH film grew on the GaN surface when the substrate was lying flat in the ionic aqueous at room temperature for 20~60 min (depending on film thickness). Then, the Li-Al LDH film was observed on the mesa region (with ITO layer on GaN:Mg), the mesa sidewall, and the ICP dry etched n-type GaN layer in the InGaN LED structure. The 1.0μm-thick Li-Al LDH film with the platelet structure provided a texture surface to increase the light scattering process. The LED structures with and without the Li-Al LDH film were defined as a LDH-LED structure and a standard LED (ST-LED). The schematic of the LDH-LED structure with a textured Li-Al LDH structure is shown in Fig. 1 . The geometric morphology of the LED structure was observed through a scanning electron microscopy (SEM). The optical properties of the LED samples were measured through the photoluminescence (PL) measurement by using a 50 mW, 150 MHz, 405 nm InGaN laser (Coherent, CUBE 405-50C) as the excited source. The electroluminescence (EL) spectra and the light-output power were characterized by an optical spectrum analyzer (Ando-6315A). The light-intensity profiles that crossed the whole LED sample were measured by a beam profiler (Spiricon: number of effective pixels: 1600 × 1200pixels).
3. Results and discussion
The SEM micrographs of the ST-LED and the LDH-LED structures were shown in Fig. 2(a) and Fig. 2(b), respectively. The Li-Al LDH film was observed on the whole LDH-LED structure. In the ST-LED structure, a smooth GaN mesa surface with top ITO layer and a smooth dry-etched n-type GaN surface were observed as shown in Fig. 2(c). In Fig. 2(d), The Li-Al LDH film was grown covering on the mesa sidewall was seen in the LDH-LED structure. The Li-Al LDH platelet structure was grown on the GaN mesa region, the mesa sidewall, the ICP-etched GaN:Si layer, and the TCL layer as a passivation layer on the LED wafer. When the LED wafer immersed in the Al3+- and Li+-containing solution, a thin Li-Al LDH layer (about 40nm) was deposited on the GaN surface and the ITO surface as a smooth seed layer. Then, the high-density LDH platelets were formed on the seed layer that each stood almost perpendicularly to the surface of the LED structures shown in Fig. 2(e). In Fig. 2(f), the dimension of the Li-Al LDH platelet structure was measured at about 2µm in width, 40nm in thick, and 1.0µm in height as shown in the SEM micrograph. The SEM micrographs in Fig. 2(f) of the Li-Al LDH platelets structure was observed at 45o bird’s-eye view. The Li-Al LDH platelet was grown perpendicular to the GaN surface, and the top of the Li-Al LDH platelet appeared to be slanted slightly shown in the SEM micrograph.
Fig. 2 The SEM micrographs of (a)(c) the ST-LED and (b)(d) the LDH-LED structures were observed. (e)(f) Larger magnification SEM micrographs of the Li-Al LDH structure were observed as the platelet structures.
In Fig. 3(a) , the operating voltage and the light-output power of the LED structures were measured by varying the injection current. At 20mA operation current, the operating voltages of both LED structures were almost the same at 3.1 V. The LED surface was covered with the Li-Al LDH film, as a passivation layer, without damage the ITO TCL layer. At 20mA operation current, the light output power of the LDH-LED structure had a 31% enhancement compared to the ST-LED structure. By increasing the injection current up to 100mA, the high light output power and the stable operation voltage were observed in the LiAl-LED structure. To analyze the thermal stability of the Li-Al platelets, the geometric and the material properties of the Li-Al platelets were almost the same after the thermal treatment at 550°C for 30 min. The Li-Al LDH film had the insulated property acted a passivation layer that had potential to replace the conventional SiO2 passivation layers. The thermal induced the light output power saturation phenomenon was not observed in both LED structure that indicated the high thermal stability property of the Li-Al platelets in the LDH-LED. In Fig. 3(b), the leakage current of both LED structures were measured under the reverse bias voltage condition. At −5V reverse bias voltage, the reverse leakage currents were measured as the values of −2.3 × 10−8 A and −1.0 × 10−10 A for the LED structures without and with the Li-Al LDH film, respectively. The low leakage current was observed in the LDH-LED structure that indicated the Li-Al LDH film had insulated property to passivate on the mesa sidewall surface and to reduce the surface leakage current. The Li-Al LDH film consisted of the high resistivity property and the textured surface to improve the electric and optical properties for the LDH-LED structure. The LDH layer was grown on the InGaN-based LED wafer through the solution process. The high adhesion properties and the insulated properties were observed on the uniformly deposited LDH layer for the InGaN LED structure. The LDH layer had been grown uniformly on the 2” InGaN LED wafer that had a high yield for the LED fabrication process. The low leakage current was observed in the LDH-LED structure that indicated the Li-Al LDH film had the high insulated and the high stabilized properties for the LED devices under a DC current injection.
Fig. 3 (a) The current-voltage (I-V) characteristics and the light-output power as a function of the operating current are measured. (b) The reverse leakage currents of both LED structure are measured at −5V bias voltage.
In Fig. 4 , the light intensity profiles of the LED structures were analyzed by a beam profiler at 20 mA operating current. In Figs. 4(a) and 4(b), the high light emission intensity was observed on the LDH-LED structure compared to the ST-LED structure that had high light scattering process on the Li-Al LDH platelet structure. The triangle-shaped light emission patterns were observed on both LED structures which was caused by the light scattering process on the truncated triangle-shaped pattern sapphire substrate as shown the Figs. 4(c) and 4(d). The high light intensity of the LDH-LED structure was observed at the triangle-shaped pattern on the mesa region and the dry-etched GaN region compared to the ST-LED structure. The textured Li-Al LDH structure was grown covering the whole LED surface that induced the uniformly high light intensity on the LDH-LED structure. The emission light from the InGaN active layer will be extracted by the textured Li-Al LDH structures to increase the external quantum efficiency. In the Fig. 4(e), the EL line intensity profiles of both LED structures were measured that the scanning-lines were labeled in the Figs. 4(c) and 4(d). In the LDH-LED structure, the line-scanning light emission intensities at the mesa region and the dry-etched GaN region were higher than the ST-LED structure. In the Fig. 4(e), the integral light intensity of the LDH-LED structure had 38% enhancement compared to the ST-LED structure at the normal detected direction.
Fig. 4 The (a)(b) low magnification and (c)(d) large magnification light-intensity profiles of the ST-LED and LDH-LED structures were analyzed by a beam profiler at a 20 mA operating current. (e) The line intensity profile of both LED structures were measured from the ICP-etched GaN:Si region to mesa region labeled in the (c)(d). The scale bar for the light intensity profile was labeled.
The far-field radiation patterns of both LED samples were measured at a 20 mA operating current shown in Fig. 5(a) . The divergent angles of the LEDs are identified as the angle of the half-maximum EL emission intensity at a 20mA operation current. The divergent angles of the LED structures with and without Li-Al LDH layer were measured on the non-encapsulated LED chip without the packaged process. The divergent angles of the ST-LED and the LDH-LED were calculated at 142° and 154°, respectively. The larger divergent angle of the LDH-LED structure was caused by the high light scattering process occurring on the Li-Al LDH platelet structure.
Fig. 5 (a) The PL spectra of both LED structure were measured at the normal and the lateral directions. (b) Far-field radiation patterns of all LED samples were analyzed by the divergent angle measurements.
At 20mA operating current, the EL spectra of both LED structures were measured at the lateral direction (0°) and the normal direction (90°) shown in Fig. 5(b). The EL peak wavelengths of both LED structures were almost the same at 450nm. In the ST-LED structure, there are no ripple signals on the EL spectra at normal and lateral detected angles. The rippling signal on the EL spectrum was observed in the LDH-LED structure at the lateral direction, which was not caused by the interference between the top GaN/interface and the bottom GaN/sapphire interfaces. The rippling signal on the EL spectrum could be caused by the lateral emission light propagating through the Li-Al LDH platelet structure on the dry etched GaN surface around the mesa structure. The irregular distributed Li-Al LDH structure perpendicular to the c-plane ICP-etch GaN surface had the imperfect periodic Li-Al LDH/air structure along the lateral direction similar to a dielectric filter structure. The light-enhanced ratios are defined as the values of the integral EL spectrum of the LDH-LED divided by the ST-LED structure. The EL light enhanced ratios were calculated as the values of 2.83 and 1.21 at the lateral direction (0°) and normal direction (90°), respectively. At the lateral direction of the LED chip, the high EL emission intensity of the LDH-LED structure observed that was caused by a higher light extraction efficiency and a filter effect on the imperfect periodic Li-Al LDH platelet/air structure. The larger divergent angle and the higher light emission intensity for the LDH-LED structures were caused by the high light extraction efficiency on the textured Li-Al LDH film.
The thin (1.0µm-thick) and thick (5.0µm-thick) Li-Al LDH films were grown on the glass substrate for the optical measurement. In Fig. 6(a) , the reflectance values at 450nm detected wavelength were measured at 9.2% and 3.2% for the thin (1.0µm-thick) and thick (3.0µm-thick) Li-Al LDH films, respectively. The reflectance was dropped rapidly at UV region (< 350nm) that was caused by the light absorption property on the Li-Al LDH film. In Fig. 6(b), the transmittances and reflectance of the flat GaN epitaxial layers (2.7µm-thick) with and without the Li-Al LDH film were measured. The absorption of the InGaN active layer and the GaN:Mg layer in the LED structure can be avoid by measuring the GaN/sapphire structure. The typical transmittance and reflectance values at 450nm were measured at 71.5%/20.6% for the GaN/sapphire structure and 73.2%/13.3% for the LDH/GaN/sapphire structure, respectively. By adding the Li-Al LDH film on the GaN surface, the transmittance was slightly increased and the reflectance was decreased. In the transmittance spectra, the oscillation of the interference spectrum of the LDH/GaN/sapphire structure was larger than the GaN/sapphire structure that indicated the Li-Al LDH film had an anti-reflection property. The reflectance of the LDH/GaN/sapphire structure had a 0.35 time decreased compared with the GaN/sapphire structure. The reflectance of the GaN/sapphire was caused by the refractive index different in the air/GaN/sapphire structure that the typical refractive index values were 1, 2.5, 1.7 for air, GaN, and sapphire substrate, respectively. The interference phenomena of the reflectance spectrum indicated that there were flat surfaces at the top p-type GaN:Mg layer and partial flat surface at the bottom GaN/patterned-sapphire interface. The low reflectance of the Li-Al LDH structure without the interference phenomena was caused by forming the platelet structure that acted an anti-reflection film. The higher transmittance and the lower reflectance were observed in the LDH/GaN/sapphire structure by adding the Li-Al LDH film on the GaN/sapphire structure. The Li-Al LDH platelet structure grown on the GaN surface acted as an anti-reflection film that can increase the light extraction efficiency in the InGaN LED structure.
Fig. 6 (a) Reflectance spectra of the thick and thin Li-Al LDH layers on the glass substrates were measured. (b) The transmittance and reflectance spectra of the GaN epitaxial layer with and without Li-Al LDH layer were analyzed.
To analyze the light extraction property of the Li-Al LDH film, the photoluminescence (PL) spectra were analyzed through angle-resolved photoluminescence measurements [31] with a 405nm excitation laser illuminated from the backside sapphire substrate. From the backside laser illumination, the InGaN active layer had been excited by the 405nm laser, however the laser light was not absorbed by the sapphire substrate, the n-type GaN:Si layer, and the p-type GaN:Mg layer. The PL emission spectra were measured by a multi-channel CCD detector with a 550 mm focal length monochromator. The PL spectra was detected at the front-side of the flat LED wafer (without chip process) with and without the Li-Al LDH platelet structure. The wavelength-resolved angular far-field patterns of the PL spectra were measured for the ST-LED and the LDH-LED structures. In Fig. 7 , the Fabry–Pérot (FP) interference line-patterns by varying the detected angles were observed in the ST-LED structure. The FP interference line-patterns were not observed clearly in the LDH-LED structure. This is the results of the PL emission light scattered by the textured Li-Al LDH platelet structure on the LDH-LED structure. The far-field emission pattern measurement can be used to measure the light scattering by the Li-Al LDH platelet structure. In the LDH-LED structure, the high emission intensity indicated the high light extraction efficiency on the Li-Al LDH platelet structure.
Fig. 7 The wavelength-resolved angular far-field patterns of the PL spectra of both LED structures were measured.
4. Conclusion
The InGaN LED structure with a Li-Al LDH platelet structure had higher light output power compared to the ST-LED structure. The insulated and textured Li-Al LDH platelet structure was grown covering the whole LED structure that had potential to replace the conventional SiO2 and Si3N4 passivation layers. High light output power, large divergent angle, and the insulated property were observed in the LDH-LED structure that had high external quantum efficiency for the Nitride-based LED applications.
Acknowledgment
The authors gratefully acknowledge the financial support for this research from the National Science Council of Taiwan under grant Nos. NSC98-2221-E-005-007-MY3, NSC100-2622-E-005-017-CC3, NSC100-3113-E-005-002-CC2, and 99-EC-17-A-07-S1-158.
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