We report on the efficiency enhancement in GaN-based light-emitting diodes (LEDs) using ZnO micro-walls grown by a hydrothermal method. The formation of ZnO micro-walls at the indium tin oxide (ITO) border on the LED structure is explained by the heterogeneous nucleation effect. The light output power of LEDs with ZnO micro-walls operated at 20 mA was found to increase by approximately 30% compared to conventional LEDs. Moreover, the finding of nearly the same current-voltage characteristics of GaN-based LEDs with and without a ZnO micro-wall shows that the ZnO micro-wall does not influence the electrical properties of the device but only leads to an increase in the light extraction efficiency. From the confocal scanning electroluminescence results, we confirm that ZnO micro-walls enhance the light output power via the photon wave-guiding effect.
© 2012 OSA
The development of high-efficiency gallium nitride (GaN) based light-emitting diodes (LEDs) is considered one of the most important topics in the area of solid-state lighting [1–3]. One reason for this interest is that the internal quantum efficiency (IQE) of GaN LEDs, which affects brightness, typically exceeds 70%, which is much higher than the 10–25% efficiency of conventional light sources such as light bulbs, incandescent electric lamps, and fluorescent lamps. Nevertheless, the light extraction efficiency (LEE) of these LEDs remains low because of the total internal reflection of the light generated in the multiple quantum wells as active layers . Note that the refractive indices in the blue region of p-GaN, indium-tin-oxide (ITO), and air are 2.52, 2.06, and 1, respectively, and according to Snell’s Law , the critical angles for the p-GaN/ITO and ITO/air interfaces are approximately 54.83° and 29.04°. Photons emitted beyond these critical angles can be reflected from the interface, reabsorbed, and internally confined. Note, however, that the critical angle at the ITO/air interface is much smaller than that at the p-GaN/ITO interface, which implies that the ITO/air interface governs the LEE. Thus, previous LEE enhancements have been achieved by incorporating geometric structures inside the LEDs, which reduces the internal reflection and increases the light extraction at the interfaces. Extensive research efforts have also been made to improve the LEE using GaN-based LEDs, with techniques such as surface roughening [5,6], corrugated Bragg gratings [7,8], GaN epilayer growth on a patterned sapphire substrate , integration of two-dimensional (2D) photonic crystal (PC) structures , and utilization of surface plasmon resonance . However, in manipulations such as dry etching, it was found that the plasma process damages the surfaces during the texturing process, which subsequently induces poor electrical properties; flip-chip bonding and laser lift-off technologies have also significantly complicated GaN LED fabrication .
Zinc oxide (ZnO) has been considered as a promising candidate material for a wide variety of applications, such as surface-acoustic-wave devices , transparent conduction electrodes , room-temperature UV lasing , and gas sensors , primarily because of its unique properties of having a wide bandgap () with a large exciton binding energy (60 meV) and a large piezoelectric constant. Since ITO and ZnO materials have similar refractive indices in the blue region ( and ), the protruded ZnO micro/nano-structures on ITO induce the decrease of Fresnel reflection between LED and air. Thus, ZnO micro/nano-structures have been employed in LEDs to enhance the light output efficiency [17,18]. In this area, a number of methods have been used to produce ZnO nano/micro-structures, such as evaporation and condensation techniques , simple gas reaction , chemical vapor deposition , and solution-based methods . Among them, the hydrothermal method is one of the most attractive candidates for industrial use because industrial processes generally require rapid, simple, and low-cost techniques, which are the main advantages of the hydrothermal method.
In this work, we show that the enhancement of light output power in GaN-based blue LEDs can indeed be achieved by ZnO micro-walls grown on the ITO contact layer using a hydrothermal method. Field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) measurements are performed to characterize the structural properties of the ZnO micro-wall on the ITO contact layer. The enhanced light extraction efficiency, which is attributed to the photon wave-guiding effect and multiple scattering by ZnO micro-walls, is well explained by confocal scanning electroluminescence microscopy (CSEM) and far-field radiation patterns.
The GaN-based LED structures on a c-oriented sapphire substrate were grown by metalorganic chemical vapor deposition (MOCVD). The LED templates consisted of a Si-doped n-type GaN layer, five pairs of InGaN/GaN multi-quantum wells (MQWs), and a Mg-doped p-type GaN layers, respectively. During the MOCVD growth, trimethylgallium, trimethylindium, and NH3 were used as precursors for the Ga, In, and N, respectively. Hydrogen was used as the carrier gas, except for the growth of the InGaN/GaN MQWs, where nitrogen gas was used. After the LED wafers were ultrasonically cleaned, a 250-nm-thick ITO layer was deposited onto the p-GaN surface of the LED wafer by electron beam evaporator. The as-deposited ITO layers were opaque and dark black in color. For the crystallization of the ITO grains, these ITO films were annealed in a N2 and O2 mixed ambient at 600 °C for 60s in a rapid thermal annealing chamber. During this process, the ITO films were converted to partially transparent films. The thickness of the Cr/Au n- and p-electrodes was 50 nm/250 nm, respectively. Then, through a conventional photolithography technique, the entire sample was covered with a photoresist, except for the ITO contact layer. Subsequently, the ZnO micro-walls were grown on the ITO contact layer via a simple non-catalytic hydrothermal method at low temperature. The experimental procedure for the ZnO micro-wall growth was designed as follows: a measured amount of zinc acetate di-hydrate [Zn(O2CCH3)2(H2O)2] was dissolved into deionized water to form a 0.05 M solution at room temperature. NH4OH was then added to the solution to create an alkaline environment (pH ~9). All hydrothermal processes were carried out for 60 min at 150 °C in autoclave. The heating rate and pressure in reactor are 3 °C/s and 4 atm, respectively. Then, the photoresist was removed by acetone solution. Figure 1 shows (a) the schematic of the procedure for fabricating a ZnO micro-wall on a GaN-based blue LED and (b) the final structure of the GaN-based LED with ZnO micro-walls. The surface morphologies were examined by FE-SEM at an acceleration voltage of 15 kV (Hitachi S-4300SE). Further, the crystal structure of the ZnO micro-wall grown on the ITO top contact layer was examined via cross-sectional TEM using a JEOL 3010 operated at 200 kV. The current-voltage (I-V) and light output-current (L-I) measurements were carried out using a probe station system. For the investigation of the spatially resolved luminescence properties of the fabricated LED, a novel fiber-optic-based confocal scanning photoluminescence/ electroluminescence microscopy was employed. In the CSEM system, a static current was applied to the sample during the scanning using a Keithley 2400s SourceMeter. Light collected from the focal planes was delivered to a monochromator through a multimode optical fiber and detected by a cooled charge-coupled device (CCD) detector. In this configuration, the optical fiber plays the role of a pinhole.
3. Results and discussion
The surface morphology of the grown ZnO micro-walls was investigated using FE-SEM. As shown in Fig. 2(a) , the ZnO micro-walls are formed at the ITO border. Figure 2(b) and 2(c) show the magnified SEM images of the ZnO micro-wall grown on the ITO. The height and width of the ZnO micro-walls on the ITO are approximately 6 and 2 μm, respectively. To explain the formation of the ZnO micro-walls at the ITO border, we consider heterogeneous nucleation effect due to the higher surface area in the corners between the ITO and the photoresist as shown in Fig. 2(d) . Since we use a negative photoresist designed for liftoff, which typically has an undercut region at an acute angle. This would allow that ZnO can be nucleated in the narrow region between photoresist and ITO while minimizing the additional Gibbs free energy of the water/ZnO interface. Consequently, the nuclei of the ZnO micro-wall are formed at the border of the ITO contact layer.
To obtain a structural characterization of the ZnO micro-walls, TEM measurements were performed. Figure 3(a) shows the low-resolution TEM (LR-TEM) image, the selected-area electron diffraction (SAED) pattern, and the high-resolution TEM (HR-TEM) image of the ZnO micro-wall. In the cross-sectional LR-TEM image, the ZnO micro-wall is successfully grown on the LED full structure with five pairs of InGaN/GaN MQWs. Moreover, both the SAED pattern and the HR-TEM picture suggest that the ZnO micro-wall has a hexagonal wurtzite structure with a high crystal quality in the observed range. The HR-TEM picture also shows that the lattice spacing of around 0.52 nm along the longitudinal axis direction corresponds to the d-spacing of ZnO (001) crystal planes.
Micro-Raman spectroscopy was then used to observe the Raman scattering spectrum from the ZnO micro-wall region on the ITO top contact layer. As shown in Fig. 3(b), the observed Raman spectrum shows the E2high optical phonon mode at 439 cm−1, excited by a 633-nm HeNe laser, suggesting that the ZnO micro-walls exhibit a hexagonal wurtzite structure. As the TEM and micro-Raman scattering results show, ZnO micro-walls with a hexagonal wurtzite structure could be grown on the ITO top contact layer using a simple non-catalytic hydrothermal method. For the optical characterization of the hydrothermally grown ZnO micro-wall, a confocal scanning photoluminescence (PL) excited by a 355-nm diode-pumped solid-state laser was conducted at room temperature. Figure 3(c) and 3(d) show the 380-nm wavelength filtered confocal scanning PL image and local PL spectrum of the ZnO micro-wall, respectively. As shown in Fig. 3(c), because the crystal quality is different in the ZnO micro-walls, the filtered confocal scanning PL image shows a different distribution of PL intensity. In Fig. 3(d), the strong peak of the near band edge emission at 380 nm and a weak yellow band having a broad feature in the 500–700 nm range are observed. A PL spectrum with a strong band edge peak compared to defects related to the yellow band peak indicates that the ZnO micro-walls have a high optical and crystalline quality.
To confirm the electric properties of the GaN-based LEDs with ZnO micro-walls, we then measured the I-V characteristics (Fig. 4(a) ). The I-V slope of the GaN-based LEDs with ZnO micro-walls at an injection current of 20 mA shows no significant difference compared to that of a GaN-based LED without ZnO micro-walls. Note that there are no discrepancies in the electrical characteristics because the ZnO micro-walls were grown on top of an ITO surface with no influence on current spreading. Figure 4(b) shows the light output power as a function of the injection current of the GaN-based LEDs with and without ZnO micro-walls. The light output power of each LED was measured from the top side of LEDs using a Si photodiode connected to an optical power meter . The distance between photodiode and LEDs was 8 cm. Compared to the GaN-based LED without ZnO micro-walls, the light output power of the GaN-based LED with ZnO micro-walls increased by ~30% when operated at 20 mA. This enhanced light output power can be attributed to the surface roughness and wave-guiding of photons via the ZnO micro-walls. To investigate the luminescence properties of the GaN-based LEDs with ZnO micro-walls, CSEM was used. Figure 4(c) shows the CSEM image of a GaN-based LED with ZnO micro-walls operated at 1 mA; in the figure, the emitted light is enhanced in the ZnO micro-wall region. Figure 4(d) shows the local EL spectra measured at two different points: a ZnO micro-wall region and an ITO top surface. The EL intensity at point A (ZnO micro-wall) is over three times larger than that at point B (ITO). We note that the EL intensity at point B is similar to that of conventional LEDs. As such, from the EL confocal scanning results, it can be suggested that ZnO micro-walls enhance the light output power from GaN-based LEDs due to the photon wave-guiding effect. Thus, we expect that higher density of ZnO micro-walls can induce the more enhancement of light output power of GaN-based LEDs.
In order to confirm this hypothesis, the far-field radiation patterns of the GaN-based LED with ZnO micro-walls were measured (Fig. 5(a) ) for a chip that was not encapsulated into epoxy . The light output pattern of the conventional GaN-based LED is also depicted for comparison. As a result, it was found that EL intensities obtained from the GaN-based LED with ZnO micro-walls were larger than those from the conventional GaN-based LED in the diagonal directions. In other words, the ZnO micro-walls affected the light propagating from the emitting surface, which resulted in a change in the observed radiation pattern. Therefore, the improvement of the emitted light extraction efficiency is considered to be a consequence of the photon wave-guiding and scattering by the ZnO micro-walls. Figure 5(b) presents a schematic of the photon escape path in a GaN-based LED with ZnO micro-walls. As previously mentioned, because photons have a greater difficulty escaping from the ITO into air than from the ITO to ZnO, we suggest that the photon wave-guiding effect and multiple scattering in ZnO micro-walls on the ITO leads to an increase in the light extracted from the ITO to air transition, as shown in Fig. 5(b).
This study proposed a simple method for enhancing the light output power of GaN-based blue LEDs by incorporating ZnO micro-walls that were grown on the ITO contact layer via a hydrothermal method. From the cross-sectional LR-TEM and HR-TEM images, the ZnO micro-walls are successfully grown on the transparent ITO contact layer. Further, through the current-voltage characteristic curve, we confirmed that the ZnO micro-walls do not influence the electrical properties of GaN-based LEDs. Finally, compared to conventional GaN-based LEDs, the light output power of GaN-based LEDs with ZnO micro-walls was enhanced by ~30%, attributed to the photon wave-guiding effect and multiple scattering through the ZnO micro-walls, which allows photons to easily escape from the devices.
This work was financially supported by Nano R&D Program (2007-02939) (2011-0019173) and the Basic Science Research Program (BRL) (2010-0019694) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology.
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