Improved light extraction of InGaN/GaN blue LEDs by GaOOH NRAs using a thin ATO seed layer
© Lee et al.; licensee Springer. 2012
Received: 18 January 2012
Accepted: 11 July 2012
Published: 16 August 2012
We investigated the effect of gallium oxide hydroxide (GaOOH) nanorod arrays (NRAs) on the light extraction of InGaN/GaN multiple quantum well blue light-emitting diodes (LEDs). GaOOH NRAs were prepared on an indium tin oxide electrode (ITO) layer of LEDs by electrochemical deposition method. The GaOOH NRAs with preferred orientations were grown on the ITO surface by sputtering a thin antimony-doped tin oxide seed layer, which enhances heterogeneous reactions. Surface density and coverage were also efficiently controlled by the different growth voltages. For LEDs with GaOOH NRAs grown at −2 V, the light output power was increased by 22% without suffering from any serious electrical degradation and wavelength shift as compared with conventional LEDs.
There has been growing interest in gallium nitride (GaN)-based light-emitting diodes (LEDs) as the most promising light source for automotive lightings, indoor/outdoor lighting products, backlight units for liquid crystal displays, and solid-state lightings. However, the light generated in the active region of LEDs undergoes Fresnel reflection loss and a narrow escape cone due to high refractive index contrast at the GaN/air or indium tin oxide (ITO)/air interface when it is emitted out from the device. To solve these problems, various techniques including surface texturing/roughening and incorporation of photonic crystal structures, subwavelength structures, ZnO nanorods have been developed for GaN-based LEDs[2–7]. However, some methods require complex process steps and cause plasma-induced damage[2, 7].
Gallium oxide hydroxide (GaOOH) is an important starting material for the synthesis of gallium oxide (Ga2O3) and gallium nitride (GaN) by simple heat treatments[8, 9]. In recent years, extensive attention has been paid to the shape- and size-controlled synthesis of GaOOH nanostructures for device applications in optoelectronics and photonics[10–13]. GaOOH exhibits a high transparency in the ultraviolet and visible wavelength ranges, and its refractive index is lower than that of Ga2O3 of 1.8 to 1.9 because the hydroxide materials are less dense[11, 12]. Thus, controllable growth in GaOOH nanorod arrays (NRAs) can efficiently provide a graded refractive index profile for GaN-based LEDs. GaOOH or Ga2O3 would also act as an efficient surface passivation layer. Several growth methods, such as sol–gel process, sonochemical reaction, and hydrothermal synthesis, have been employed to synthesize the rodlike GaOOH structures[8, 14, 15]. However, the rod-shaped GaOOH in powder form, made by homogeneous nucleation from chemical solutions, was usually prepared. For more efficient device applications, it is essential to grow vertically aligned NRAs on substrates. It was found that the vertically aligned GaOOH NRAs can be grown on rigid substrates by chemical solution deposition (CSD) using thin metal oxide (SnO2 and MgO) seed layer. These materials indicate the low lattice mismatch with GaOOH for a-axis (aGaOOH = 4.58 Å, aSnO2 = 4.738 Å, aMgO = 4.212 Å). However, the CSD process takes a long growth time, and it is not easy to control the growth process. Meanwhile, electrochemical deposition (ED) process is a simple, cost-effective, low-temperature, and fast growth method. In this work, we fabricated GaN-based blue LEDs with electrochemically grown GaOOH NRAs to enhance the light extraction efficiency. The GaOOH NRAs were successfully synthesized on the ITO surface of LEDs by the use of a thin antimony (Sb)-doped SnO2 (ATO) seed layer, which is one of the transparent conducting oxide materials and indicates a refractive index of 1.8 to 2 in the visible wavelength range. Compared with conventional LEDs, the optical and electrical characteristics of LEDs with GaOOH NRAs were investigated.
GaN-based blue LED structures were grown on sapphire (Al2O3) substrate by metal organic chemical vapor deposition. The epilayer structure consisted of a 3.5-μm-thick undoped GaN layer, a 4-μm-thick Si-doped n-type GaN layer, five pairs of InGaN/GaN multiple quantum wells (MQWs), and a 210-nm-thick Mg-doped p-type GaN layer. The LEDs were fabricated with a mesa size of 400 × 450 μm2 in a lateral device using conventional fabrication processes. After mesa etching process, a 200-nm-thick ITO layer was deposited on the p-type GaN layer using an e-beam evaporator, and it was annealed at 600°C for 60 s in air ambient. The Cr/Au (10/500 nm) layers were used as the p- and n-metal electrodes. In the fabricated LEDs, GaOOH NRAs were synthesized on the ITO surface by ED method.
Results and discussion
At high temperature, the OH− ion is sufficiently generated in water, and GaOOH can be easily synthesized by homogeneous nucleation. In the ED method, similarly, the growth processes were carried out in aqueous gallium nitrate solution at 80°C. Thus, the white-colored GaOOH precipitates were naturally formed in the chemical solution during the reaction time. For ED process, the growth of GaOOH rods is mainly based on the generation of OH− ions at the cathode electrode. When the certain voltage is applied, the OH− ions are produced near the cathode electrode by electrochemical reduction of precursors such as NO3− and O2 in aqueous gallium nitrate solution. The Ga3+ ions then react with the OH− ions, and GaOOH rods are formed. First, the influence of the seed layer on the growth property of GaOOH rods was investigated. In the case of the growth process without the seed layer, only a few GaOOH rods were observed on the substrate, and the rods were located parallel to the surface, as can be seen in Figure 2a. This means that the GaOOH rods were synthesized through homogeneous nucleation without the influence of a substrate. The synthesized GaOOH rods indicated the rhombus-shaped structures with an average rod length of approximately 1.9 μm and lateral dimensions of 200 to 600 nm. Under a cathodic voltage, the heterogeneous nucleation was enhanced by depositing a thin ATO seed layer on Si substrate. As shown in Figure 2b, the GaOOH NRAs were vertically and uniformly synthesized on the ATO/Si surface. The morphology of GaOOH NRAs indicated a rhombus-shaped structure. The GaOOH NRAs had an average nanorod length of approximately 520 nm, and the lateral dimension was roughly varied from 50 to 220 nm.
GaN-based LEDs with GaOOH NRAs on ITO electrode were fabricated by the ED method. The oriented growth of GaOOH NRAs on the ITO electrode was successfully obtained by the use of a thin sputtered ATO seed layer. The surface coverage and density of GaOOH NRAs were controlled by the cathodic voltage. In comparison with the conventional LEDs, the enhancement of 22% in light output power was achieved by the incorporation of GaOOH NRAs without any radiative defects. The electrical properties also were not distinctly affected by the growth process in GaOOH NRAs.
This research was supported partially by a grant from the Ministry of Knowledge Economy of the Republic of Korea, under the project no. 10039151 and by Basic Science Research Program through the NRF funded by the MEST (no. 2011–0026393).
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