In recent years, 1D nanorods, nanocolumns, and nanowires have been synthesized, while their electrical and material properties have been studied. Among various semiconductor materials employed for nanostructures, gallium nitride (GaN) is one of the most popular choices for solid-state lighting. The sidewall areas of the nanorods that form LED arrays (see Figure 1) give them an increased effective surface. This leads to better external extraction efficiencies. The nanorods may also act as waveguides in which optical modes propagate vertically, which affects their radiation profiles. In addition, the light-emitting efficiency of GaN-based LEDs can be improved by decreasing their defect density. This can be achieved using nanorods with diameters of less than 100nm.
Figure 1. Scanning-electron-microscopy image of the nanorod array.
There are two approaches to fabricating nanorod-based LED arrays. These include the bottom-up method of growing nanostructures directly on the samples and the alternative, top-down technique of growing planar epi-structures that are subsequently subject to an etching process. Fabrication of nanostructured devices using the latter approach presents several technical and economic challenges. The commercial viability of nanostructures depends on the ability to obtain an affordable nanopattern definition covering large chip sizes. In addition, the nanorod-sidewall damage needs to be repaired. Otherwise, this will lead to sidewall leakage currents and nonradiative recombination, which will seriously degrade device performance. Moreover, since each nanorod forms a vertically stacked p-i-n multilayer structure (see Figure 2)—where the intrinsic (i) part is usually composed of multiple quantum wells—a suitable planarization layer is required to prevent the p-type contact metal from shorting the underlying n-type semiconductor.
Figure 2. Schematic diagram of our gallium nitride (GaN)-based nanorod LED array. The silicon dioxide (SiO2) passivation layer is shown in yellow, and the p/n-type gold (Au)/titanium (Ti) contact electrodes (P/N-pads) are represented by brown rectangles. The white horizontal lines in the blue GaN-based nanorods indicate a structure of multiple quantum wells (MQWs). The thin, p-type metallic (P-thin metal) layer coated on top of the p-type tips of the GaN nanorods is indicated in orange.
Our group has developed techniques to address these issues. We established nanosphere lithography, a technique that makes surface nanopatterns by spin coating a monolayer of nanospheres on the device surface.1 Depending on their type, nanosphere diameters range from nano- to micrometers, and nanorods are formed on these GaN-based epi-structures using silica nanoparticles as the hard etching mask. Figure 1 shows a scanning-electron-microscopy image of the resulting nanorod arrays. We deposited a silicon dioxide (SiO2) layer—grown by plasma-enhanced chemical-vapor deposition—over the nanorods to passivate the sidewall defects created during nanorod formation. Finally, we exposed the p-type GaN nanorod tips using the chemical-mechanical polishing process.2
In the presence of the SiO2 passivation layer, the nanorod LED array shows an ideality factor of 7.35 and a small reverse-leakage current of 4.77nA at −5V. This indicates that the sidewall damage has been successfully repaired. Moreover, with an injection-current density of 32A/cm2, the diode exhibits an optical output intensity of 6807mW/cm2. Finally, the array's far-field radiation pattern has a Lambertian profile. This indicates that the LED device consist mainly of point light sources that emit the same quality of light in all directions (see Figure 3).
Figure 3. Light emission from nanorod LED devices.
We are currently working on improving the metal contacts on the nanorods' p-type GaN tips. If the metal-evaporation process is not properly adjusted, the metal is deposited on the semiconductor as nanoscale-sized grains. This results in a poor metal-nanorod contact that degrades the electrical and optical behavior of the LEDs. We are addressing this issue from the perspective of device physics and its applications.
Jian Jang Huang, Liang-Yi Chen, Ying-Yuan Huang, Chun-Hsiang Chang
National Taiwan University
Jian Jang Huang is a professor at the Graduate Institute of Photonics and Optoelectronics. He received his PhD in electrical engineering from the University of Illinois at Urbana-Champaign in 2002.
1. M.-Y. Hsieh, C.-Y. Wang, L.-Y. Chen, T.-P. Lin, M.-Y. Ke, Y.-W. Cheng, Y.-C. Yu, C.-P. Chen, D.-M. Yeh, C.-F. Lu, C.-F. Huang, C. C. Yang, J. J. Huang, Improvement of external extraction efficiency in GaN-based LEDs by SiO2 nanosphere lithography, IEEE Electron Dev. Lett. 29
, pp. 658-660, 2008. doi:10.1109/LED.2008.2000918
2. L.-Y. Chen, Y.-Y. Huang, C.-H. Chang, Y.-H. Sun, Y.-W. Cheng, M.-Y. Ke, C.-P. Chen, J. J. Huang, High performance InGaN/GaN nanorod light emitting diode arrays fabricated by nanosphere lithography and chemical mechanical polishing processes, Opt. Express 18
, no. 8, pp. 7664-7669, 2010. doi:10.1364/OE.18.007664