Nanorods on surface of GaN-based thin-film LEDs deposited by post-annealing after photo-assisted chemical etching
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This study investigates the optoelectronic characteristics of gallium nitride (GaN)-based thin-film light-emitting diodes (TF-LEDs) that are formed by a two-step transfer process that involves wet etching and post-annealing. In the two-step transfer process, GaN LEDs were stripped from sapphire substrates by the laser lift-off (LLO) method using a KrF laser and then transferred onto ceramic substrates. Ga-K nanorods were formed on the surface of the GaN-based TF-LEDs following photo-assisted chemical etching and photo-enhanced post-annealing at 100 °C for 1 min. As a result, the light output power of GaN-based TF-LEDs with wet etching and post-annealing was over 72% more than that of LEDs that did not undergo these treatments.
KeywordsPhoto-enhanced post-annealing Laser lift-off KOH Nanorods
photo-assisted chemical etching
scanning electron microscope
Thin-film light-emitting diode
Flip-chip LEDs have been extensively utilized for backlighting in LCD televisions because they effectively increase the intensity of back-light source and reduce the cost of fabrication from LCD [1, 2, 3, 4, 5]. In the past, their use has been limited by their high unit price. The aims of the experimental stage are high reliability and low equipment cost. However, with the evolution of metal electrode materials and constant improvement in relevant processes, flip-chip LED technology has matured into something quite different from traditional LED technology [6, 7, 8, 9, 10]. Recently, thin film LEDs have attracted considerable attention as components in light sources for portable electronic devices because they are flat.
Gallium nitride (GaN)-based LEDs are stripped from sapphire substrates using the laser lift-off (LLO) method because sapphire exhibits poor heat dissipation. The substrates are fabricated by utilizing wafer bonding (or electroplating) and LLO methods [11, 12, 13, 14, 15]. After sapphire substrate has been removed from the GaN epitaxial layer, the exposed GaN surface is roughened by chemical etching using dilute aqueous KOH to improve its light extraction efficiency [16, 17, 18, 19, 20]. In this study, after wet etching and post-annealing at 100 °C for 1 min, nanorods grew on the surface of n-GaN. The hexagonal pyramid structure differed from that generally obtained using the KOH wet etching method. Finally, the optoelectronic characteristics of the GaN-based thin-film light-emitting diodes (TF-LEDs) that were fabricated with various etching times were examined.
The GaN-based LED epitaxial wafer consisted of a 3-μm-thick GaN:Si layer, seven pairs of undoped InGaN/GaN multiple quantum wells, and a 0.25-μm-thick layer of GaN:Mg, in that order, on a (0001)-oriented patterned sapphire substrate with a 30-nm-thick GaN buffer layer that had been grown by metal-organic chemical vapor deposition (MOCVD). LED chips were then fabricated. First, the wafer was etched using the inductively coupled plasma reactive ion etching (ICP-RIE) method until the sapphire substrate formed a mesa array of size 20 mil × 40 mil. Then, the surface of the p-type GaN layer was partially etched using the ICP-RIE method until the n-type GaN layer was exposed. Thereafter, a reflective conductive ITO/Ag (100 nm/500 nm) film layer was formed on the p-type GaN layer. Cr/Au (50 nm/2000 nm) electrodes were formed simultaneously on the ITO/Ag film and the exposed n-type GaN layer on the front of the wafer. The thickness of the wafer was lapped down to approximately 120 μm. GaN-based LEDs were mounted on a silicone plate (OE-6650) that was used as the temporary substrate, and this silicone substrate was heated to 120 °C for 60 min to cure LED chips. Subsequently, a KrF excimer laser with a wavelength of 248 nm and a pulse energy of 100~700 mJ was used to separate the sapphire substrate from the epitaxial LED structure. The laser beam was incident from the polished back surface of the sapphire substrate onto the sapphire/GaN epilayer interface. This process is the so-called laser lift-off (LLO) process. The sapphire/GaN epilayer interface absorbed the energy of the KrF laser beam, producing a high temperature such that the GaN buffer layer between the sapphire substrate and the GaN epilayer dissolved and the GaN epilayer was stripped from the sapphire substrate. The GaN-based thin-film LEDs (TF-LEDs) were thus formed. Finally, the GaN-based TF-LEDs were transferred onto a ceramic substrate, and then, the silicone temporary substrate was removed using acetone.
Results and discussion
Therefore, GaN-based TF-LEDs must undergo a surface roughing process to improve their light output power. The GaN-based TF-LEDs herein were placed in a beaker that contained KOH solution for various periods to produce a rough surface, as presented in Fig. 2a–c. The light output power of the GaN-based TF-LEDs following the PCE process for 30 min at an injection current of 100 mA was 25.9 mW, which was around 19.4% greater than that of flip LEDs.
Photo-enhanced PA at 100 °C was conducted for 1 min after the PCE process. Numerous nanorods are observed on the surface of the n-GaN epilayer, as presented in Fig. 2d–f. These nanorods were crystalline Ga-K, as revealed by EDX spectroscopy. The TF-LED that underwent the PCE process for 30 min and the PA process had the highest light output power, which was 37.4 mW at an injection current of 100 mA, which was around 72% more than that of the GaN-based TF-LEDs that had not undergone PCE and PA following the LLO process. In contrast, the light output power of the TF-LED that underwent the PCE process for 30 min and not underwent the PA process was 25.94 mW at an injection current of 100 mA. Therefore, nanorods on the surface of the LEDs are estimated to yield a light output power that is approximately 44% greater than that achieved with hexagonal pyramids, revealing better light scattering and light extraction efficiency.
In summary, a process for the fabrication of a thin-film LED using laser lift-off and surface roughening processes was presented. GaN-based TF-LEDs were etched using a photo-assisted chemical (PEC) process to remove the residues from the surface. At an injection current of 100 mA, the light output power of the GaN-based flip LEDs that had not undergone the LLO process was approximately 21.7 mW. The LLO process reduced the light output power of the GaN-based TF-LED to 11.5 mW. The TF-LED that underwent the PCE process for 30 min and the PA process had the highest light output power of 37.4 mW at an injection current of 100 mA, which was approximately 72% greater than that of GaN-based TF-LEDs that had not undergone PCE or PA following the LLO process. The Ga-K nanorods on the surfaces of the GaN-based TF-LEDs that were prepared by the PCE process and PA process improved their light output power.
The authors gratefully acknowledge the financial support from the Ministry of Science and Technology of the Republic of China under Contract No. NSC 105-2221-E-027-055.
This work was supported by the financial plan of the Ministry of Science and Technology of the Republic of China.
The authors declare that they have no competing interests.
LCC wrote the paper, designed the experiments, and analyzed the data. WWL and TYL prepared the samples and did all the measurements. All authors read and approved the final manuscript.
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