Research Review: Chemical Etching Improves Thin-film LEDs
Photochemical etching with potassium hydroxide creates LEDs with reduced strain and higher efficiencies.
TAIWANESE RESEARCHERS claim to have developed a superior alternative to the widely used laser lift-off process employed for the manufacture of thinfilm, high-brightness LEDs.
“Conventional laser lift-off processes on InGaN LED structures increase dislocation density," explains corresponding author Chia-Feng Lin from National Chung Hsing University. According to him, this results in an increase in reverse bias leakage current and destruction of GaN by local heating during laser treatment. The approach of Lin and his co-workers addresses all these issues and begins with the insertion of a sacrificial silicon-doped superlattice of alternating, 3 nm-thick layers of InGaN and GaN. This superlattice is removed by a photochemical etch in potassium hydroxide, before the buffer and substrate are separated from all of the rest of the device by a mechanical lift-off process employing adhesive tape.
A 50 mW, 405 nm laser diode provides the light source for the 10-minute photochemical etching step. “The lateral wet-etching rate of the InGaN/GaN sacrifical layer is higher than that for a heavily silicon-doped GaN layer," says Lin, who adds that a +1 V bias was applied to the structure during etching to prevent damage to the top InGaN layer.
One of the great strengths of the chemical mechanical lift-off process is that, in addition to yielding thin-film devices, it produces high-quality GaNon- sapphire templates that can be used for subsequent device growth. Lin estimates that using these substrates can increase MOCVD reactor throughput from three to four runs per day.
The Taiwanese team is not the only one to have developed a chemical lift-off process – other research groups have had success with CrN, ZnO and tungsten. However, Lin argues that the approach he and his co-workers take is more attractive, because it uses materials that are associated with the growth of nitride LEDs.
A photochemical lift-off process can create a thin-film LED with reduced strain in the device.
Another strength of this particular approach is that it leads to a partial release of the strain in the device. Reducing the strain, which stems from the lattice mismatch between the nitride layers and the sapphire substrate, leads to a blue shift in emission from 526 nm to 511 nm. This shift, which results from a reduction in the distortion of the conduction band and valence band profiles, also promises to increase the internal quantum efficiency of the LED. What’s more, the etching process leads to an increase in extraction efficiency, because it creates cone-shaped structures that reduce internal reflection within the chip.
Lin told Compound Semiconductor that the team has compared the performance of conventional LEDs that have not been subjected to any laser-based processing to those made with its chemical lift-off process. At a drive current of 20 mA, the latter are brighter and have a shorter peak emission wavelength. “The forward voltage and leakage current of both structures are almost the same," adds Lin.
One of the team’s next goals is to apply its process to high-power LEDs on electroplated copper substrates. In addition, the researchers want to develop InGaN-LED/Ga2O3/GaN templates. “The un doped GaN epitaxial layer can be oxidized as a Ga2O3 layer through the photoelectrochemical oxidation process," explains Lin. “And after the epitaxial re-growth process in the MOCVD system, the Ga2O3 layer can be etched in a dilute hydrochloric acid solution as a sacrificial layer for the chemical lift-off process."
M.-S. Lin et. al. Appl. Phys. Express 4 062101 (2011)