Heavily Doped Silicon Enables Chemical Lift Off
Etching a sacrificial GaN layer with oxalic acid boosts LED light extraction
Fabrication of the LED involves: laser scribing; lateral wet etching, which leads to the formation of nanoporous GaN; and self-separation to yield chips.
Removing the sapphire substrate from a GaN LED is a tried-and-tested route to increasing light extraction efficiency. This process normally involves a laser lift-off step to separate the substrate from the epilayer, but this can damage the material, so several research groups are now pursuing chemical lift-off techniques. These are being refined, and are now in their simplest form yet, thanks to the recent work of researchers in China.
This team of scientists from National Chung Hsing University and Feng Chia University has pioneered a chemical lift-off technique that involves etching a thin, heavily silicon-doped layer of GaN in oxalic acid solution. Other groups, in comparison, have used more radical sacrificial layers made from the likes of CrN, ZnO and AlN.
Team member Chia-Feng Lin from Feng Chia University says that the undoped-GaN-on-sapphire templates liberated by the etching process can be used to regrow the InGaN LED and silicon-doped GaN structures in an MOCVD system, reducing the time and cost for depositing a full LED structure.
To separate the LED from the substrate, the researchers begin by growing a fairly conventional device on top of a heavily doped, 0.1 μm-thick sacrificial GaN layer. Specifically, the MOCVD growth that is carried out on 2-inch sapphire to produce the epiwafer involves the deposition of: a 30 nm-thick buffer; a 1.4 μm-thick, unintentionally doped GaN layer; a 0.1 μm-thick GaN layer with a silicon doping level of 2 x 1019 cm-3; a 1.0 μm-thick undoped GaN layer; a 3 μm-thick n-doped GaN layer; a multiple quantum well region with nine In0.2Ga0.8N quantum wells; and a 30 nm-thick p-GaN cap.
Dry etching with a plasma to a depth of 2.2 μm defines the mesa regions, before a 250 nm film of indium tin-oxide is deposited, to provide a transparent conductive layer. The pairing of chromium and gold provides the metallic contacts for the LEDs, which are isolated from one another – but still attached to the substrate – using a laser-scribing process.
To remove the sacrificial layer, the samples are immersed in 0.5 M oxalic acid solution for 40 minutes, while they illuminated by a 400 W mercury lamp and subjected to a 15 V DC bias.
“The oxalic acid attacks the silicon-doped GaN layer to form a nanoporous structure," says Lin. According to him, the higher the silicon-doping concentration, the faster the GaN etching.
Lateral etching in the sacrificial layer takes place at a rate of 315 μm/hr to yield 240 μm by 180 μm chips featuring a 30 μm-wide nanoporous structure in the thicker n-type silicon layer, which has a lower doping level than the sacrificial layer.
This nanoporous, n-type silicon layer is not an impediment to high performance, however. “By forming nanoporous GaN close to the mesa region, high light scattering occurs in the InGaN LED, leading to high light extraction efficiency in the normal direction of the LED chips," says Lin.
The team have compared the performance of their LED to a control sample that had not been subjected to a chemical lift-off process. Electroluminescence spectra for both samples featured Fabry-Pérot interference fringes, indicating that etching had not had a major impact of the flatness of the bottom surface of the LED.
Driven at 20 mA, the sapphire-free LED produced an output power 2.1 times higher than that of the control. The team attribute the superior brightness to an increase in the light scattering process at the bottom lift-off GaN surface and the GaN nanoporous structure around the LED chip.
Lin believes that the team’s process is compatible with high-volume manufacturing. He argues that the modification to the epitaxial structure is minimal, and points out that it is compatible with a vertical LED process, because a patterned metal structure can be electroplated onto the chip.
The researchers are planning to fabricate this type of device, and also increase the lateral wet etching rate by increasing the silicon doping concentration. The latter will impact the crystalline quality of the LED structure above, but this can be addressed by turning to an alternative heterostructure, according to Lin.
K.-C. Wu App. Phys. Express 6 086501 (2013)