Sapphire interlayer aids GaN-on-Si LED output
Silicon substrates are a desirable platform for LED production due to their low cost. However, successful growth of LEDs on silicon requires overcoming issues related to the thermal and lattice mismatch between the two materials.
Ferguson’s team has addressed this by adding a 5–20 nm thick Al2O3 interlayer by atomic layer deposition, before growing nitride epilayers in an Emcore D-125 MOCVD system.
The silicon-based devices emit at a longer wavelength than their sapphire counterparts, due to tensile strain and higher indium composition in the active layer. Two emission peaks are produced by this LED and the researchers believe that this may be caused by non-uniformity in the composition of the InGaN in the quantum wells.
The researchers fabricated their LEDs on 2 inch silicon substrates that were cut into halves or quarters. “The size limitation stems from our oxide deposition system, which until recently could only accommodate these smaller wafer sizes,” said lead author William Fenwick.
Reducing LED manufacturing costs will require the growth of devices on far larger silicon substrates, where issues such as wafer bow and thin-film uniformity come into play. “However, our initial attempts to scale the process to larger substrates have shown promise,” said Fenwick, “and we are hopeful that we can demonstrate a process on larger silicon substrates in the near future.”
He believes that the particular architecture of the team’s LEDs will allow novel approaches to device processing. “The use of the oxide layer will allow for wet etch removal of the substrate, opening the door to new approaches to light extraction and thermal management.”
