Alternative process could cut GaN template costs
Researchers from the à‰cole Poytechnique Fédérale de Lausanne (EPFL), Switzerland, have grown high-quality GaN-on-sapphire templates by HVPE that have a GaN thickness of just 8 µm (Appl. Phys. Lett. 88, 241914). This compares with values of 20 µm to over 300 µm for GaN films for other high-quality GaN-on-sapphire templates.
The new process could cut the cost of GaN templates used to produce lasers and transistors due to a reduction in GaN material deposited and the removal of an MOCVD growth-step that is typically used prior to HVPE deposition.
The Swiss team fabricates its templates with a two-stage process that involves an ex-situ surface preparation technique, followed by HVPE growth of a GaN layer in a commercially-available Aixtron reactor.
The templates are produced by cleaning sapphire in hydrogen and then exposing it to ammonia gas at 1050°C for 4 minutes. A nucleation layer is then deposited at 600°C, before the reactor's temperature is ramped to 1045 °C for the remaining GaN growth.
According to the team, the GaN material quality and polarity is strongly influenced by sapphire substrate preparation.
Layers with gallium-polarity are formed when the sapphire is cooled to room-temperature and exposed to ambient air after nitridation, while either mixed or nitrogen-polarity films are produced if a continuous process is used from cleaning to high-temperature GaN growth.
Researcher Raphaël Butte told Compound Semiconductor.net that the main advantage of gallium-polarity layers is that they can be grown with a very smooth morphology, which allows subsequent devices to be grown without surface processing.
Atomic force microscopy (AFM) measurements across 5 µm x 5 µm areas of this material revealed a surface roughness of 0.54 nm, and optical microscopy images in transmission mode revealed no cracks in the wafer. Hydrochloric acid etching into the templates, followed by AFM measurements, determined that the threading dislocation densities are as low as 1 x 10 8 cm-2.
"One disadvantage of the nitrogen-polarity layers, besides surface quality issues, is the higher impurity incorporation that makes doping control difficult," added Butte.
The researchers have also grown an LED structure by MOCVD on top of their 8 µm-thick HVPE templates. Butte says that this structure has a comparable quality to LEDs grown on MOCVD templates.
The EPFL group currently has no plans to commercialize its growth technology, but several GaN template manufacturers are showing interest in the team's work.
