Effects of phosphoric acid on the surface morphology and reflectance of AlN grown by MBE under Al-rich conditions

• Phosphoric acid was used to remove excess Al from the AlN surface.
• Phosphoric acid heated to 70 °C created hexagonal etch pits in the AlN films.
• Excess surface Al reduced the specular component of reflectance.
• Excess surface Al introduced an error of up to 7.6% in the measured AlN thickness.

Aluminium nitride (AlN) films grown by molecular beam epitaxy (MBE) were exposed to phosphoric acid (H3PO4) heated to 70 °C for 10 min. The H3PO4 treatment removed excess aluminium (Al) from the surface of AlN grown under Al-rich conditions. A side effect of the H3PO4 process was the formation of hexagonal etch pits up to 180 nm in size. Reflectance measurements were performed before and after the removal of excess Al from the film surface. Excess surface Al in the form of droplets was found to strongly influence the reflectance of the AlN films. Consequently the Al surface droplets (AlSD) introduced an error into film thickness values derived from Lorentz oscillator model fitting of the specular reflectance. The film thickness measurement error due to excess surface Al was quantified as a function of the AlSD surface coverage percentage. The effect of AlSD on reflectance-based film thickness measurements must be taken into account when using AlN grown in the Al droplet regime to characterize the nitrogen (N) growth rate.