Focused ion beam micromilling of GaN and related substrate materials (sapphire, SiC, and Si)

Micromilling of GaN films has been obtained using a ${\mathrm{Ga}}^{\mathrm{+}}$ focused ion beam (FIB). The GaN micromilling has been investigated over a range of energies (15–70 keV), incident angles $\text{(0°–30°),}$ and number of scans (10–50). At normal incidence, increasing the ${\mathrm{Ga}}^{\mathrm{+}}$ energy up to 50 keV increases the milling rate, while higher energies produce the same (or a slightly decreased) milling rate. Increasing the angle of incidence increases the milling rate at all energies. The highest GaN milling rate of $\text{0.6\hspace{0.05em}μ}{\mathrm{m}}^{\mathrm{3}}\mathrm{/nA s}$ (corresponding to an average yield of 6.6 atoms/ion) has been obtained at 50 keV, $\text{30°}$ incidence, and 50 scans. The milling rate of current substrate materials (sapphire, Si and SiC) for GaN thin film growth is shown to be 2–5 times lower. The sputtering yield is found to vary inversely with the strength of the chemical bond in the materials investigated. Distributed Bragg reflection air/GaN gratings for short cavity lasers were fabricated to show the capability of FIB micromilling to produce optoelectronic devices based on GaN.