GaN pitch-variable grating fabricated on Si substrate

A tunable grating is fabricated by micromachining a GaN crystal layer grown on Si substrate. The tunable grating consists of grating lines, electro-static comb-drive actuator and connection springs. The grating consists of 85nm wide, 12μm long 24 grating lines with 674nm period. The crystallization stress of an HfO₂ layer deposited on the GaN crystal is used to compensate the residual stress of the GaN crystal grown on Si substrate. The freestanding GaN structure consisting of the grating and the actuator is fabricated by etching the Si substrate with XeF₂ gas. Applying the voltage of 140V, the grating is expanded by 600nm corresponding to the period change of 3.7%.