A GaN Electromechanical Tunable Grating on Si Substrate

An electromechanical tunable grating was fabricated by micromachining a GaN crystal grown on a Si substrate. The tunable grating consisted of an expandable freestanding grating and an electrostatic comb-drive actuator. In order to compensate the residual stress of the GaN crystal grown on the Si substrate, crystallization stress of an HfO₂ layer deposited on the GaN crystal was utilized. The freestanding GaN structure of the grating was fabricated by etching the Si substrate with XeF₂ gas. The freestanding grating consisted of twenty-four 12-μm-long and 85-nm-wide grating lines with a 720-nm period. By applying a voltage of 140 V, the grating was expanded by 0.6 μm to change the period by 3.5%. The proposed tunable grating can be used for monolithic integration of a GaN light source and a micro-spectrometer.