Two-step temperature ramping technique in MOCVD GaN films with high electromechanical coupling coefficients

Undoped GaN films for high electromechanical coefficient were grown by controlling the size of nucleation sites through a special two step growth method. The film grown by this sequence exhibited the sheet resistance of up to 10⁹ Ω/sq with mirror-like surface morphology. The high resistance of undoped GaN film is thought to be due to the promoted misorientation of nuclei in a or c axis and the formation of deep trap levels in the bandgap when a GaN film was grown during ramping temperature from 950 to 1020 °C for 3 min in the initial growth stage. The fabricated saw filter on semi-insulating GaN exhibited a high velocity of 5342 m/s at center frequencies of 133.57 MHz and an electromechanical coupling coefficient (k²) of about 0.763% which was enhanced due to the improvement of surface morphology with high sheet resistance by the two step ramping technique.