Increase in the piezoelectric response of scandium-doped gallium nitride thin films sputtered using a metal interlayer for piezo MEMS

Abstract

Wurtzite gallium nitride (GaN) has a polarity along the c-axis and piezoelectric properties the same as aluminum nitride. Since it has a high mechanical quality factor and high output sensitivity, it is expected to perform well in piezo micro-electro-mechanical system devices. This paper demonstrates that Hf and Mo interlayers enable the preparation of highly (001)-oriented GaN films via conventional sputtering at a low temperature (400 °C). The piezoelectric coefficient d33 of the prepared undoped GaN films is equivalent to that of a single-crystal GaN. Furthermore, the results demonstrate that the piezoelectric response of GaN films increases significantly when they are doped with scandium (Sc). Although this enhancement was predicted theoretically, the piezoelectric response of Sc-doped GaN films prepared on Hf and Mo interlayers has shown great improvement. Moreover, bulk acoustic wave resonators constructed using Sc-doped GaN films show a piezoelectric coupling factor that is three times larger than that of a single-crystal GaN.

Lattice Distortion and Piezoelectricity Enhancement in GaN by Alloying with Group III and Rare‐Earth Elements: A Comparative Experimental Study

“…We prepared M x Ga 1 −x N alloyed films (M = Y, Dy, and Yb) via a cosputtering method following a procedure previously reported for Sc x Ga 1− x N, [ 4 ] which was also prepared for comparative purposes. Accordingly, the sputtering conditions were 400 °C of substrate temperature, a nitrogen/argon mixture at a ratio of 4:6 as the sputtering gas, and 0.25 Pa of pressure.…”

“…M x Ga 1− x N alloyed films were prepared via reactive cosputtering according to a previously reported procedure using GaN and the corresponding metal targets. [ 4 ] The substrate was n‐type (100) silicon (Si). Before M x Ga 1− x N deposition, a hafnium interlayer was prepared by sputtering at 400 °C.…”

“…[1,2] Moreover, their piezoelectricity can be largely enhanced by alloying with Sc. [3][4][5] In fact, Sc x Al 1Àx N alloys have been applied for the RF filter of 5G smartphones. Theoretical calculations suggest that the piezoelectric enhancement achieved via Sc alloying is caused by an increase in the strain sensitivity of the wurtzite internal u-parameter and the softening of the elastic constant.…”

“…[13,14] Sc x Al 1Àx N and Sc x Ga 1Àx N with high remanent polarization are attractive for ferroelectric random-access memory and ferroelectric tunnel junction devices. [15][16][17] Although many studies have reported the occurrence of lattice distortion due to alloying, [3][4][5][6][7]9,10,[12][13][14][18][19][20][21][22][23][24][25][26] systematic studies comparing the effects of different alloying elements are scarce and limited to theoretical investigations. Daoust systematically investigated the effect of alloying rare-earth elements into AlN on piezoelectricity using density-functional theory (DFT) calculations, [27] and Winiarski investigated the influence of alloying group III elements such as Sc, Y, and Lu on the electronic structure of AlN, GaN, and InN using DFT.…”

Fabrication of AlGaN High Frequency Bulk Acoustic Resonator by Reactive RF Magnetron Co-sputtering System

“…In recent years, some literature has studied the applications of AlGaN and AlScN in acoustic wave devices [ 18 , 19 ]. Both Wurtzite AlN and GaN have c-axis-oriented polarities and similar piezoelectric characteristics [ 20 ]. Therefore, GaN and AlGaN are also suitable for the applications of acoustic wave devices.…”