Abstract
Simultaneously inducing preferred crystalline orientation with a strong piezoelectric response in polycrystalline aluminum nitride (AlN) thin films by atomic layer deposition is a technical challenge due to the upscaling of the integration of piezoelectric functionalities, such as sensing and actuation, in micro-devices without any poling process. Utilizing low-temperature plasma-enhanced atomic layer deposition (PE-ALD), highly c-axis-oriented AlN films have been prepared with precise control over the relative composition, purity levels, and chemical states of constituent elements. Tailoring thermodynamic parameters, such as the growth temperature and purging time after the trimethylaluminum precursor pulsing before the N2:H2:Ar plasma reaction, provide the possibility of modulating the texture coefficient and the relative piezoelectric response. The effective transverse piezoelectric e31,f coefficient of 0.37 C/m2 was achieved on the AlN film grown at 250 °C and 30 s with the highest texture coefficient TC(002) of 2.75 along the c-axis orientation. The process proposed, at a low temperature with the highly conformal growth of aluminum nitride thin films by PE-ALD, opens up pathways to design novel piezoelectric functional materials for micro-electro-mechanic system devices with complementary metal oxide semiconductor process temperature compatibility.
Highlights
Aluminum nitride (AlN) thin films have attracted significant attention in optoelectronics1 and micro-electro-mechanic system (MEMS) applications, for example, energy harvesting,2–4 and surface-acoustic-wave transducers and resonators,2,4 due to their excellent properties such as a wide direct bandgap of 6.2 eV, piezoelectricity along the c-axis, high phase velocities, and high thermal stability.2 In addition, the aluminum nitride (AlN) film does not require any poling processes due to its oriented structure that facilitates its integration into micro-systems
Utilizing low-temperature plasma-enhanced atomic layer deposition (PE-ALD), highly c-axis-oriented AlN films have been prepared with precise control over the relative composition, purity levels, and chemical states of constituent elements
(002)-oriented AlN films can be synthesized by radio frequency (RF) sputtering,6–8 pulsed laser deposition (PLD),9–11 chemical vapor deposition (CVD),12–14 and molecular beam epitaxy
Summary
Aluminum nitride (AlN) thin films have attracted significant attention in optoelectronics and micro-electro-mechanic system (MEMS) applications, for example, energy harvesting, and surface-acoustic-wave transducers and resonators, due to their excellent properties such as a wide direct bandgap of 6.2 eV, piezoelectricity along the c-axis, high phase velocities, and high thermal stability. In addition, the AlN film does not require any poling processes due to its oriented structure that facilitates its integration into micro-systems. The AlN film does not require any poling processes due to its oriented structure that facilitates its integration into micro-systems. These applications require a (002) crystalline structure of the AlN film to achieve the piezoelectric effect and highest surface acoustic velocity because it is well known that the hexagonal wurtzite AlN has a spontaneous polarization along the c-axis.. The abovementioned techniques present severe drawbacks: either the anisotropy of the deposited film prevents the conformal coating of sophisticated micro-systems or they often require high temperatures of over 350 ○C to deposit AlN films that may lead to incompatibility with complementary metal oxide semiconductor (CMOS) and MEMS processing. The recent emergence of electronics based on organic flexible substrates demand low-temperature processes to enable integration and functionalization.
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