Tensile-strained crystalline aluminium nitride nanomechanical resonators

Abstract

High-Q_m nanomechanical resonators have proven to be a promising platform for advancing quantum technology. Resonators with Q_m×f_m products exceeding 6.2×10^12 Hz can sustain at least one coherent oscillation at room temperature, enabling their use in emerging quantum applications such as engineering long-lived quantum states and quantum sensing. Silicon nitride has become the favored material in this regard due to its great mechanical properties. However, it is an amorphous material that lacks additional functionalization capabilities beyond its admirable mechanical characteristics. We therefore explore crystalline aluminum nitride (AlN) as a promising alternative platform for high-Q_m nanomechanical resonators. Like other crystalline nitride materials, we expect AlN to possess robust mechanical properties. Moreover, the lack of centrosymmetry in its crystal structure gives rise to its piezoelectricity, making it a particularly versatile material for electromechanical applications. In this thesis, we studied four tensile-strained crystalline aluminum nitride samples with thickness ranging from 90nm to 295 nm. We extracted their elastic properties, including Young’s modulus, residual stress, and intrinsic quality factor. We then designed and realized phononically-shielded high-Q_m nanomechanical resonators out of them. Our bestperforming device achieved a quality factor of 8.6 × 10^6 and a Q_m × f_m product as high as 1.5 × 10^13, sufficient to provide a coherent oscillation at room temperature.