Effect of long-term stability of the aluminium nitride - silicon interface for microwave-frequency electronic devices

The long term stability of the AlN/Si interface is of paramount importance for reliability and energy efficiency of a wide range of microwave frequency devices. The propagation loss of high power amplifiers, the bandwidth of 5G telecommunication devices, the quantum efficiency of light emitting diodes and conversion efficiency of energy harvesters depend on this interface. We demonstrate that tensile biaxial strain is directly associated with an increase in electromechanical performance by comparing the piezoelectric modulus of different AlN films grown on Si (1 1 1) by three different methods. Only one sample showed a uniquely high d33 of 8.3 pmV−1. However, the d33 degrades almost linearly with storage time to a value of only 4.7 pmV−1 two years after sample preparation, due to the degradation of the interface by pressure-induced phase transformation of the crystalline Si substrate to amorphous Si. Hence, the time-dependent electromechanical performance of AlN is characteristic for the stability of the AlN/Si interface. We show that variations of measured d33 values in literature can directly be attributed to strain variations at the interface. Our AlN/Si interface comparison highlights that device performance for high frequency applications represents a compromise between energy efficiency and reliability.