Ultra-Wide Bandgap AlN-based Semiconductor Materials for Optoelectronic and Sensing Devices

The III-N deep UV LED operating at 250‒280 nm has huge potentials of revolutionizing multiple critical applications such as water sterilization, sensing, and communication. However, it suffers inadequately low external quantum efficiency (EQE). The main challenges associate with low efficiency in DUV LEDs are (i) tensile strain exist in quantum wells (QWs) due to lattice mismatch between QWs and quantum well barriers (QWBs); (ii) polarization fields exist at heterostructure interfaces that leads to spatial separation of electrons and holes wave functions; and (iii) poor carrier confinements in QWs due to carrier leakage from active region. These challenges lead to low radiative efficiency and injection efficiency. Moreover, generated photons experience total internal reflections due to transverse magnetic emission mode dominance with higher AlN mole fraction in AlGaN QW active region, which is the case in DUV LEDs. Consequently, emitted photos are not able to extracted into air efficiently. In this dissertation, we studied the effect of external strain on the performance of DUV LEDs. The overall result is that all of the three components of the EQE are simultaneously enhanced when QW is experiencing external compressive strain. In addition, we found that the diode requires lower power when compressive strain is applied. Extreme environments including high temperature and pressure, and corrosive and radiation conditions are often faced in energy, transportation, aerospace, and defense applications and pose a technical challenge in sensing. To address the challenge, we develop a piezoelectric sensor based on single-crystalline AlN transducer having ultrawide bandgap energy of 6.2 eV. The pressure sensor, consisting of stainless-steel diaphragm and mechanically flexible 1.5-μm AlN thin film, shows high sensitivity of 0.4‒0.5 mV/psi up to 900 °C, producing output voltages from 73.3 mV to 143.2 mV for the input gas pressure range of 50 to 200 psi at 800 °C. The sensitivity and output voltage also show the dependence on temperature due to two different origins. A decrease in elastic modulus of stainless-steel diaphragm slightly enhances the sensitivity and the generation of free carriers degrades the voltage output beyond 800 °C, which also matches well with theoretical estimation. The performance characteristics of the sensor are also compared with polycrystalline AlN film and single-crystalline GaN thin film. The Polycrystalline AlN sensor shows significantly lower output voltage and sensitivity, suggesting the importance of single crystallinity in the piezoelectric effect. Single-crystalline GaN provides similar high sensitivity but only in a limited temperature range up to 300 °C, confirming the importance of bandgap energy in piezoelectric devices for high-temperature operation. Long-term stability and reliability of the AlN sensor operated at elevated temperature are also confirmed experimentally. The demonstration of the sensor at 900 °C, which we believe is the highest operating temperature among the pressure sensors, and inherent properties of AlN including chemical and thermal stability and radiation resistance offer a new solution for sensing in extreme environments.