Extreme temperature and radiation-harsh environments, such as those involving nuclear power, advanced military applications (e.g., electronically guided missiles) or space exploration have created a high demand for more robust electronics. The ultra-wide bandgap (UWBG) semiconductor aluminum nitride (AlN) is a highly promising platform to fill this need. Commanding the largest bandgap Eg (6.2 eV) out of any semiconductor, AlN has a critical field Ec of 12 mV/cm, electron and hole mobilities of 1090 and 14 cm2/V-s, and a maximum operating temperature of 690 °C. The ultra-wide bandgap, coupled with the 11.52 eV/atom chemical bond strength and small lattice constants give AlN a natural advantage against impact ionization and lattice displacement, the two most fundamental radiation effects.
To this end, lateral Pd/n-AlN Schottky diodes were fabricated by metal organic chemical vapor deposition (MOCVD) on (0001) sapphire substrates and their current–voltage (I–V) characteristics were studied across varying temperatures and gamma-ray irradiation doses. Multiple Schottky diodes with varying geometries were fabricated. Large ideality factors, ranging from 25 at 300 K to 20 at 475 K, were obtained at forward bias. Diode breakdown voltage exhibited a negative temperature dependence at reverse breakdown conditions, leading the investigators to conclude that the breakdown mechanism is related to surface states instead of impact ionization.
The devices were bombarded with protons at 3 MeV from various: 5×109 /cm2, 5×1011 /cm2 and 5×1013 /cm2. Neither forward- nor reverse-bias currents change appreciably until after exposure the highest dose. At the highest forward bias of 10 V, the currents reach 16.26 μA prior to radiation exposure and decrease to 13.96 μA after exposure to 5×1013 /cm2. The devices were subjected to the high-temperature I-V tests under the following temperatures: 20 C, 40 C, 60 C, … 120 C. Forward currents increase consistently with temperature, reaching over 10 μA, and reverse-bias currents consistently decrease. Radiation exposure at the aforementioned doses does not appreciably alter the high-temperature I-V characteristics generated. Ideality factors were obtained as a function of temperature and radiation exposure. The diode ideality factors consistently decrease with temperature and radiation dose, ranging from ~7 to 3. High-resolution x-ray diffraction (HR-XRD) tests were performed after each radiation dose, four in total (including one before any exposure). In increasing order with respect to dose, each rocking curve has a full width half maximum (FWHM) of 0.04409, 0.04691, 0.06605 and 0.0563 for the (0002) plane of AlN. For the (20-24) plane, the FWHM values are 0.05895, 0.069, 0.912, and 0.268.