Bipolar conductivity AlN with ultra-low contact resistance

Recent advances in achieving conducting n- and p-type AlN films have pioneered a new technology suitable for high-voltage/power/temperature applications. However, the development of low contact resistances to AlN films is still a major research problem attributed to extremely high bandgap and exceptionally small electron affinity. In this work, Pt/Pd/Au contacts are applied to metal modulated epitaxy grown undoped, Be-doped (p-type), and Si-doped (n-type) AlN films for Hall and circular transfer line method (CTLM) measurements. The optimal annealing temperature of 850–1000 °C was found to be dependent on the amount of metal used in the mask and on dopant type. From Hall effect measurements, the AlN:Be samples showed p-type conductivity with a hole concentration of 8 × 1017 cm−3 and a doping activation efficiency of 11.4%, while the AlN:Si sample showed an electron concentration of 1 × 1018 cm−3 and a doping activation efficiency of 14.3%. From CTLM, record low contact resistances of ∼3.45 × 10−5 and 4.87 × 10−4 Ω cm2 were achieved for metal contacts in direct contact to n- and p-type AlN films, respectively, with no use of compositional grades. Temperature-dependent-CTLM-extracted resistivity measurements in the range of 80–340 K demonstrated activation energies in the range of 5–15 meV, indicating degenerately doped films with minimal carrier freeze out, but compensation values remained high attributed to oxygen impurities. These conduction figures of merit demonstrate significant improvement over prior state-of-the-art, highlighting the importance of high doping in reducing the contact resistance of doped AlN films while compensation is the primary challenge for further advances.