Space-charge-controlled field emission analysis of current conduction in amorphous and crystallized atomic-layer-deposited Al_2O_3 on GaN

摘要

As previously reported, postdeposition annealing at 800 °C and higher simultaneously crystallizes atomic-layer-deposited (ALD) Al_2O_3 films and reduces the current in Al/ALD-Al_2O_3/(0001) GaN capacitors by two orders of magnitude. This current reduction is caused by the enhancement of conduction band offset from 1.4 to 1.8 eV, as revealed by the space-charge-controlled field emission analysis. Selected area electron diffraction (SAED) patterns demonstrate that the crystallized films consist of twinned (111)-oriented cubic γ-Al_2O_3 with an epitaxial relation of Al_2O_3 <0(1)1> ‖GaN <2(11)0>. The SAED patterns additionally include spots that are specific to triaxially tripled γ-Al_2O_3. The aforementioned epitaxy is due to the similarity of hexagonal close-packed sublattices between oxygen on a (111) γ-Al_2O_3 plane and nitrogen on a (0001) GaN plane. However, the hexagonal close-packed lattice constant of γ-Al_2O_3 is 12% smaller than that of GaN, necessitating domain matching epitaxy. The thickness of the interfacial transition layer caused by the large misfit is estimated to be thinner than four monolayers of oxygen sublattice, by using the methodology developed here. Based on these results, the effect of Al_2O_3 crystallinity on the characteristics of Al_2O_3/GaN capacitors, such as conduction current, dielectric breakdown, interface states, and bias instability, was comprehensively captured. According to x-ray diffraction analyses, Al_2O_3 films crystallize at 700 °C, which is ~100 °C lower than the threshold temperature estimated by transmission electron microscope observations. This difference was possibly caused by locally crystallized Al_2O_3 films, as confirmed by the slightly reduced current. These findings form a basis for improving ALD-Al_2O_3 films as gate insulator.