SiN_x/(Al,Ga)N interface barrier in N-polar III-nitride transistor structures studied by modulation spectroscopy

Contactless electroreflectance studies coupled with numerical calculations are performed on in-situ SiN_x capped N-polar III-nitride high electron mobility transistor (HEMT) structures with a scaled channel thickness in order to analyse the built-in electric field in the GaN channel layer. The experimentally obtained field values are compared with the calculated field versus channel thickness curves. Furthermore, the experimental and theoretical sheet carrier densities, n_s, are evaluated. While a gradual decrease in carrier concentration with decreasing channel thickness is expected for N-polar structures, experimentally a sudden drop in the ns values is observed for samples with very thin channels. The additional loss in charge was associated with a change in the SiN_x/AlGaN interface Fermi level at very thin channel thicknesses.