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Proposed existence of acceptor-like traps at positive polarization interfaces in p-type III-nitride semiconductors

We propose the existence of an acceptor-like trap at positive polarization interfaces in p-type III-nitride semiconductor heterostructures, using N-polar p-type GaN/AlN/AlGaN superlattices as a demonstration platform. Metal Organic Vapor Phase Epitaxy was used to grow all samples, with a p-type modulation doping scheme using Mg as the dopant. The samples were characterized using x-ray diffraction and room-temperature Hall measurements, and energy band-diagram simulations were carried out using STR FETIS® and Silvaco packages. For higher doped samples (Mg > 1.5 × 1019 cm−3) with thinner AlN interlayers (≤0.7 nm), the total sheet charge measured using Hall measurements agreed with the value observed in standard simulations without invoking any traps, whereas for lower doped samples (Mg < 1.5 × 1019 cm−3) and those with thicker AlN interlayers (≥ 0.7 nm), the measured charge was very high compared to the value obtained from simulations and higher than the Mg doping in the films. The higher charge was attributed to the existence of an acceptor trap at positive polarization interfaces, which became ionized at lower doping and/or at higher AlN thicknesses. A consistent ionization energy of the trap was obtained by comparing the energy band diagram with and without acceptor traps with the experimental results. This work also elucidates the source of charge balance in p-type samples with insufficient or no Mg doping.