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A1N/GaN resonant tunneling diodes (RTDs) were grown separately on freestanding GaN (FS-GaN) substrates and sapphire substrates by plasma-assisted molecular-beam epitaxy (PA-MBE).Room temperature negative differential resis-tance (NDR) was obtained under forward bias for the RTDs grown on FS-GaN substrates,with the peak current densities(Jp) of 175-700 kA/cm2 and peak-to-valley current ratios (PVCRs) of 1.01-1.21.Two resonant peaks were also observed for some RTDs at room temperature.The effects of two types of substrates on epitaxy quality and device performance of GaN-based RTDs were firstly investigated systematically,showing that lower dislocation densities,flatter surface morphol-ogy,and steeper heterogeneous interfaces were the key factors to achieving NDR for RTDs.