Growth and characterization of plasma-assisted molecular beam epitaxial-grown AlGaN/GaN heterostructures on free-standing hydride vapor phase epitaxy GaN substrates

We have grown AlGaN/GaN high electron mobility transistor (HEMT) structures by plasma-assisted molecular beam epitaxy on free-standing n -GaN substrates grown by hydride vapor phase epitaxy. Reflection high energy electron diffraction patterns of the as-loaded wafers exhibit narrow streaks which persist throughout the growth. Atomic force microscopy shows smooth AlGaN surfaces with root-mean-square roughness of 10 Å over a 20×20 μm2 area. High resolution x-ray diffractometry indicates that the AlGaN peak is ∼20% narrower than for similar structures grown on SiC. Hall mobilities, electron sheet densities, and sheet resistances were measured on ten 60×60 μm2 Hall test patterns defined photolithographically across the surface of the 10×10 mm2 sample. Buffer leakage measurements demonstrate that a Be:GaN layer effectively isolates the channel from the conductive substrate. Average sheet resistances and sheet densities were 380 Ω/□ and 0.94×1013 cm-2, respectively. These HEMT structures exhibit room-temperature Hall mobilities in excess of 1900 cm2V s. In addition, devices on these structures exhibit excellent pinch-off, low gate leakage, and saturated drain current densities of almost 700 mA/mm. Further details regarding the structural and electrical properties will be described along with device testing.