Mechanism behind post-fabrication increases in 2DEG density and mobility in AlBN/GaN

Determine the physical mechanism by which device processing—particularly the 830 °C, 65 s N₂ anneal used for Ta/Al/Ni/Au ohmic contact formation—alters the polarization-induced two-dimensional electron gas in molecular beam epitaxy grown AlBN/GaN heterostructures to produce higher sheet electron density and Hall mobility (and thus lower sheet resistance) compared to the as-grown state. Quantify the contributions of the annealing steps to changes in the heterojunction and transport properties and establish whether annealing is the primary cause of the observed improvements.

Background

The authors grow AlBN/GaN heterostructures by MBE and observe a polarization-induced 2DEG. After fabricating HEMTs—including BCl3 ICP isolation and alloyed Ta/Al/Ni/Au ohmic contacts annealed at 830 °C for 65 s—they remeasure Hall parameters on processed samples.

Compared to the as-grown wafer (e.g., Ns ≈ 9.25×10^12 cm^-2, μ ≈ 524 cm^2/V·s, Rs ≈ 1290 Ω/□), the processed devices show increased Ns (≈1.41–1.48×10^13 cm^-2), increased μ (≈700–707 cm^2/V·s), and reduced Rs (≈598–630 Ω/□). The authors attribute this improvement presumably to annealing during processing and explicitly state that the underlying cause needs to be understood better in future work.