Table 1. Details of Optimized Barrier Layers.

ID	barrier parameters
barrier A	negatively graded AlGaN
barrier A1	tb. n = 20 nm, Xb. bottom = 20%
arrier A2	tb. n = 20 nm, Xb. bottom = 25%

ID	upper layer parameters	lower layer parameters
barrier B	negatively graded AlGaN, Xb. bottom = 25%	positively graded AlGaN, Xb. top = 25%
barrier B1	tb. n = 5 nm	tb. p = 15 nm
barrier B2	tb. n = 10 nm	tb. p = 10 nm
barrier B3	tb. n =1 5 nm	tb. p = 5 nm