Patent ID: 8872226

Claim:
An epitaxial substrate for a semiconductor device, comprising: a base substrate; a channel layer formed of a first group III nitride having a composition of In x1 Al y1 Ga z1 N (x1+y1+z1=1); and a barrier layer formed of a second group III nitride containing at least In and Al and having a composition of In x2 Al y2 Ga z2 N (x2+y2+z2=1), wherein: the composition of said first group III nitride is in a range determined by x1=0 and 0≦y1≦0.3; and the composition of said second group III nitride is, in a ternary phase diagram with InN, AlN and GaN being vertices, in a range surrounded by straight lines respectively represented by the following expressions determined in accordance with the composition of said first group III nitride: x ⁢ ⁢ 2 = y ⁢ ⁢ 2 - ( 0.36 + 0.5 ⁢ y ⁢ ⁢ 1 ) 1.78 = - z ⁢ ⁢ 2 - ( 0.64 - 0.5 ⁢ y ⁢ ⁢ 1 ) 2.78 ; x ⁢ ⁢ 2 = y ⁢ ⁢ 2 - ( 0.45 + y ⁢ ⁢ 1 ) 4.5 = - z ⁢ ⁢ 2 - ( 0.55 - y ⁢ ⁢ 1 ) 5.5 ; x ⁢ ⁢ 2 = - y ⁢ ⁢ 2 - ( 0.9 - 0.7 ⁢ y ⁢ ⁢ 1 ) 4.5 = z ⁢ ⁢ 2 - ( 0.1 + 0.7 ⁢ y ⁢ ⁢ 1 ) 3.5 ; z ⁢ ⁢ 2 = 0 ; and x ⁢ ⁢ 2 = 0 ⁢ ⁢ ( where ⁢ ⁢ 9 34 ≤ y ⁢ ⁢ 1 ≤ 0.3 ) ; and the mean surface roughness at the interface between the channel layer and the barrier layer is within a range of 0.1 nm to 3 nm.