We present surface properties of buffer films (AlN and GaN) and Al0.3Ga0.7N/Al2O3-High Electron Mobility Transistor (HEMT) structures with/without AlN interlayer grown on High Temperature (HT)-AlN buffer/Al2O3 substrate and Al2O3 substrate. We have found that the GaN surface morphology is step-flow in character and the density of dislocations was about 108–109 cm−2. The AFM measurements also exhibited that the presence of atomic steps with large lateral step dimension and the surface of samples was smooth. The lateral step sizes are in the range of 100–250 nm. The typical rms values of HEMT structures were found as 0.27, 0.30, and 0.70 nm. HT-AlN buffer layer can have a significant impact on the surface morphology of Al0.3Ga0.7N/Al2O3-HEMT structures.