The surface morphologies of AlGaN/AlN/GaN HEMT structures were examined by using the atomic force microscopy (AFM).These HEMT structures have been grown by metalorganic chemical vapor deposition (MOCVD) onto the sapphire (0001) substrates where the thicknesses of AlN interlayers were varied from 0 to 5 nm. After the growth of GaN buffer layer, the AlN intermediate and AlGaN top layers were subsequently deposited at 1130°C in an trimethylaluminum (TMAl) and ammonia (NH3) atmosphere. The surface of AlGaN layers shows the thick- and fine-thread patterns.
It was found that the root-mean-square (RMS) roughness of the samples with 0, 0.5, 2.5 and 5nm AlN interlayer thickness are 0.503, 0.534, 0.534 and 0.601nm, respectively. Although the cracks and rougher surfaces show that the qualities of AlGaN barrier layers have slightly degraded in the samples with thicker AlN layer. These phenomena could be attributed to the lattice mismatch and the growth temperature. In addition, the room-temperature two-dimensional electron gas (2DEG) mobility and density analysis were performed on the AlGaN surfaces and the measured results were discussed in detail.