14 CHAPTER 1. STRUCTURAL PROPERTIES OF SEMICONDUCTORS
(a)
(b)
Figure 1.10: (a) Cross-sectional TEM images of a typical AlGaN/GaN HFET structure grown on
a SiC substrate. First, a 50 nm AlN nucleation layer is grown, followed by a 450 nm GaN buffer
layer and a 29 nm AlGaN cap. A large number of defects are formed at the AlN/GaN interface,
but many of the defects are annihilated as the GaN layer is thickened. The AlGaN cap layer is
coherently strained on top of the bulk GaN. No new defects are formed at this interface. (b) High
resolution X-ray diffraction scan of this structure. The close match between the data and theory
indicates the high crystalline quality of the structure. Images courtesy of Prof. J. Speck, UCSB.