SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

1.4. STRAINED EPITAXY: STRAIN TENSOR 23

surface

Al 2 O 3

GaN

interface

dislocations

(a)

(b) (c)

Figure 1.18: (a) Cross-sectional TEM image of GaN grown heteroepitaxially on sapphire, in-
dicating the highly defective interface and the dislocations that propagate upwards. (b) AFM
surface image of the dislocated GaN , showing the atomic step structure which is typical of GaN
surfaces. The black dots are dislocations that have propagated upwards to the surface. (c) AFM
surface image of non-dislocated GaN, exhibiting a smooth and continuous step structure. Images
courtesy of P. Fini and H. Marchand of UCSB.