360
12 Heterostructures
Fig. 12.12 The
morphology of a GaP
grown on a Ge hemisphere
with [111]-pole and b
GaAs grown in a Ge(001)
spherical depression.
Adapted from [1129]
(a)
(b)
10µm
Fig. 12.13 Side view of
the heterointerfaces
between AlN and
c-oriented sapphire with
nitrogen (a) and Al (b)
being the first layer.
Adapted from [1143]. c
Phase image of
piezoresponse force
microscopy (PFM) of
lateral polarity GaN
heterostructure and d
linescan of phase signal
along white line in part (c).
Adapted from [1146], part
(c) reprinted with
permission, ©2002 AIP
(b)
(a)
(d)
(c)
Returning to the discussion of [00.1] or [00. ¯
1] growth of wurtzites, on a sapphire substrate with
lateral AlN patterns, laterally orientation-modulated GaN can be grown (Fig. 12.13). At the juncture
of the phases an inversion domain boundary forms [1145].
The defect density can be reduced in parts of the structure using pendeo-epitaxy [1147] or epitaxial
lateral overgrowth (ELO) [1148]. The defects thread only from the limited contact area of the layer
with the substrate and the part of the layer away from the mask (‘wing’) is free of defects (Fig. 12.14).
While the patterning of a mask may be cumbersome and require interruption of growth, the insitu deposition of a random SiN mask with small holes has been found beneficial in the epitaxy of
GaN [1149]. GaN islands nucleate selectively in the mask openings (Fig. 12.15a). The nucleation and
subsequent coalescence of islands (Fig. 12.15b) leads to defect annihilation and eventually again to a
flat film (Fig. 12.15c). The reduction of defect density is evident (Fig. 12.15d) and leads to improved
optical properties [1150].