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9
EFFECT OF THE ALLOY COMPOSITION
IN THE InAlAs METAMORPHIC
BUFFER LAYERS
The present results confirm the primary impor-
tance of surface steps for the nucleation of the islands
and the role of surface mass transport in the growth
of the islands. To further investigate the role of the
mismatch strain as well as the composition of the
buffer layer, a second set of samples has been grown
on metamorphic InAlAs buffer layers with indium
molar fraction varying from 33% to 52%. The layers
were grown by solid source molecular beam epitaxy
(SSMBE). The graded composition buffer grown at
400°C was followed by a 0.3 µm thick uniform compo-
sition layer. During the growth of the uniform compo-
sition layer the substrate temperature was elevated
to 500°C prior to the deposition of InAs. This compo-
sition profile allows to achieve similar relaxation
rates, i.e. about 85% of the initial mismatch strain
between the buffer layer and the GaAs substrate.
5 As
a result, the mismatch strain between InAs and the
templates varies from 4.9% (InAs on MM-InAlAs with
33% In content) to 3.8% (InAs on MM-InAlAs with
52% In content). After the InAs deposition at 500°C,
the substrate temperature was kept at 500°C for456 Cordier, Miska, and Ferre
30 sec before cooling and removing the sample from
the growth chamber. For comparison, two reference
samples were grown, one with a GaAs buffer layer,
the other with a InAlAs buffer lattice matched to an
InP substrate (LM-InAlAs with 52% indium content).
The evolution of the intensity of RHEED pattern
Bragg spot is reported in Fig. 5. As in the previous
experiments, the onset of the transition from 2D to 3D
growth mode is 1.7 ML for InAs on GaAs and occurs
at about 0.9–1 ML for InAs on the 33% In content MM-
InAlAs buffer. As shown in Fig. 5, the transition is
delayed when the indium molar fraction in the buffer
layer is increased due to the lowering of the mismatch
strain with InAs. For 52% In content buffers, the 2D-
3D transition occurs at 1.7 ML on the MM-InAlAs
layer that is earlier than on the LM-InAlAs (onset at
about 2.1 MLs); this is probably due to the roughness
of the cross-hatched surface of the metamorphic layer
associated with a larger lattice mismatch (da/a = 3.8%
vs. 3% for InAs on LM-InAlAs).
The surface of the InAs covered InAlAs buffer
layers has been studied with atomic force microscopy.
The density and the height of InAs islands grown on the
metamorphic InAlAs templates are shown in Fig. 6.
As for previous experiments, the density and the size
of the islands depends on the InAs coverage. The high
densities and the small sizes of the islands measured
on the 33% and the 42% In content MM-InAlAs
buffers confirm the previous results obtained with
GSMBE. However, the increase of the density with
the InAs coverage is less on the 52% In content buffer.
In fact, as shown on 1 µm × 1 µm AFM images (Fig. 7),
the InAs islands elongates along the [1