2. Outline
• Management of the strain/relaxation in the thick AlGaN layer sample.
• Objective is achieved by the introduction of GaN interlayer.
• The thickness of the GaN interlayer is varied to get the desired strain in the AlGaN layer.
2
3. • High strain results in bowing and cracking of the layer.
• Insertion of stress relieving GaN interlayer eliminates these
problem by the strain management.
• The GaN interlayer thickness was varied and the effect of the
same on the AlGaN strain was analysed.
• The result of this interlayer is no bowing due to no strain and
also the thick (~2 µm) AlGaN with Al content of 50% was grown
on the AlN substrate.
Thick AlGaN Layer
Thin GaN Interlayer
AlN Substrate
Sapphire Substrate
4. Variation of AlGaN layer relaxation with the change in
the GaN interlayer thickness
160 170 180 190 200 210
-120
-80
-40
0
40
80
120
160
200
240
280 5
4
3
1
GaN
5 nm
8.75 nm
10 nm
20 nm
QW
AlGaN
Wafercurvature(m
-1
)
Time (s)
AlN growth
QB
2
8
12
16
20
R(%)
RSM showing strained (left) (5nm interlayer)
and relaxed (right) (12nm interlayer)
In-situ (during growth) curvature
monitor signal for samples grown
with different GaN thicknesses
If the GaN interlayer thicker than 5 nm is introduced in between the AlGaN layer and the AlN substrate, the
burger vectors of the induced dislocations becomes anti-parallel to those which are already in the layer. Thus,
there is the generation of the dislocations in the layer but the direction of the burger vectors of them are in
opposite direction which cancels out the strain in the layer and thus the layer becomes relaxed.
Relaxation variation with GaN interlayer thickness
5. Variation of AlGaN layer edge dislocation density with
the change in the GaN interlayer thickness
Edge dislocation density variation with
GaN interlayer thickness
AlGaN layer’s relaxation increased with increase in the edge
dislocation density.
Addition of the new dislocation densities with burger vectors’
in anti-parallel direction.
a lattice parameter reached from the fully strained AlN value to
the almost fully relaxed Al0.5Ga0.5N value.
To compensate for this thing, the edge dislocations increased
with the increase in the GaN interlayer thickness.
6. Variation of the a lattice parameter with the change in
GaN interlayer thickness
a Lattice parameter variation with GaN interlayer thickness
With the increase in the GaN interlayer thickness, the a lattice
parameter of the AlGaN layer increased following the relaxation
trend.
As the relaxation increased, the a lattice parameter also
increased from the strained AlN value to the theoretical
Al0.5Ga0.5N value.
The a lattice parameter value of AlN is 3.111 Å and that of the
Al0.5Ga0.5N is 3.15013 Å.
7. Variation of the AlGaN layer composition with the
change in GaN interlayer thickness
Al content variation with GaN interlayer thickness
With the change in the GaN interlayer thickness, it was
observed that the composition of the AlGaN layer remained
fairly the same to the value of 50%.
The a lattice parameter reached approximately to the fully
relaxed value at Al0.5Ga0.5N from the almost fully strained value
at Al0.5Ga0.5N.
8. Conclusion
With increase in the thickness of the GaN interlayer, the relaxation of the
AlGaN layer can be increased even to 100% relaxation as well before it
reaches the over relaxation state when layer cracks eventually.
The composition of the AlGaN layer remained approximately the same.
The relaxation in the layer occurs for the account of generation of numerous
misfit edge type dislocations.
Thickness of GaN interlayer of 10 nm was found to be a good compromise
between reducing the strain in consequent AlGaN layer and not causing its
cracking at the same time.
