Diploma Thesis - Erich Schmid Institute

13.4. Discussion:
Results of GaN/GaBN/Al2O3(0 0 0 1)
Sapphire is no perfectly suitable substrate for the growth of GaN layer due to the
enormous lattice mismatch. For that reason, attempts are made to reduce the
mismatch by introducing a buffer layer to minimize the defect density which would
degrade the GaN layer’s quality.
In the present case a GaN buffer layer containing about 3% boron nitride was
deposited on the sapphire substrate. If the anion sublattices of the sapphire and the
BGaN are continuous than the deposited buffer layer should be under compressive
stress [33], which cannot be shown by the experimental data.
The simple approach, by only considering geometrical aspects, is not sufficient to
explain the transition region between sapphire and GaN buffer layer that is
influencing the intrinsic stresses. Not only the structural difference between GaN and
sapphire that seems to be responsible for a tensile stress state, caused by island
coalescence [34] of the growing GaN layer and surface tension, but also the
substitution of GaN by BN, which has a smaller unit cell than GaN, contributes to
tensile stress in the buffer layer. An extrapolation of buffer layer’s in-plane
stresses/temperature dependence shows a value of 766MPa at the deposition
temperature at 700°C. The tensile stresses of the buffe r layer affects the GaN layer
that is, if the smoothing function is extrapolated, in a tensile stress state of 128MPa at
the deposition temperature. Therefore the intrinsic stresses of the buffer and GaN
layer are supposed to have a value of 766MPa and 128MPa, respectively.
Extrinsic stresses do not exist at a temperature of 700°C, because they can only
originate from a heating or cooling after the thin film formation. A cooling procedure
starting from the deposition temperature will cause a higher contraction of the
substrate than of the layer materials. For that reason the substrate will induce
compressive in-plane stresses in the BGaN and GaN layer, as is observed in the
experiment where the stresses at room temperature were much lower than at
elevated temperatures.
High resolution X-ray diffraction stress measurements were performed during the
cooling to provide information about an eventually plastic flow of the material.
However, the results showed a good correspondence between heating and cooling of
the sample, so it is assumed that the sample is only elastically deformed.
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