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Substrate effects on the strain relaxation in GaN/AlN short-period superlattices.

Kladko V, Kuchuk A, Lytvyn P, Yefanov O, Safriuk N, Belyaev A, Mazur YI, Decuir EA, Ware ME, Salamo GJ - Nanoscale Res Lett (2012)

Bottom Line: Three-dimensional growth of the GaN cap layer in samples with pseudomorphly grown SLs on the AlN template is observed.At the same time, two-dimensional step-flow growth of the cap layer was observed for structures with non-pseudomorphly grown SLs on the GaN template with a significant density of large cracks appearing on the surface.The growth mode of the GaN cap layer is predefined by relaxation degree of top SL layers.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Physics, University of Arkansas, Fayetteville, AR, 72701, USA. ymazur@uark.edu.

ABSTRACT
We present a comparative study of the strain relaxation of GaN/AlN short-period superlattices (SLs) grown on two different III-nitride substrates introducing different amounts of compensating strain into the films. We grow by plasma-assisted molecular beam epitaxy (0001)-oriented SLs on a GaN buffer deposited on GaN(thick)-on-sapphire template and on AlN(thin)-on-sapphire template. The ex-situ analysis of strain, crack formation, dislocation density, and microstructure of the SL layers has established that the mechanism of strain relaxation in these structures depends on the residual strain in substrate and is determined mainly by the lattice mismatch between layers. For growth on the AlN film, the compensating strain introduced by this film on the layer prevented cracking; however, the densities of surface pits and dislocations were increased as compared with growth on the GaN template. Three-dimensional growth of the GaN cap layer in samples with pseudomorphly grown SLs on the AlN template is observed. At the same time, two-dimensional step-flow growth of the cap layer was observed for structures with non-pseudomorphly grown SLs on the GaN template with a significant density of large cracks appearing on the surface. The growth mode of the GaN cap layer is predefined by relaxation degree of top SL layers.

No MeSH data available.


Related in: MedlinePlus

AFM topography maps of S1 (a to c) and S2 (d to f) samples. Insets (a,d) illustrate FFT of corresponding AFM maps. Surface height profiles along dashed lines are shown on (c,f). Maps (a,d) have enhanced, contrasted for convenience.
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Figure 3: AFM topography maps of S1 (a to c) and S2 (d to f) samples. Insets (a,d) illustrate FFT of corresponding AFM maps. Surface height profiles along dashed lines are shown on (c,f). Maps (a,d) have enhanced, contrasted for convenience.

Mentions: The principal difference in the mechanical stresses and the resulting relaxation in samples S1 and S2 is also illustrated by a detailed AFM analysis of the typical morphological defects that occurred in the GaN cap layers. This is shown in Figure 3. Here, we see that the surface of S1 is covered with a net of microcracks that run along the <2-1-10 > crystallographic directions. Strict adherence of the cracks to these directions is shown by the fast Fourier transform (FFT) of the AFM images (see Figure 3a inset). This demonstrates a sharp sixfold symmetry as would be expected, with a deviation in direction of not more than ± 2°. The intensity of FFT bands qualitatively indicates anisotropy in the crack density along two equal crystallographic directions on the surface with values of Ncr. of 1.1 and 1.7 × 103 cm−1. Note that on the surface, there appear microcrack clusters (lateral size between 10 and 20 μm) where a local density of cracks Ncr. rises up to 9 × 103 cm−1.


Substrate effects on the strain relaxation in GaN/AlN short-period superlattices.

Kladko V, Kuchuk A, Lytvyn P, Yefanov O, Safriuk N, Belyaev A, Mazur YI, Decuir EA, Ware ME, Salamo GJ - Nanoscale Res Lett (2012)

AFM topography maps of S1 (a to c) and S2 (d to f) samples. Insets (a,d) illustrate FFT of corresponding AFM maps. Surface height profiles along dashed lines are shown on (c,f). Maps (a,d) have enhanced, contrasted for convenience.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3814589&req=5

Figure 3: AFM topography maps of S1 (a to c) and S2 (d to f) samples. Insets (a,d) illustrate FFT of corresponding AFM maps. Surface height profiles along dashed lines are shown on (c,f). Maps (a,d) have enhanced, contrasted for convenience.
Mentions: The principal difference in the mechanical stresses and the resulting relaxation in samples S1 and S2 is also illustrated by a detailed AFM analysis of the typical morphological defects that occurred in the GaN cap layers. This is shown in Figure 3. Here, we see that the surface of S1 is covered with a net of microcracks that run along the <2-1-10 > crystallographic directions. Strict adherence of the cracks to these directions is shown by the fast Fourier transform (FFT) of the AFM images (see Figure 3a inset). This demonstrates a sharp sixfold symmetry as would be expected, with a deviation in direction of not more than ± 2°. The intensity of FFT bands qualitatively indicates anisotropy in the crack density along two equal crystallographic directions on the surface with values of Ncr. of 1.1 and 1.7 × 103 cm−1. Note that on the surface, there appear microcrack clusters (lateral size between 10 and 20 μm) where a local density of cracks Ncr. rises up to 9 × 103 cm−1.

Bottom Line: Three-dimensional growth of the GaN cap layer in samples with pseudomorphly grown SLs on the AlN template is observed.At the same time, two-dimensional step-flow growth of the cap layer was observed for structures with non-pseudomorphly grown SLs on the GaN template with a significant density of large cracks appearing on the surface.The growth mode of the GaN cap layer is predefined by relaxation degree of top SL layers.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Physics, University of Arkansas, Fayetteville, AR, 72701, USA. ymazur@uark.edu.

ABSTRACT
We present a comparative study of the strain relaxation of GaN/AlN short-period superlattices (SLs) grown on two different III-nitride substrates introducing different amounts of compensating strain into the films. We grow by plasma-assisted molecular beam epitaxy (0001)-oriented SLs on a GaN buffer deposited on GaN(thick)-on-sapphire template and on AlN(thin)-on-sapphire template. The ex-situ analysis of strain, crack formation, dislocation density, and microstructure of the SL layers has established that the mechanism of strain relaxation in these structures depends on the residual strain in substrate and is determined mainly by the lattice mismatch between layers. For growth on the AlN film, the compensating strain introduced by this film on the layer prevented cracking; however, the densities of surface pits and dislocations were increased as compared with growth on the GaN template. Three-dimensional growth of the GaN cap layer in samples with pseudomorphly grown SLs on the AlN template is observed. At the same time, two-dimensional step-flow growth of the cap layer was observed for structures with non-pseudomorphly grown SLs on the GaN template with a significant density of large cracks appearing on the surface. The growth mode of the GaN cap layer is predefined by relaxation degree of top SL layers.

No MeSH data available.


Related in: MedlinePlus