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Nanostrukturen und Grenzflächen Poster: Do., 13:00–15:30 D-P311
Characterization of InGaN/GaN nano-islands by grazing incidence x-ray
diffraction
Th. Schmidt 1 , M. Siebert 1 , J. I. Flege 1 , S. Figge 1 , T. Yamaguchi 1 , D.
Hommel 1 , J. Falta 1
1 Inst. of Solid State Physics, University of Bremen, Otto-Hahn-Allee 1, 28359 Bremen
InGaN quantum dots (QDs) embedded in GaN offer a great potential for applications
in light emitting devices. Depending on the In concentration, a wide spectral range
can be covered. In comparison to InGaN/GaN quantum-well based structures, InGaN
quantum dots embedded in GaN are especially advantageous, e. g. because a smaller
laser threshold is expected. In this context, information about the size, shape, chemical
composition, and the strain state of such islands is necessary. Grazing incidence x-ray
diffraction (GIXRD) is ideally suited for this purpose, as has already been shown for
different material systems such as GaN/AlN [1], CdZnSe/ZnSe [2], or InGaAs/GaAs
[3]. Here, we present the first grazing incidence x-ray characterization of InGaN/GaN
nano-islands.
GIXRD experiments were performed ex-situ in the so-called z-axis setup [4] at the BW1
undulator beamline at HASYLAB (Hamburg, Germany), using a focussed monochromatic
beam at 10 keV photon energy. Reciprocal space maps were recorded in the
vicinity of different reflections. InGaN/GaN(0001) samples were grown on sapphire
(0001) substrates by metal-organic vapor pressure epitaxy (MOVPE) at our institute
in Bremen. Samples with different InGaN deposition temperatures were investigated.
For InGaN growth at T = 650 ◦ C, we find that the InxGa1−xN islands are virtually
completely relaxed, and based on Vegard’s law, an In concentration of x ≈ 0.60 follows
for these islands. For lower growth temperature (T = 600 ◦ C), broad reflections
indicate the presence of small islands which are found to be coherently strained.
In addition, samples grown by molecular beam epitaxy (MBE) on MOVPE templates
were investigated. For an InGaN growth temperature of T = 450 ◦ C, fully relaxed
islands are observed. The In concentration of these islands is as high as x ≈ 0.86. This
value is found to be significantly reduced after MBE overgrowth with GaN cap layers.
In comparison to as-grown QD layers, additional reflections occur for the overgrown
samples. Especially for stacked QD layers, these reflections become intense. This is
explained by the progressive formation of stacking faults and of regions with cubic
crystal structure during MBE overgrowth.
[1] V. Chamard et al., Appl. Phys. Lett. 79 (2001) 1971
[2] T. Passow, H. Heinke, Th. Schmidt, J. Falta, A. Stockmann, H. Selke, P. L. Ryder,
K. Leonardi, and D. Hommel, Phys. Rev. B 64 (2001) 193311
[3] K. Zhang, J. Falta, Th. Schmidt, Ch. Heyn, G. Materlik, and W. Hansen, Appl.
Phys. Lett. 72 (2000) 199
[4] M. Lohmann and E. Vlieg, J. Appl. Cryst. 26 (1993) 706