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Nanostrukturen und Grenzflächen Poster: Do., 13:00–15:30 D-P316
X-ray standing waves on GaN: crystal defects and non-dipole effects
M. Siebert 1 , Th. Schmidt 1 , J. I. Flege 1 , S. Figge 1 , J. Zegenhagen 2 , T.-L.
Lee 2 , D. Hommel 1 , J. Falta 1
1 Inst. of Solid State Physics, Univ. of Bremen, Germany – 2 ESRF, Grenoble, France
Magnesium and silicon are most commonly used as p-dopant and n-dopant material,
respectively, in GaN based devices such as laser diodes. The incorporation of
these dopants is known to lead to a variety of defects [1] which significantly decrease
the performance of the devices. Hence, detailed knowledge about the interplay between
dopant concentration and defect formation is required. The samples were grown
by metal-organic vapor phase epitaxy on sapphire (0001) substrates. In order to determine
the dopant incorporation sites, we performed x-ray standing wave measurements
(XSW) in (0002) backscattering geometry at a photon energy of E=2390 eV at the
beamlines BW 1 (HASYLAB/DESY) and ID 32 (ESRF). As secondary signals, Ga
2p photoelectrons, Mg KLL Auger electrons, and Si K fluorescence were recorded. In
backscattering geometry, the intrinsic width of the rocking curve may exceed its defect
induced broadening. Hence, the data can be evaluated directly applying the dynamical
theory of diffraction. However, in 3 µm thick Mg doped GaN films, the defect induced
broadening cannot be sufficiently compensated by choosing a backscattering setup.
This could be compensated by an optimizing the sample design: A 3 µm thick high
quality undoped GaN film was used to establish a standing wave field for the investigation
of a 300-400 nm thin doped film deposited on top. By this approach, the width of
the rocking curve can be significantly reduced. For XSW investigations using photoelectrons
as secondary signal, contributions of non-dipole effects to the photoelectron
yield need to be taken into account. In backscattering geometry, these effects become
especially pronounced. For electrons emitted from an s state, the influence of nondipole
contributions on the XSW-signal can be accounted for by applying calculated
correction paramters [2]. However, such parameters are not yet available for other initial
states, as required for the Ga 2p electrons used here. Therefore, we used a method
that allows to determine non-dipole parameters with high precision, by comparing the
XSW signal of Ga 2p photoelectrons and Ga LMM-Auger electrons from high quality
undoped GaN films. In the setup we used, the difference of the kinetic energies of
these two types of electrons is very small, hence both signals probe the same crystal
volume. The parameters obtained from these experiments were then used for the data
evaluation of XSW measurements of doped films with decreased crystal quality. The
results confirm a prevalent incorporated of Mg on substitutional sites. With increasing
dopant concentration, however, additional sites are occupated by Mg, which can
be related to inversion domains. Different results have been obtained for Si doping,
pointing to a general degradation of the crystal structure without a significant increase
of non-substitutional Si atoms with increasing dopant concentration.
[1] Z. Liliental-Weber et al., Physica B, 273–274 (1999) 124
[2] I. Vartanyants et al., Nucl. Instrum. Methods. Phys. Res. 547 (2005) 196