1. Short range ordering in InxGa1-xAs1-yNy, a dilute III-V nitride
G. Ciatto,1 F. d'Acapito,2 F. Boscherini,3 S. Mobilio,4 D. Batchelor,5 D. De Salvador,6 G. Bisognin,6 R. Carboni,3 L. Floreano,7 R. Gotter,7 L. Grenouillet,8 and H. Mariette9
GILDA – CRG
General purpose Italian beam Line
for X-ray Diffraction and Absorption
 1CNR c/o ESRF - Gilda CRG, Grenoble, France; 2INFM c/o ESRF - Gilda CRG, Grenoble, France; 3INFM and Physics Dept., University of Bologna, Italy. 4 Physics Dept, University of Rome III, Italy; 5 BESSY, Berlin, Germany;6 INFM and Physics Dept., Univ. of Padova, Italy; 7TASC-INFM, Trieste, Italy; 8CEA/LETI, Grenoble, France; 9 CEA-CNRS, University J.Fourier, Grenoble, France
What do we mean for short range ordering (SRO) ?
Why is it important ? N In As Ga
In a quaternary alloy the relative number of bonds is not fixed by the composition, a particular bonding can be energetically favored with respect to an other. In InxGa1-xAs1-yNy an excess of In-N bonds with respect to the random case has been predicted at the equilibrium by Monte Carlo simulations (Kim and Zunger, PRL, 2001).
SRO causes a larger statistical presence of N-centered In3Ga1 clusters, whose energy level is higher than that of N-centered In0Ga4 clusters. This leads to a diminished downward repulsion of the conduction band, so to a larger band gap. Since the technological interest of these alloys resides in the emission at very high wavelengths what predicted would result in an intrinsic material limitation.
What do we do ?
By In K-edge EXAFS we measure the relative In-N coordination, which we can compare with the random one: this allows us to quantify the actual SRO. Due to N dilution and low backscattering amplitude N-related paths have a small amplitude and a statistical analysis following standard criteria (Sayers, IXS 2000) is necessary in order to demonstrate their reliability. The results indicate a weak ordering in annealed samples.
By N K-edge XANES we qualitative exclude the presence of strong ordering, even in annealed (closer to equilibrium) samples. Simulations performed in the Full Multiple Scattering approach (in dots) show that N K-edge XANES cannot exclude the presence of weak ordering and are compatible with In K-edge results.
By exploiting the polarization dependency of XAFS we can demonstrate that N is totally substitutional to As. Spectra taken at different incidence angles, in fact, are virtually identical and XANES resonances do not show any angular dependence, which is a fingerprint of the cubic geometry of the zinc blend structure (G. Ciatto et. al., Phys. Scripta, 2004).
What have we found out ?
We gave the first quantitative determination of SRO in InxGa1-xAs1-yNy..
Virgin samples are virtually random alloys while annealed samples (closer to the equilibrium condition of the predictions) show a weak ordering, to which we are sensitive with our techniques.
Even if the probability of an N atom to occupy a In-nearest neighbor site is up to three times the random one, our measured order parameters are about one order of magnitude smaller than the predicted (G. Ciatto et al., PRB - RC , 2003).