Lattice parameter dependence versus composition in semiconductor alloys: the InGaAs case
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Lattice parameter dependence versus composition in semiconductor alloys: the InGaAs case C. Ferrari 1, E. Villaggi1 , N. Armani1, G. Carta2, G. Rossetto2 1 CNR Maspec Institute, Parco Area delle Scienze 37/A, 43010 Fontanini, Parma, Italy; 2 CNR Ictima Institute, Area della Ricerca di Padova, Corso Stati Uniti 4, 35127 Padova, Italy ABSTRACT Following recent works that report a non linear dependence of the lattice parameter versus composition in some semiconductor alloys the InGaAs/InP system has been investigated. The lattice parameter and the composition of InGaAs/InP lattice matched heterostructures have been independently determined by measuring the high resolution X-ray diffraction profile and the absorption of the X-ray beam diffracted from the InP substrate. In contrast with previous results that stated a linear dependence of the lattice parameter with composition, a 6% larger In content in the InGaAs/InP lattice matched alloy is found. Such result has been confirmed by the analysis of the X-ray fluorescence induced by an electron beam on the layer and on standards made of InAs and GaAs fine ground crystals. The result is in good agreement with the predictions of models based on the elasticity theory applied on a microscopic scale.
INTRODUCTION The accurate knowledge of the lattice parameter-composition dependence in semiconductor alloys is important since it allows a non destructive determination of composition in the alloy by using X-ray diffraction methods. This in turn will permit to establish with better accuracy the dependence between important properties of the alloys, such as the bandgap, and the composition rather than the lattice parameter. Although simple models [1-3] based on the elasticity theory predict a deviation from Vegard law in the alloys depending on the elasticity constants of the compounds, in lack of more accurate experimental data, the Vegard law is still assumed. Experimental determinations of the lattice parameter dependence evidenced a quadratic deviation from the linear relationship in SiGe,[4], GaAlSb [5], GaAlAs [6-7] and SiC [8] alloys with a maximum of several percents of the total lattice parameter range near the stoichiometric value x=0.5. On the contrary as for the InxGa1-xAs alloy, measurement based on Electron Probe Microanalysis [9], Rutherford Backscattering [10] and X-ray fluorescence [11] report a linear dependence in the full composition range with an error of ±1% near x=0.5. To verify such old results in the present work the composition of single InGaAs/InP lattice matched heterostructures with composition x near 0.5 is measured by a method based on the absorption of an X-ray beam diffracted by the InP substrate.
EXPERIMENTAL Two InGaAs/InP heterostructures have been grown on an exactly oriented (001) Fe-doped InP substrates by Low-Pressure Metal Organic Chemical Vapour Epitaxy technique using a commercial reactor (AIXTRON AIX 200), prior to the deposition of the ternary layer, AA4.1.1
CuKα (004) reflection InP substrate InGaAs layer
Reflectivity
0.3
0.2
0.1
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