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Distribution of strain in laterally overgrown GaAs layers determined by x-ray diffraction

Jaroslaw Domagala ,  Aleksandra Czyzak ,  Zbigniew R. Zytkiewicz 

Polish Academy of Sciences, Institute of Physics, al. Lotników 32/46, Warszawa 02-668, Poland


Continuous improvement in quality of semiconductor structures and miniaturization of electronic devices leads to an increasing demand for techniques that allow detection and visualization of crystalline lattice microdefects. From many techniques available the Rocking Curve Imaging (RCI) based on synchrotron x-ray diffraction has gained much attention recently as the useful method of wafer defect analysis and industrial wafer quality inspection [1]. Using it quantitative information on crystallographic misorientations and lattice quality can be obtained by direct imaging of large sample area with high spatial resolution. However, for most crystal growers an access to the synchrotron radiation sources is still difficult and laboratory techniques are preferred. Therefore, we have developed a simple RCI version that makes use of conventional high resolution x-ray diffractometer.

Briefly, by specially designed set of slits and masks the size of the incident x-ray beam of the diffractometer is reduced to 6 µm × 0.5 mm. Then, this narrow beam is precisely scanned in small steps across the sample surface and x-ray diffraction area map is collected. Both the ω and 2θ/ω scans could be used, that allows mapping of crystallographic misorientation and lattice parameter distributions along the sample area. The technique was used in this work to study crystallographic perfection of epitaxial laterally overgrown (ELO) GaAs layers grown by the liquid phase epitaxy on SiO2-masked GaAs substrates. Since ELO layer consists of monocrystalline stripes regularly arranged on a substrate such samples are the best to demonstrate potential of the technique.

We will show, in agreement with our previous report [2], that laterally parts of ELO layers (wings) are tilted downwards due to their interaction with underlying mask. By using x-ray mapping this phenomenon can be easily distinguished from macroscopic sample curvature. Direction of the tilt and distribution of tilt magnitude across width of each wing can also be readily determined. The later allows us to measure the shape of lattice planes in individual ELO stripe and to compare it with the shape of layer surface. If large area of the sample is mapped fluctuation of wing tilts of various ELO stripes are monitored. In fully overgrown GaAs samples additional strain field is found at the plane where ELO stripes grown from adjacent seeds merge. This strain is due to coalescence of two wings tilted in opposite direction.

It is mentioning worthy that a common procedure applied for structural analysis of ELO layers is to measure x-ray diffraction curves under standard conditions of a wide x-ray beam. Such measurements are quite easy since the diffracted x-ray beam is highly intensive. Then, however, diffracted signal is integrated over many ELO stripes. Our results indicate some fluctuation of local wing tilts across the sample surface. This phenomenon, as well as macroscopic curvature of the sample, lead to broadening of diffraction curves obtained in the standard procedure and consequently to overestimation of the tilt angle value. In this way we show that local x-ray diffraction, e.g. x-ray mapping, must be used if precise information on wing tilt and its spatial distribution is required.

Acknowledgements: This work was partially supported by the Polish Committee for Scientific Research under grant No. 3T08A 021 26.

[1] D. Lubbert, T. Baumbach, J. Hartwig, E. Boller, E. Pernot, Nucl. Instrum. Methods B 160 (2000) 521.

[2] Z.R. Zytkiewicz, J. Domagala, D. Dobosz, J. Bak-Misiuk, J. Appl. Phys. 84 (1998) 6937.


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Submitted: 2007-01-18 12:56
Revised:   2009-06-07 00:44