STRAIN RELAXATION IN SURFACE AND BURIED III-V MULTILAYER GRATINGS STUDIED BY HIGH RESOLUTION X-RAY DIFFRACTION AND ELASTICITY THEORY

T. Baumbach,¹D. Lübbert¹, L.Leprince², J.Schneck², A.Talneau², R. Felici³, A. Mazuelas³

¹Fraunhofer-Institut für Zerstörungsfreie Prüfverfahren, Saarbrücken and Dresden , Germany
present address: ESRF, BP 220, F-38043 Grenoble Cedex, France
²France Télécom, CNET/DTD, BP 107, F-92225 Bagneux Cedex, France
³European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble Cedex, France

Technological patterning of periodic gratings at the surface of III-V semiconductor strained layers is frequently required for the engeneering of oproelectronic devices for telecommunication application. The corrugation shape of the gratings is expected to induce strain relaxation generating non-homogeneous strain distribution which in turn may influence the electronic and optical properties of the device. In this aspect, the experimental and theoretical investigation of the strain status before and after the technological steps of etching a surface grating into a planar multilayer and later of burying the grating under an embedding layer is of great importance.

High resolution X-ray diffraction is shown to be a valuable tool for the non-destructive investigation of the strain state in epitaxially grown, mesoscopically patterned semiconductor structures.

It is the purpose of the present investigation to clarify the influence of the these fabrication steps on the evolution of the strain field.

We have applied conventional symmetrical and asymmetrical diffraction, and for the first time also non-coplanar grazing incidence diffraction (GID), to probe the strain field in mulitlayered gratings. The measurements were performed at the beamlines ID32 and ID 19 of the European Synchrotron Radiation Facility Grenoble. Multiple crystal-arrangements permit high-resolution reciprocal space mapping. The different scattering geometries provided complementary information on different components of the strain tensor. GID additionally allows to control the penetration depth of the X-ray beam via the angle of incidence, and in this way to investigate the strain field directly as a function of depth below the sample surface. By an appropriate choice of reflections, we were able to separate the influence of the grating shape on the diffraction pattern from that of the strain field.

Our samples consisted of low strained simple layer gratings and strongly symmetrically strained multilayer gratings, both of the material system (GaIn)(AsP)/InP. The compositions of the quaternary layers in the multilayered grating were chosen such that they exhibit a low average misfit with respect to the substrate.

At present, X-ray diffraction is the only non-destructive method of investigation for buried structures. Investigating the gratings before and after burying under an InP capping layer, we could identify the role of the technologically important process of burying in the evolution of the strain field.

We compare our experimental results with numerical simulations of the strain field in the samples, finding a good agreement. The simulations are based on an elasticity model, taking into account the interaction of different crystalline media at their interfaces. The numerical solution is done with a Finite Element Method.

Our results show a laterally periodic strain field which has its origin at the substrate/layer interface. After etching, the effect of the increased sample surface allows the grating region to relax inhomogeneously. After burying, the embedding layer partially counterbalances this relaxation effect. However, there remains a strain field which extends both into the substrate and into the embedding layer and reaches the free planar sample surface. Additionally the intrinsic antisymmetric vertical strain modulation in the planar mutlilayer leads to antisymmetric elastic strain relaxation in the mutlilayer near the side walls of the grating.