Paper EM-WeM10
Probing the Effect of Interaction and Thermal Expansion Mismatch between Ge and Templated Mask on Defects during Selective Molecular Beam Epitaxy of Ge on Si

Wednesday, October 17, 2007, 11:00 am, Room 612

We have previously demonstrated that high-quality, single-crystalline Ge can be grown on Si by epitaxial lateral overgrowth (ELO) as well as by touchdown where nanoscale windows (~7 nm in diameter) are created through a thin chemical SiO₂ layer. These techniques have been successfully used to reduce threading dislocations in the Ge-Si lattice mismatched system. Despite the improvement, dislocations are generated in the epilayer above the templated mask. To test the hypothesis that these defects occur due to varying level of epilayer-template interaction energy (e.g., bond strength and diffusion activation barrier) and thermal expansion differences between the epilayer and template, in addition to coalescence events, we have created masks of SiO₂, Al₂O₃, and Ta having thermal expansion coefficients smaller, larger, and equal to Ge epilayers selectively grown on Si in the mask openings. The windows in the masks are created by spin-coating a disperse layer of polystyrene spheres on the Si substrate followed by mask deposition and lift-off. Ge islands are selectively grown in the mask openings using molecular beam epitaxy (MBE), and further grown laterally over the mask until coalescence occurs. The Ge layers are then characterized by x-ray diffraction, transmission electron microscopy, and etch pit density after chemical-mechanical polishing to determine the effect of interaction energy and thermal expansion differences on the resulting film quality. Furthermore, we quantitatively measure the desorption activation energy of Ge adspecies from the SiO₂, Al₂O₃, and Ta masks to determine the characteristic diffusion lengths of Ge adspecies on the mask surfaces. We will then present the impact of inter-distance of windows in the masks on dislocation density in the Ge epilayer.