Low-defect metamorphic Si (Ge) epilayers on Si (001) with a buried template of nanocavities for multiple-junction solar cells
Résumé
Si (001)-oriented substrates (p-type) were implanted at 10 keV with He ion doses in the 5 x 10(15) cm(-2) range. They were annealed at 973 K for 1 h to create a buried layer of nano-sized bubbles. Layers of Si0.37Ge0.23 about 215 nm thick were grown by low pressure chemical vapor deposition at 848 K on these silicon wafers. The surface roughness and morphology were checked by atomic force microscopy and optical microscopy before and after etching with a modified Shimmel recipe to reveal etch pit dislocations. The thickness, composition, crystalline quality and relaxation-state of the SiGe layer were assessed by Rutherford backscattering spectroscopy and high-resolution X-ray diffraction. A fully relaxed strain-state was obtained for Si0.77Ge0.23 layers that did not exhibit the classical cross-hatched pattern morphology while having a threading-dislocations density below 10(3) cm(-2). This low dislocation density was confirmed by photoluminescence spectroscopy. From high resolution transmission electron microscopy observations, the quasi-total strain-relief is assumed to take place by emission of dislocation loops protruding down into the Si substrate from the SiGe/Si interface and terminating at void surfaces of the buried-nanoporous layer. Possible annihilation of dislocation segments coming from either the SiGe/Si interface or punched out in glide planes by overpressurized nanobubbles may also occur. This near surface porous Si is well adapted for multiple junction solar cell manufacturing. (C) 2014 Elsevier B.V. All rights reserved.