Low-defect metamorphic Si (Ge) epilayers on Si (001) with a buried template of nanocavities for multiple-junction solar cells
Fully relaxed Si(Ge) pseudo-substrates (PSs) grown on implanted Si(001).
Formations of half-loops of dislocations having 60°segments at the Si(Ge)/Si interface.
Low threading dislocations densities are observed on HRTEM and AFM images for SiGe PSs.
Using this technique for manufacture of Ge PS for multi-junction solar cells.
Si (001)-oriented substrates (p-type) were implanted at 10 keV with He+ ion doses in the 5×1015 cm−2 range. They were annealed at 973 K for 1 h to create a buried layer of nano-sized bubbles. Layers of Si0.77Ge0.23 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 Si0.77Ge0.23layers that did not exhibit the classical cross-hatched pattern morphology while having a threading-dislocations density below 103 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.