Low-defect metamorphic Si (Ge) epilayers on Si (001) with a buried template of nanocavities for multiple-junction solar cells

Highlights

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Fully relaxed Si(Ge) pseudo-substrates (PSs) grown on implanted Si(001).

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Formations of half-loops of dislocations having 60°segments at the Si(Ge)/Si interface.

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Low threading dislocations densities are observed on HRTEM and AFM images for SiGe PSs.

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Using this technique for manufacture of Ge PS for multi-junction solar cells.

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Abstract

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 Si_0.77Ge_0.23 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 Si_0.77Ge_0.23layers 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.

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Graphical abstract

Keywords

• Implantation; 
• He⁺; 
• Nano-bubbles; 
• Ge(Si); 
• LPCVD; 
• Strain relaxation
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    • SOURCE:Sciencedirect
      
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