Metamorphic GaAsP buffers for growth of wide-bandgap InGaP solar cells

Abstract

GaAs x P 1 − x graded buffers were grown via solid source molecular beam epitaxy (MBE) to enable the fabrication of wide-bandgap InyGa1−yP solar cells. Tensile-strained GaAsxP1−x buffers grown on GaAs using unoptimized conditions exhibited asymmetric strain relaxation along with formation of faceted trenches, 100–300 nm deep, running parallel to the [01¯1] direction. We engineered a 6 μm thick grading structure to minimize the faceted trench density and achieve symmetric strain relaxation while maintaining a threading dislocation density of ≤106 cm−2. In comparison, compressively-strained graded GaAsxP1−x buffers on GaP showed nearly-complete strain relaxation of the top layers and no evidence of trenches but possessed threading dislocation densities that were one order of magnitude higher. We subsequently grew and fabricated wide-bandgap InyGa1−yP solar cells on our GaAsxP1−x buffers. Transmission electron microscopy measurements gave no indication of CuPt ordering. We obtained open circuit voltage as high as 1.42 V for In0.39Ga0.61P with a bandgap of 2.0 eV. Our results indicate MBE-grown InyGa1−yP is a promising material for the top junction of a future multijunction solar cell.

Elemental and magnetic sensitive imaging using x-ray excited luminescence microscopy

“…To grow it on a conventional substrate such as GaAs, the use of an intermediate step-graded buffer layer of GaAs x P 1-x is required. [10][11][12] Thus, knowledge of the luminescence efficiency and defect structure is critical for advancing devices based on these materials. Figure 3 were incident at an angle of 45 • .…”

Threading dislocation density characterization in III–V photovoltaic materials by electron channeling contrast imaging

“…Dislocations in III-V metamorphic materials are often inhomogeneously distributed into regions of high TDD (i.e. clusters or pileups) separated by large areas with relatively low TDD [6][7][8][9]. This presents challenges for accurate TDD quantification and hinders understanding of the effect of dislocations on device performance.…”

Heteroepitaxy, an Amazing Contribution of Crystal Growth to the World of Optics and Electronics

“…In the indicated case (negative mismatch; tensile layer growth) the strain will be released as plastic via dislocation nucleation and glide and form two-dimensional (2D) layers [80,91]. That is why plastic relaxation is encouraged if the goal is to grow metamorphic buffers [92]. In contrast, when the strain is elastic (positive misfit; compressive layer growth) there will be an elastic relaxation that will result in surface roughening, which can be used to drive quantum dot self-assembly processes [80,93,94].…”