BITS Faculty Publications
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Item Single crystalline Ge thin film growth on c-plane sapphire substrates by molecular beam epitaxy (MBE)(RSC, 2022-04) Kumar, RahulSingle crystalline Ge has been grown on c-plane sapphire substrates by molecular beam epitaxy. Direct growth of Ge on sapphire results in three-dimensional (3D) Ge islands, two growth directions, more than one primary domain, and twinned crystals. The introduction of a thin AlAs nucleation layer significantly improved the surface and material quality, which is evident from a smoother surface, single epitaxial orientation, sharper rocking curve, and a single domain. The AlAs nucleation layer thickness was also investigated, and a 10 nm AlAs layer resulted in the lowest surface roughness of 3.9 nm. We have been able to achieve a single primary domain and reduced twinning relative to previous works. A high-quality Ge buffer on sapphire has the potential as an effective platform for the subsequent growth of GeSn and SiGeSn for microwave photonics.Item Comparison of different pathways in metamorphic graded buffers on GaAs substrate: Indium incorporation with surface roughness(Elsevier, 2015-01) Kumar, RahulIn this work, compositionally graded In(Al,Ga)As metamorphic buffers (MBs) on GaAs substrate have been grown by MBE through three different paths. A comparative study has been done to comprehend the effect of underlying MB on the constant composition InAlAs healing layer by analyzing the relaxation behaviour, composition and surface morphology of the grown structures. The compositional variation between the constant composition healing layers on top of graded MB has been observed in all three samples although the growth conditions have been kept same. Indium incorporation rate has been found to be dependent on underlying MB. By combining the result of atomic force microscopy, photo-luminescence and X-ray reciprocal space mapping, varying surface roughness has been proposed as the probable driving force behind different Indium incorporation rate.Item InAs nanostructures for solar cell: Improved efficiency by submonolayer quantum dot(Elsevier, 2021-06) Kumar, RahulThe effect of different quantum structures in the intrinsic region of a pin junction solar cell (SC) on the optical and electrical properties have been investigated. SCs with different quantum structures, such as, Stranski-Krastanov (SK) quantum dots (QDs), quantum well (QW), submonolayer (SML) QDs (0.25 ML, 0.5 ML and 0.75 ML) and a quasi-monolayer (1 ML) InAs stack, were fabricated while keeping the total InAs content the same in all SCs. In a comparison of performance, the SML-QD sample delivered superior performance (Almost 23% relative efficiency improvement compared to reference SC) compared to the other devices. Moreover, different coverages of SML InAs have been tested for optimum performance improvement of solar cell and near 0.25 ML InAs deposition was found best for solar cell application. These findings present a promising alternative to SK-QDs as intermediate band in photovoltaic applicationsItem Single crystalline Ge thin film growth on c-plane sapphire substrates by molecular beam epitaxy (MBE)(RSC, 2022) Kumar, RahulSingle crystalline Ge has been grown on c-plane sapphire substrates by molecular beam epitaxy. Direct growth of Ge on sapphire results in three-dimensional (3D) Ge islands, two growth directions, more than one primary domain, and twinned crystals. The introduction of a thin AlAs nucleation layer significantly improved the surface and material quality, which is evident from a smoother surface, single epitaxial orientation, sharper rocking curve, and a single domain. The AlAs nucleation layer thickness was also investigated, and a 10 nm AlAs layer resulted in the lowest surface roughness of 3.9 nm. We have been able to achieve a single primary domain and reduced twinning relative to previous works. A high-quality Ge buffer on sapphire has the potential as an effective platform for the subsequent growth of GeSn and SiGeSn for microwave photonics.