Marina Manganaro

InAlAs solar cell on a GaAs substrate employing a graded InxGa1−xAs–InP metamorphic buffer layer

Single junction In0.52Al0.48As solar cells have been grown on a (100) GaAs substrate by employing a 1 μm thick compositionally graded InxGa1−xAs/InP metamorphic buffer layer to accommodate the 3.9% mismatch. Cells processed from the 0.8 μm thick InAlAs layers had photovoltaic conversion efficiency of 5% with an open circuit voltage of 0.72 V, short-circuit current density of 9.3 mA/cm2, and a fill factor of 74.5% under standard air mass 1.5 illumination. The threading dislocation density was estimated to be 3 × 108 cm−2.

Surface organization of homoepitaxial InP films grown by metalorganic vapor-phase epitaxy

We present a systematic study of the morphology of homoepitaxial InP films grown by metalorganic vapor-phase epitaxy which are imaged with ex situ atomic force microscopy. These films show a dramatic range of different surface morphologies as a function of the growth conditions and substrate (growth temperature, V/III ratio, and miscut angle < 0.6deg and orientation toward A or B sites), ranging from stable step flow to previously unreported strong step bunching, over 10 nm in height. These observations suggest a window of growth parameters for optimal quality epitaxial layers. We also present a theoretical model for these growth modes that takes account of deposition, diffusion, and dissociation of molecular precursors, and the diffusion and step incorporation of atoms released by the precursors. The experimental conditions for step flow and step bunching are reproduced by this model, with the step bunching instability caused by the difference in molecular dissociation from above and below step edges, as was discussed previously for GaAs (001).

Suppression of threading defects formation during Sb-assisted metamorphic buffer growth in InAs/InGaAs/InP structure

A virtual substrate for high quality InAs epitaxial layer has been attained via metalorganic vapor-phase epitaxy growth of Sb-assisted InxGa1−xAs metamorphic buffers, following a convex compositional continuous gradient of the In content from x = 53% to 100%. The use of trimethylantimony (or its decomposition products) as a surfactant has been found to crucially enable the control over the defect formation during the relaxation process. Moreover, an investigation of the wafer offcut-dependence of the defect formation and surface morphology has enabled the achievement of a reliably uniform growth on crystals with offcut towards the [111]B direction.

Unusual nanostructures of “lattice matched” InP on AlInAs

We show that the morphology of the initial monolayers of InP on Al0.48In0.52As grown by metalorganic vapor-phase epitaxy does not follow the expected layer-by-layer growth mode of lattice-matched systems, but instead develops a number of low-dimensional structures, e.g., quantum dots and wires. We discuss how the macroscopically strain-free heteroepitaxy might be strongly affected by local phase separation/alloying-induced strain and that the preferred aggregation of adatom species on the substrate surface and reduced wettability of InP on AlInAs surfaces might be the cause of the unusual (step) organization and morphology.

SiNx-induced intermixing in AlInGaAs/InP quantum well through interdiffusion of group III atoms

We analyze the composition profiles within intermixed and non-intermixed AlInGaAs-based multiple quantum wells structures by secondary ion mass spectrometry and observe that the band gap blue shift is mainly attributed to the interdiffusion of In and Ga atoms between the quantum wells and the barriers. Based on these results, several AlInGaAs-based single quantum well (SQW) structures with various compressive strain (CS) levels were grown and their photoluminescence spectra were investigated after the intermixing process involving the encapsulation of thin SiNx dielectric films on the surface followed by rapid thermal annealing. In addition to the annealing temperature, we report that the band gap shift can be also enhanced by increasing the CS level in the SQW. For instance, at an annealing temperature of 850 °C, the photoluminescence blue shift is found to reach more than 110 nm for the sample with 1.2%-CS SQW, but only 35 nm with 0.4%-CS SQW. We expect that this relatively larger atomic compositional g...

Evaluation of defect density by top-view large scale AFM on metamorphic structures grown by MOVPE

Abstract We demonstrate an atomic force microscopy based method for estimation of defect density by identification of threading dislocations on a non-flat surface resulting from metamorphic growth. The discussed technique can be applied as an everyday evaluation tool for the quality of epitaxial structures and allow for cost reduction, as it lessens the amount of the transmission electron microscopy analysis required at the early stages of projects. Metamorphic structures with low surface defectivities (below 106) were developed successfully with the application of the technique, proving its usefulness in process optimisation.

Inp-alinas “strain free” early stages heteroepitaxy leading to nanostructure formation by MOVPE

We show that heteroepitaxy of thin films of InP on Al0.48In0.52As by MOVPE does not result in continuous flat layer growth, but instead develops a number of low dimensional structures, e.g. quantum dots and rings. We consider local phase separation/alloying-induced strain and preferred aggregation of adatom species on the substrate surface together with reduced wettability of InP on AlInAs to be the cause of the observed surface organization. This behavior has not been previously reported, and it opens a new application window for creating strain-free type-II staggered QD structures for possible applications in optical memories, detectors, and solar cells.

Surfactant role of (TM)Sb in MOVPE growth of metamorphic InGaAs graded buffers

We present a virtual substrate for high quality InAs epitaxial layer, obtained via metalorganic vapor-phase epitaxy growth of Sb-assisted InxGa1−xAs metamorphic buffers, following a convex compositional continuous gradient of the In content from x = 53 % to 100 %. The use of Trimethylantimony (or its decomposition products) as a surfactant was found to be a suitable way to control defect formation during the relaxation process. Reliably uniform growth with good morphology was obtained on crystals with misorientation toward [111]B direction.

High speed AlInGaAs/InGaAs quantum well waveguide photodiode for wavelengths around 2 microns

A high speed waveguide photodiode fabricated with AlInGaAs/InGaAs multiple quantum wells for 2 micron wavelength detection is reported. The fabricated photodiode shows a photoresponsivity of 0.3 A/W at around 2μm wavelength and a 3 dB bandwidth around 7 GHz.

Quantum well intermixing in AlInGaAs QW structures through the interdiffusion of group III atoms

In this paper, the composition profiles within intermixed AlInGaAs-based multiple quantum wells structure are analyzed by secondary ion mass spectrometry and the bandgap blue shift is found to be mainly attributed to the interdiffusion of In and Ga between the quantum wells and barriers. Based on these results, AlInGaAs-based single quantum well structures with various compressive strain (CS) levels are then investigated and we report an enhancement of the bandgap shift by increasing the compressive strain level in the SQW. For instance, at an annealing temperature of 850°C, the photoluminescence blue shift can reach more than 110 nm for the sample with 1.2%-CS SQW, but only 35 nm with 0.4%-CS SQW. The indium composition ratios are designed to be 0.59 and 0.71 for the 0.4% and 1.2%-CS quantum wells, respectively, as opposed to 0.53 for the lattice-matched barrier. This relatively larger atomic compositional gradient between the CS quantum well and barrier is expected to facilitate the atomic interdiffusion and lead to the more pronounced bandgap shift.