Study end surfaces of GaAsSb NW grown on graphene by electron microscopy
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- Institutt for fysikk 
III-V nanowires (NWs) are attractive for solar cells. Through the compositionof the NWs, their bandgap and hence their electronic properties can be tuned, forexample, to optimise absorption so that most of the Sun's radiation is used.In this study, compositional analysis of Ga-As-Sb, Al-Ga-N, In-Ga-N systems hasbeen done using Scanning transmission electron microscopy (STEM) and EnergyDispersive X-ray Spectroscopy (EDS) with the aid of the Python-based Hyperspylibrary for a more transparent and possibly better compositional analysis. Thisincludes setting up an analysis routine based on the quantication method calledzeta method intruduced by Watanabe & Williams, but not available yet in com-mercial software packages.The required -factor has been estimated experimentally for the elements Ga, As,Sb, Al, N, In, C, O, Si on the used JEOL JEM2100F with an 80 mm2 silicondrift detector and the -method compared to the classical Cli-Lorimer method.Dierent reference samples with known thickness and composition were used forthis step.Tilt eects on the zeta-factor quantication were studied with GaAs NWs andthe Ga catalyst droplet of GaAsSb NWs. When the holder was tilted to negativeX-tilts, the -factor increased. In the range between 0o and 35o tilt, the -factorswere constant indicating no detector shadowing.For calibration and mapping, a positive tilt was used to avoid shadowing eects.Quantitative compositional maps have been calculated of areas within GaAsSb,AlGaN and InGaN NWs that contain a heterostructure. These heterostructurescould have a varying thickness or composition in the direction of the electron beamUsing the EDS spectra images, thickness maps could be constructed for areas wherethe composition was known.Further renements to the zeta method have to be made, especially nding goodcalibration specimens with exactly known thicknesses.