This dataset provides the experimental data used to generate the figures in the paper entitled "Spatially-resolved optical and structural properties of semi-polar (11-22) Al_xGa_(1-x)N with x up to 0.56".
The cathodoluminescence (CL) data discussed and presented in the paper was recorded using a variable pressure field emission scanning electron microscope (SEM, FEI Quanta 250) which is equipped with a custom-built CL hyperspectral imaging system. The CL system collects the emitted light at an angle of 45° with respect to the incident electron beam using a Cassegrain reflecting objective. The light is then dispersed using a 125 mm focal length spectrograph (Oriel MS125) and detected using an electron-multiplying charge-coupled device (Andor Newton). As the electron beam scans across the sample surface, a whole CL spectrum is recorded per pixel building up the 3D hyperspectral data set. 2D CL images can then be extracted from the hyperspectral data set, such as peak energy, intensity or half width.
Electron channelling contrast imaging is a non-destructive, diffraction technique performed in the SEM. ECC images are generally constructed by measuring the intensity of the backscattered electrons (BSEs) as the electron beam scans across the surface of a suitably-orientated sample. Any changes in crystallographic orientation and local strain can be monitored by the variation in the BSE intensity causing a change in contrast in an ECC image. This allows the imaging of low-angle tilt and rotation boundaries, atomic steps and extended defects (e.g. TDs). ECCI is carried out in a forward scattering geometry in a field emission SEM (FEI Sirion 200), equipped with an electron-sensitive diode and a custom-built signal amplifier.
Characterisation of the surface morphology was performed using atomic force microscopy (AFM, Bruker Dimension with Icon scanner) in PeakForce tapping mode with ScanAsyst Air probes.
Abstract of the paper:
Pushing the emission wavelength of efficient ultraviolet (UV) emitters further into the deep-UV requires material with high crystal quality, while also reducing the detrimental effects of built-in electric fields. Crack-free semi-polar (11-22) Al_xGa_(1-x)N epilayers with AlN contents up to x=0.56 and high crystal quality were achieved using an overgrowth method employing GaN microrods on m-sapphire. Two dominant emission peaks were identified using cathodoluminescence hyperspectral imaging. The longer wavelength peak originates near and around chevron-shaped features, whose density is greatly increased for higher contents. The emission from the majority of the surface is dominated by the shorter wavelength peak, influenced by the presence of basal-plane stacking faults (BSFs). Due to the overgrowth technique BSFs are bunched up in parallel stripes where the lower wavelength peak is broadened and hence appears slightly redshifted compared with the higher quality regions in-between. Additionally, the density of threading dislocations in these region is one order of magnitude lower compared with areas affected by BSFs as ascertained by electron channelling contrast imaging. Overall, the luminescence properties of semi-polar AlGaN epilayers are strongly influenced by the overgrowth method, which shows that reducing the density of extended defects improves the optical performance of high AlN content AlGaN structures.