Data for "Strain and luminescence properties of hexagonal hillocks in N-polar GaN"

This dataset provides the experimental data used to generate the figures in the paper entitled "Strain and luminescence properties of hexagonal hillocks in N-polar GaN".

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. The room temperature CL measurements were acquired with a beam voltage of 5 kV.

Electron backscatter diffraction (EBSD) measurements were performed using a Nordlys and Symmetry S2 EBSD detector from Oxford Instruments attached to an FEI Quanta 250 and Versa 3D field emission scanning electron microscope (SEM), respectively. The EBSD data was acquired at 20 kV and at a sample tilt of 70° with respect to the normal of the incident electron beam.

Raman spectroscopy measurements were performed at room temperature using a Renishaw Invia spectrometer and a 488 nm solid-state laser. The laser beam was focused on the sample surface using 100× microscope lens (NA= 0.9) to a spot of ≈0.5 μm in diameter. The unpolarised signal was collected in a confocal mode in the quasi-backscattering geometry. The sample was mounted on an automated XYZ microscope stage allowing the scanning with ≈0.05 μm step resolution in XY-plane. Raman images were obtained from peak fitting of the E2 (high) phonon mode in GaN at each measured location to extract peak position, total peak area and peak full width at half maximum (FWHM).

Abstract of the paper:
Owing to its unique properties, N-polar GaN offers several advantages over Ga-polar GaN, particularly for applications in high power electronics. However, the growth of high-quality N-polar material is challenging. One dominant issue is the increased surface roughness, due to the occurrence of hexagonal-shaped hillocks, referred to as hexagons, on the material’s surface. Although, there are different methods to reduce the density of these hillocks, such as the use of vicinal substrates or optimum growth conditions, the properties of such hillocks are not extensively studied. Here, we investigate the crystallographic and luminescence properties of these hexagonal features using the techniques of electron backscatter diffraction (EBSD) and cathodoluminescence (CL) hyperspectral imaging in the scanning electron microscope combined with micro-Raman mapping. CL revealed increased light emission from the top of the hexagons compared with the surrounding material. Additionally, dark spots in intensity images, associated with non-radiative recombination at threading dislocations, could be resolved on top of the hexagons, but not in the surrounding area, implying improved material quality of the hexagons. Extensive strain analysis using EBSD revealed that the hexagons are composed of equivalent triangular segments with tensile strain along symmetrically equivalent <11-20> directions. As the hexagons become larger, this strain increases with distance from the centre. This was confirmed by mapping the Raman E2 (high) mode. Overall this provides crucial insight into the strain state of these hexagonal features.