Metrology of crystal defects through intensity variations in secondary electrons from the diffraction of primary electrons in a scanning electron microscope

Understanding defects and their roles in plastic deformation and device reliability is important for the development of a wide range of novel materials for the next generation of electronic and optoelectronic devices. We introduce the use of gaseous secondary electron detectors in a variable pressure scanning electron microscope for non-destructive imaging of extended defects using electron channelling contrast imaging. We demonstrate that all scattered electrons, including the secondary electrons, can provide diffraction contrast as long as the sample is positioned appropriately with respect to the incident electron beam. Extracting diffraction information through monitoring the modulation of the intensity of secondary electrons as a result of diffraction of the incident electron beam, opens up the possibility of performing low energy electron channelling contrast imaging to characterise low atomic weight and ultra-thin film materials. Our methodology can be adopted for large area, nanoscale structural characterisation of a wide range of crystalline materials including metals and semiconductors, and we illustrate this using the examples of aluminium nitride and gallium nitride. The capability of performing electron channelling contrast imaging, using the variable pressure mode, extends the application of this technique to insulators, which usually require conducting coatings on the sample surface for traditional scanning electron microscope based microstructural characterisation.

Determining GaN Nanowire Polarity and its Influence on Light Emission in the Scanning Electron Microscope

The crystal polarity of noncentrosymmetric wurtzite GaN nanowires is determined nondestructively in the scanning electron microscope using electron backscatter diffraction (EBSD). The impact of the nanowire polarity on light emission is then investigated using cathodoluminescence (CL) spectroscopy. EBSD can determine polarity of noncentrosymmetric crystals by interrogating differences in the intensity distribution of bands of the EBSD pattern associated with semipolar planes. Experimental EBSD patterns from an array of GaN nanowires are compared with theoretical patterns produced using dynamical electron simulations to reveal whether they are Ga- or N-polar or, as in several cases, of mixed polarity. CL spectroscopy demonstrates the effect of the polarity on light emission, with spectra obtained from nanowires of known polarity revealing a small but measurable shift (≈28 meV) in the GaN near band edge emission energy between those with Ga and N polarity. We attributed this energy shift to a difference in impurity incorporation in nanowires of different crystal polarity. This approach can be employed to nondestructively identify polarity in a wide range of noncentrosymmetric nanoscale material systems and provide direct comparison with their luminescence.

Spatially-resolved optical and structural properties of semi-polar [Formula: see text] Al x Ga1-x N with x up to 0.56

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 [Formula: see text] Al x Ga1-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.

Quantitative imaging of anti-phase domains by polarity sensitive orientation mapping using electron backscatter diffraction

Advanced structural characterisation techniques which are rapid to use, non-destructive and structurally definitive on the nanoscale are in demand, especially for a detailed understanding of extended-defects and their influence on the properties of materials. We have applied the electron backscatter diffraction (EBSD) technique in a scanning electron microscope to non-destructively characterise and quantify antiphase domains (APDs) in GaP thin films grown on different (001) Si substrates with different offcuts. We were able to image and quantify APDs by relating the asymmetrical intensity distributions observed in the EBSD patterns acquired experimentally and comparing the same with the dynamical electron diffraction simulations. Additionally mean angular error maps were also plotted using automated cross-correlation based approaches to image APDs. Samples grown on substrates with a 4° offcut from the [110] do not show any APDs, whereas samples grown on the exactly oriented substrates contain APDs. The procedures described in our work can be adopted for characterising a wide range of other material systems possessing non-centrosymmetric point groups.

Rapid nondestructive analysis of threading dislocations in wurtzite materials using the scanning electron microscope

We describe the use of electron channeling contrast imaging in the scanning electron microscope to rapidly and reliably image and identify threading dislocations (TDs) in materials with the wurtzite crystal structure. In electron channeling contrast imaging, vertical TDs are revealed as spots with black-white contrast. We have developed a simple geometric procedure which exploits the differences observed in the direction of this black-white contrast for screw, edge, and mixed dislocations for two electron channeling contrast images acquired from two symmetrically equivalent crystal planes whose g vectors are at 120° to each other. Our approach allows unambiguous identification of all TDs without the need to compare results with dynamical simulations of channeling contrast.