Spatially-resolved optical and structural properties of semi-polar $$\mathrm{(11}\bar{2}\mathrm{2)}$$ Al x Ga1−x N with x up to 0.56

Abstract

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 AlxGa1−xN 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.

Untwinned semipolar (101̅3) Al x Ga1-x N layers grown on m-plane sapphire

“…This hinders two-dimensional growth of high Al-content layers. The rough surface morphology of the (101̅ 3) layers might enhance alloy fluctuations, as previously reported for (112̅ 2) AlGaN overgrown on GaN microrods on m-plane sapphire [20]. Symmetric reciprocal space map (RSM) scans were measured to investigate crystal twinning and in-plane relationship of the (101̅ 3) AlGaN/AlN layers with respect to mplane sapphire.…”

Cathodoluminescence studies of chevron features in semi-polar (112¯2) InGaN/GaN multiple quantum well structures

“…5,6 However, semi-polar growth introduces a new range of growth imperfections, including arrow-head features often referred to as "chevrons." [7][8][9] Both planar and patterned substrates have been used to achieve semi-polar growth, and these different growth techniques have an impact on the chevron formation. 10,11 For growth on planar substrates, the formation of chevrons has been attributed to interference between undulations along the ½11 2 3 and ½1 100 directions, which in turn occur due to anisotropic surface diffusion.…”

Structural and luminescence imaging and characterisation of semiconductors in the scanning electron microscope

“…UV LEDs produced from semi-polar nitride semiconductor thin films promise higher performance than those produced from their polar counterparts, due to reduction of piezoelectric and spontaneous polarisation fields. Unfortunately, semi-polar nitride semiconductor thin films are often of poor quality with a high density of structural defects; in particular basal-plane stacking faults, in addition to TDs [62][63][64][65][66]. Here we report the study of the structural and luminescence properties of a 5μm thick (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22) GaN epilayer overgrown by MOVPE on a regular array of microrods on an m-plane sapphire substrate using ECCI and room temperature CL imaging.…”