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Mapping light emission, elemental composition and polarity of GaN wires with synchrotron radiation

​​​The properties of hard X-ray to excite light emission can be combined simultaneously with X-ray fluorescence techniques to study nanoscale devices as demonstrated for multiple quantum well (MQW) InGaN/GaN core/shell wires integrated in blue LED emitters. This study allowed us to understand the effect of growth conditions on GaN polarity in wires and the consequences on InGaN deposition and MQWs quality.

Published on 21 July 2018
Noncentrosymmetric one-dimensional structures are key driving forces behind advanced nanodevices. Owing to the critical role of silane injection in Metal-Organic Vapour Phase Epitaxy in creating nanosized architectures, it has become a challenge to investigate the induced local lattice polarity in the volume of single GaN wires. By combining synchrotron X-ray fluorescence and X-ray excited optical luminescence, we show here experimental evidence of the role of silane to promote the N-polarity, light emission and elemental incorporation within single wires.

The use of focused beams (about 60x60 nm2 in size) provides an accurate tool to study heterostructures, i.e. radial and axial MQW visible emission and also a complete mapping of the device to study polarity and band defect features. This experiment demonstrates the ability to spatially examine carrier diffusion phenomena without electrical contacts, opening new avenues for further studies with simultaneous optical and elemental sensitivity at the nanoscale. The collection of light emission excited by X-rays with its spectral analysis (near UV- visible range) can be combined to other techniques, in particular absorption spectroscopy and µLaue diffraction to get access to local order and strain/orientation and to link optical and structural properties. In a similar way, It can be applied to understanding many other complex semiconductor heterostructures of the optoelectronics and photovoltaics fields.

(a) Geometry of the excitation of InGaN/GaN wires (b) with X-rays nanobeam to collect emitted light and X-ray fluorescence (d). (c) definition of the Ga polarity in the non-centrosymmetric GaN crystal and (e) emission spectra.

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