Abstract
A novel overgrowth approach has been developed in order to create a multiple-facet structure consisting of only non-polar and semi-polar GaN facets without involving any c-plane facets, allowing the major drawbacks of utilising c-plane GaN for the growth of III-nitride optoelectronics to be eliminated. Such a multiple-facet structure can be achieved by means of overgrowth on non-polar GaN micro-rod arrays on r-plane sapphire. InGaN multiple quantum wells (MQWs) are then grown on the multiple-facet templates. Due to the different efficiencies of indium incorporation on non-polar and semi-polar GaN facets, multiple-colour InGaN/GaN MQWs have been obtained. Photoluminescence (PL) measurements have demonstrated that the multiple-colour emissions with a tunable intensity ratio of different wavelength emissions can be achieved simply through controlling the overgrowth conditions. Detailed cathodoluminescence measurements and excitation-power dependent PL measurements have been performed, further validating the approach of employing the multiple facet templates for the growth of multiple colour InGaN/GaN MQWs. It is worth highlighting that the approach potentially paves the way for the growth of monolithic phosphor-free white emitters in the future.
Highlights
The last two decades have seen tremendous progress in developing III-nitride semiconductor visible light emitters for solid-state lighting
Phosphor-free white light emitting diodes (LEDs) developed so far are primarily grown on c-plane substrates, i.e., by introducing InGaN multiple quantum wells (MQWs) with different indium composition as an active region9–12
Due to the well-known polarisation induced quantum confined Stark effect (QCSE), it is difficult to achieve a phosphor-free white LED with high optical efficiency on a c-plane substrate13. Another great challenge in developing a monolithic white LED is due to the fundamental limitation in incorporating high indium content into GaN on c-plane substrates, while InGaN with high indium content is crucial for achieving green or yellow emission, the major components for phosphor-free white LEDs
Summary
By means of a standard photolithography approach and a reactive ion etching (RIE) technique, the SiO2 film is fabricated into regularly arrayed micro-rods with a micro-rod diameter of 2.5 μm. The regularly arrayed SiO2 micro-rod arrays are used as a second mask to etch the GaN layer underneath into regular GaN micro-rod arrays by using a standard inductively coupled etching (ICP) system. Cathodoluminescence hyperspectral imaging was carried out at room temperature using a variable pressure field emission scanning electron microscope (FEI Quanta 250). The emitted light was collected by a Cassegrain reflecting objective, focussed on the entrance slit of a 1/8 m focal length spectrometer (Oriel MS125) and detected using an electron multiplying charge-coupled device (Andor Newton). The CL was collected in hyperspectral imaging mode, meaning a full CL spectrum was collected per pixel in an image
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