Strain-induced yellow to blue emission tailoring of axial InGaN/GaN quantum wells in GaN nanorods synthesized by nanoimprint lithography

www.nature.com/scientificreports OPEN Strain‑induced yellow to blue emission tailoring of axial InGaN/GaN quantum wells in GaN nanorods synthesized by nanoimprint lithography Geoffrey Avit1,2*, Yoann Robin1, Yaqiang Liao1, Hu Nan1, Markus Pristovsek1 & Hiroshi Amano1 GaN nanorods (NRds) with axial InGaN/GaN MQWs insertions are synthesized by an original costeffective and large-scale nanoimprint-lithography process from an InGaN/GaN MQWs layer grown on c-sapphire substrates. By design, such NRds exhibit a single emission due to the c-axis MQWs. A systematic study of the emission of the NRds by time-resolved luminescence (TR-PL) and power dependence PL shows a diameter-controlled luminescence without significant degradation of the recombination rate thanks to the diameter-controlled strain tuning and QSCE. A blueshift up to 0.26 eV from 2.28 to 2.54 eV (543 nm to 488 nm) is observed for 3.2 nm thick InGaN/GaN QWs with an In composition of 19% when the NRds radius is reduced from 650 to 80 nm. The results are consistent with a 1-D based strain relaxation model. By combining state of the art knowledge of c-axis growth and the strong strain relieving capability of NRds, this process enables multiple and independent single-color emission from a single uniform InGaN/GaN MQWs layer in a single patterning step, then solving color mixing issue in InGaN based nanorods LED devices. InGaN based semiconductors have a direct bandgap that can be tuned across the entire visible spectrum, from 0.7 eV for InN to 3.4 eV for GaN. Efficient blue and green emitting lasers and light emitting diodes (LEDs) have been achieved for many y ears1–3. However, InGaN must be grown at relatively low temperature which results in poor crystalline quality when the InGaN/GaN quantum wells (QWs) reach high In composition (> 20%)4–7. This hinders the development of efficient red emitting diodes. Furthermore, the compressive strain due to the lattice mismatch between InGaN and GaN induces a large piezoelectric field, which increases with the In composition. This reduces the radiative recombination rate due to spatially separating electron and hole wavefunctions, and induces a shift of the emission towards longer wavelength—the quantum confined Stark effect (QCSE). QSCE together with non-radiative recombinations at defects is also assumed as main causes for the reduced internal quantum efficiency (IQE)4 in InGaN based LEDs towards longer emission wavelengths. Interestingly, despite the strong QCSE, the luminescence of InGaN heterostructures on (0001) planes shows the highest IQE compared to m-axis or semipolar o rientations3,8,9. Recently, incorporating AlGaN layers into the QW barriers increased red emitting InGaN based LEDs more likely5,10,11. Hwang et al.12 demonstrated strong red luminescence with a peak wavelength at 629 nm and FWHM of 60 nm using an InGaN/AlGaN QW structure. Delta growth of AlN and InN has also shown some promising results13. Another approach is the use of nanostructures and nanorods (NRds), which are promising for the integration of high efficiency LEDs d evices14–19, for instance into micro-displays. NRds relieve the strain of vertical InGaN QWs at their sidewalls; and core–shell structures offer large active surface. Nevertheless, standard core/shell structures obtained by selective area growth (SAG) on masked substrates often have three or more type of facets (m-planes, c-plane and semi-polar planes)8,20,21, and each facet has its own emission properties due to different kinetic of incorporation of In and different piezoelectric fields8,22. Moreover, the IQE of these non- or semi-polar QWs remained much lower than expected. Finally, the independent current injection into each separate facet to control the emission color is a severe technological challenge for applications such as RGB displays. 1 IMaSS, Nagoya University, Furo‑cho, Chikusa‑ku, Nagoya 464‑8601, Japan. 2Present address: CNRS, UMR6602, Institut Pascal, 4 Avenue blaise Pascal, 63178 Aubière, France. *email: Scientific Reports | (2021) 11:6754 | https://doi.org/10.1038/s41598-021-86139-9 1 Vol.:(0123456789) www.nature.com/scientificreports/ Sample A B C D QWs In composition 17 19 21 21 QWs thickness 3.5 nm 3.2 nm 2.1 nm 1.3 nm E0 (eV) 2.8 2.78 2.82 3.0 Bm (eV) 0.32 0.42 0.34 – κ−1 (nm) 27 22 19 – Table 1.  Composition, thickness of the different QWs of this study derived by XRD measurements and values of E0, Bm and κ for sample A, B, C and D. The NRds in this study were fabricated by a combination of nano-imprint lithography (NIL) and a mixed dry–wet etching process of GaN wafers with axial InGaN/GaN multiple quantum wells (MQWs). Contrary to bottom-up processes, the top down process enables NRds with exclusively axial InGaN/GaN MQWs from etching of InGaN/GaN MQWs layers grown under optimized condition on planar GaN. Therefore, such NRds show a single emission due to the c-plane uniform InGaN/GaN MQWs. We use a nanoimprint lithography process which is a very powerful tool because it enables the patterning of 2-in. substrates in a few minutes compared to the patterning of 1 c m2 in a few hours by electron or focus ion beam lithography methods used in previous studies23–26 which have a very high resolution (50 nm) but are expensive and time consuming and therefore not suitable for mass production. The size, position and density of the NRds are also govern by the NIL mask which allows a good homogeneity of the physical properties compared to methods based on the self-assembling of metallic nano-islands27 or direct deposition of silica n anoparticles28,29 that are fast processes but present some irregularities and dispersion in the shape and size of the NRds. Furthermore, a metal mask is preferred to obtain high aspect ratio structures by deep plasma etching. While displacement talbot lithography can pattern thick resist at the n anoscale30, it is still an emerging technique with a resolution limited by the wavelength illumination source, often 365 nm30 or 266 nm31, while NIL resolution can be scaled further down. In order to change the emission wavelength, the strain relief is controlled via the diameter using a single InGaN/GaN MQWs set. Such control is achieved from 650 to 90 nm thanks to lateral wet etching in a AZ400K solution. Power dependence luminescence study is performed to show the influence of the piezoelectric field on the emission wavelength, while time-resolved (TR) luminescence is performed to enlighten any effect of the etching process on the recombination time of the charge carriers. The results are then interpreted by a phenomenological 1-D relaxation model based on an exciton potential at the center of the NRds dominated by the strain-induced piezo-electric field32. In the presented approach, providing the appropriate NIL mask design, the simultaneous patterning of NRds with different diameters could achieve multi-color emission from a single InGaN/GaN MQWs lay (...truncated)