Strain analysis of InGaN/GaN multi quantum well LED structures

Strain Relaxation Effect on the Peak Wavelength of Blue InGaN/GaN Multi-Quantum Well Micro-LEDs

In this paper, the edge strain relaxation of InGaN/GaN MQW micro-pillars is studied. Micro-pillar arrays with a diameter of 3–20 μm were prepared on a blue GaN LED wafer by inductively coupled plasma (ICP) etching. The peak wavelength shift caused by edge strain relaxation was tested using micro-LED pillar array room temperature photoluminescence (PL) spectrum measurements. The results show that there is a nearly 3 nm peak wavelength shift between the micro-pillar arrays, caused by a high range of the strain relaxation region in the small size LED pillar. Furthermore, a 19 μm micro-LED pillar’s Raman spectrum was employed to observe the pillar strain relaxation. It was found that the Raman E2H mode at the edge of the micro-LED pillar moved to high frequency, which verified an edge strain relaxation of = 0.1%. Then, the exact strain and peak wavelength distribution of the InGaN quantum wells were simulated by the finite element method, which provides effective verification of our PL and Raman strain relaxation analysis. The results and methods in this paper provide good references for the design and analysis of small-size micro-LED devices.

Enhanced Photoluminescence of Flexible InGaN/GaN Multiple Quantum Wells on Fabric by Piezo-Phototronic Effect

Fabric-based wearable electronics are showing advantages in emerging applications in wearable devices, Internet of everything, and artificial intelligence. Compared to the one with organic materials, devices based on inorganic semiconductors (e.g., GaN) commonly show advantages of superior characteristics and high stability. Upon the transfer of GaN-based heterogeneous films from their rigid substrates onto flexible/fabric substrates, changes in strain would influence the device performance. Here, we demonstrate the transfer of InGaN/GaN multiple quantum well (MQW) films onto flexible/fabric substrates with an effective lift-off technique. The physical properties of the InGaN/GaN MQWs film are characterized by atomic force microscopy and high-resolution X-ray diffraction, indicating that the transferred film does not suffer from huge damage. Excellent flexible properties are observed in the film transferred on fabric, and the photoluminescence (PL) intensity is enhanced by the piezo-phototronic effect, which shows an increase of about 10% by applying an external strain with increasing the film curvature to 6.25 mm^-1. Moreover, energy band diagrams of the GaN/InGaN/GaN heterojunction at different strains are illustrated to clarify the internal modulation mechanism by the piezo-phototronic effect. This work would facilitate the guidance of constructing high-performance devices on fabrics and also push forward the rapid development of flexible and wearable electronics.

Strain-stress study of AlxGa1-xN/AlN heterostructures on c-plane sapphire and related optical properties

This work presents a systematic study of stress and strain of Al_xGa_1−xN/AlN with composition ranging from GaN to AlN, grown on a c-plane sapphire by metal-organic chemical vapor deposition, using synchrotron radiation high-resolution X-ray diffraction and reciprocal space mapping. The c-plane of the Al_xGa_1−xN epitaxial layers exhibits compressive strain, while the a-plane exhibits tensile strain. The biaxial stress and strain are found to increase with increasing Al composition, although the lattice mismatch between the Al_xGa_1−xN and the buffer layer AlN gets smaller. A reduction in the lateral coherence lengths and an increase in the edge and screw dislocations are seen as the Al_xGa_1−xN composition is varied from GaN to AlN, exhibiting a clear dependence of the crystal properties of Al_xGa_1−xN on the Al content. The bandgap of the epitaxial layers is slightly lower than predicted value due to a larger tensile strain effect on the a-axis compared to the compressive strain on the c-axis. Raman characteristics of the Al_xGa_1−xN samples exhibit a shift in the phonon peaks with the Al composition. The effect of strain on the optical phonon energies of the epitaxial layers is also discussed.