Figure 1: Microfabrication of bulk GaN crystal into single crystalline GaN microbridges. An optical photograph showing the bulk GaN crystal (top left) from which the samples are microfabricated. SEM images show the microfabricated GaN microbridge (top right) and the in situ tensile straining process (bottom), demonstrating an ultralarge elastic strain of up to 6.8% before fracture.
A research team from the Faculty of Engineering at the University of Hong Kong (HKU) has recently achieved a significant scientific breakthrough. Researchers have successfully used mechanical stretching technology to dynamically control the emission colour of gallium nitride (GaN) material from "ultraviolet (UV) to blue light." This technological breakthrough provides a new semiconductor material control solution for future advanced power transistors, optoelectronic components, radio frequency components, and micro-LED displays.
Led by Professor Yang Lu from the Department of Mechanical Engineering, the team utilised micro-nano processing technology to fabricate single-crystalline GaN material into tiny bridge-like structures (Figure 1). Through precise mechanical stretching, the material achieved an elastic deformation of up to 6.8%, with a tensile strength of approximately 11 GPa. This demonstrates the extraordinary elastic deformation capability brought by the size effect, offering broad prospects for deep strain engineering.
This physical stretching not only did not damage the material but also successfully shifted the emission colour of GaN from the originally invisible ultraviolet light to a visible blue light. In experiments combining in-situ mechanical stretching with cathodoluminescence (CL) systems, researchers monitored optical property changes in real-time during the straining process. When the stretching degree reached 3.9%, a significant change in emission colour was observed. The bandgap of GaN continuously redshifted from 3.41 eV to 3.08 eV (Figure 2), and the emission wavelength correspondingly shifted from the ultraviolet region into the visible light region. Under maximum strain conditions, the bandgap could be further reduced to 2.96 eV (wavelength shifting from approximately 365 nm to 420 nm).
As the core material for blue LEDs, which led to the Nobel Prize in Physics 2014, scientists previously needed to add different chemical elements to adjust the emission colour of GaN. However, this research from HKU demonstrates a purely physical control method. The uniqueness of this technology lies in its "reversibility"—when the stretching force is removed, the material returns to its original state, and the emission colour reverts to ultraviolet light. The luminescent properties of GaN change completely reversibly with the strain state. This dynamic control method, distinct from traditional approaches that require altering the material's chemical composition, introduces a new direction for semiconductor optoelectronic technology.
To demonstrate the potential for practical device applications, the research team further designed and microfabricated a mechanically strain-fixed GaN device with a push-to-pull structure (Figure 3). By locking in a tensile strain of ~3%, the device successfully achieved a stable wavelength redshift from 363 to 371nm, maintaining the strained light emission state without requiring continuous external force, making this design more practical for applications. In the future, this technology is expected to be applied in micro-displays, intelligent lighting, and even biosensing fields, bringing more innovative possibilities to people's lives.
The research findings have been published in top physics journal Physical Review X under the title "Deep Elastic Strain Engineering of Free-Standing GaN Microbridge."
About Professor Yang Lu
Professor Yang Lu is currently Chair Professor of Nanomechanics in the Department of Mechanical Engineering and Kingboard Professor in Materials Engineering, and serves as Associate Dean (Mainland Affairs) of the Faculty of Engineering at HKU. Professor Lu is a leading expert in experimental nanomechanics and its interdisciplinary applications in materials engineering, advanced manufacturing, and semiconductor technologies. He is recognized for his groundbreaking works on elucidating the extreme mechanical properties of crystalline solids at micro- and nanoscale, and his innovative use of nanomechanics to manipulate micro/optoelectronics and their applications in a diverse set of engineering areas. Professor Lu has published more than 300 journal articles in peer-reviewed academic journals including Science, Nature Materials, Nature Nanotechnology etc., and 7 US patents granted.
