Bong Won-jun, Ph.D. candidate (Department of Electrical Engineering); Professor Park Jun-Seok (Department of Electrical Engineering); Adjunct Professor Ahn Hyung-soo (Department of Quantum Security and Next-Generation Communications); Adjunct Professor Lee Jae Hak (Department of Quantum Security and Next-Generation Communications)

Bong Won-jun (Ph.D. candidate) and Professor Park Jun-Seok from the Quantum Communications Research Lab (formerly the Millimeter-Wave Circuit Research Lab) in the Department of Electrical Engineering at Kookmin University, along with researchers from the Electronics and Telecommunications Research Institute (ETRI), LNBS, Korea National University of Ocean Science and Technology, and Nagoya University in Japan have jointly demonstrated that semiconductor core-shell crystals—comprising a silicon (Si) or germanium (Ge) core and an aluminum nitride (AlN) shell—grow in a manner contrary to conventional wisdom, with the “shell forming first and the core filling in afterward.” Furthermore, the researchers observed luminescence in the E-band (approximately 1,372–1,392 nm), which corresponds to the optical communication band, at room temperature in these crystals. This study was published in the international journal *ACS Omega*, published by the American Chemical Society (ACS), on October 8, 2026.
 
Silicon and germanium are key materials widely used in semiconductor chips, solar cells, and optoelectronic devices. The core-shell structure—in which these materials are layered as a core and a shell within a single crystal—has garnered attention as a material for next-generation devices because it allows for the control of interfacial properties, deformation, and composition. Until now, such structures have been understood within the framework that “the crystal core forms first, and the shell is then deposited on top of it.” However, the exact sequence in which micrometer-scale crystals are actually formed had not been clearly elucidated.
 
The research team found a clue in the natural phenomenon of dew forming as beads at regular intervals on morning spiderwebs. This phenomenon, in which a long, thin liquid column breaks into droplets at regular intervals, is explained by the “Plateau-Rayleigh Instability (PRI).” The research team placed aluminum (Al), gallium (Ga), germanium, and silicon into a single graphite boat and grew crystals by flowing hydrogen chloride (HCl) and ammonia (NH₃) gases through them—a process known as mixed-source hydride vapor phase epitaxy (HVPE). By interrupting the ammonia supply midway through the process, they first created long, thin aluminum-based “nanoabsorbents.” As a result, thin aluminum films spaced approximately 2–6 micrometers apart—resembling dewdrops on a spiderweb—formed regularly along the nano-absorbents, and it was observed that germanium droplets became trapped in the voids between the films, gradually filling them from the inside out.
 
To verify this process, the research team employed electron microscopy (FE-SEM, HRTEM), energy-dispersive X-ray spectroscopy (EDS), and Raman spectroscopy, in addition to nano-focus X-ray tomography (nano-CT). A three-dimensional examination of the interior of the crystals using nano-CT revealed, in sequence under a single growth condition, completely hollow needle-like structures, partially filled structures, and fully filled core-shell structures.
 These results support the “shell-first” growth model, in which an aluminum nitride-based shell framework is formed first, followed by the gradual filling of that shell with germanium or silicon-germanium. In silicon-rich crystals, regions resembling the hexagonal (2H) structure—which differs from the typical cubic silicon structure—were also observed locally.
 
In particular, the research team observed emission at wavelengths of approximately 1,371.9 nm, 1,377.0 nm, 1,381.1 nm, and 1,391.8 nm—corresponding to the E-band for optical communications—through room-temperature photoluminescence (PL) measurements of the crystals grown in this manner. The fact that silicon-germanium-based crystals emit light at room temperature within the wavelength band used for fiber-optic communications demonstrates their potential for applications in light sources and quantum devices. Using first-principles calculations, the research team examined carbon-oxygen composite defects as candidate models for light emission and presented their sensitivity as a function of calculation conditions. At this stage, they did not conclusively identify a specific defect as the source of light emission but used it as a reference point for subsequent low-temperature measurements and photon correlation measurements.

This study is significant in that it confirmed, through 3D imaging, a growth path that reverses the conventional “core-first, shell-later” sequence in the field of semiconductor crystal growth, and observed light emission in the communication band at room temperature from that crystal. The Kookmin University research team is currently leading a government-funded project to develop a quantum memory platform that operates directly in the telecommunications band without the need for Gwang-frequency conversion, and the hexagonal silicon region and telecommunications-band luminescence confirmed in this study are expected to serve as a starting point for validating candidate core materials.
 
Professor Park Jun-Seok stated, “This research is highly significant in that it directly confirmed the crystal growth sequence in three dimensions and demonstrated that the crystal emits light at optical communication wavelengths at room temperature,” adding, “We will continue our research on silicon-based communication-band quantum light sources and quantum memory platforms.”
 
This research was conducted with support from the Institute for Information & Communications Technology Planning & Evaluation (IITP)’s project “Development of Core Technologies for a Communication-Band Dual-Band Quantum Memory Platform without Gwang-Frequency Conversion (QFC)” (Ministry of Science and ICT), the Seoul ANCHOR Project funded by the Ministry of Education and the Seoul Metropolitan Government, and the IMaSS Joint Use and Joint Research Program at Nagoya University.
 
Meanwhile, the research paper was authored by Choi, S., Kim, K. H., Bong, W. J., Park, J.-S., Kwon, E., Kim, S., Kim, M., Jeong, D., Lee, J. H., Kato, T., Honda, Y., Honda, A., and Ahn, H. S., titled “Plateau-Rayleigh Instability-Driven Shell-First Growth of (Ge, Si1-xGex)-AlN Core-Shell Crystals Exhibiting Telecom E-Band Photoluminescence,” was published in *ACS Omega*, Vol. 11, No. 41 (2026), pp. 63138–63150.