Department of Material Science Research Team
Achieved Major Breakthrough
Opto-electrical Materials Results Published in the
Internationally Renowned Journal "Nanotechnology"
Department of Material Science Professor Miln Jang Chen's research team achieved a major breakthrough in opto-electrical materials. His research team did a study on "highly efficient hetero-structured silicon light-emitting diodes" and that paper was selected as the "highlighted article" by the internationally renowned journal "Nanotechnology."
Silicon is a non-direct energy band (indirect bandgap) semiconductor, so its luminescent efficiency is very low. Nevertheless, the Department of Material Science research team achieved a major breakthrough in using nano-silicons for opto-electrical materials. Led by Professor Miln-Jang Chen and Professor Jer-Ren Yang, the research team used the hetero-structures of n-type zinc oxide /silicon dioxide, silicon nano crystals, and n-ZnO/SiO2 nanocrystals-SiO2/pSi, and successfully produced highly efficient silicon light-emitting diodes. This achievement has been published in the internationally renowned journal "Nanotechnology" and was chosen as the "highlighted article" by nanotechweb.org and reported on its website. This technology can be applied in the future to produce the highly efficient silicon light-emitting diodes needed in integrated circuits for optical interconnection. The following chart illustrates how the silicon light emitting diodes work:
The research team used low pressure chemical vapor deposition (LPCVD) technology in the p-type silicon board to grow nano silicon crystals, then used thermal oxidation method to bury the nano silicon crystals in the layer of silicon dioxides. Then, the team used atomic layer deposition (ALD) technology to produce high quality n-type ZnO as a transparent conducive layer, electron injection layer, and an anti-reflective layer which could improve the efficiency of optical out. The ALD technology is one of today's advanced technology for nano thin film deposition, and possessed the advantages of the atomic level control of materials. It could control the thickness and ingredients of the thin film, had excellent uniformity, excellent three dimensional coating, low defect density, a large size and batch-type production capacity, as well as low temperature for film deposition. From the picture taken by high resolution transmission electron microscope, we can see how the silicon anno-crystals are embedded in the silicon dioxide. The diameter of the silicon crystal is about 24nm, and the thickness of the silicon dioxide layer is about 9.2 nm. The electrons and the electron hole entered the silicon crystals separately through P-type ZnO thin film and silicon plate by the tunneling effect. Since the electrons and the electron hole were confined in the narrow space of the silicon crystals, and the silicon dioxides had a repair effect toward the defects on the surface of the nano silicon crystals, so the chances of electrons and electron holes creating radioactive recombination were greatly increased. This coupled with the fact that zinc oxide thin films played the role of an anti-reflective layer, the light emitting efficiency of the silicon diodes was thus significantly enhanced.
The spectrum of light emitting silicon diodes under room temperature. The wavelength was at 1140 nm, very close to the silicon bandgap semiconductors (bandgap) energy, which corresponds to the physical mechanism of phonon assisted indirect carrier recombination. The external quantum efficiency of this light emitting diode could reach up to 4.3 x 10-4, which was 100 times the efficiency of bulk Si. The internal quantum efficiency was estimated to be around 10-3, which completely smashed the limitations of non-direct energy band of semiconductors. Also worth mentioning is that the manufacturing process and the structure of this component is totally compatible with the techniques used in producing large sized integrated circuits, and therefore can be directly applied to the modern day integrated circuits. This research achievement can be applied to the highly efficient light emitting silicon diodes that are needed for optical interconnection and photonic integrated circuits.