Efficient silicon heterostructure LEDs unveiled

Silicon is an indirect semiconductor and consequently exhibits very low light emission efficiency, but there are ways to get around the problem. Researchers have managed to fabricate efficient silicon LEDs by using an n-type ZnO/SiO2-silicon nanocrystals-SiO2/p-type silicon heterostructure. The result could contribute to the development of efficient silicon LEDs for future photonic applications.

Scientists at National Taiwan University (NTU) have successfully fabricated efficient silicon light-emitting diodes (LEDs). Silicon nanocrystals were embedded in a SiO₂ matrix and combined with an n-type Al-doped ZnO (n-ZnO) layer to improve the external quantum efficiency of silicon in the n-ZnO/SiO₂-silicon nanocrystals-SiO₂/p-silicon LEDs.

The silicon nanocrystals embedded in a SiO₂ matrix were prepared by low-pressure chemical vapour deposition (LPCVD) and post-deposition thermal oxidation. Figure (a) shows a high-resolution transmission electron microscope (HRTEM) view of the silicon nanocrystals enclosed by SiO₂. Owing to the spatial confinement of carriers and surface passivation by the surrounding SiO₂, the silicon nanocrystals embedded in a SiO₂matrix lead to significant enhancement of the light emission efficiency from silicon.

 Atomic layer deposition (ALD) was used to prepare the n-type ZnO:Al film, which plays the roles of electron injection layer, transparent conductive window and anti-reflection coating (ARC) to increase the light extraction efficiency. The electrons and holes tunnel through the SiO₂ layers from the n-ZnO layer and the p-silicon substrate into the silicon nanocrystals, where the carriers radiatively recombine to produce the electroluminescence (EL).

Figure (b) shows the room-temperature EL spectra with a spectral peak at 1140 nm, corresponding to the phonon-assisted indirect carrier recombination in silicon. Figure (c) gives the measured optical power as a function of the DC injection current from the LED at room temperature.

External quantum efficiency up to 4.3 × 10^–4 at room temperature was achieved, which is two orders of magnitude greater than that of bulk silicon. The internal quantum efficiency was estimated to be of the order of 10^–3. In addition, the structures and fabrication processes are fully compatible with silicon-based Ultra-Large Scale Integration (ULSI) technology. These findings could contribute to the fabrication of efficient silicon LEDs for silicon-based photonic integrated circuits.

The researchers presented their work in Nanotechnology.

About the author

The work was performed at the Department of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan, Republic of China. Prof. Miin-Jang Chen currently focuses on the science and novel technology associated with atomic layer deposition (ALD), high-K/metal gate, nanoscaled materials, ZnO-based photonic devices and efficient silicon light-emitting diodes. Prof. Jer-Ren Yang's research interests include phase transformation of alloys, crystal geometry, crystal defects and transmission electron microscopy of semiconductor materials. From August 2007, he has been appointed as chairperson of the Department of Materials Science and Engineering, National Taiwan University.

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