(November 16, 2010)
— Laura Peters, contributing editor, covers a team led by the National Nano Device Laboratories (Hsinchu, Taiwan), which has achieved a record external quantum efficiency (QE) of >80% for photodiodes in the visible regime. This QE was achieved by harnessing a local surface plasmon resonance (LSPR) effect, which greatly enhances transmission efficiency in an otherwise conventional silicon photodiode. Quantum efficiencies in the 80% or higher range are needed for applications such as artificial eyes. The team, which includes researchers from National Taiwan University (Taipei) and National Cheng Kung University (Tainan), will present their results at the upcoming IEDM in San Francisco, CA, December 6-8.
|Figure 1. The optical array patterns that enable the local surface plasmon resonance effect.
LSPR involves the collective oscillations of conduction electrons in metallic nanostructures excited by electromagnetic energy. The team compared the improvement in quantum efficiency and polarized light energy enabled by the LSPR effects from four metallic optical array patterns (Figure 1) — C-shaped, square, striped, and chiral-shaped pattern — deposited over the metal finger electrodes in the photodiode device (Figure 2). The test device (control) does not contain the optical arrays. Each pattern has a period of 2.4µm and unit area of 1.44µm2. In this process, the p/n junctions are formed, contacts are deposited and etched, the top metal finger electrodes and metallic optical patterns are formed, followed by backside metal electrode deposition and sintering.
|Figure 2. Illustrations of the proposed device, with optical structures and the control photodiode.
The team determined that the devices with the chiral-shaped optical arrays demonstrated the greatest increase in current density with increasing illumination. The external quantum efficiency was also greatest with the chiral-shaped arrays, while demonstrating greater QE than previously reported organic devices or Schottky solar cells (Figure 3).
|Figure 3. External quantum efficiency of full device structures reaches 80% with the chiral-shaped optical structures.
The researchers observed that by having more corners of pattern above the metal electrodes, photocurrent density is enhanced due to self-accumulated local electric field in these corners. In addition to the reliance on LSPR pattern, transmission spectra are greatly influenced by the direction of the induced electric field, so the array layout must consider these polarity effects.
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