CHEMISTRY & MATERIALS
Photodetectors A ferroelectric boost
The introduction of a ferroelectric layer into an ultraviolet photodetector enhances its output signals
Ultraviolet (UV) radiation is a hazardous form of electromag- netic radiation. It can pose threats to the human body by causing genetic mutations and inducing cancer. Despite its undesirable effects, however, UV radiation is useful for a wide range of industrial and commercial applications that can make a differ- ence in our daily lives, including laser engraving, optical storage and bug zappers. Unfortunately, UV radiation is invisible to the human eye.
People working in an environment where UV radiation is present might not be aware of the health risks they are exposed to. Photodetectors based on semiconductor technology are the standard tool for detecting or measuring the amount of UV radia- tion in the environment. Kui Yao and co-workers at the A*STAR Institute of Materials Research and Engineering have now improved the performance of semiconductor UV photodetectors with the aid of a ferroelectric thin film1
. Semiconductor photodetectors are often based on the principle
of the photovoltaic effect. Typically photodetectors consist of a junction between two materials comprised of either two differ- ent types of semiconductors, or a semiconductor and a metal, so that an electric field can be established at the interface between the two materials. When UV radiation hits the semiconductor materials, electron-hole pairs are generated and separated by the electric field, giving rise to an output signal — the photovoltage. The photovoltage is fundamentally limited by the electric field at the interface. Yao and his team broke such a constraint by fol- lowing a different route. They designed and fabricated a photo- detector consisting of a platinum layer and a conductive metal oxide thin film electrode, and further inserted a ferroelectric layer — in this case, lead zirconate titanate (PLZT) — between them. When the PLZT is electrically polarized in the right direction, it provides an extra electric field that complements the field at the interface, thus enhancing the output. The ferroelectric layer gives a marked effect: when the PLZT
layer is electrically polarized, the photovoltage is more than three times that of an unpolarized PLZT layer. This is clearly a substan- tial gain for the team’s handheld PLZT-layer UV photodetector (see image), although its output is not as high as the theoretical
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A photo of the UV photodetector incorporated with a ferroelectric layer
structure for the device because the device prototype has not yet been fully optimized. Yao explains that such UV detectors not only output large
voltage signals for driving electronic circuits and displays, but they also hold great potential for achieving improved durability and stability under strong UV intensity because of the use of a stable PLZT thin film.
1. Gan, B. K., Yao, K., Lai, S. C., Goh, P. C. & Chen, Y. F. A ferroelectric ultraviolet (UV) detector with constructive photovoltaic outputs. IEEE Electron Device Letters 32, 665–667 (2011).
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A*STAR RESEARCH OCTOBER 2011– MARCH 2012
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