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Solar-blind AlGaN Photodetectors With Record-low Dark Current

Aluminium gallium nitride photodectors have been fabricated by Chinese researchers using a proprietary high temperature MOCVD process and a special surface treatment

A collaboration between researchers from Nanjing University and Nanjing Electron Devices Institute, China has developed a high quantum efficiency AlGaN-based solar-blind photodetector with a very low dark current.

Despite their significant applications including missile plume sensing, environmental monitoring, chemical/biological agent detection, and solar physics, AlGaN-based solar-blind photodetectors commonly suffer from low quantum efficiency and a high dark current. This significantly limits their weak signal detection capability.

But for use in harsh environments, such as flame detection for gas turbines or in complex battle field applications, photodetectors capable of operating at high temperature are very much needed.

To address this problem, the research team, led by Hai Lu, a professor at Nanjing University, has fabricated a novel AlGaN-based solar-blind MSM photodetector. Using a proprietary high temperature epitaxial growth process and a special surface treatment technique, they have created a device that they say has a record low dark current - not only at room temperature, but also at elevated temperatures.

The photodiode structure, grown on sapphire by MOCVD, consists of a high-temperature AlN buffer layer and an undoped Al0.4 Ga0.6N active layer. Standard optical lithography and lift-off techniques have been used to define the Ni/Au semi-transparent interdigitated contact electrodes. Before the deposition of the Schottky metalby e-beam evaporation, a special surface chemical treatment was conducted.

The researchers point out that dark current measurement of the photodetector at room temperature is limited by the experimental setup. Theminimum current level the scientists' current-voltage measurement setup can resolve is 1 fA, which is the noise floor of the high-end setup. If the actual dark current is below 1 fA, what can be seen is only an irregular current noise signal around 1 fA.

One femtoamp corresponds to an ultra-low dark current density of less than 1.25 × 10−12A/cm2. Even at a temperature of 150 °C, the dark current of the photodetector is still in the fA range.

The photodetector also exhibits a sharp spectral cut-off in the solar-blind wavelength region and a high quantum efficiency. It had a maximum room-temperature quantum efficiency of 64 percent at about 275 nm, with a solar-blind/UV rejection ratio up to 4 orders of magnitude under a 10 V bias.

Record low dark current is achieved in the AlGaN-based solar-blind photodetector at room temperature as well as in a high temperature environment

At a temperature of 150 °C, the maximum quantum efficiency of the photodetector was still more than 50% with a reasonable rejection ratio of more than 8000, suggesting it has potential applications in high temperature deep-UV detection.

“This is very nice result from the technology point of view and is the first high temperature characterisation of AlGaN-based solar-blind photodetectors" says team leader Hai Lu.

Using the material growth and processing techniques developed in this work, the research team has now fabricated a high performance large area MSM solar-blind photodetector. The maximum device area is 5 × 5 mm2, and the scientists say this is the largest AlGaN-based solar-blind photodetector reported so far. They also say the epi-layer has excellent uniformity.

“If we can make high performance large area PDs, theoretically we can make large-scale solar-blind focal plane arrays for deep UV imaging applications, which would be the next step of the research team," adds Lu.   

Further details of this work can be found in their paper published in IEEE Sensors J. 12, 2086 (2012).

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