PHOTONIC SENSORS: Ozone sensor integrates LEDs and nanoparticle thin films

Ozone is a harmful gas that can be released by past-generation photocopiers and laser printers and by some air purifiers that actually use ozone to remove airborne pollutants like odors, bacteria, and mold; in addition, ground-level ozone is the chief component of smog. Although ozone can be sensed by classical metal oxide thin films whose sensing layers rely on conventional thermal reactivation from a built-in heating system, the high operating temperature and high power consumption of these heating systems precludes the use of such sensors in mobile or handheld devices.

Some recent research has replaced those heating systems with an external UV light source, paving the way for researchers at the Technical University Ilmenau (Ilmenau, Germany) and the Fraunhofer-Institut für Angewandte Festkörperphysik (IAF; Freiburg, Germany) to demonstrate an ozone sensor that integrates thin-film nanoparticle layers with blue light-emitting-diode (LED) structures on a single sensor chip.¹ The resulting sensor can detect ozone concentrations as small as 40 parts per billion (ppb), and is compact and robust enough for integration into mobile devices.

Low-temperature-grown body-centered-cubic indium oxide (In₂O₃) nanoparticle thin films having a thickness of 15 nm were illuminated with light from a gallium indium nitride/gallium nitride (GaInN/GaN) blue LED operating at a wavelength of 400 nm. The sensor response toward ozone was determined from the changes in resistivity of the In₂O₃ layer, caused by a periodic switching between gas exposure and illumination. In the presence of different ozone concentrations, the ozone response increased as a function of LED optical power (see figure). These test results were sufficient to determine that the blue LED would efficiently activate the ozone sensing layer, and that an LED power of approximately 0.25 mW was necessary to activate the ozone-sensing layer.

Chip fabrication consisted of GaInN quantum-well blue LED arrays grown by low-pressure metal-organic chemical-vapor deposition (MOCVD) on a sapphire wafer substrate that was polished on both sides. The In₂O₃ films were grown on the back side of the wafer in a horizontal MOCVD reactor. The finished package consisted of an integrated LED-array/sapphire-substrate/In₂O₃ chip that was exposed to various ozone concentrations through a 3 × 3 mm laser-cut window. The top side, with eight blue LEDs in a square array, was wire bonded to the top contacts of the chip.

“The combination of blue light-emitting GaN/GaInN LEDs and In₂O₃ nanolayers grown and processed on the two sides of a single sapphire substrate enables the realization of fast, cheap, and extremely selective ozone sensors,” says researcher Oliver Ambacher. “The possibility to measure a difference signal enabled by switching on and off the LED under ozone exposure enables an extremely high sensitivity. This compact chip is a demonstrator for a new generation of miniaturized gas and fluidic sensors combining GaN-based light-emitting devices with active metal oxide nanolayers.”

REFERENCE

1. Ch. Y. Wang et al., Applied Phys. Lett. 91, 103509 (2007).