Electrical engineers in Debdeep Jena's lab at the University of Notre Dame have found a way to make two nitride semiconductors conduct electricity better, which may make them useful for building more effective ultraviolet (UV) lasers and light-emitting diodes (LEDs). These devices could enable a wide range of applications such as high-density optical data storage, water treatment, sterilization of medical equipment, UV-enabled security marks on credit cards and paper money, and biological imaging.

A prototype UV LED made using Jena's technique for growing nitride semiconductors. Credit: AAAS

Nitride semiconductors such as aluminum gallium nitride and gallium nitride have the widest spectral range of band gaps--the energy required to move electrons through the material--among all semiconductors, ranging from the infrared through the visible and into the deep UV range. This makes them excellent for use in short-wavelength lasers and in LEDs for solid-state lighting, but it also makes it hard for engineers to design energy-efficient devices.

Like all semiconductors, nitrides need to be "doped" with foreign materials to conduct electricity efficiently. This either provides the material with charge-carrying electrons, or electron vacancies--called holes--that allow electrons to move freely. But the energy barriers in gallium nitride (GaN), for instance, are so large that even devices made with magnesium (the most commonly used hole-dopant for GaN) don't work well at room temperature, making them extremely inefficient.

In a paper published in the January 1, 2010, issue of Science, Jena and his colleagues describe growing graded layers of aluminum gallium nitride (doped with magnesium) on the nitride surface of gallium nitride crystals. This means that the proportion of aluminum to gallium in the top layer increased as its thickness grew. Experiments testing this material's conductivity showed that making the semiconductor this way efficiently activated the magnesium doping atoms at room temperature.

Jena's group also built prototype UV LEDs using both the graded aluminum gallium nitride (AlGaN) material and regular maginesium-doped GaN. The AlGaN LEDs were both more efficient and brighter than the GaN devices. Jena believes that this should make nitride semiconductors much more practical alternatives for any device requiring UV light.