MURRAY HILL, N.J. An electrically activated laser device with an active region formed from an organic material, rather than typical compound semiconductors such as gallium arsenide or indium phosphide, has been demonstrated here for the first time at Lucent Technologies Bell Laboratories. The new material, called tetracene, forms a regular cystalline structure at 200 degrees Celsius and has ideal electronic and optical characteristics for creating the spontaneous emission underlying laser action.
While laser emission has been observed in tetracene and a variety of other carbon-based materials, their operation has depended on stimulation by bursts of light. The Bell Labs device has a novel, dual-gate FET structure, and intense yellow-green laser light is emitted when a potential difference of 5 volts is maintained across the source and drain regions.
"The electrical quality of this material is indeed a key to the success of this work," said Bertram Batlogg, a researcher on the project and director of the solid-state physics research department. The high-quality, defect-free crystalline films that the Bell Labs team has created help in achieving the breakthrough, said Batlogg. But perhaps more significant are the intrinsic properties of the material itself. "An important factor is the chemical purity of the material it's an electrically pure material," he said.
Specifically, tetracene supports high mobilities for holes on an equal basis with electrons, which is important for making the best use of the injected carriers. In these materials, holes and electrons pair up to produce what are known as "excitons," which serve as the source of coherent radiation when they decay. Typically, holes have more difficulty moving through materials to link up with electrons and, therefore, fewer excitons are formed.
The other barrier to electrically activated injection lasers has been the contact problem. Essentially, the interface between metal contacts and polymer or crystalline organic systems is poor, making it difficult to generate enough electrons and holes for laser action.
Device engineers try to balance the electronic properties of organic semiconductors with dopants, but these can then affect the behavior of electrons and also reduce the optical quality of the material. With tetracene, the additional handicaps introduced by dopants were not present, and the Bell Labs team hit on the idea of using the field effect to boost the injection of electrons and holes.
Though he called the field effect "an important factor," Batlogg said that "there may be other ways to inject carriers into the film" as well.
The work with tetracene began about two years ago, stimulated by team member Christian Kloc's discovery of a simple way of depositing chemically pure films using a vapor deposition technique at 200 degrees Celsius. During the vapor phase, impurities have a chance to escape, creating an ideal crystal for studies of the material's electronic properties.
"This gave us the opportunity to study the intrinsic electronic properties of the material not just tetracene, but also other members of the family such as pentacene and anthracene," said Batlogg.
The tetracene crystal is held together with weaker chemical bonds than those in inorganic semiconductors such as silicon. As a result, they are more flexible, both in the ability to form on dissimilar substrates and in their mechanical resilience.
"It's conceivable that we could get these films to grow on plastic substrates and although they are ordered crystals, they could bend without disrupting the lattice structure," said Richart Slusher, director of the optical physics research department at Bell Labs. "That would allow us to create very low-cost electronic and photonic circuits on plastic substrates."
The tetracene family is characterized by a regular arrangement of benzene rings to form a regular lattice. For example, pentacene has five benzene rings linked together to form a unit cell of the lattice, while tetracene has four rings.
Exactly where to apply the new capability to create low-cost lasers is not obvious at this early stage. "The fact that these are efficient light emitters that can be driven electrically certainly makes them interesting for displays," said Slusher. "I know some people are pursuing polymer LEDs for display, and uncovering this new class of materials might certainly boost those sorts of applications."