Diffusion injected light emitting diode

Lighting plays a major role in consumption of electrical energy in the world. Thus, increasing the efficiency of light sources is one key element in reducing the green house gas emissions. Light emitting diodes (LEDs) are gaining a foothold in general lighting. Despite their rapid development in light output and their superior efficiency compared to other light sources, LEDs still need improvements in order to become the ultimate lighting technology. A typical LED is a double heterojunction (DHJ) structure, in which the active region fabricated from a lower band gap material is sandwiched between higher band gap p- and n-doped regions. By biasing such a structure electrons and holes are transferred by current into the active region, where they recombine releasing energy as photons. The carrier injection in a conventional LED structure is typically efficient. However, in more exotic novel structures based on nanowires or near surface nanostructures, fabricating a DHJ becomes difficult. This thesis presents the experimental studies on a novel current injection method for light emitting applications. The method is based on bipolar diffusion of charge carriers. Unlike in the conventional method, the active region does not have to placed between the p- and n-layers of the pn-junction. The diffusion injection method is experimentally demonstrated by two types of prototype structures. The first prototype was fabricated using a multi quantum well (MQW) stack buried under the pn-junction. The second prototype was fabricated using a near surface quantum well (QW) placed on top of the pn-junction. The first prototype showed that the diffusion current components can be used to excite an active region outside of the pn-junction. The second prototype showed a large improvement in injection efficiency as well as the suitability of the method for exciting surface structures. The applications of diffusion injection can be found in blue galliun nitride based LEDs studied in this thesis as well as in green solid-state light sources, light sources integrated into silicon technology and devices based on nanostructures and plasmonics.