Novel LED structures
Novel methods for current injection in non-conventional LED structures
For decades, current transport in light emitting diodes (LEDs) and solar cells has been primarily based on a structure where the active region is sandwiched within a pn-junction. Our recent results suggest that better exploitation of carrier diffusion allows more flexible carrier injection to next generation light emitting diode (LED) structures. Based on simulations a team from Engineered nanosystems group, department of Biomedical Engineering and Computational Science (BECS), School of Science recently proposed a novel diffusion injected LED (DILED) structure where charge carriers are transported from a pn-junction by bipolar diffusion to an active region located outside the pn-junction. This idea was transformed into reality as we recently presented a working prototype of an indium gallium nitride DILED structure.
Conventionally minority carrier diffusion is considered a problem in LED structures, since it is generally one of the main reasons for leakage current in devices. However, exploiting diffusion for charge carrier injection provides a promising method to electrically excite novel structures such as nanowires and surface quantum wells (QWs) whose electrical excitation using conventional methods is challenging.
FIGURE 1: Schematic illustration of the fabricated DILED
The layer structure of the fabricated device was grown by metalorganic vapour phase epitaxy (MOVPE) on c-plane sapphire substrate in the MNT laboratories located in Micronova. The device structure was processed using standard LED microfabrication methods into a fingered mesa leaving the buried MQW stack intact. Under forward biasing a blue emission from the InGaN QWs was observed at room temperature. Since the QWs are located below the pn-junction, this indicates that both electrons and holes are transported to the QWs from the same side of the active region through bipolar diffusion. Input current dependent peak shift, which is typical to conventional LEDs, was not observed. This suggests that the electric field inside the QWs is much less dependent on the external bias voltage than in conventional III-N LEDs. Also contrary to conventional LEDs the optical power of the device was found to increase with increasing chip temperature.
FIGURE 2: a) Microscope image and b) emission spectrum of a fabricated DILED under forward bias at room temperature.
The external quantum efficiency (EQE) in the presently studied prototype structure is still modest. However simulations predict that near unity injection efficiencies are expected with more optimized structures. The poor injection efficiency of the prototype device is mainly expected to result from inefficient hole transport. According the simulations, the hole diffusion is limited by the distance and potential barriers between p-type layer and the active region. The team is positive that with device structure optimization the DILED EQE can approach and even exceed the EQE of conventional LED structures.
The diffusion driven current injection opens up new engineering possibilities and challenges the conventional device structures in selected applications. In particular, it enables a new way to inject charge carriers to nanowires and near surface QW structures with emission enhancing surface plasmon gratings.
The first experimental DILED structure was presented in Applied Physics Letters vol. 104 p. 081102 (2014), while the original DILED concept and simulations were presented in Applied Physics Letters vol. 103 p. 031103 (2013). The discovery was also covered by Semiconductor Today on 14th of March 2014.
L. Riuttanen, P. Kivisaari, H. Nykänen, O. Svensk, S. Suihkonen, J. Oksanen, J. Tulkki and M. Sopanen. Diffusion injected multi-quantum well light-emitting diode structure, Applied Physics Letters 104, 081102 (2014).
P. Kivisaari, J. Oksanen, and J. Tulkki. Current injection to free-standing III-N nanowires by bipolar diffusion, Applied Physics Letters 103, 031103 (2013).
http://www.semiconductor-today.com/news_items/2014/MAR/AALTO_140314.shtml