III-nitride tunnel junctions and its application in bidirectional 
light-emitting diodes

The application of tunnel junctions (TJs) into gallium nitride (GaN) based optoelectronic structures provides new opportunities for the design of novel devices such as vertically integrated multicolor light-emitting arrays or high-power laser diode stacks. For a long time, high tunneling currents could not be achieved due to the high GaN energy gap of 3.4 eV. Polarization field engineering in TJs (mainly by introducing a thin InGaN layer), combined with heavy doping, enabled tunneling currents to be significantly increased. Furthermore, by developing a high-quality TJ, we were able to demonstrate a bidirectional light-emitting diode (BD LED). This is an innovative structure in which the active region is surrounded on both sides by p-type regions followed by tunnel junctions. This allows carriers to be injected to the same quantum well regardless of the BD LED's bias - with both positive and negative voltages. Thanks to this unique property, BD LEDs can be considered as a new group of optoelectronic devices powered directly with alternating current (AC). During the seminar, I will describe our contribution to the development of III-nitride TJs from an experimental and theoretical perspective. I will also present the properties of symmetrical BD LEDs and discuss the prospects behind these devices.