Fabrication and characterisation of RGB LEDs based on nanowire technology

Abstract: The white light LEDs of today are usually based on a blue LED and a phosphor, converting the blue light to longer wavelengths. While these phosphor-converted LEDs are extremely efficient compared to incandescent light or fluorescent light, there is still plenty of room for improvement. In the conversion between blue light and light of lower energy approximately 25-45 % of the radiant power is lost as heat [1]. A more efficient solution is to use white light sources based on RGB LEDs. Since these LEDs do not require phosphor conversion the efficiency has potential to be much higher compared to today’s white light sources. High efficiency blue LEDs are based on an InGaN quantum well and barrier layers of GaN. By altering the In content in InGaN the bandgap can be tuned for emission from UV to red light, which makes the material system a good candidate to be used in fabrication of RGB LEDs. In green and red LEDs the In content in the quantum well is higher compared to blue LEDs which gives rise to more strain between the quantum well and GaN barrier layers. The strain induces piezoelectric charges which spatially separates electrons and holes in the quantum well leading to lower radiative recombination efficiency (quantum-confined Stark effect). The use of InGaN barrier layers would thus enable fabrication of red and green LEDs of higher efficiency, but the main problem is that it is very hard to synthesise thick layers of InGaN with high material quality. The poor crystal quality is partly due to strain induced dislocations and partly due to phase separation and In content fluctuations [2], which will reduce the efficiency of the device [3]. However it has been shown that thick InGaN layers of high material quality can be synthesised in MOCVD grown nanocrystals of InGaN. These nanocrystals, or platelets (truncated pyramids with a flat c-plane), can be used as dislocation free substrates for growth of LEDs, which enables the use of InGaN barrier layers instead of GaN. The strain between the quantum well and barriers in In rich quantum wells is thus reduced which makes it possible to fabricate red and green LEDs of high efficiency. The reduced strain between the barriers and the quantum well leads to a decrease of the quantum-confined Stark effect and therefore potential of higher efficiency LEDs. In this thesis the potential of nano RGB LEDs based on InGaN and GaN platelets is evaluated and the results are promising. During the project an LED device design for LEDs based on (In)GaN platelets was developed, characterised (electrically and electroluminescence) and optimised (spacer layer thicknesses, contact materials). The implemented device design works well. Parasitic currents outside the active area are negligible and the devices show much improved stability compared to previous designs. The device design is based on parallel connected platelets in the device area and lifted bond pads outside of the device area. The bond pads are lifted by a thick spacer layer (resist) in order to enable easy bonding and avoid shunt current. A range of devices of different size were studied, from single platelets (<1μm) to tens of thousands of platelets (400μmx400μm) and the crystal homogeneity is shown to be very important for the device performance. The current rectification is excellent for single platelet devices but decreases with increased number of platelets in the device. While work lies ahead in improving homogeneity, efficiency and device design, this technology is unique in achieving high quality material with very high In content and an extremely small light emission area. Not only are these structures interesting for high-efficiency RGB illumination, but also for microLED displays - potentially with pixel size an order of magnitude smaller than what is possible today.