UC Santa Barbara

With the invention of efficient Gallium Nitride (GaN) based blue light emitting diodes (LEDs), the lighting sector has been revolutionized. The blue light is converted into white using a yellow phosphor coating. These white light sources are currently being heavily utilized in the backlight units of liquid crystal displays (LCDs) in residential and commercial settings. This white light goes through various stages such as multiple polarizers, liquid crystals, and color filters, leading to a reduced light output. Recently, organic LEDs have emerged as a premium display technology due to their self-emissive nature leading to extremely high contrast ratio and improved overall efficiency. However, due to their organic nature, they are susceptible to degradation over time and require sophisticated encapsulation due to their sensitivity to moisture and oxygen. Hence, the use of self-emissive inorganic LEDs is highly desired which will have extremely high contrast ratio and at the same time have very high lifetime, brightness, and efficiency. For the three basic colors red, green and blue, different materials are preferred. The Indium Gallium Nitride (InGaN) alloy system spans a bandgap of 0.7 – 3.4 eV covering the full visible spectrum. However, only blue and green emitting InGaN LEDs are preferred by display manufacturers, as the efficiency of InGaN LEDs emitting beyond green in the visible spectrum reduces drastically. The higher indium content necessary to emit light in the longer wavelength of emission is accompanied by a lot of defects. As InN and GaN have a 10% lattice mismatch, the increased indium content increases the strain in the epitaxial structure causing defect formation. Hence, Aluminum Indium Gallium phosphide (AlInGaP) based red emitters are preferred as they offer very high efficiency in the red emission range. To facilitate cost effective commercialization and provide an immersive user experience in near eye displays, the inorganic LED sizes must be scaled to dimensions < 10 µm. Due to unfavorable material properties, scaling of the phosphide based red emitters is met with a lot of challenges. Hence, InGaN based scaled red emitters are needed. Our approach was to target the strain in the epitaxial structure by utilizing a flexible underlayer that can be visualized as a strain reducing layer. The flexible material of our choice was porous GaN, which could be obtained using a doping selective electrochemical etching process. This etching process could convert the GaN:Si films into porous GaN films. Using this flexible underlayer, elastic strain relaxation was demonstrated for InGaN films in the micrometer scale regime, leading to very high-quality epitaxial layer growth. Prior demonstrations of elastic strain relaxation were in the nanometer scale regime, posing challenges in device fabrication. With this substrate technology, the first red micro-LED sized < 10 µm was demonstrated with measurable efficiency. Beyond InGaN, the growth of relaxed AlGaN films was also pursued. 1.3 µm thick crack-free relaxed Al0.2Ga0.8N was grown using these substrates. With continued development large area strain relaxed AlGaN with arbitrary lattice constant can be demonstrated, that can be utilized not only for AlGaN based UV-LEDs but also for AlGaN electronics applications.