UC Santa Barbara

Micro-size light-emitting diodes (μLEDs) have attracted huge attentions as the next-generation display technology for the wide applications, such as wearable watches, virtual/augmented reality, micro-projectors, and ultra-large televisions. InGaN LEDs or laser diodes with epitaxial tunnel junctions (TJs) are attractive due to several advantages, such as simple fabrication process, higher efficiency, and lower efficiency droop by improved current spreading, less loss. In the first part, high performance InGaN µLEDs with MOCVD-grown TJs were successfully demonstrated using selective area growth (SAG) and p+GaN/InGaN/n+GaN structure. SAG apertures provide paths for the out diffusion of H+ atoms, which passivated Mg atoms in the p-type GaN during the overgrowth of TJs. The forward voltage (Vf) in the SAG TJ µLEDs is independent on the device sizes, suggesting that the H+ atoms are effectively removed through the holes on top of the p-GaN surface. The Vf @ 20 A/cm2 in the TJ µLEDs utilizing SAG is significantly reduced to 3.24 V. Moreover, the output power of TJ µLEDs with SAG is ~10% higher than the common µLEDs with indium tin oxide (ITO) contact. The InGaN TJs µLEDs show a significant reduction of Vf by ~0.6 V compared to the common n-GaN TJs µLEDs. The external quantum efficiency (EQE) of the packaged TJ µLEDs was improved by 6% compared to the common µLEDs with common ITO contact. These demonstrations solve the key challenges of MOCVD-grown TJs. Then, we present fully MOCVD-grown InGaN cascaded µLEDs with independent junction control. The cascaded µLEDs consisted of a blue emitting diode, a TJ, and a green emitting diode. We can control the injection of carriers into blue, green, and blue/green junctions in the same device independently. Finally, we present the state-of-the-art InGaN-based red μLEDs, which have become research focus now in the whole nitride community. The common AlInGaP red μLEDs show a dramatical reduction of the EQE with decreasing the devices area and poor thermal property. We demonstrated several remarkable InGaN device performances with emission wavelength greater than 600 nm. First, we reported that the EQE of InGaN red µLEDs has less influence from the size effect due to the lower surface recombination velocity, compared to conventional AlInGaP red µLEDs. Moreover, we investigated the temperature-dependent electrical luminesce characteristics of InGaN red µLEDs, and we found that InGaN red µLEDs have superior device performances even operating at high temperature up to 400 K. We realized 5×5 µm2 InGaN devices with a peak EQE value of 2.6%. We also present red InGaN 60×60 µm2 µLED with SAG TJ contact, which show a peak EQE of 4.5% at 623 nm. These values about InGaN red µLEDs are one of the best reported among the literatures. A 568 nm stimulated optically lasing from our InGaN red MQWs was also achieved.