UC Santa Barbara

High efficiency III-nitride light-emitting diodes (LEDs) have drastically improved solid-state lighting. They are sold in stores and are gradually replacing compact fluorescent lightbulbs because they use less power and last longer. III-nitrides are on the cusp of entering another market. As the size of mobile electronics shrink over time, display technologies must also move to smaller form factors while maintaining high efficiencies. To achieve these goals, III-nitride LEDs are once again a candidate to overtake the state of the art.

Incumbent technologies, such as liquid crystal displays (LCDs) and organic LED (OLED) displays, have major issues with power efficiency. A new technology, termed the micro-LED (uLED) display, is poised to enter the market in the next few years. A uLED display is made of inorganic LEDs (such as InGaN or AlGaInP) with dimensions typically below 40 um. uLED displays are promising due to higher luminance (brightness) than OLEDs, wider viewing angles, and significantly higher energy efficiencies.

In this thesis, advances in InGaN uLED epitaxial design, nanofabrication, and mass transfer are discussed. Chapter 1 introduces the III-V families (arsenides/phosphides/nitrides) and provides insight into uLED design. While the majority of IngaN uLEDs are grown on sapphire or silicon, there are many reasons to explore homoepitaxial growth on freestanding GaN (particularly on semipolar planes). Chapter 2 presents a comparison of external quantum efficiency (EQE) amongst various sized uLEDs and shows that high EQEs (40-45%) are sustained as the size of the uLED drops. Reasons for efficiency loss are presented and designs aimed at improving uLED efficiencies are highlighted. Chapter 3 discusses the incorporation of tunnel junction contacts to uLEDs to enable new design space (n-type mirrors and multiple active region growths). Chapter 4 examines ideas to eliminate the drop of efficiency with decreasing uLED size that incorporates a current aperture and reduction of dry etch damage at the active region sidewall. Chapter 5 highlights a new mass transfer method that is applicable to uLEDs grown on freestanding GaN, sapphire, or other substrates. Most of the commercial mass transfer techniques today use laser lift-off (LLO), which is incompatible with uLEDs grown on freestanding GaN. The technique in Chapter 5 combines photoelectrochemical (PEC) etching and transfer printing. With this method, red, green, and blue InGaN uLEDs have been transferred from their growth substrates (sapphire and freestanding GaN) to the same transparent and/or flexible substrates such as glass or acrylic without damage to the uLED. This thesis reports the first demonstration of red, green, and blue uLEDs all with an InGaN active region that have been transferred to the same substrate.