Fabrication of high efficiency micro-light-emitting diodes and its application to light-activated gas sensors

Abstract

A paradigm change in the field of displays has occurred as a result of technological advancement. Demand for low-power technology has increased due to rising global power consumption. In addition, displays are required to be of high resolution, high stability, and small size for various applications, including ultra-high-definition displays, head-up displays, flexible or wearable displays, augmented reality, virtual reality, and mixed reality displays. Conventional display technologies such as liquid crystal displays (LCD) or organic light-emitting diodes (OLED) are not appropriate for such applications, since LCDs suffer from narrow viewing angles, high energy consumption, and the necessity for backlight units, whereas OLEDs also have narrow viewing angles and low environmental stability. Consequently, as the next-generation display technology, micro light-emitting diodes (micro-LEDs have drawn considerable interests from academia and industry. To meet the increasing demands of displays, researchers and companies are focusing on micro-LEDs. Micro-LED technology has been developing ever since it has been first introduced by professor Jiang in the early 2000s. Many researchers from institutes and companies tried to implement displays using micro-LED technology. A passive matrix display was first attempted, followed by an active matrix display and a full color display using micro-LED. Recently, several companies have released large-area micro-LED TV. Micro-LEDs are utilized not only in displays but also in various fields. Owing to its small configuration and high stability, various studies are being conducted to apply the micro-LEDs in the bio-electronic filed, such as neuron stimulation, or smart contact lens. However, there are still technical issues in realizing micro-LED display. Micro-LEDs are normally fabricated by etching a LED film into micron scale. As the size of each chip decreases, the etching loss increases, affecting the production cost. Moreover, it is found that the dry etching process damages the active layer of the LED, degrading the quantum efficiency of micro-LED devices. Furthermore, conventional transfer processes including laser lift-off or chemical lift-off techniques are costly and time-consuming for micro-LED application. Lastly, emission peak shift induced by quantum confined screening effect (QCSE) also could be a problem for a display device. In this thesis, sapphire nano-membrane (SNM) technology has been introduced to solve the abovementioned issues. The fabrication process and consequent growth of micro-LEDs on it was investigated. The fabrication of SNM template was carried out through the following process. It started with photolithography to make photoresist (PR) pattern on a sapphire substrate. Then, an amorphous Al2O3 layer with a thickness of 120 nm was deposited by atomic layer deposition. Second photolithography and wet etching by H3PO4 solution was conducted to make a discrete SNM array. After removal of PR using acetone, the template was annealed at 1100 °C for 2 hours in air to crystallize the amorphous Al2O3 into single crystalline Al2O3 through solid phase epitaxy. Micro-LEDs were grown on the SNM using metal organic vapor deposition (MOCVD). By controlling the space between the nano-membranes, micro-LEDs with various sizes and shapes could be obtained. Micro-LEDs grown on SNM template was in self-passivated structure and showed enhanced properties including reduced internal strain and threading dislocation density, and enhanced internal quantum efficiency and photoluminescence intensity. The micro-LEDs were transferred to Si substrate using mechanical lift-off at 300 °C and under 10 kgf for eutectic bonding of Au-Sn alloy. The fabricated device showed enhanced leakage current level compared to the dry etched reference sample. Moreover, the micro-LED device grown on SNM showed almost no shift of emission wavelength even though the injection current was increased up to 100 A/cm2, while the dry etched reference sample showed significant peak shift. This is because of reduced QCSE screening of our sample, which is owing to the compliant substrate effect of SNM that reduced the internal strain of micro-LEDs grown on it. Secondly, the crystallization process of SNM was investigated and utilized for novel selective area growth (SAG) of GaN. The crystallization of amorphous alumina to single crystalline sapphire was conducted through solid phase epitaxy (SPE). SPE starts at the interface between the single crystalline and amorphous layer since the activation energy of crystallization at the interface is lower than the random nucleation and growth (RNG) energy at the middle of amorphous layer. In addition to the SNM, homogeneous growth mask for GaN was invented using the energy difference between SPE and RNG, by depositing thick amorphous alumina on a sapphire. Growth of GaN on the sample started before the thick amorphous alumina was completely transformed into single crystalline, resulting in poly crystalline GaN islands instead of GaN film. By selectively depositing the thick amorphous alumina, selective area growth of GaN was accomplished. The selectively grown GaN array showed much faster growth rate compared to the reference sample since the Ga adatoms were diffused to the exposed growth sites. Moreover, using the principles of crystallization of SNM and SAG of GaN, high efficiency hexagonal GaN array was fabricated. First, hexagonal shaped SNM was fabricated for growth of hexagonal GaN. By controlling thermal treatment time, crystallization of hexagonal SNM was stopped in the middle. Since GaN film was not grown on the region where the crystallization was not complete, selective growth GaN on the single crystalline region was achieved. GaN film grown on the outer hexagonal then grew inside laterally, filling the empty space. After growth of 90 minute, fully grown hexagonal GaN array was achieved. The hexagonal GaN showed reduced strain and TDDs since the threading dislocations were not generated at the laterally grown region. For the application of micro-LEDs, light activated chemoresistive gas sensor was selected. Chemoresistive gas sensor is one type of gas sensor which detects the change of resistance according to the reaction with the target gas. Conventional gas sensors required external heater for activation of sensing material with the target gas, which induced complicated structure and high production cost of the device. To overcome the disadvantages of the conventional gas sensors, light activated gas sensors are investigated. Using the light activation principle with micro-LEDs, the device with lower power consumption can be fabricated with lower production cost. For such purpose, micro-LED platform for gas sensor application was fabricated with interdigitated electrodes (IDEs) on the micro-LEDs. SnO2 nanoparticle was chosen for sensing material since it showed good responsibility and stability with high selectivity by metal doping or decorating. SnO2 nanoparticle solution was drop casted on the micro-LED platform and sensing property to NO2 was tested using gas sensing system. With increasing the light intensity, base resistance of the samples were decreased due to increased photo-generated electron-hole pairs. However, the response of the SnO2 nanoparticle was reduced due to increased recombination of electron-hole pairs. Optimal value of light intensity was chosen considering the base resistance and response of the gas sensor. Under optimal light intensity, various gas sensing properties including reliability, linearity, and detection limit was analyzed. Under blue light illumination, the sample showed constant base resistance and responsibility after 4 repeating pulses of NO2. In addition, the sampled showed a linear increase in response with an increase in the gas concentration. From the linear relation of the response and gas concentration, detection limit of the SnO2 nanoparticle sample was calculated to be 2.71 ppt, meaning that the sample can detect the NO2 gas with such low value.