(Invited) Vertical-Geometry GaN-Based Light-Emitting Diodes: Improving Current Injection and Light Extraction Efficiencies

For GaN-based light-emitting diodes (LEDs) that are expected to replace incandescent and fluorescent lighting lamps, chip-area and driving current are best optimized. To use such LEDs as a lighting source, effective heat dissipation is one of the major problems to be solved. The heat generated during operation can accelerate the aging degradation of LEDs. LEDs for solid-state lighting applications could be realized by vertical-type configuration having a robust conductive support where emitted light is extracted through the n-type semiconductor rather than the p-type semiconductor. Compared to conventional lateral LEDs, vertical LEDs (VLEDs) were known to show better performance in particular at high current injection due to the excellent heat dissipation capability. To fabricate wafer-level GaN-based vertical LEDs, the formation of a robust conductive supporter is vital. A great deal of efforts has been so far made to fabricate a robust conductive supporter using Si, Cu, and Ni mainly by wafer-bonding and electroplating techniques. In this study, a multi-functional bonding material system, which consists of thick Cu layer and a Sn-based bonding layer, was employed to fabricate wafer level GaN-based VLEDs. It is shown that fully packaged VLEDs fabricated using the multi-functional bonding material system give an operating voltage of <3.35 V at 350 mA and after 1500 hrs, their reverse currents remain stable.

Furthermore, VLEDs require the formation of low resistance and thermally stable ohmic contacts to N-polar n-type GaN and to Ga-polar p-type GaN. However, unlike n-ohmic contacts to Ga-polar n-GaN, n-ohmic contacts to N-polar GaN are usually known to produce poor electrical characteristics. Thus, in this study, we investigated the electrical characteristics of Ohmic contacts to N-polar n-GaN. To form reliable n-type electrodes to N-polar n-GaN, different fabrication processes, such as laser-annealing, use of a blocking layer were employed. The results of the designed n-type electrodes are much more stable than conventional untreated samples. Based on the X-ray photoemission spectroscopy, Auger electron spectroscopy, transmission electron microscopy, and secondary ion mass spectroscopy results, Ohmic formation mechanisms for the N-polar contact are described and discussed.

In addition, to enhance the light extraction, the formation of Ga-polar p-type reflectors with high reflectance and low contact resistance is essential. Because of the reasonable electrical property, Ag is the most frequently used reflector. However, Ag only reflector suffers from thermal degradation when annealed above 300ºC in air. Thus, in this study, we presented ways of improving both the thermal and electrical properties of Ag-based reflectors. It is shown that the use of interlayers, middle layers, and capping layers significantly improves the optical reflectance as well as the electrical properties. The combined Ag-based reflectors give better electrical property than the Ag only sample. Blue LEDs fabricated with the annealed combined Ag-based reflectors show a lower forward voltage at 20 mA than LEDs with the annealed Ag only contacts. On the basis of scanning electron microscopy and X-ray pole figure and phi scan results, possible mechanisms for the electrical and thermal improvement are described and discussed.