No abstract

No abstract

The authors present a light extraction improvement at a low operation voltage from p-GaN patterned InGaN/GaN light-emitting diodes (LEDs) fabricated using size-controllable nanosphere lithography and subsequent inductively coupled plasma etching. A 300-nm polystyrene (PS) nanosphere array was used as an etching mask in order to produce ordered pillar patterns on the p-GaN layer, during which the top and bottom size of the pillars were tailored to optimize the electrical and optical properties by varying the diameter of the PS nanosphere masks. Three LEDs, without patterns and with pillar patterns of 210 nm and 240 nm diameter, were compared with each other, in which the LED with 240 nm diameter pillar patterns showed the highest output power (32.6% higher than that of the LEDs without patterns) in both its electroluminescence and photoluminescence measurements.

We demonstrate improved electrical and optical properties of indium tin oxide ͑ITO͒/Al reflectors, via SF 6 plasma treatments, for vertical-type GaN light emitting diodes. A 200 nm thick ITO film was deposited and annealed at 650°C to obtain ohmic contacts with p-GaN, after which SF 6 and O 2 plasma were applied on ITO before the Al deposition to effectively increase the work function of ITO and decrease the roughness of the ITO surface. The SF 6 plasma-treated ITO/Al reflector showed the lowest specific contact resistance of 9.83 ϫ 10 −4 ⍀ cm 2 and the highest reflectance of 91% at 460 nm, compared to as-deposited and O 2 plasma-treated ITO/Al reflectors.Due to the advantages in large-area, high power applications, GaN-based vertical-type light emitting diodes ͑V-LEDs͒ have been studied extensively in recent years. 1,2 The V-LEDs utilize top-down electrode schemes, unlike the conventional top-emitting LEDs, so that uniform currents can be injected across the LED chip, which eventually increases the output power level that can be achieved from the device. 1,2 A typical chip manufacturing process for GaNbased V-LEDs involves a flip-chip bonding with submounting materials, such as Si substrates, to remove the sapphire substrate from the n-type GaN. 3 Therefore, it is important to achieve a highly reflective ohmic contact to the p-GaN to increase the light extraction efficiency. The reflector on the p-GaN should effectively reflect a downward emitted light back toward the emitting substrate without absorption losses. Ag-based alloys such as Ni/Au/indium tin oxide ͑ITO͒/Ag, 4 NiO/Ag/Ni, 5 and Ni/Ag/Al 6 have been widely used for the p-GaN reflector because of their high conductivity and high reflectance ͑Ͼ90%͒ for visible light. However, Ag-based reflectors suffer from a large amount of leakage current, 7 particularly under high power operation, due to their poor thermal stability and adhesion with p-GaN, and therefore, they might not be suitable materials for a p-GaN reflector in a V-LED.Because of high reflectance ͑Ͼ90%͒ for both visible and UV light, Al-based electrodes have also been developed along with Agbased electrodes as p-GaN reflectors. However, an Al reflector cannot be applied directly on a p-type GaN for ohmic contact because it is an n-type ohmic material. Therefore, it has been developed in combination with a transparent electrode such as ITO so that the possible light absorption at the interface between ITO and the p-type GaN can be minimized as much as possible. 7,8 However, the work function of ITO ͑4.7 eV͒ is still lower than that of p-GaN ͑7.5 eV͒, which makes it difficult to obtain a good ohmic contact to the p-type GaN using the ITO/Al metal schemes.In this article, the effect of plasma treatment on the ohmic properties between ITO and p-type GaN is investigated, and then the electrical and optical properties of the plasma-treated ITO/Al reflector are investigated. ExperimentalFor the experiment, 200 nm thick ITO films were deposited on p-type GaN using an electron-beam evaporator and annealed ...

No abstract