Low Resistance High Reflectance ITO/Al Ohmic Contacts to p-Type GaN Via SF[sub 6] Plasma Treatments

Abstract

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 ...

Domain Matching Epitaxy of Mg-Containing Ag Contact on p-Type GaN

“…GaN-based light emitting diodes (LEDs) are promising solid state lighting devices as an alternative to the fluorescent light lamp. − Vertical-structure LEDs by laser lift-off (LLO) were exploited in GaN-based LEDs to improve the light extraction efficiency and thermal management of the devices . In these configurations, Ohmic contacts with low contact resistance and high reflectance are essential in improving the electrical and optical performance of the devices.…”

Improved light output power of GaN-based ultraviolet light-emitting diode using a mesh-type GaN/SiO2 /Al omnidirectional reflector

“…Thus, Al‐based reflectors have increasingly attracted interest as an alternative, because they have high reflectance in the visible and UV spectra. However, there is a critical problem with Al‐based reflectors; Al has a low work function, and therefore, Al can form ohmic contacts only to n‐type GaN, but not to p‐type GaN . To solve this problem, nanopixel contacts (size: 1 × 1 µm 2 ) were combined with Al reflectors to enhance the light extraction in UV LEDs .…”

Indium Tin Oxide (ITO) based Ohmic Contacts on Bulk n-GaN Substrate

“…Additionally, Lee et al, demonstrated ITO/Al contacts to p-GaN epitaxial film with SF 6 plasma treatments on sapphire. 27 D. Selvanathan et al reported SiCl 4 plasma treatment for formation of low-resistance metal based ohmic contacts on AlGaN/GaN heterostructure field effect transistors. 28 On the other hand, Li et al, reported the role of N 2 and BCl 3 /Cl 2 plasma treatments on GaN surfaces for the formation of metal-based ohmic contact to n-GaN thin film on sapphire.…”