Jae Seong Park

Near ultraviolet InGaN/AlGaN-based light-emitting diodes with highly reflective tin-doped indium oxide/Al-based reflectors

The enhanced light output power of a InGaN/AlGaN-based light-emitting diodes (LEDs) using three different types of highly reflective Sn-doped indium oxide (ITO)/Al-based p-type reflectors, namely, ITO/Al, Cu-doped indium oxide (CIO)/s-ITO(sputtered)/Al, and Ag nano-dots(n-Ag)/CIO/s-ITO/Al, is presented. The ITO/Al-based reflectors exhibit lower reflectance (76 - 84% at 365 nm) than Al only reflector (91.1%). However, unlike Al only n-type contact, the ITO/Al-based contacts to p-GaN show good ohmic characteristics. Near-UV (365 nm) InGaN/AlGaN-based LEDs with ITO/Al, CIO/s-ITO/Al, and n-Ag/CIO/s-ITO/Al reflectors exhibit forward-bias voltages of 3.55, 3.48, and 3.34 V at 20 mA, respectively. The LEDs with the ITO/Al and CIO/s-ITO/Al reflectors exhibit 9.5% and 13.5% higher light output power (at 20 mA), respectively, than the LEDs with the n-Ag/CIO/s-ITO/Al reflector. The improved performance of near UV LEDs is attributed to the high reflectance and low contact resistivity of the ITO/Al-based reflectors, which are better than those of conventional Al-based reflectors.

Graphene diffusion barrier for forming ohmic contact on N-polar n-type GaN for high-power vertical-geometry light-emitting diodes

Graphene sheets were employed as a diffusion barrier to form highly reliable ohmic contacts to N-polar n-GaN for high-power vertical-configuration LEDs (VLEDs). Before annealing, both Ti/Al and Ti/graphene/Al contacts exhibit ohmic behavior. After annealing at 250 °C, unlike the Ti/Al contact, the Ti/graphene/Al contacts remain ohmic with a contact resistivity of 7.5 × 10−4 Ω cm2. Secondary ion mass spectroscopy (SIMS) results show that the graphene sheets serve as a barrier to the outdiffusion of Ga atoms into the electrode. On the basis of SIMS and X-ray photoemission spectroscopy results, the ohmic and degradation behaviors of the Ti-based contacts are described and discussed.

A tantalum diffusion barrier layer for improving the output performance of AlGaInP-based light-emitting diodes

We have investigated the effect of a Ta diffusion barrier layer on the electrical characteristics of AuBe/Au contacts on a p-GaP window layer for AlGaInP-based light-emitting diodes (LEDs). It was shown that after annealing at 500 °C, the AuBe/Ta/Au contacts exhibited nearly 2 orders of magnitude lower specific contact resistance (2.8 × 10−6 Ωcm2) than the AuBe/Au contacts (1.0 × 10−4 Ωcm2). The LEDs with and without the Ta diffusion barrier layer showed an external quantum efficiency of 14.03 and 13.5% at 50 mA, respectively. After annealing at 500 °C, the AuBe/Ta/Au contacts showed a higher reflectance (92.8% at 617 nm) than that of the AuBe/Au contacts (87.7%). X-ray photoemission spectroscopy (XPS) results showed that the Ga 2p core level for the annealed AuBe/Au samples shifted to higher binding energies, while this level shifted towards lower binding energies for the AuBe/Ta/Au samples. Depth profiles using Auger electron spectroscopy (AES) showed that annealing of the AuBe/Au samples caused the outdiffusion of both Be and P atoms into the metal contact, while for the AuBe/Ta/Au samples, the outdiffusion of Be atoms was blocked by the Ta barrier layer and more Be atoms were indiffused into GaP. The annealing-induced electrical degradation and ohmic contact formation are described and discussed based on the XPS and electrical results.

Polarity dependence of the electrical characteristics of Ag reflectors for high-power GaN-based light emitting diodes

We report on the polarity dependence of the electrical properties of Ag reflectors for high-power GaN-based light-emitting diodes. The (0001) c-plane samples become ohmic after annealing in air. However, the (11–22) semi-polar samples are non-ohmic after annealing, although the 300 °C-annealed sample shows the lowest contact resistivity. The X-ray photoemission spectroscopy (XPS) results show that the Ga 2p core level for the c-plane samples experiences larger shift toward the valence band than that for the semi-polar samples. The XPS depth profile results show that unlike the c-plane samples, the semi-polar samples contain some amounts of oxygen at the Ag/GaN interface regions. The outdiffusion of Ga atoms is far more significant in the c-plane samples than in the semi-polar samples, whereas the outdiffusion of N atoms is relatively less significant in the c-plane samples. On the basis of the electrical and XPS results, the polarity dependence of the electrical properties is described and discussed

Use of a patterned current blocking layer to enhance the light output power of InGaN-based light-emitting diodes

We employed a patterned current blocking layer (CBL) to enhance light output power of GaN-based light-emitting diodes (LEDs). Nanoimprint lithography (NIL) was used to form patterned CBLs (a diameter of 260 nm, a period of 600, and a height of 180 nm). LEDs (chip size: 300 × 800 µm2) fabricated with no CBL, a conventional SiO2 CBL, and a patterned SiO2 CBL, respectively, exhibited forward-bias voltages of 3.02, 3.1 and 3.1 V at an injection current of 20 mA. The LEDs without and with CBLs gave series resistances of 9.8 and 11.0 Ω, respectively. The LEDs with a patterned SiO2 CBL yielded 39.6 and 11.9% higher light output powers at 20 mA, respectively, than the LEDs with no CBL and conventional SiO2 CBL. On the basis of emission images and angular transmittance results, the patterned CBL-induced output enhancement is attributed to the enhanced light extraction and current spreading.

Highly reliable Ti-based ohmic contact to N-polar n-type GaN for vertical-geometry light-emitting diodes by using a Ta barrier layer

The formation of thermally stable and low resistance Ti/Al-based ohmic contacts to N-polar n-GaN for high-power vertical light-emitting diodes (VLEDs) using a Ta diffusion barrier is presented. Before annealing, both Ti/Al/Au and Ti/Ta/Al/Au contacts reveal ohmic behavior with specific contact resistances of 2.4 × 10⁻⁴ and 1.2 × 10⁻⁴ Ωcm², respectively. However, unlike the Ti/Al/Au samples that are electrically degraded with increasing annealing time at 250 °C, the Ti/Ta/Al/Au samples remain thermally stable even after annealing for 600 min. LEDs fabricated with the Ti/Ta/Al/Au contacts yield 8.3% higher output power (at 300 mA) than LEDs with the Ti/Al/Au contact. X-ray photoemission spectroscopy results show that the Ta layer serves as an efficient barrier to the indiffusion of oxygen toward the GaN. On the basis of the XPS and electrical results, the annealing dependence of the electrical characteristics of Ti/Al-based contacts are described and discussed.