Joon Woo Jeon

Electrical characteristics of contacts to thin film N-polar n-type GaN

The electrical characteristics of metallization contacts to a thin film N-polar n-type GaN layer fabricated by a laser lift-off process combined with a dry etching are investigated. It is shown that for Pt Schottky contacts, the Schottky barrier height of the N-polar GaN is 1.27eV, which is larger than that (1.23eV) of reference Ga-polar GaN. Ti∕Al Ohmic contacts to the N-polar GaN experience thermal degradation even at 400°C. Such annealing-induced degradation is explained in terms of the presence of the complex surface states of the N-polar GaN, which consists of impurities and process-induced donorlike and acceptorlike defects.

TiN/Al Ohmic contacts to N-face n-type GaN for high-performance vertical light-emitting diodes

We report on the electrical properties of TiN(30 nm)/Al(200 nm) Ohmic contacts to N-face n-type GaN for high-performance vertical light-emitting diodes and compare them with those of Ti(30 nm)/Al(200 nm) contacts. Both the as-deposited samples show Ohmic behaviors with contact resistivity of (6.0–7.2)×10−4 Ω cm2. However, annealing the samples at 300 °C causes the degradation of their electrical properties. Furthermore, unlike the TiN/Al contacts, the Ti/Al contacts suffer from aging degradation when exposed to air. Based on the x-ray photoemission spectroscopy and secondary ion mass spectrometry results, Ohmic formation and degradation mechanisms are briefly described and discussed.

Formation of low-resistance Ohmic contacts to N-face n-GaN for high-power GaN-based vertical light-emitting diodes

We report on the formation of low-resistance Ohmic contacts to N-face n-GaN for high-power vertical light-emitting diodes using an Al–Ga solid solution (50 nm)/Ti(30 nm)/Al(200 nm) scheme and compare them with Ti(30 nm)/Al(200 nm) contacts. The Al–Ga solid solution layer is introduced to minimize the formation of Ga vacancies near the N-face n-GaN surface. It is shown that, unlike the Ti/Al contacts, the Al–Ga solid solution/Ti/Al contacts exhibit Ohmic behavior with a resistivity of 4.1×10−4 Ω cm2, even after annealing at 250 °C. X-ray photoemission spectroscopy and secondary ion mass spectrometry examinations are performed to understand the temperature dependence of the electrical properties.

Highly reliable Ag/Zn/Ag ohmic reflector for high-power GaN-based vertical light-emitting diode.

We report the improved performance of InGaN/GaN-based light-emitting diodes (LEDs) through Ag reflectors combined with a Zn middle layer. It is shown that the Zn middle layer (5 nm thick) suppresses the agglomeration of Ag reflectors by forming ZnO and dissolving into Ag. The Ag/Zn/Ag contacts show a specific contact resistance of 6.2 × 10(-5) Ωcm(2) and reflectance of ~83% at a wavelength of 440 nm when annealed at 500 °C, which are much better than those of Ag only contacts. Blue LEDs fabricated with the 500 °C-annealed Ag/Zn/Ag reflectors show a forward voltage of 2.98 V at an injection current of 20 mA, which is lower than that (3.02 V) of LEDs with the annealed Ag only contacts. LEDs with the 500 °C-annealed Ag/Zn/Ag contacts exhibit 34% higher output power (at 20 mA) than LEDs with the annealed Ag only contacts.

Possible Ohmic Mechanisms of Ag/Indium Tin Oxide p-Type Contacts for High-Brightness GaN -Based Light Emitting Diodes

We have investigated the Ag (1 nm)/indium tin oxide (ITO) (200 nm) contacts by scanning transmission electron microscopy (STEM), Auger electron spectroscopy (AES), and X-ray photoemission spectroscopy (XPS) to understand its ohmic mechanism. The Ag/ITO contacts exhibit ohmic behaviors, when annealed at 400-600°C. The effective Schottky barrier heights depend on the annealing temperatures. STEM and AES results reveal the formation of Ag nanodots (5-35 nm across) and Ga-Ag solid solution. Based on the STEM, AES, and XPS results, the ohmic contact formation is described in terms of the formation of the Ga-Ag solid solution and the inhomogeneous interfaces with nanodots.

Electrical Characteristics of V/Ti/Au Contacts to Ga-Polar and N-Polar n-GaN Prepared by Different Methods

We have investigated the electrical properties of V (20 nm)/Ti (60 nm)/Au (20 nm) contacts to Ga- and N-polar n-GaN. Regardless of the crystal polarities, all the samples exhibit similar electrical characteristics. The as-deposited samples are ohmic. However, they become nonohmic when annealed at 300-500°C. The samples are ohmic again at 700°C. Based on the X-ray photoemission spectroscopy and Auger electron spectroscopy results, the ohmic and degradation behaviors are explained in terms of the formation of donorlike surface defects and Ga vacancies, which are generated by dry etching, the out-diffusion of Ga, and the formation of nitride phases.

Improved Light Output Power of GaN-Based Light Emitting Diodes by Enhancing Current Spreading Using Single-Wall Carbon Nanotubes

Single-wall carbon nanotubes (SWCNTs) have been combined with indium tin oxide (ITO) to improve the output power of GaN-based light emitting diodes (LEDs). LEDs fabricated with the SWCNT/ITO contacts give a forward voltage of 3.61 V at 350 mA, which is slightly higher than that of LEDs with ITO-only contacts. The SWCNT/ITO and ITO-only contacts produce transmittance values of 91.5 and 94.4% at 460 nm, respectively. However, LEDs with SWCNTs show a higher output power by 60% at 20 mA compared to those without SWCNTs. Photoemission microscope analyses show that the well-dispersed SWCNT bundle efficiently serves as a current spreader.

Nanostructure Ag dots for improving thermal stability of Ag reflector for GaN-based light-emitting diodes

We report the improved thermal stability of Ag reflectors for GaN-based light-emitting diodes (LEDs) using Ag nano-dots (∼65–∼190 nm in size). The nano-dot Ag samples show much higher reflectance than the Ag only samples. The annealed nano-dot Ag samples exhibit a smoother surface, where the grains contain numerous micro-twins. 〈111〉 texture becomes more dominantly evolved in the nano-dot Ag samples than in the Ag only samples after annealing. LEDs with the 300 °C-annealed nano-dot Ag reflectors exhibit 15%–36% higher output power (at 20 mA) than LEDs with the 300 and 400 °C-annealed Ag only reflectors.

Highly Reliable Ohmic Contacts to N-Polar n-Type GaN for High-Power Vertical Light-Emitting Diodes

We report on the formation of highly reliable Ti/Al-based Ohmic contacts to N-polar n-GaN for vertical light-emitting diodes via laser-annealing. All as-deposited samples are Ohmic with specific contact resistances of 1.1 - 4.3 × 10^-4 Ω cm². After annealing at 250°C, unlike the untreated sample, the laser-annealed samples remain Ohmic with specific contact resistances of 2.6- 3.9 × 10^-4 Ω cm². The laser-annealed samples remain electrically stable up to 60 min at 300°C. Laser-annealing causes the formation of interfacial TiN/β -AlN phases with rock salt structure. Based on X-ray photoemission spectroscopy and scanning transmission electron microscopy results, possible Ohmic mechanisms are described.

Low Resistance CrB2 / Ti / Al Ohmic Contacts to N-face n-GaN for High Power GaN-Based Vertical Light Emitting Diodes

We investigated the effect of a CrB 2 interlayer on the electrical properties of Ti/Al contacts to N-face n-type GaN for high performance vertical light emitting diodes. Before annealing, both CrB 2 (30 nm)/Ti (30 nm)/Al (200 nm) and Ti (30 nm)/Al (200 nm) contacts produce ohmic behaviors with a contact resistivity of 1.92 × 10- 4 and 1.99 × 10 ―4 Ω cm 2 , respectively. Unlike the Ti/Al contacts, however, the CrB 2 /Ti/Al contacts remain ohmic with a contact resistivity of 8.30 × 10- 4 Ω cm 2 even after annealing at 250°C for 1 min in N 2 ambient. X-ray photoemission spectroscopy and secondary-ion mass spectrometry examinations are performed to describe the ohmic formation behavior.