Dong Seok Leem

Low resistance and reflective Mg-doped indium oxide-Ag ohmic contacts for flip-chip light-emitting diodes

We have investigated an Mg-doped In/sub x/O/sub y/(MIO)-Ag scheme for the formation of high-quality ohmic contacts to p-type GaN for flip-chip light-emitting diodes (LEDs). The as-deposited sample shows nonlinear current-voltage (I--V) characteristics. However, annealing the contacts at temperatures of 330/spl deg/C-530/spl deg/C for 1 min in air ambient results in linear I--V behaviors, producing specific contact resistances of 10/sup -4/--10/sup -5/ /spl Omega//spl middot/cm/sup 2/. In addition, blue LEDs fabricated with the MIO-Ag contact layers give forward-bias voltages of 3.13-3.15 V at an injection current of 20 mA. It is further shown that LEDs made with the MIO-Ag contact layers give higher output power compared with that with the Ag contact layer. This result strongly indicates that the MIO-Ag can be a promising scheme for the realization of high brightness LEDs for solid-state lighting application.

Formation of low resistance and transparent ohmic contacts to p-type GaN using Ni-Mg solid solution

We report on the formation of Ni–Mg solid solution/Au ohmic contacts on p-GaN (5×1017 cm−3). The as-deposited Ni–Mg solid solution (8 nm)/Au (8 nm) contact shows near-linear I–V characteristics. However, oxidizing the contacts at 450 and 550 °C for 1 min in air results in a dramatic improvement in their I–V behaviors, producing specific contact resistance of ∼10−6 Ω cm2, which is much better than the conventional oxidized Ni/Au contacts. The light transmittance of the Ni–Mg solid solution/Au contacts annealed at 550 °C is measured to be better than 79% at a wavelength of 460 nm. Based on the I–V measurements, Auger electron spectroscopy, and x-ray photoemission spectroscopy results, possible ohmic formation mechanisms are described.

Improvement of the luminous intensity of light-emitting diodes by using highly transparent Ag-indium tin oxide p-type ohmic contacts

We have investigated Ag-indium tin oxide (ITO) scheme for obtaining high-quality p-type ohmic contacts for GaN-based light-emitting diodes (LEDs). The Ag(1 nm)-ITO(200 nm) contacts exhibit greatly improved electrical characteristics when annealed at temperatures in the range 400/spl deg/C-600/spl deg/C for 1 min in air, yielding specific contact resistances of /spl sim/10/sup -4/ /spl Omega//spl middot/cm/sup 2/. In addition, the contacts give transmittance of about 96% at 460 nm, which is far better than that of the conventionally used oxidized Ni-Au contacts. It is shown that the luminous intensity of blue LEDs fabricated with the Ag-ITO contacts is about three times higher than that of LEDs with oxidized Ni-Au contacts. This result strongly indicates that the Ag-ITO scheme can serve as a highly promising p-type ohmic contact for the realization of high brightness near ultraviolet LEDs.

Low-resistance and highly-reflective Zn–Ni solid solution/Ag ohmic contacts for flip-chip light-emitting diodes

We have investigated a Zn–Ni solid solution/Ag scheme for use in producing high-quality ohmic contacts for flip-chip light-emitting diodes (LEDs). The as-deposited contact shows nonlinear I–V characteristics. However, oxidizing the contacts at temperatures of 350–550 °C for 1 min in air ambient results in linear I–V behaviors, yielding specific contact resistances of 10−4–10−5 Ω cm2. In addition, LEDs are fabricated with the oxidized Zn–Ni solid solution/Ag contacts and Ag single contacts. The typical I–V characteristics of the LEDs with the annealed Zn–Ni solid solution (2.5 nm)/Ag (200 nm) p-type contact layers reveal a forward-bias voltage of 3.25 V at an injection current of 20 mA, which is much better than that of the LEDs with the Ag (200 nm) contact layers.

Light-output enhancement of GaN-based light-emitting diodes by using hole-patterned transparent indium tin oxide electrodes

We have demonstrated the improvement of the light-output power of GaN-based light-emitting diodes (LEDs) using hole-patterned indium tin oxide (ITO) p-type electrodes. Hole patterns were defined by a laser holographic lithography combined with a postlithography deposition process. It is shown that near-UV LEDs made with the patterned ITO with a hole period of 710nm and a size of 320nm give 23% and 67% higher light-output power (at 20mA) than those of LEDs with unpatterned ITO and Ni∕Au contacts, respectively. It is further shown that the reduction of the hole period results in an additional improvement of light-output power.

High-quality nonalloyed rhodium-based ohmic contacts to p-type GaN

We report on a promising Rh-based scheme for high-quality ohmic contacts to surface-treated p-GaN:Mg (4×1017 cm−3). It is shown that the two-step surface-treated Rh contacts (10 nm) produce a specific contact resistance of 1.7×10−5 Ω cm2. It is also shown that the two-step treated Rh/Ni (5/5 nm) and Rh/Au (5/5 nm) contacts yield 6.0×10−5 and 9.3×10−6 Ω cm2, respectively. Based on the current–voltage measurement, x-ray photoemission spectroscopy, and Auger electron spectroscopy results, the mechanisms for the formation of the nonalloyed Rh-based ohmic contacts is described and discussed.

Formation of Nonalloyed Low Resistance Ni/Au Ohmic Contacts to p-Type GaN Using Au Nanodots

The effects of the insertion of Au nanodots (4-10 nm in diameter) at the Ni/GaN interface on the electrical properties of Ni/Au ohmic contacts to p-type GaN have been investigated. As-deposited Ni/Au contacts with Au nanodots show better electrical behavior than contacts without Au nanodots. Nanodot contacts produce a specific contact resistance of 8.4 X 10 - 4 Ω cm 2 . The multiquantum-well light-emitting diodes (LEDs) are fabricated with the nanodot Ni/Au contact layers. Nanodot LEDs show a lower operating voltage compared with LEDs made with a conventional Ni/Au contact layer.

Formation mechanism of cerium oxide-doped indium oxide/Ag Ohmic contacts on p-type GaN

The authors report on the formation of cerium oxide-doped indium oxide(2.5nm)∕Ag(250nm) contacts to p-GaN. The contacts become Ohmic with a specific contact resistance of 3.42×10−4Ωcm2 upon annealing at 530°C in air. X-ray photoemission spectroscopy (XPS) Ga 3d core levels obtained from the interface regions before and after annealing indicate a large band bending of p-GaN (about 1.7–1.8eV), namely, an increase of Schottky barrier height. Based on the XPS, secondary ion mass spectroscopy, and capacitance-voltage data, possible transport mechanisms for the annealed contacts are described and discussed.

Low resistance and transparent Ni–La solid solution/Au ohmic contacts to p-type GaN

We report on the formation of reliable Ni–La solid solution (8 nm)/Au (8 nm) ohmic contacts to p-type GaN (Na=5×1017 cm−3). The as-deposited contact reveals nonlinear current–voltage (I–V) characteristics. However, the contacts show considerably improved I–V behavior when annealed at temperatures of 350–550 °C for 1 min in air ambient. For example, the specific contact resistance as low as 7.2×10−5 Ω cm2 is obtained from the samples annealed at 550 °C, whose transmittance is measured to be 82.5% at a wavelength of 470 nm. Based on Auger electron spectroscopy and x-ray photoemission spectroscopy results, possible ohmic formation mechanisms for the annealed contacts are described and discussed.

Low resistance and highly reflective Sb-doped SnO2/Ag ohmic contacts to p-type GaN for flip-chip LEDs

We have investigated high-quality Sb-doped SnO 2 /Ag ohmic contacts to p-GaN for use in flip-chip light emitting diodes (LEDs). The Sb-doped SnO 2 /Ag contacts produce specific contact resistances of ∼10 - 4 Ω cm 2 upon annealing at 430 and 530°C for 1 min in air. It is shown that InGaN blue LEDs fabricated with the Sb-doped SnO 2 /Ag contacts give a forward-bias voltage of 3.18 V at 20 mA, while LEDs with Ag contacts show 3.36 V. It is further shown that the LEDs made with the Sb-doped SnO 2 /Ag contact layers show higher light output power compared with the LEDs with the Ag contacts.