Joon Seop Kwak

Ohmic and degradation mechanisms of Ag contacts on p-type GaN

The electrical and optical properties of the Ag contacts have been investigated as a function of the annealing temperature. The as-deposited contact becomes good Ohmic with contact resistivity of 2.47×10−4Ωcm2 and produces reflectance of ∼84% when annealed at 330°C for 1mm in air ambient. However, annealing at 530°C results in nonlinear current–voltage behavior and degraded reflectance. The light output of InGaN∕GaN blue light emitting diodes (LEDs) fabricated with the as-deposited Ag contact shows somewhat better output performance than those of LEDs with the annealed Ag contacts. Based on the transmission electron microscopy-energy dispersive x-ray analysis, synchrotron x-ray diffraction, and x-ray photoemission spectroscopy results, possible mechanisms for the ohmic formation and degradation of the Ag contacts are described.

Crystal-polarity dependence of Ti/Al contacts to freestanding n-GaN substrate

The effect of crystal polarity on the electrical properties of Ti/Al contacts to n-GaN substrate has been investigated. The Ti/Al contacts prepared on Ga-face n-GaN substrate became ohmic with a contact resistivity of 2×10−5 Ω cm2 after annealing at temperatures higher than 600 °C for 30 s. On the contrary, the contacts on N-face n-GaN substrate exhibited nonlinear current–voltage curve and high Schottky barrier heights over 1 eV were measured at the same annealing conditions. These results could be explained by opposite piezoelectric-field at GaN/AlN heterostructure resulted from different polarity of the GaN substrate.

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.

Light-extraction enhancement of vertical-injection GaN-based light-emitting diodes fabricated with highly integrated surface textures

We report on the fabrication of high-efficiency vertical-injection GaN-based light-emitting diodes (LEDs) fabricated with integrated surface textures. An optical ray-tracing simulation shows that the high integration of surface textures can effectively enhance the light-extraction efficiency. The integrated surface textures are fabricated on the top surface of LEDs by generating hexagonal cones on the periodically corrugated surfaces of n-GaN. Compared to reference LEDs without textures, LEDs fabricated with integrated surface textures show an enhancement of the output power by a factor of 2.59, which is in agreement with the calculated results.

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.

Cu-doped indium oxide∕Ag ohmic contacts for high-power flip-chip light-emitting diodes

We have investigated Cu-doped In2O3(CIO)(3nm)∕Ag(250nm) schemes for forming high-quality ohmic contacts to p-type GaN for high-power flip-chip light-emitting diodes (FCLEDs). It is shown that the CIO∕Ag contacts produce specific contact resistance of 1.28×10−5Ωcm2 and reflectance of about 90% at a wavelength of 460nm when annealed at 530°C for 1min in air ambient. It is also shown that unlike single Ag contacts, the CIO∕Ag contacts are fairly stable without surface and interface degradation although annealed at 530°C for 1min in air ambient. In addition, blue multiquantum-well InGaN∕GaN LEDs fabricated with the annealed CIO∕Ag contact layers give forward-bias voltages of around 3.0V at an injection current of 20mA. The results strongly indicate that the CIO∕Ag scheme can be a highly promising p-type contact for high-power GaN-based FCLEDs for solid-state lighting application.

Abnormal dependence of contact resistivity on hole concentration in nonalloyed ohmic contacts to p-GaN

The dependence of contact resistivity on hole concentration has been investigated for nonalloyed Pd contacts to p-GaN. The hole concentration was varied by changing the Mg concentration, [Mg], in p-GaN. The p-GaN having the [Mg] of 4.5×1019 cm−3 showed the hole concentration of 2.2×1017 cm−3, where contact resistivity was measured as 8.9×10−2 Ω cm2. When the [Mg] increased to 1.0×1020 cm−3, the hole concentration was significantly reduced to 2.0×1016 cm−3. Nevertheless, the Pd contacts on the p-GaN displayed contact resistivity as low as 5.5×10−4 Ω cm2. The abnormal dependence of contact resistivity on hole concentration may be explained by predominant current flow at the Pd/p-GaN interface through a deep level defect band, rather than the valence band.

Improvement of the light output of InGaN-based light-emitting diodes using Cu-doped indium oxide/indium tin oxide p-type electrodes

We report on the formation of highly transparent and low-resistance Cu-doped indium oxide(CIO)(3 nm)/indium tin oxide (ITO) (400 nm) ohmic contact to p-type GaN for high-brightness light-emitting diodes (LEDs) for solid-state lighting. The CIO/ITO contacts become ohmic with specific contact resistances of ∼10−4Ωcm2 and give transmittance higher than 98.7% at a wavelength of 405 nm when annealed at 630 °C for 1 min in air. Near UV LEDs fabricated with the annealed CIO/ITO p-type contact layers give a forward-bias voltage of 3.25 V at injection current of 20 mA. It is further shown that the output power of the LEDs with the CIO/ITO contacts is enhanced 78% at 20 mA as compared with that of LEDs with the conventional Ni∕Au contacts.

Low resistance Al/Ti/n-GaN ohmic contacts with improved surface morphology and thermal stability

The Al/Ti/n-GaN ohmic contact and multilayer variations on this contact are widely used because of the low contact resistivities they provide to n-GaN. However, there are few reports that reveal the influence of the atomic ratio of Al to Ti on the contact resistivity, thermal stability and surface morphology of the contacts. This study reveals that the ratio of Al to Ti strongly influences all of these characteristics. All contacts in this study had atomic ratios of Al to Ti higher than 1. Those contacts with ratios of Al to Ti less than 3 required higher annealing temperatures or longer annealing times to reach comparable contact resistivities compared to the more Al-rich contacts. On the other hand, the less Al-rich contacts provided several advantages. They exhibited smooth surface morphologies even after they were annealed at temperatures near 1000 °C, and they suffered much less severe degradation during long-term aging at 600 °C. These findings are explained by differences in the phases formed after annealing.