June O. Song

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.

Wafer-level fabrication of GaN-based vertical light-emitting diodes using a multi-functional bonding material system

We first report on the fabrication of 2 inch wafer-level GaN-based vertical light-emitting diodes (LEDs) by using a multi-functional bonding material system, which is composed of a thick Cu diffusion barrier and a bonding layer. The bonding material system superbly absorbs laser-induced stress and also effectively serves as a barrier to the indiffusion of Sn to the active region. Fully packaged vertical LEDs fabricated with indium tin oxide (ITO)/AgCu contact and the bonding material system give an operating voltage of 3.35 V at 350 mA. After over 1800 h, the operating voltages remain stable, and the reverse currents are in the range 3–8 × 10−7 A at −5 V.

Ohmic-Contact Technology for GaN-Based Light-Emitting Diodes: Role of P-Type Contact

GaN-based semiconductors are of great technological importance for the fabrication of optoelectronic devices, such as light-emitting diodes (LEDs) and laser diodes. The further improvement of LED performance can be achieved through the enhancement of external quantum efficiency. In this regard, high-quality p-type ohmic electrodes having low contact resistance and high transmittance (or reflectivity), along with thermal stability, must be developed because p-type ohmic contacts play a key role in the performance of LEDs. In this paper, we review recent advances in p-type ohmic-contact technology for GaN-based LEDs. A variety of methods for forming transparent and reflective ohmic contacts are introduced.

High electron concentration and mobility in Al-doped n-ZnO epilayer achieved via dopant activation using rapid-thermal annealing

We report on the growth of very high-quality Al-doped n-type ZnO epilayers on sapphire substrates using a radio-frequency (rf) magnetron sputtering technique combined with a rapid-thermal annealing. Photoluminescence (PL) and Hall measurements show that both the optical and electrical properties of the ZnO layers are significantly improved with an increasing annealing temperature up to 900 °C. For example, the samples that are grown at 600 °C and a rf power of 100 W with an Ar∕O2 gas ratio of 1 give an electron concentration of 1.83×1020cm3 and a mobility of 65.6cm2∕Vs, when annealed at 900 °C for 3 min in a nitrogen ambient. Furthermore, x-ray diffraction measurements show that both the as-grown and annealed samples are of excellent crystallinity.

Highly low resistance and transparent Ni/ZnO ohmic contacts to p-type GaN

We report on a promising Ni (5 nm)/Al-doped ZnO (AZO) (450 nm) metallization scheme for low resistance and transparent ohmic contacts to p-GaN (5×1017 cm−3). It is shown that the as-deposited Ni/AZO contact shows a nonohmic characteristic due to the insulating nature of the as-deposited AZO. However, annealing the contacts at 450 and 550 °C for 2 min in air ambient results in linear current–voltage characteristics, giving a specific contact resistance of 1.01×10−5 and 8.46×10−6 Ω cm2, respectively. It is further shown that annealing the contact at 550 °C for 5 min produces a specific contact resistance of 6.23×10−6 Ω cm2. The light transmittance of the contacts annealed at 550 °C for 2 min is measured to be higher than 76% at wavelengths in the range of 400–550 nm. It is shown that the Ni/AZO contact could be a suitable scheme for high-performance optical devices.

Design of high-efficiency GaN-based light emitting diodes with vertical injection geometry

The authors report on the design and fabrication of high-efficiency GaN-based light emitting diodes (LEDs) with vertical-injection geometry. Based on the analyses of LED test patterns fabricated with various n-electrode dimensions, a design rule for vertical LEDs is proposed. It is found that the suppression of the vertical current under n electrodes and the efficient injection of the spreading current across the n layers are essential to fabricate high-efficiency LEDs. Introduction of the current blocking layer along with well-designed branched n electrodes results in a large enhancement of power efficiency by a factor of 1.9, compared with that of reference LEDs.

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.

Effect of rapid thermal annealing on Al doped n-ZnO films grown by RF-magnetron sputtering

High-quality Al-doped n-type ZnO (n-ZnO:Al) epilayers have been grown by an rf-magnetron sputtering technique combined with a rapid thermal annealing (RTA) process. The electrical and optical properties of as-deposited samples are considerably improved upon annealing at 900°C for 3 min in nitrogen ambient. The improvement is attributed to the deoxidation of Al-oxides, i.e., the activation of Al dopants. The samples annealed at 900°C produce a mobility of 65.5 cm2/Vs and a carrier concentration of 1.03×1020 cm-3. It is also shown that the sample surface becomes significantly smoother after annealing. The results show that the RTA process effectively improves the electrical and optical properties of the Al-doped ZnO films.

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.