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.

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

Polarity issues affecting III-V nitride semiconductors are reviewed with respect to their determination and control. A set of conditions crucial to the polarity control of GaN is provided for each of the following growth techniques; molecular beam epitaxy (MBE), pulsed laser deposition (PLD) and hydride vapor phase epitaxy (HVPE). Although GaN films might have been deposited by identical growth methods using the same buffer layer technologies, there is often a conflict between the resulting polarities achieved by different research groups. In this paper, we present the implications of the conditions used in each of the processes used for two-step metalorganic chemical vapor deposition (MOCVD), demonstrating systematic control of the polarity of GaN films on sapphire substrates. The potential for confusion in polarity control will be explained, taking into account the implications clarified in our studies. The correlation between the polarity and the growth conditions will be discussed in order to provide a mechanism for the determination and control of the crystal polarity during the growth of GaN films.

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Review of polarity determination and control of GaN

Contact mechanisms and design principles of alloyed ohmic contacts to n-GaN are investigated. For the investigation, both tunnel contacts and thermionic contacts are considered. While the tunnel contacts include the Ti/Al/Ti/Au, Ti/Al/Ni/Au, Ti/Al/Pd/Au, (Ta,Ti)/Ni/Au, Ti/Al/Mo/Au, and Ti/Au/Pd/Au contacts, the thermionic contacts include the Ni/Au contacts. The proposed design principles correctly dictate the characteristics of all these contacts. At present, tunneling is believed to be the primary mechanism for low resistivity of the tunnel contacts. The present study demonstrates that both tunneling and thermionic emission are equally important for the low resistivity of these contacts. Band-gap narrowing and/or image force lowering due to heavy doping also contribute to the resistivity reduction of these contacts. An exciting feature of the present study is the observation of a very low work function intermetallic alloy formed during annealing of an appropriate combination of large work function metal...

Contact mechanisms and design principles for alloyed ohmic contacts to n-GaN

The change of band banding with the crystal polarity of GaN films was investigated using high-resolution photoemission spectroscopy. Compared with a N-face sample, the Ga-face sample exhibited higher Schottky barrier height and lower contact resistivity of a Ti/Al-based Ohmic contact. It was found that Ga-face GaN has a larger surface band bending than N-face GaN by 1.4 eV due to spontaneous polarization, resulting in higher Schottky barrier height. The lower Ohmic contact resistivity on Ga-face GaN originated from the formation of polarization-induced two-dimensional electron gas at the interface of AlN with GaN.

Characterization of band bendings on Ga-face and N-face GaN films grown by metalorganic chemical-vapor deposition

A vertical-cavity surface-emitting laser (VCSEL) comprising a low-loss thin metal contact and current spreading layer within the optical cavity that provides for improved ohmic contact and lateral current distribution, a substrate including a plano-concave optical cavity, a (Ga,In,Al)N multiple quantum well (MQW) active region contained within the optical cavity that generates light when injected by an electrical current, and an integrated micromirror fabricated onto the substrate that provides for optical mode control of the light generated by the active region. A relatively simple process is used to fabricate the VCSEL.

Electrically-pumped (Ga,In,Al)N vertical-cavity surface-emitting laser

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.

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

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.

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

The effect of an overlayer in the Ni-based ohmic contacts on contact resistivity, light transmittance and the performance of light emitting diodes (LEDs) has been investigated. The ohmic contact composed of an Au overlayer on Ni resulted in a low contact resistivity of 9×10-4 Ωcm2 and a high transmittance of 92% after annealing at 500°C in O2 ambient. The LED with a Ni/Au contact showed a threshold voltage and an output power of 4.1 V and 1.7 mW at 20 mA, respectively. In the case of a Pt overlayer on Ni, the minimum contact resistivity was comparable to that of the Ni/Au contacts. However, the maximum transmittance was only 50%, which reduced the output power of the fabricated LED to 1.0 mW. For a Pd overlayer on Ni, the transmittance of 72% was obtained after annealing at 500°C in O2 ambient, while the contacts showed nonohmic behavior. The LED with the Ni/Pd contact resulted in a high threshold voltage of 7.5 V at 20 mA. These results were explained by the differences in the thermodynamic properties between the overlayer and Ni.

The Role of an Overlayer in the Formation of Ni-based Transparent Ohmic Contacts to p-GaN

An electrically injected InGaN/GaN-based distributed Bragg reflector (DBR) laser was demonstrated. Surface grating was formed on both sides of ridge waveguide by chemically assisted ion beam etching technique. The observed threshold current was 375 mA with threshold voltage of 15.1 V for 500×3 μm2 devices. The emission of the DBR laser occurred in a single longitudinal mode at a wavelength of 401.3 nm. The ratio of sidemode suppression was found to be more than 13 dB until a current injection of 1 A.

InGaN/GaN multi-quantum well distributed Bragg reflector laser diode

Interfacial microstructure and elemental diffusion of Cu-doped indium oxide (CIO)/indium tin oxide (ITO) ohmic contacts to p-type GaN for light-emitting diodes (LEDs) were investigated using cross-sectional transmission electron microscopy (XTEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction. The CIO/ITO contacts gave specific contact resistances of approximately 10(-4) omegacm2 and transmittance greater than 95% at a wavelength of 405 nm when annealed at 630 degrees C for 1 min in air. After annealing at 630 degrees C, multi-component oxides composed of Ga2O3-In2O3, Ga2O3-CuO, and In2O3-CuO formed at the interface between p-GaN and ITO. Formation of multi-component oxides reduced the barrier height between p-GaN and ITO due to their higher work functions than that of ITO, and caused Ga in the GaN to diffuse into the CIO/ITO layer, followed by generation of acceptor-like Ga vacancies near the GaN surface, which lowered contact resistivity of the CIO/ITO contacts to p-GaN after the annealing.

S. Chae

Papers

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

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

The Role of an Overlayer in the Formation of Ni-based Transparent Ohmic Contacts to p-GaN

InGaN/GaN multi-quantum well distributed Bragg reflector laser diode

Interfacial reactions of nano-structured Cu-doped indium oxide/indium tin oxide ohmic contacts to p-GaN.