Very-low-specific-resistance Pd/Ag/Au/Ti/Au alloyed ohmic contact to p GaN for high-current devices (original) (raw)
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Formation and electronic properties of oxygen annealed Au/Ni and Pt/Ni contacts to p-type GaN
Semiconductor Science and Technology, 2007
The formation of Au/Ni and Pt/Ni contacts to p-GaN has been examined with Auger depth profiling and x-ray photoelectron spectroscopy analysis and compared to circular transmission line measurements of their respective transport properties. The metal bilayers were deposited by radio frequency argon ion sputtering, and the samples were annealed at different temperatures in a 10 −3 mbar oxygen ambient. Both contacts were distinctly non-ohmic as deposited. Ohmic behaviour was achieved after oxidation at temperatures above 400 • C, and the two contacts were found to have comparable performance, with a lowest specific contact resistance ρ c = 1.0 × 10 −2 cm 2 for the Pt/Ni contact after annealing at 400 • C and ρ c = 1.3 × 10 −2 cm 2 for the best Au/Ni contact annealed at 600 • C. Au and Pt both serve to ensure a highly conducting contact layer after annealing in oxygen and help lower the Schottky barrier at the metal/p-GaN interface by virtue of their large work functions. Distinct differences were observed in the alloying properties of the Pt/Ni and the Au/Ni contact layers, however. While Pt and Ni interdiffuse to form a uniform alloy upon annealing in oxygen at 600 • C, Ni diffuses outwards to form surface nickel oxide for the Au/Ni metallization. Trace amounts of Ga were observed at the surface after annealing at 600 • C for the Pt/Ni and the Au/Ni bilayers alike, albeit in considerably larger measure for Pt/Ni. We believe that this surface Ga derives from a thermally activated reaction between Ni and GaN, which adversely affects the contact performance. The outward diffusion of Ni upon thermal oxidation of the Au/Ni contact appears to inhibit the rate of this interface reaction, which speaks in favour of using Au over Pt in ohmic contacts to p-GaN. Preconditioning of the p-GaN surface by deposition and subsequent removal of a thin coating of Ni are found to improve the contact performance, provided the coating is not annealed before removal.
Low-resistance and thermally stable Pd/Ru ohmic contacts to p-type GaN
Journal of Electronic Materials, 2002
We report on low-resistance and thermally stable Pd/Ru ohmic contacts to surface-treated p-GaN (3 ϫ 10 17 cm Ϫ3 ). It is shown that annealing at 500°C for 2 min in a N 2 ambient improves ohmic contact properties. Specific contact resistance is measured to be 9.2(Ϯ0.2) ϫ 10 Ϫ4 and 2.4(Ϯ0.2) ϫ 10 Ϫ5 ⍀cm 2 for the as-deposited and annealed samples, respectively. Atomic force microscopy results show that the surfaces of both the contacts are remarkably smooth with a root-mean-square (rms) roughness of about 0.6 nm. The current-voltagetemperature (I-V-T) and calculation results indicate that, for the as-deposited contact, thermionic field emission is dominant, while for the annealed contact, field emission dominates the current flow.
Study of Au nanoparticles/ITO ohmic contacts to p‐type GaN
physica status solidi (a), 2006
P‐type ohmic contact between Au nanoparticles/ITO and p‐GaN during heat treatment is reported. Optimal conditions are selected to minimize the lowest specific contact resistance to 1.66 × 10–3 Ω cm2, as determined by the transmission line model (TLM) after heat treatment process at an alloying temperature of 600 °C for 10 min in air. Au nanoparticles/ITO bi‐layers are also applied to GaN‐based LEDs to form an electrode with a p‐type ohmic contact. Typical I–V characteristics of the GaN‐based LEDs with an ohmic contact layer of Au nanoparticles/ITO exhibit a forward‐bias voltage of 3.43 V at an injection current of 20 mA. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Electrical and structural properties of low-resistance Ti/Al/Re/Au ohmic contacts to n-type GaN
Journal of Electronic Materials, 2004
Titanium (15 nm)/aluminum (60 nm)/rhenium (20 nm)/gold (50 nm) ohmic contacts to moderately doped n-type GaN (4.07 ϫ 10 18 cm Ϫ3) have been investigated as a function of annealing temperature. It is shown that the currentvoltage (I-V) characteristics of the contacts are improved upon annealing at temperatures in the range of 550-750°C. Specific contact resistance as low as 1.3 ϫ 10 Ϫ6 Ωcm 2 is obtained after annealing at 750°C for 1 min in a nitrogen ambient. X-ray photoemission spectroscopy (XPS) results show that the Ga 2p core level for the sample annealed at 750°C shifts toward the high binding side by 0.71 eV compared with that of the as-deposited one. It is also shown that the contact does not seriously suffer from thermal degradation even when annealed at 750°C for 30 min. Based on Auger electron spectroscopy (AES), glancing angle x-ray diffraction (GXRD), and XPS results, possible explanations for the annealing-induced improvement of the ohmic behavior are described and discussed.
We present the results of structural and morphological investigations of interactions between phases in the layers of Au-Pd-Ti-Pd-n + -GaN contact metallization that appear at rapid thermal annealing (RTA). It is shown that formation of ohmic contact occurs in the course of RTA at Т = 900 C due to formation of titanium nitride. We studied experimentally and explained theoretically the temperature dependence of contact resistivity ρ с (Т) of ohmic contacts in the 4.2-380 K temperature range. The ρ с (Т) curve was shown to flatten out in the 4.2-50 K range. As temperature grew, ρ с decreased exponentially. The results obtained enabled us to conclude that current flow has field nature at saturation of ρ с (Т) and the thermofield nature in the exponential part of ρ с (Т) curve.
Mechanism of Ohmic Cr/Ni/Au contact formation on p-GaN
Journal of Vacuum Science & Technology B, Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena, 2012
Detailed investigation of Ohmic Cr/Ni/Au based contact formation to p-GaN was realized by scanning transmission electron microscopy and energy-dispersive x-ray spectroscopy and compared to the Cr/Au bilayer metallization scheme. The authors found that it is essential to introduce a nickel film in Cr-based contacts and anneal the trilayer structure in air in order to suppress the Shottky barrier and thus obtain the Ohmic contact. Our findings also indicate that oxygen behaves as a dopant dispersed in chromium nitride matrix. Thus Ohmic trilayer Cr/Ni/Au contact to p-GaN annealed in air is formed by Ni–Ga–Au alloy mixed with Au–Ga-doped Cr2N crystalline composites. Possible ways for improvement of such types of contact are discussed.
Journal of Electrical Engineering
The work investigates an increase of the density of free charge carriers in the sub-surface region of p-GaN by adding p-type dopants into the Ni-O layer of an Au/Ni-O metallization structure. We have examined electrical properties and concentration depth profiles of contact structures Au/Ni-Mg-O/p-GaN and Au/Ni-Zn-O/p-GaN, thus with magnesium and zinc as p-type dopants. The metallization layers were deposited on p-GaN by DC reactive magnetron sputtering in an atmosphere with a low concentration of oxygen (0.2 at%). The contacts were annealed in N2 . We have found that the structures containing magnesium or zinc exhibit lower values of contact resistivity in comparison with otherwise identical contacts without Mg or Zn dopants. In our opinion, the lower values of contact resistivity of the structures containing of Mg or Zn are caused by an increased density of holes in the sub-surface region of p-GaN due to diffusion of Mg or Zn from the deposited doped contact layers.
The role of Al on Ohmic contact formation on n-type GaN and AlGaN∕GaN
Applied Physics Letters, 2005
A standard metallization scheme for the formation of Ohmic contacts on n-type GaN does exist. It has the following multilayer structure: Ti∕Al∕metal∕Au. Ti is known to extract N out of the GaN. This leaves a high density of N vacancies (donors) near the interface pinning the Fermi level. The created tunnel junction is responsible for an Ohmic contact behavior. Au is deposited as the final metal layer to exclude oxidation of the contact and the metal should limit the diffusion of Au into the layers below and vice versa. Al in the metallization scheme is known to improve the contact resistance, but the reason why has not been reported yet. We studied Ti and Ti∕Al contacts on GaN and AlGaN∕GaN as a function of annealing temperature by transmission electron microscopy. The role of Al in the metal multilayer, and of Al in the AlGaN on the Ohmic contact formation, has been determined. The latter result indicates that the standard metallization scheme for GaN cannot be simply transferred t...
Metal contacts to p-type GaN by electroless deposition
Proceedings of SPIE - The International Society for Optical Engineering, 2007
Initial results are presented on the electroless deposition of metal contacts to p-type gallium nitride (GaN). Deposition procedures were developed for the deposition of both nickel and tungsten-cobalt (W-Co) contacts onto p-type GaN. Attempts to deposit platinum on p-type GaN failed, despite the fact that electroless platinum deposition was successfully achieved on other substrate types. Nickel contacts were overlaid with gold and annealed in oxygen ambient to form ohmic contacts with specific contact resistivity values down to 2x10 -2 Ωcm 2 . Measurements at elevated temperatures up to 140 0 C showed that the specific contact resistivity was almost independent of temperature. The tungsten-cobalt contacts showed rectifying behaviour even after annealing at 650 0 C. This makes this contact type a possible candidate for Schottky contacts in high temperature applications.