Effect of Pt barrier on thermal stability of Ti/Al/Pt/Au in ohmic contact with Si-implanted n-type GaN layers (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.
Electrical, microstructural, and thermal stability characteristics of Ta/Ti/Ni/Au contacts to n-GaN
Journal of Applied Physics, 2004
A metallization technique has been developed for obtaining low resistance Ohmic contact to n-GaN. The metallization technique involves the deposition of a metal layer combination Ta/Ti/ Ni/Au on an n-GaN epilayer. It is observed that annealing at 750°C for 45 s leads to low contact resistivity. Corresponding to a doping level of 5ϫ10 17 cm Ϫ3 , the contact resistivity of the contact S ϭ5.0ϫ10 Ϫ6 ⍀ cm 2 . The physical mechanisms underlying the realization of low contact resistivity is investigated using current-voltage characteristics, x-ray diffraction, Auger electron spectroscopy, transmission electron microscopy, and energy dispersive x-ray spectrometry.
Thermally-stable low-resistance Ti/Al/Mo/Au multilayer ohmic contacts on n–GaN
Journal of Applied Physics, 2002
A metallization scheme consisting of Ti/Al/Mo/Au with excellent edge acuity has been developed for obtaining low-resistance ohmic contacts to n-GaN. Excellent ohmic characteristics with a specific contact resistivity as low as 4.7ϫ10 Ϫ7 ⍀-cm 2 were obtained by rapid thermal annealing of evaporated Ti/Al/Mo/Au at 850°C for 30 sec in a N 2 ambient. Additionally, no degradation in specific contact resistivity was observed for these contacts subjected to long-term annealing at 500°C for 360 h.
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.
Effects of cap layer on ohmic Ti/Al contacts to Si + implanted GaN
Applied Surface Science, 2009
A low resistivity ohmic contact to Si-implanted GaN was achieved using a metal combination of Ti/Al. The effect of a protection cap during post-implantation annealing is investigated, and how it affects the specific contact resistivity (ρc). Relevant differences between the protected (PR) sample with SiO2 and unprotected (UP) sample during the post-implantation annealing were observed after metal alloying at 700 °C. The lower values of ρc have been obtained for UP sample, but with very low reproducibility. In contrast, SiO2 cap layer has demonstrated its relevance in yielding a much more uniformity of a relatively low ρc around 10−5 Ω cm2. Related mechanism for the uniformity in ρc was discussed based on the results obtained from electrical measurements, XRD (X-ray diffraction) analysis, AFM (atomic force microscopy) and SEM (scanning electron microscopy) observations.
Microstructural properties of thermally stable Ti/W/Au ohmic contacts on n-type GaN
Microelectronic Engineering, 2006
We have investigated the microstructural and electrical characteristics of Ti/W/Au ohmic contacts on n-type GaN (4.0 • 10 18 cm À3) using Auger electron spectroscopy (AES) and transmission electron microscopy (TEM) after annealing at 900°C. It is shown that the electrical properties are improved upon annealing at 900°C for 1 min in nitrogen ambient. The 900°C annealed contact produced a specific contact resistance of 8.4 • 10 À6 X cm 2. It is further shown that the contact exhibits thermal stability during annealing at 900°C. Based on the Auger electron microscopy and transmission electron microscopy studies, the formation of TiN layer results in an excess of N vacancies near the surface of the GaN layer, which could be the reason for the low-resistance of the Ti/W/Au contact.
Journal of Materials Science: Materials in Electronics, 2019
Formation of ohmic contacts to GaN is of high practical importance for device fabrication. Due to the wide band gap, formation of multilayer metal structures is required to make electrical connections with low contact resistance. The paper presents a study on structure, composition, adhesion and electrical properties of Ti/Al/Ti/Au and Ta/Al/Ta metal stacks fabricated by e-beam evaporation and thermal annealing in order to provide ohmic contacts to n-type GaN films grown on Si. For the Ti-based case, an interdiffusion of Au and Ga into the stack is found, which is probably caused by a granular structure of the top Ti layer making no proper barrier. Ti of the bottom layer is observed to diffuse into GaN, forming a thin layer of titanium nitride with a low Schottky barrier at GaN interface allowing ohmic contact as shown by electrical measurements.
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...