The intermetallic compound NiAl ͑50:50 at. %͒ has been shown to be a low-resistance ohmic contact to n-GaN and n-AlGaN. NiAl contacts on n-GaN (nϭ2.5ϫ10 17 cm Ϫ3 ) had a specific contact resistance of 9.4ϫ10 Ϫ6 ⍀ cm 2 upon annealing at 850°C for 5 min. NiAl contacts annealed at 900°C for 5 min in n-Al 0.12 Ga 0.88 N (nϭ2.4ϫ10 18 cm Ϫ3 ) and n-Al 0.18 Ga 0.82 N (nϭ2.7 ϫ10 18 cm Ϫ3 ) had specific contact resistances of 2.1ϫ10 Ϫ5 ⍀ cm 2 and 4.7ϫ10 Ϫ5 ⍀ cm 2 , respectively. Additionally, these contacts were subjected to long-term annealing at 600°C for 100 h. On n-GaN, the contact specific contact resistance degraded from 9.4ϫ10 Ϫ6 ⍀ cm 2 to 5.3 ϫ10 Ϫ5 ⍀ cm 2 after the long-term anneal. Contacts to n-Al 0.18 Ga 0.82 N showed only slight degradation with a change in contact resistance, from 4.7ϫ10 Ϫ5 ⍀ cm 2 to 9.2ϫ10 Ϫ5 ⍀ cm 2 . These results demonstrate the NiAl has great promise as a stable, low-resistance contact, particularly to n-AlGaN used in high-temperature applications.

Phase equilibria were investigated in the Ga-Ni-As ternary system, with particular emphasis on the regions of technological importance to Ni/GaAs electrical contacts. A 600°C Gibbs isotherm was constructed using x-ray-diffraction analysis and electron probe microanalysis of annealed samples. Additionally, three isopleths ͑NiAs-GaAs, NiGa-NiAs, and NiGa-GaAs͒ and a partial liquidus projection were established using differential thermal analysis and metallography. These data were utilized to clarify some discrepancies in the literature pertaining to the constitution of the Ga-NiAs system, particularly questions about the existence of ternary phases. It was demonstrated that at 600°C, previously reported ternary phases were actually specific compositions of the binary phase, NiAs, which exhibits significant ternary solubility. Additional x-ray-diffraction and differential thermal analysis experiments suggested that superlattice structures based on the NiAs structure may become stable at lower temperatures. A ternary eutectic reaction was shown to occur at 810Ϯ5°C, with eutectic point at the composition Ni 0.48 Ga 0.30 As 0.22 . The existence of this eutectic reaction has important ramifications for the development of Ni-based electrical contacts to GaAs because any metallization scheme with a composition within the region bounded by NiGa, NiAs, and GaAs, as well as elemental Ni, will experience at least partial liquid formation at temperatures greater than 810°C.

A new metallization scheme has been developed to form Ohmic contacts to n-GaN. Contacts were fabricated by sputtering the intermetallic compound, PtIn 2 on metal-organic vapor phase epitaxy grown n-GaN (nϳ5ϫ10 17 cm Ϫ3 ) with some of the contacts subjected to rapid thermal annealing. Contacts in the as-deposited state exhibited nearly Ohmic behavior with a specific contact resistance of 1.2ϫ10 Ϫ2 ⍀ cm 2 . Contacts subjected to rapid thermal annealing at 800°C for 1 min exhibited linear current-voltage characteristics and had specific contact resistances less than 1 ϫ10 Ϫ3 ⍀ cm 2 . Auger depth profiling and glancing angle x-ray diffraction were used to examine the interfacial reactions of the PtIn 2 /n-GaN contacts. Consistent with estimated phase diagram information, the results from Auger depth profiling and glancing angle x-ray diffraction indicated the formation of (In x Ga 1Ϫx )N at the contact interface, which could be responsible for the Ohmic behavior of PtIn 2 contacts. © 1997 American Institute of Physics. ͓S0003-6951͑97͒00201-5͔GaN is a III-V compound semiconductor with a wurtzite crystal structure having a 3.4 eV direct energy band gap at room temperature. When alloyed with the other group III nitrides, GaN can form a continuous alloy system whose room temperature band gaps range from 1.9 eV ͑InN͒ to 6.2 eV ͑AlN͒. 1 This makes GaN a very suitable material for optoelectronic devices, such as light emitting diodes ͑LEDs͒, performing in the blue and ultraviolet regions.Despite its promise, GaN was useless for high efficiency optical devices due to the inability to grow p-type GaN films. That is until recently, when success in obtaining p-type GaN films lead to renewed and intense interest in GaN. 2 Blue and blue-green LEDs are commercially available from Nichia Chemical Industries with a luminosity of 2 cd and central wavelengths from 450 to 500 nm. 3 Nichia has also demonstrated a nitride laser operating at 416 nm. 4 Additionally, Cree Research announced the development of GaN LEDs that operate at 430 nm and produce 500 W output at 20 mA. 5 GaN has also been used for junction field effect transistors 6 and high electron mobility transistors. 7 Even with the successes of GaN devices there is much more work to be done. High contact resistance can substantially reduce the performance of GaN optical and electrical devices; therefore, to obtain optimum performance the contact resistance should be minimized. The present technology for the formation of Ohmic contacts to n-GaN usually involves the use of Ti or Al metallization schemes. [8][9][10] In this letter, we report the results of a different type of metallization scheme, one utilizing PtIn 2 . PtIn 2 is a intermetallic compound with a CaF 2 ͑C1͒ crystal structure, good chemical stability, and a peritectic melting point at 1039°C. 11 Based on binary phase diagram data and the estimated ternary phase diagrams of the Pt-In-Ga-N system, PtIn 2 /GaN meets the thermodynamic criteria formulated by Jan 12 and Chang 13 for participating in the exchange reaction. ...

We reply to the comment by Guerin and Guvarc’h concerning the phase equilibria of the Ga–Ni–As ternary system. We refute their position that the quench rate of the phase diagram samples is not important and that the solid-state phase equilibria of the Ga–Ni–As system does not change significantly between the temperatures of 25 and 800 °C. It is also demonstrated that the occurrence of ordered superlattice structures is not inconsistent with our work. We conclude that an adequate number of samples were prepared in our study to justify the modifications proposed to the Ga–Ni–As isothermal section and it is an accurate representation of the phase equilibria at 600 °C.

PdIn was used as a contact material to n-type and p-type GaP. On n-type GaP it forms a low resistance ohmic contact upon rapid thermal annealing. PdIn/n-GaP (S doped at 2–3 ×1018 cm−3) contacts annealed at 600 °C for 1 min had specific contact resistance’s lower than 1×10−4 Ω cm2. Unlike the contacts to n-GaP, PdIn contacts to p-GaP (Zn doped 1–2×1018 cm−3) show rectifying behavior at all annealing conditions. However, the effective Schottky barrier height seems to decrease significantly with thermal annealing. In addition to the electrical measurements, glancing angle x-ray diffraction was used to characterize the contacts. The glancing angle x-ray diffusion pattern of PdIn/n-GaP, annealed at 600 °C for 1 min, is consistent with the formation of an (InyGa1−y)P phase due to the thermal annealing. The ohmic behavior of the PdIn contacts to n-type GaP and the decrease in the contact’s Schottky barrier height on p-type GaP is attributed to the formation of this (InyGa1−y)P phase at the contact’s interface.