B. P. Luther

Investigation of the mechanism for Ohmic contact formation in Al and Ti/Al contacts to n-type GaN

We report on a study of Al and Ti/Al contacts to n-type GaN. Al contacts on n-GaN (7×1017 cm−3) annealed in forming gas at 600 °C reached a minimum contact resistivity of 8×10−6 Ω cm2 and had much better thermal stability than reported by previous researchers. Ti/Al (35/115 nm) contacts on n-GaN (5×1017 cm−3) had resistivities of 7×10−6 and 5×10−6 Ω cm2 after annealing in Ar at 400 °C for 5 min and 600 °C for 15 s, respectively. Depth profiles of Ti/Al contacts annealed at 400 °C showed that low contact resistance was only achieved after Al diffused to the GaN interface. We propose that the mechanism for Ohmic contact formation in Ti/Al contacts annealed in the 400–600 °C range includes Ti reducing the GaN native oxide and an Al–Ti intermetallic coming into intimate contact with the GaN.

X-ray photoelectron spectroscopy and x-ray diffraction study of the thermal oxide on gallium nitride

The oxidation of single crystal gallium nitride in dry air has been investigated. X-ray photoelectron spectroscopy (XPS) revealed minimal oxide growth at 450 and 750 °C for up to 25 h. However, at 900 °C the growth of an oxide approximately 5000 A thick was observed after 25 h. This oxide was determined to be the monoclinic β-Ga2O3 using glancing angle x-ray diffraction. XPS spectra of the Ga 3d and Ga 2p core levels indicated peak shifts of 1.2 and 1.3 eV, respectively, from Ga–O to Ga–N bonding. The Ga L3M45M45 core level binding energy was also investigated and β-Ga2O3 and GaN each presented a characteristic peak shape.

High temperature Pt Schottky diode gas sensors on n-type GaN

The characteristics of Pt Schottky diodes on n-type GaN in hydrogen and propane are reported for the first time. This response from 200–400°C has been characterized by current–voltage measurements, revealing that the diodes are able to detect hydrogen from 200–400°C and propane from 300–400°C. The high temperature stability of Pt diodes on GaN has been investigated by long term annealing at 400°C in Ar or 20% O2 in Ar. The diodes have been held at 400°C for 500 h without degradation of their electrical characteristics or response to hydrogen-containing gases.

Analysis of a thin AlN interfacial layer in Ti/Al and Pd/Al ohmic contacts to n-type GaN

Using high resolution transmission electron microscopy, a thin pseudomorphic AlN layer (2–3 nm) has been observed at the metal/GaN interfaces of Ti/Al (35/115 nm) and Pd/Al (25/125 nm) ohmic contacts to n-type GaN annealed in Ar at 600 °C for 15 and 30 s, respectively. The interfacial layer has a c-plane (002) lattice spacing of 2.48±.03 A and an a-plane (100) spacing matching that of GaN (2.76 A), and the identification of this layer as AlN is consistent with chemical analysis by energy dispersive x-ray spectroscopy. The formation of this interfacial AlN layer coincides with the onset of ohmic behavior in Ti/Al and Pd/Al contacts annealed at 600 °C and may be the cause.

Titanium and titanium nitride contacts to n-type gallium nitride

Ti (150 nm), TiN (200 nm) and Ti (5 nm)/TiN (200 nm) contacts have been fabricated on n-type GaN (Si doped , ) and characterized by specific contact resistance measurements and by x-ray photoelectron spectroscopy depth profiling. Ti contacts were annealed between 400 and C in Ar and to observe the effects of annealing environment on ohmic contact formation. Ti contacts annealed in became ohmic after 1 min at C and reached a minimum specific contact resistance of after 1 min at C. Ti contacts annealed in Ar required 20-25 min at C to become ohmic and also reached a minimum specific contact resistance of after 1 min at C. Depth profiles of Ti contacts showed that the presence of TiN at the metal-GaN interface was a necessary condition for ohmic contact formation. TiN and Ti/TiN contacts were annealed between 400 and C in Ar. TiN contacts became ohmic after 1 min at C but only reached a minimum specific contact resistance of after being annealed at C. Ti/TiN contacts became ohmic after being annealed for 1 min at C, similar to Ti contacts annealed in . Measurements at room temperature and 77 K revealed that annealed TiN ohmic contacts displayed increased specific contact resistance at 77 K (characteristic of thermionic or thermionic-field emission), while Ti/TiN and Ti contacts annealed in Ar showed no temperature dependence (characteristic of field emission). Based on the results of this study, we conclude that having TiN in intimate contact with n-type GaN was necessary for these contacts to be ohmic, while some reaction between Ti and GaN during the contact anneal caused the ohmic contacts to operate by field emission and to reach a lower specific contact resistance.

Interfacial reactions between nickel thin films and GaN

Thin Ni films on GaN were annealed at temperatures between 400 and 900 °C in N2, Ar, and forming gas and were analyzed using glancing angle x-ray diffraction and Auger depth profiling. The first indication of an interfacial reaction was found after an anneal at 600 °C for 1 h, after which Ga was observed to be dissolved in the face-centered cubic Ni film. The extent of dissolution increased with continued annealing. After annealing at 750 °C for 1 hr in either N2 or Ar, greater intermixing occurred. The reaction product was either Ni3Ga or face-centered cubic Ni with dissolved Ga. Annealing at 900 °C resulted in the formation of the B2 phase NiGa. It was clear from Auger depth profiles that the reacted film contained significantly more Ga than N and that N2 gas was released to the annealing environment, even when the samples were annealed in N2 gas at 1 atm. Thus, a trend of increasing Ga content in the reacted films was observed with increasing temperature. The observed reactions are consistent with the ...

Nearly isotropic etching of 6H‐SiC in NF3 and O2 using a remote plasma

Nearly isotropic etching of the 6H‐SiC carbon face has been achieved in a remote plasma at 330 °C using a mixture of O2 and NF3 in argon. Using evaporated aluminum as a mask, undercutting has been observed to a distance equal to the etch depth. The etch rate is a function of the ratio of O2 to NF3 flow rates and of temperature, peaking strongly to 220 nm/min at 82% oxygen for 330 °C. Smooth surfaces were obtained for gas ratios leading to the maximum etch rate, and also for a NF3‐argon mixture, with significant roughening observed for other O2‐NF3‐argon mixtures. In the absence of a practical wet etch for SiC, this procedure is promising for isotropic etching in SiC device processing.

Phase Formation and Morphology in Nickel and Nickel/Gold Contacts to Gallium Nitride

Metallurgical reactions between Ni and GaN have been explored at temperatures between 400 and 900 °C in N 2 , Ar, and forming gas. A trend of increasing Ga content in the reacted films was observed with increasing temperature. The reactions are consistent with the thermodynamics of the Ni-Ga-N system. Changes in the film morphology on annealing were also examined. Metal island formation and a corresponding deep, non-uniform metal penetration into GaN were observed at high temperatures. The relevance of the observed nature of phase formation and morphology in these thin films to electrical properties of Ni/GaN and Au/Ni/GaN contacts is also considered.

Environmental aging of Schottky contacts to n-AlGaN

Gold Schottky contacts to n-AlGaN were fabricated, and the influence of the semiconductor surface preparation on the electrical performance of the diodes was examined. More significantly, the electrical characteristics of the diodes were found to be sensitive to the environment in which they were exposed. Diodes stored in vacuum had stable but poor electrical characteristics, exhibiting the same high reverse leakage currents, low barrier heights, and high ideality factors as the freshly prepared diodes. On the other hand, didoes exposed to air changed over the course of days, in some cases with decreases in the reverse leakage currents by four or more orders of magnitude and increases in the barrier height by 0.3–0.5 eV. Further study of this change in electrical properties showed that the effect was reversible with exposure to N2 gas or vacuum and adequate temperature. In addition, the effect was more pronounced when the metal contact was thin, indicating that diffusion of gases through the metal was sig...

Morphology of nickel and nickel/gold contacts to gallium nitride

Changes in film morphology upon annealing Ni and Ni/Au contacts to GaN were examined using scanning electron microscopy and atomic force microscopy. Atomic force microscopy was performed on the GaN surface that was exposed by etching away the metal film. This technique was demonstrated to be a convenient and effective way to evaluate the morphology of the contact/semiconductor interface, rapidly providing spatial information in three dimensions. Metal island formation and a corresponding deep, nonuniform metal penetration into GaN were observed after annealing Ni/GaN and Au/Ni/GaN contacts above 800 and 700 °C, respectively.