High Work Function Metallizations on Gallium Nitride for Schottky Diodes
Authors: Molina Alex , Dail Steven P. , Campbell Ian E. , Walter Timothy N. , Thomas Michael W. , Mughal Asad J. , Mohney Suzanne E. 
Published: July 2021

Although silicon (Si) currently dominates the semiconductor industry, its small 1.1 eV band gap limits its maximum operating temperature, which restricts its use in high-temperature, high-power devices. Gallium nitride (GaN) is an attractive semiconductor with its wide bandgap (3.4 eV), high electron mobility (1700 cm2/Vs), high electron saturation velocity (3 x 107 cm/s), large critical breakdown field (2 MV/cm), and thermal stability. The high-power capabilities of GaN allow for a reduction in device size, which can conserve physical space if used to replace conventional Si power devices. While the semiconductor itself can endure harsh operating conditions, the reliability of the metal/semiconductor contacts can be a limiting factor for its use. Schottky contacts should provide a high barrier height and low reverse leakage current, and they must be electrically stable over the lifetime of the device.

In this study, three materials reported to have high work functions are compared as Schottky diodes to n-type GaN, each selected for its anticipated thermodynamic stability with GaN1 or the potential to minimize process-induced defects in the diodes or both. Rhenium (Re) diodes fabricated via electron beam deposition, molybdenum nitride (MoNx) diodes via remote plasma atomic layer deposition (ALD), and palladium (Pd) diodes via electroless deposition were investigated. Ti/Al-based ohmic contacts were employed.

The Re/n-GaN Schottky diode was chosen for study because of its thermodynamic stability against metallurgical reactions1 and high work function (4.96 eV)2. The barrier heights were investigated by current–voltage (I-V) and capacitance–voltage (C-V) measurements at room temperature. Both techniques demonstrated that the barrier height increased after an anneal at 400°C for 5 min, yielding a barrier height of 0.88 eV and ideality factor of 1.02 from by I-V measurements, while the C-V measurements revealed a barrier height of 0.91 eV. These barrier heights and reverse leakage currents remained stable upon annealing in N2 at 600°C.

The MoNx/n-GaN Schottky diode was chosen for study because of the reported high work function of MoNx (5.33 eV)3, its conductive and refractory nature, and its thermodynamic equilibrium with GaN4. Films were deposited with bis (tert-butylimido)-bis (dimethylamido) molybdenum and a 300 W remote N2/H2 plasma. Four-point probe measurements and x-ray photoelectron spectroscopy (XPS) were used to measure sheet resistance and composition. Barrier heights from the I-V measurements were 0.41 eV, and ideality factors were 1.4, with good stability upon annealing at 600°C.

The Pd/n-GaN Schottky diode was chosen because of its high work function (5.12 eV)2 and its potential to be electrolessly deposited, offering a gentle technique to minimize process-induced defects in the GaN, although Pd is not expected to be in thermodynamic equilibrium with GaN. A new method for electroless deposition of Pd films onto GaN surfaces used palladium dichloride with sodium L-ascorbate as the reducing agent was developed. Profilometry, four-point probe measurements, energy dispersive x-ray spectroscopy, and XPS were used to measure film thickness, sheet resistance, resistivity, morphology, and composition. The process provided conductive and pure Pd films, but some challenges with nucleation of the film on GaN makes the process less robust. Barrier heights from the I-V measurements were 1.13–1.26 eV, and ideality factors were 1.02–1.05. However, Pd is not in thermodynamic equilibrium with GaN.

Among the candidates tested so far, the Re diodes were overall the strongest candidate. Future work will involve stress testing followed by materials characterization to provide more information on stable metallizations for high-power GaN devices.

The authors are grateful to Sandia National Laboratories (Andrew Allerman) for providing GaN epilayers. This work was funded by the Office of Naval Research under Grant N000141812360, distribution A, approved for public release, distribution is unlimited (DCN# 43-7434-20).

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