(Invited) Material Considerations for the Development of III-Nitride Power Devices

Monday, 2 October 2017: 11:20
Chesapeake B (Gaylord National Resort and Convention Center)
B. Sarkar (North Carolina State University), P. Reddy (Adroit Materials Inc., North Carolina State University), F. Kaess, B. Haidet (North Carolina State University), J. Tweedie, S. Mita, R. Kirste (Adroit Materials Inc.), E. Kohn, R. Collazo, and Z. Sitar (North Carolina State University)
AlGaN is of high interest for high-power electronics based on vertical structures due to its high breakdown field and electron mobility. Recent studies in GaN based p-n junctions indicate a dielectric strength in the range of 3.3–3.75 MVcm-1, resulting in a Baliga’s figure of merit several times larger than that of SiC. However, high breakdown voltages (>4kV) are currently confined to GaN p-n junctions indicating GaN and AlGaN surface instabilities. Consequently, defect-free metal-nitride interface and passivation of Al/GaN are necessary for breakthroughs in high power vertical Al/GaN Schottky diodes.

Accordingly, we report defect-free behavior in Ni/GaN Schottky diodes with unity ideality factor and fabricated by photolithography. They also exhibit high temperature stability (>600 oC) and low leakage with forward and reverse I-V-T characteristics successfully modeled by a single homogeneous barrier (0.7 eV) by ATLAS simulations. It is significant that the forward and reverse characteristics could be modeled by a common set of parameters without a defect based second diode to model the reverse leakage. XPS chemical and electronic analysis was employed to determine the surface treatment necessary to obtain such a Schottky interface. Consequently, we demonstrate the suitability of GaN surface and interface with metals for high power electronics.

Although a near-ideal diode may be obtained on GaN, the relatively low barrier height limits the GaN Schottky diode to operating voltages <1000V. Increasing the barrier by controlling the Fermi level pinning and producing a camel junction by surface Mg doping is possible.

Passivation is the next challenge to implementing Al/GaN based high power electronics. Silicon nitride has emerged as the standard passivation material for GaN and low Al composition AlxGa1-xN (x<0.3). However, the bandgap of silicon nitride is lower than AlGaN for x>0.75 and the feasibility of silicon nitride functioning as an insulating dielectric (by providing appropriate electron/hole barriers) in Al rich AlGaN and AlN is uncertain. Similarly, reduction of surface states by silicon nitride passivation has not been reported for Al rich AlGaN (x>0.3).

In this work, we have employed X-ray photoelectron spectroscopy to determine the band offsets and interface Fermi level at the hetero-junction formed by stoichiometric silicon nitride deposited on metal polar AlxGa1-xN (of varying Al composition ‘x’) via high temperature low pressure chemical vapor deposition. Silicon nitride is found to form a type II staggered band alignment with AlGaN for all Al compositions (0<x<1) and presents an electron barrier into AlGaN even at higher Al compositions where Eg(AlGaN)>Eg(Si3N4). No band bending is observed in AlGaN for x<0.6 and indicating excellent passivation beyond x=0.3. Further a reduced band bending (by 1 eV relative to free surface) is observed for x>0.6. The Fermi level in silicon nitride is found to be at 3 eV with respect to its valence band and is likely due to silicon (≡Si0/-1) dangling bonds. The presence of band bending for x>0.6 is seen as a likely consequence of Fermi level alignment at Si3N4/AlGaN hetero-structure and not due to interface states indicating likely passivation on AlGaN beyond x=0.6. Photoelectron spectroscopy is corroborated by capacitance-voltage measurements. A shift in the interface Fermi level from the conduction band in Si3N4/n-GaN to the valence band in Si3N4/p-GaN is observed which strongly indicates no or greatly reduced mid-gap interface states. Further, required compensation of spontaneous polarization charge is typically performed by oppositely charged surface states/defects on free surface is hypothesized to be replaced by oppositely charged silicon dangling bonds (≡Si0/-1) after passivation. This hypothesis is supported by characterization of silicon nitride on N-polar GaN. In conclusion, HT LPCVD silicon nitride is found to be a suitable passivation for Al rich AlGaN.