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Open Access Publications from the University of California

Thermal Stability of Plasma-enhanced chemical vapor deposition Silicon Nitride Passivation on AlGaN/GaN High-electron-mobility transistors

  • Author(s): Ha, Minh-Trang Teresa
  • Advisor(s): Streit, Dwight C.
  • et al.

AlGaN/GaN HEMTs are the most promising high power switching devices. The material properties of III-nitrides are exceptionally better than that of Si and GaAs. GaN-based devices have been recorded to have higher operating temperatures and higher breakdown field due to the wide bandgap. AlGaN/GaN heterostructures forms 2DEG without doping due to the spontaneous polarization. The performance and reliability of AlGaN/GaN HEMTs are dependent on the structure of the AlGaN/GaN heterostructures.

Surface passivation has been proven to improve the 2DEG conductivity and device performance. 20 nm of plasma-enhanced chemical vapor deposition (PECVD) SiN was deposited on AlGaN/GaN HEMTs, and the PECVD SiN passivated sample demonstrated higher carrier concentration of 9.88 ? 1012 cm-2 compared to the un-passivated sample, 8.08 ? 1012 cm-2.

High temperature annealing is an important processing step in the fabrication of the devices, and the effects have shown to improve the DC and RF performance. High temperature annealing may affect the structure and the 2DEG conductivity. The annealing effects modifies the AlGaN layer and the AlGaN/GaN interface. Herein, we present the a study on the thermal stability of the PECVD SiN passivation layer on AlGaN/GaN HEMT structures at high temperature anneals. High-resolution x-ray diffraction (HRXRD) measurements were used to investigate the strain of AlGaN layer, and Hall measurements were used to investigate the 2DEG conductivity.

PECVD SiN passivated and un-passivated AGaN/GaN HEMTs structure underwent high temperature thermal anneals for 30 minutes in N2. The starting temperature of the annealing is 400?C with step of 50?C until degradation. Degradation was determined through Hall sheet resistivity and mobility measurements. The ending annealed temperature is 1000 ?C and 700 ?C for passivated and control samples, respectively. From no anneal to degradation temperature, the 2DEG conductivity dropped by 15% and 34% for passivated and un-passivated samples, respectively. The HRXRD measurements found the change in-plane strain of the AlGaN layer after high temperature anneals. Higher in-plane strain showed higher 2DEG conductivity. In-plane strain from no anneal to degradation temperature dropped from 2% and 7% for passivated and un-passivated samples, respectively. Therefore, the passivated sample demonstrated to be more stable at high temperatures. The SiN passivation layer adds tensile stress to the AlGaN layer thus increased the piezoelectric effect and 2DEG conductivity.

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