287749 Atomic Layer Deposition of AlN Thin Films As Gate Dielectrics for Wide Bandgap Semiconductors
Aluminum nitride (AlN) is a potential dielectric layer for wide bandgap semiconductor based power electronic devices, such as those demanded in radio frequency, high-speed and high-temperature communication, because of its wide bandgap and high dielectric constant. In particular, for 4H-SiC, AlN is also a promising interfacial layer due to their similar atomic arrangement, small lattice mismatch (1.3%) and comparable thermal expansion coefficients. Although various deposition techniques have been investigated to synthesize AlN thin films with atomic controllability over a large substrate remains a challenge. Atomic layer deposition (ALD) was thus used in this work to grow AlN thin films.
AlN deposition was performed in an ultra-high vacuum chamber with base pressure of 10-7 Torr using trimethylaluminum (TMA) and ammonia (NH3) as precursors. It was discovered that ALD of AlN is possible only when the minute amount of moisture in NH3, which competed with and inhibited the nitride growth, was completely eliminated. The ALD window was found to be 500-570 oC with a growth rate of 1.5 Å/cycle. The deposited film composition was evaluated via in-situ x-ray photoelectron spectroscopy (XPS) with Al/N determined to be 1.2. In-situ reflective high-energy electron diffraction (RHEED) measurements showed as-deposited AlN films were crystalline, which was confirmed by x-ray diffraction (XRD). AlN/4H-SiC MIS capacitors were fabricated to examine the electrical properties with the dielectric constant of AlN determined to be 8.3 and a leakage current density of 10-3 A/cm2 at 4.3 MV/cm. The ALD AlN passivated AlGaN/GaN hetero-structure demonstrated an increase in the carrier density and a 3% decrease in mobility compared to those of non-passivated hetero-structure. With amorphous Al2O3 surface passivation, the mobility decreases by 22% with increased carrier density, showing that the crystalline AlN providing a superior property on passivating the hetero-structures.