(711d) 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.