(213f) Engineering Epitaxial AlN Thin Films On Wide Bandgap Semiconductors

Nitride materials have drawn lots of attention because of their wide bandgaps and high breakdown voltages, making them suitable for operations at high temperatures and serve as materials for high power and RF devices. Aluminum nitride, a wide bandgap material (6.2 eV), is a promising interfacial layer or dielectric material on wide bandgap semiconductors in MISFET and MIS-HEMT devices. Specifically, AlN has a small lattice mismatch to SiC and Al_xGa_1-xN/GaN (1.3% and <2%)¹ and a similar thermal expansion coefficient to SiC and Al_xGa_1-xN/GaN. Conventionally, AlN thin films are deposited by molecular beam epitaxy (MBE) on these wide bandgap materials. In this work, ALD is chosen to assess its ability to grow ultra-thin, uniform, and conformal AlN on these substrates, as a potential alternative for synthesizing epitaxial materials over a larger substrate and at lower temperatures.

Atomic layer deposition (ALD) has been utilized to synthesize AlN thin films by using trimethyl aluminum (TMA) and ammonia (NH₃) as precursors at 400~500^oC. The deposition rate of AlN on SiC and AlGaN were determined to be about 0.1 nm/cycle. The composition, microstructure, and surface morphology were determined by x-ray photoelectron spectroscopy, transmission electron microscopy, x-ray diffraction and atomic force microscope. Fourier Transform infrared spectroscopy is implemented to study the change of surface functional groups during TMA and ammonia pulses, in an effort to affirm the mechanism leading to the growth of stoichiometric AlN. The as-deposited AlN was amorphous, as monitored by in situ by RHEED analysis but can be transformed into an epitaxial layer on SiC and AlGaN by a high temperature rapid thermal annealing process at 900^oC. By synchrotron based XRD, we determined the epitaxial relationship between AlN and SiC to be AlN(11-20)//SiC(11-20) and AlN(0004)//SiC(0008). Similarly, the epitaxial relation to AlGaN is AlN(11-20)//AlGaN(11-20) and AlN(0002)//AlGaN(0002). This process is also combined with an ALD Al₂O₃ process to synthesize aluminum oxinitride as a graded interfacial layer between AlN and Al₂O₃, to realize the fabrication and testing of viable MIS-HEMT structures.

¹ H. Morkoc, S. Strite, G. B. Gao et al., Journal Of Applied Physics 76 (3), 1363 (1994).