Wide bandgap semiconductors have been interesting for high temperatures operation and serving as materials using in high power and RF devices because of their high breakdown voltages. Aluminum nitride, a wide bandgap material (6.2 eV), is a promising interfacial layer or dielectric material on wide bandgap semiconductors, especially SiC and AlxGa1-xN/GaN. Because of its small lattice mismatch to SiC and AlxGa1-xN/GaN (1.3% and <2%)1 and a similar thermal expansion coefficient, it could potentially lower the interface state densities between the materials. 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 400~500oC under high vacuum as 10-4~10-6 torr. The deposition rate of AlN on SiC and AlGaN were determined to be about 0.08nm/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 900oC. 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). Capacitance-voltage and conductance -voltage characteristics are used to determine the interface states density between the thin film and wide bandgap substrate. 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).