Process and Properties of Nitride-based Thin Film Heterostructures

Abstract

The goals of this work were to synthesize nitride-based thin film heterostructures by Pulsed Laser Deposition, study the structural, mechanical, electrical and optical properties of these heterostructures and establish structure-property relations for these materials in order to further improve their properties and design new structures. Domain matching epitaxy was explored in most of these heterostructures and studied in detail for each case.

Mechanical and electrical properties of TiN as a function of microstructure varying from nanocrystalline to single crystal TiN films deposited on (100) silicon substrates were investigated. By varying the substrate temperature from 25°C to 700°C during PLD, the microstructure of TiN films changed from nanocrystalline (having uniform grain size of 8 nm) to a single crystal epitaxial film on the silicon (100) substrate. The hardness of TiN films decreased with decreasing grain size. The dependence of resistivity of TiN as a function of the substrate temperature is discussed and correlated with hardness results.

High-quality epitaxial B1 NaCl-structured TaN films were deposited on Si(100) and Si(111) substrates with TiN as buffer layer, using pulsed laser deposition. Our method exploits the concept of lattice-matching epitaxy between TiN and TaN and domain-matching epitaxy between TiN and silicon. XRD, TEM, and STEM experiments confirmed the single-crystalline nature of the films with cube-on-cube epitaxy. The stoichiometry of TaN films was determined to be nitrogen deficient (TaN[subscript 0.95]) by RBS. Resistivity of the TaN films was found to be 220μΩ-cm at room temperature with temperature coefficient of resistivity of -0.005K⁻¹.

Diffusivity of copper in single-crystal (NaCl-structured) and polycrystalline TaN thin films grown by PLD was investigated. The polycrystalline TaN films were grown directly on Si(100), while single-crystal films were grown with TiN buffer layers. The diffusion distances in epitaxial TaN are found to be about 5nm at 650°C for 30 min annealing. Cu diffusion in polycrystalline TaN thin films is found to be nonuniform with enhanced diffusivities along the grain boundary.

By PLD, TiN and TaN targets were arranged in a special configuration that they can be ablated in a sequential manner to obtain TiN-TaN alloy or TiN/TaN superlattice structure. The 60% TaN resulted in superlattice of TaN(3nm) /TiN(2nm), while 30% and 70% TaN generated uniform TaXTi1-XN alloys. TiN buffer layers were deposited first to achieve those epitaxial binary components. XRD and TEM analysis showed the epitaxial nature of these films. Microstructure and uniformity of the superlattice and alloy structures were studied by TEM and STEM. Nanoindentation results suggested high hardness and future hard coating applications for these TiN-TaN composites. Four point probe electrical resistivity measurements and Cu diffusion characteristics study prove that TiN-TaN binary components provide a superior diffusion barrier for copper.

Uniform AlxTi1-xN alloys (x up to 70%) and highly aligned TiN/AlN superlattices were deposited by PLD. Microstructure and uniformity for the superlattice structures and alloys were studied by TEM and STEM. Nanoindentation results suggested high hardness for these new structures and four point probe electrical resistivity measurements showed overall insulating behavior for both alloys and superlattices.

The eptaxial wurtzite AlN thin films were grown on (0001) &alpha-Al2O3 substrates by PLD. XRD and SAD in TEM revealed the epitaxial growth of AlN on (0001) α -Al2O3 substrate. These AlN films were post-deposition annealed at 1300°C for 30mins. Bright field and dark field TEM and transmittance spectra for the samples before and after annealing prove the annealing can effectively improve the quality of the film. Post-deposition annealing for AlN on α-Al2O3 substrates could be a very promising procedure for high quality optical device fabrications.

The eptaxial wurtzite AlN thin films were grown on (111) Si substrates by PLD and Laser-MBE. XRD and SAD in TEM revealed the epitaxial growth of AlN on Si(111) substrate. The interface structure and growth mechanism were studied by high-resolution TEM. Fourier filtered image of cross-sectional AlN/Si(111) samples from both Si (112) zone axes revealed the domain matching epitaxy of 4:5 ratio between a[subscript Si(110)] and a[subscript AlN(2110)].