Lattice-Matched Buffer Layers for InGaN Growth

Transitional nitrides, ZrTiN alloys, are strong candidate buffer layers for AlxGa1-xN (0≤x≤1) and InxGa1-xN (0≤x≤0.14) on sapphire substrates. ZrTiN thin films are metallic, highly reflective in the important UV-NIR range, have high melting points, and are chemically resistant materials. They form under the cubic crystal structure with an effective lattice constant in the (111) plane matching the c-plane oriented AlGaN and InxGa1-xN within the above mentioned compositional range.

In this research, we deposited (111)-oriented ZrTiN thin films across the whole compositional range by DC magnetron sputtering. The effect of growth parameters such as target power, chamber pressure, growth temperature, and N2 flow rate on thin films orientation and crystal quality were investigated. The resulting ZrTiN thin films are ofhigh crystallinity, as indicated by low rocking curve X-ray diffraction (111) peaks with FWHM values of 14~22 arcsec. ZrTiN thin films have 0.3~ 3 nm roughness with wellordered surfaces, indicated by streaky or spotty reflection high-energy electron diffraction patterns.

As an attractive lower-cost substitute plasmonic material for Au and Si, the optical properties of ZrxTi1-xN thin films were characterized using multiple wavelength spectroscopic ellipsometry at various incidence angles. A Drude model plus Lorentz oscillators are employed to extract the optical constants for our thin films. ZrxTi1-xN optical properties are found to be dependent on film thickness, growth temperature, and film composition. Our ZrxTi1-xN thin films are highly reflective with 90% reflection in the visible to near IR region and very conductive with a near bulk low resistivity of ~ 14 μcm.

GaN thin films with a thickness of ~ 0.5 micron are grown on developed ZrN, TiN, and lattice matched ZrxTi1-xN (x=0.8) buffer layers by plasma-assisted molecular beam epitaxy. Smooth GaN layers with 2D RHEED patterns are achieved. We show that In flux during GaN deposition acts as a strong surfactant resulting in films with improved crystallinity quality with XRD (0002) and (1ത01) peaks having RC FWHM of 572 and 1915 arcsec respectively. PL measurements from GaN less than 300 nm thick have a FWHM of 106 meV, matching that from 5~7 μm HVPE grown GaN templates.