Sputter Epitaxy of ZnO Films on Sapphire Substrates Using 3D MgO Buffer Layers

Area of Science:

• Materials Science
• Thin Film Epitaxy
• Semiconductor Physics

Background:

• Epitaxial growth of zinc oxide (ZnO) on sapphire is challenging due to significant lattice mismatch.
• Achieving atomically flat and high-quality ZnO films requires effective strain management and substrate templating.

Purpose of the Study:

• To investigate the use of ultrathin magnesium oxide (MgO) buffers composed of three-dimensional (3D) islands for epitaxy of ZnO on sapphire.
• To analyze the impact of MgO buffer structure (3D islands vs. continuous film) on ZnO crystalline quality and properties.

Main Methods:

• Sputter epitaxy of ultrathin MgO buffer layers on sapphire substrates.
• Growth of 500 nm-thick ZnO films on the prepared MgO buffers.
• Characterization using X-ray rocking curve (XRC) for crystalline quality and atomic force microscopy (AFM) for surface morphology.
• Low-temperature cathodoluminescence (CL) spectroscopy for optical properties.

Main Results:

• Ultrathin 3D MgO island buffers enabled epitaxy of atomically flat ZnO films with an exceptionally narrow XRC-fwhm of 21 arcsec and RMS roughness of 0.11 nm.
• The 3D MgO islands, in their metastable wurtzite phase, relaxed strain and provided a template with a 1-2% mismatch for ZnO.
• ZnO films grown on thermodynamically stable rocksalt MgO buffers showed drastically degraded crystalline quality (XRC-fwhm > 1000 arcsec) due to a 7% mismatch.

Conclusions:

• Three-dimensional wurtzite MgO buffers are an effective strategy for high-quality ZnO epitaxy on lattice-mismatched substrates like sapphire.
• The findings demonstrate the critical role of buffer layer nanostructure and phase in managing strain and achieving superior film quality.
• This approach is potentially extendable to the epitaxy of other wurtzite materials on mismatched substrates.