Oxide dielectrics are attractive for a variety of microelectronic device architectures, but their integration beyond silicon-based systems is relatively limited.  Interfacial electronic structure and chemical stability are often limiting factors.  Wide bandgap semiconductors like GaN and SiC offer opportunities for operation at high power, high temperatures, and high speeds.  Integrating a dielectric layer for passivation or as a FET gate could dramatically improve device performance.    However, the hexagonal crystal structures of GaN and SiC present unique challenges to heteroepitaxial growth of desired cubic oxides.  For example, cubic rocksalt oxides having large bandgaps (6 - 8 eV) are of interest for blocking electron injection at wide-gap semiconductor interfaces, and cubic ferroelectric perovskites present unique opportunities for dipole interactions with fixed polarity GaN or SiC substrates.  The critical challenges to heteroepitaxial growth of cubic oxides are symmetry and surface energies leading to twinning (Fig. 1) and faceting (Fig. 2) of the oxide epilayer.  Cubic oxides like MgO epitaxially grow with (111) orientation on (0001) hexagonal GaN surfaces.  Simply considering dangling bond density, the (111) rocksalt oxide surface has a high surface energy.  This high surface energy leads to {100} faceting and promotes twinning in the epilayer.  However, observations of differences in twin boundary density for MgO and CaO epilayers (Fig 3a) led to my realization that surface hydroxyls can lower the (111) surface energy.  This critical realization led to the concept of using a water flux during molecular beam epitaxy (MBE) growth of cubic oxides to act as a surfactant stabilizing the (111) surface.  This work has been continued by Beth Paisley, a graduate student whom I trained prior to finishing my Ph.D. thesis.  Water surfactant enabled MBE growth has subsequently been demonstrated to lower DC leakage currents for these Oxide||GaN heterostructures by several orders of magnitude (Fig. 3 and Ref. 2).


Fig. 1: Twinning of (111) cubic oxide epilayers on (0001) hexagonal GaN.  (a) Near site coincident lattices showing the two possible in-plane twin orientations for (111) cubic MgO epitaxial crystals on (0001) hexagonal GaN.  (b) Off-axis x-ray diffraction scans showing the 6-fold symmetry of twinned MgO and its alignment along the m-plane of hexagonal GaN.  RHEED patterns collected during MBE growth also confirm alignment: (c) bare GaN substrate, (d) after ~1 monolayer of MgO growth.

Interfacing Epitaxial Oxides to Gallium Nitride

Related Publications


1. E. A. Paisley, H. S. Craft, M. D. Losego, H. Lu, A. Gruverman, R. Collazo, Z. Sitar, and J. P. Maria, “Epitaxial PbxZr1-xTiO3 on GaN” J. Appl. Phys. 113 074107 (2013). DOI 1. E. A. Paisley, M. D. Losego, B. E. Gaddy, A. L. Rice, R. Collazo, Z. Sitar, D. L. Irving, and J-P. Maria, “Surfactant-enabled epitaxy through control of growth mode with chemical boundary conditions” Nat. Comm. 2 461 (2011). DOI


2. E. A. Paisley, M. D. Losego, B. E. Gaddy, A. L. Rice, R. Collazo, Z. Sitar, D. L. Irving, and J-P. Maria, “Surfactant-enabled epitaxy through control of growth mode with chemical boundary conditions” Nat. Comm. 2 461 (2011). DOI


3. M. D. Losego, H. S. Craft, E. A. Paisley, S. Mita, R. Collazo, Z. Sitar, J-P. Maria, “Critical examination of growth rate for magnesium oxide (MgO) thin films deposited by molecular beam epitaxy with a molecular oxygen flux.” J. Mater. Res. 25 670 (2010). DOI


5. M. D. Losego, L. F. Kourkoutis, S. Mita, H. S. Craft, D. Muller, R. Collazo, Z. Sitar, J-P. Maria, “Epitaxial Ba0.5Sr0.5TiO3-GaN heterostructures with abrupt interfaces.” J. Cryst. Gr. 311 1106 (2009). DOI


6. H. S. Craft, R. Collazo, M. D. Losego, Z. Sitar, J-P. Maria, “Surface water reactivity of polycrystalline MgO and CaO films investigated using x-ray photoelectron spectroscopy.” J Vac. Sci. Technol. A 26 1507 (2008). DOI

7. T. Goodrich, Z. Cai, M. Losego, J-P. Maria, L. Kourkoutis, D. Muller, K. Ziemer, “Improved epitaxy of barium titanate by molecular beam epitaxy through a single crystalline magnesium oxide template for integration on hexagonal silicon carbide.” J. Vac. Sci. Technol. B 26 1110 (2008). DOI


  1. 8. H. S. Craft, R. Collazo, M. Losego, S. Mita, Z. Sitar, J-P. Maria, “Spectroscopic analysis of the epitaxial CaO (111)-GaN (0002) interface.” Appl. Phys. Lett. 92 082907 (2008). DOI


9. M. Losego, S. Mita, R. Collazo, Z. Sitar, J-P. Maria, “Epitaxial growth of the metastable phase ytterbium monoxide on gallium nitride surfaces.” J. Cryst. Growth 310 51 (2008). DOI


10. H. Craft, R. Collazo, M. Losego, S. Mita, Z. Sitar, J-P. Maria, “Band offsets and growth mode of molecular beam epitaxy grown MgO (111) on GaN (0002) by x-ray photoelectron spectroscopy.” J. Appl. Phys. 102 (2007). DOI


11. M. Losego, S. Mita, R. Collazo, Z. Sitar, J-P. Maria, “Epitaxial calcium oxide films deposited on gallium nitride surfaces.” J. Vac. Sci. Technol. B. 25 (2007). DOI


12. T. Goodrich, Z. Cai, M. Losego, J-P. Maria, K. Ziemer, “Thin crystalline MgO on hexagonal 6H-SiC(0001) by molecular beam epitaxy for functional oxide integration.” J. Vac. Sci. Technol. B 25 1033 (2007). DOI


13. H. Craft, J. Ihlefeld, M. Losego, R. Collazo, Z. Sitar, J-P. Maria, “MgO epitaxy on GaN (0002) surfaces by molecular beam epitaxy.” Appl. Phys. Lett. 88 212906 (2006). DOI

Fig. 2: Faceting of (111) cubic oxide epilayers on (0001) hexagonal GaN.  (a) 3D image of twinned MgO nuclei that choose to facet along their {100} surfaces when grown epitaxially on hexagonal (0001) GaN.      (b) SEM images showing this faceting with clearly delineated twin boundaries between the two orientations. 

Fig. 3: Water surfactant growth of epitaxial oxides by MBE.  (a) SEM showing more twinning in MgO than CaO.  Realizing that the reduction in twinning resulted from CaO’s higher reactivity with water present in the growth chamber led to the idea of using H2O as a surfactant during MBE growth of oxides.  (b) In-situ RHEED monitoring of crystal diffraction during MBE growth.  Oscillations indicative of smooth growth are observed when an H2O flux is used during growth.  (c) AFM images showing the smoother morphology possible when a water surfactant is introduced. (d) I-V measurements for ~5 nm CaO epilayers on n-type GaN showing three orders of magnitude reduction in DC leakage for epilayers grown with an H2O surfactant.  

School of Materials Science & Engineering

Last Updated: June 20, 2014

© 2012 M. D. Losego