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We succeeded in fabricating 10-period coherent γ-Al2O3/Ga2O3 superlattices (SLs) on MgAl2O4 substrates by molecular beam epitaxy. By varying each layer thickness, we tuned the average Al composition (x ave) of the coherent SLs from 0.20 to 0.85, and obtained nearly-lattice-matched SLs to the substrate at x ave ~ 0.5. The lattice-matched SLs maintained coherent interfaces up to a period length of 7.2 nm in spite of a large lattice mismatch between the end members. Our results suggest lots of flexibility in designing γ-(Al x Ga1−x )2O3-based heterostructures for future functional heterojunction devices.

Ten-period binary α-Al2O3/Ga2O3 superlattices were fabricated on r-plane sapphire substrates by molecular beam epitaxy. By systematic variation of α-Ga2O3 thickness and evaluation through X-ray reflectivity and diffraction measurements and scanning transmission electron microscopy, we verified that the superlattice with α-Ga2O3 thickness up to ~1 nm had coherent interfaces without misfit dislocation in spite of the large lattice mismatches. This successful fabrication of coherent α-Al2O3/Ga2O3 superlattices will encourage further development of α-(Al x Ga1− x )2O3-based heterostructures including superlattices.

We have fabricated sputter-induced graphite nanoneedle field emitters by the RF magnetron sputtering of a carbon disk with H2 gas and investigated their detailed structure with a high-resolution transmission electron microscopy (HRTEM) system. The nanoneedle emitters show good emission characteristics with an emission current of 1.1 mA (8.8 mA/cm2) at an average field of 12 V/µm and a stability of 2.9% at an average field of 10 V/µm. HRTEM observation of the nanoneedles revealed that the graphite nanoneedles are composed of graphite fibers and graphite nanocapsules enclosing an Fe or Ni particle. A plausible formation mechanism of the graphite nanoneedles is also discussed.

The spontaneous formation of a two-layered structure in p-InGaN is observed with increasing Mg doping level. X-ray reciprocal space mappings reveal that the two layers are resulted from phase separation with different In contents and strain states. The bottom p-InGaN layer near the GaN template is totally strained with low-density dislocations, while the top layer is relaxed with high-density dislocations. The average In content in the relaxed layer is much higher than that in the totally strained one. The layered structure caused by phase separation is interpreted in terms of the compressive strain increase induced by Mg doping and the strain relaxation by dislocations.

Ion beams are widely used for micro/nanostructure fabrication with unique features of the energetic processes. In this work, we present a self-assembled nanostructure fabrication by ion irradiation with a patterned strain/stress field. The patterned strain/stress field is introduced by micro-Vickers indentation to amorphous SiO(2) (load = 100 mN). After irradiation of 60 keV Cu(-) ions, nanogrooves (width from 150 to 250 nm) were created at the indentation edge. Under the ion irradiation of a constant ion flux, the nanogroove size linearly increased with ion fluence from 2 × 10(16) to 1.2 × 10(17) ions cm(-2). Comparison of indented samples after irradiation and thermal annealing suggests that the nanogrooving originates from ion beam interactions with residual stress/strain by indentation and that a non-thermal effect results from the ion irradiation processes. Cross-sectional TEM analysis showed that the grooves extend over 200 nm below the surface, implying that the groove growth is not caused by surface diffusion. It is concluded from the results that the grooving is due to irradiation-induced mass transport accompanied by vacancy aggregation at the interface between the indented and non-indented areas.