Synthesis of Honeycomb-Structured Beryllium Oxide via Graphene Liquid Cells.

Using high-resolution transmission electron microscopy and electron energy-loss spectroscopy, we show that beryllium oxide crystallizes in the planar hexagonal structure in a graphene liquid cell by a wet-chemistry approach. These liquid cells can feature van-der-Waals pressures up to 1 GPa, producing a miniaturized high-pressure container for the crystallization in solution. The thickness of as-received crystals is beyond the thermodynamic ultra-thin limit above which the wurtzite phase is energetically more favorable according to the theoretical prediction. The crystallization of the planar phase is ascribed to the near-free-standing condition afforded by the graphene surface. Our calculations show that the energy barrier of the phase transition is responsible for the observed thickness beyond the previously predicted limit. These findings open a new door for exploring aqueous-solution approaches of more metal-oxide semiconductors with exotic phase structures and properties in graphene-encapsulated confined cells.

Chemical Imprinting of Crystalline Silicon with Catalytic Metal Stamp in Etch Bath.

Conventional lithography using photons and electrons continues to evolve to scale down three-dimensional nanoscale patterns, but the complexity of technology and equipment is increasing due to diffraction and scattering problems. Physical contact lithography methods, such as nanoimprint and soft lithography, have been developed as an alternative technique. These techniques imprint predefined structures on a stamp to the polymer resist and use the polymer resist as a mask to dry etch the nanostructure on the substrate. In this study, we introduce a method of chemically imprinting crystalline silicon (Si) with a catalytic stamp to enable the direct etching of the Si without using a polymer mask. A metal catalyst is deposited on the predefined structure of the stamp. The stamp physically contacts the Si in the etching bath, and metal-assisted chemical etching occurs on the semiconductor surface. Since the metal catalyst is mounted on a stamp, it can be used repeatedly. This is a technology that combines conventional lithography and etching without using a polymer resist. This technology not only produced nano/microscale arrays of circular and square holes and trench structures but also successfully produced complex eagle-shaped structures that contained such structures.

Atomic-Layer Deposition of Single-Crystalline BeO Epitaxially Grown on GaN Substrates.

We have grown a single-crystal beryllium oxide (BeO) thin film on a gallium nitride (GaN) substrate by atomic-layer deposition (ALD) for the first time. BeO has a higher thermal conductivity, bandgap energy, and dielectric constant than SiO2. As an electrical insulator, diamond is the only material on earth whose thermal conductivity exceeds that of BeO. Despite these advantages, there is no chemical-vapor-deposition technique for BeO-thin-film deposition, and thus, it is not used in nanoscale-semiconductor-device processing. In this study, the BeO thin films grown on a GaN substrate with a single crystal showed excellent interface and thermal stability. Transmission electron microscopy showed clear diffraction patterns, and the Raman shifts associated with soft phonon modes verified the high thermal conductivity. The X-ray scan confirmed the out-of-plane single-crystal growth direction and the in-plane, 6-fold, symmetrical wurtzite structure. Single-crystalline BeO was grown on GaN despite the large lattice mismatch, which suggested a model that accommodated the strain of hexagonal-on-hexagonal epitaxy with 5/6 and 6/7 domain matching. BeO has a good dielectric constant and good thermal conductivity, bandgap energy, and single-crystal characteristics, so it is suitable for the gate dielectric of power semiconductor devices. The capacitance-voltage (C-V) results of BeO on a GaN-metal-oxide semiconductor exhibited low frequency dispersion, hysteresis, and interface-defect density.