No abstract

In this report, chemical mechanical polishing (CMP) of gallium nitride (GaN) with colloidal silica was studied. It was confirmed that colloidal silica based slurry could be used for gallium face of GaN. Removal rate of GaN was 17 nm/h under typical polishing conditions. An atomically flat surface with Ra = 0.1 nm was achieved after CMP. Detailed observation of the scratch density was done during the CMP process. Cathode luminescence (CL) imaging was used to understand the sub-surface damage induced by mechanical polishing process and its removal by CMP with colloidal silica based slurry. Combining the optical microscope, atomic force microscope (AFM), CL imaging, and reflection high energy electron diffraction (RHEED) observations before, after and at intermediate stages of the CMP process, the schematic model of the removal of scratch and damage layer from Ga-faced GaN substrate by CMP process was proposed.

The possibility of chemical mechanical polishing (CMP) as an intermediate ex-situ surface planarization process for thin-film epitaxy devices has been investigated. The surface quality of the homoepitaxial GaN after a surface pretreatment of CMP on the GaN-on-sapphire template drastically improved as compared to that with the regular homoepitaxial process. In addition, it was found that CMP contributed to a reduction in the dislocation densities in the subsequent homoepitaxial GaN layers. The CMP-treated GaN surface exhibited pits, with an average diameter of 0.3 μm, due to the chemical etching effect of the CMP slurry. These pits are thought to enhance the epitaxial lateral over growth of the GaN thin films, leading to a reduction in the dislocation densities in the homoepitaxial GaN layers.

The direct integration of gallium nitride (GaN) and diamond holds much promise for high‐power devices. However, it is a big challenge to grow GaN on diamond due to the large lattice and thermal‐expansion coefficient mismatch between GaN and diamond. In this work, the fabrication of a GaN/diamond heterointerface is successfully achieved by a surface activated bonding (SAB) method at room temperature. A small compressive stress exists in the GaN/diamond heterointerface, which is significantly smaller than that of the GaN‐on‐diamond structure with a transition layer formed by crystal growth. A 5.3 nm‐thick intermediate layer composed of amorphous carbon and diamond is formed at the as‐bonded heterointerface. Ga and N atoms are distributed in the intermediate layer by diffusion during the bonding process. Both the thickness and the sp2‐bonded carbon ratio of the intermediate layer decrease as the annealing temperature increases, which indicates that the amorphous carbon is directly converted into diamond after annealing. The diamond of the intermediate layer acts as a seed crystal. After annealing at 1000 °C, the thickness of the intermediate layer is decreased to approximately 1.5 nm, where lattice fringes of the diamond (220) plane are observed.

The fabrication of a high-quality freestanding diamond substrate was successfully demonstrated via heteroepitaxy by introducing diamond micropatterns and microneedles in the early stage of growth. Micropatterns contributed to a marked reduction in the number of dislocations induced by epitaxial lateral overgrowth, and microneedles relaxed heteroepitaxial strain. Raman spectroscopy indicated the absence of nondiamond carbon inclusions in the obtained freestanding substrate. The full width at half maximum of the X-ray rocking curve for diamond (004) reflections was 0.07°, the lowest value for heteroepitaxial diamond that has been reported so far. The results provide novel insights toward realizing large-diameter single-crystalline diamond substrates.

One-inch free-standing (001) diamond layers on a (112¯0) (a-plane) sapphire substrate with an Ir buffer layer (Kenzan Diamond®) were grown. The full-width at half maximum values of (004) and (311) x-ray rocking curves were 113.4 and 234.0 arc sec, respectively. The dislocation density of the substrates was 1.4 × 107 cm−2, determined by plan-view transmission electron microscopy observation. These values are much lower than the reported values among heteroepitaxial diamonds. Furthermore, x-ray pole figure measurements showed four symmetry of the crystal, showing single crystallinity without any twinning. The curvature radius of diamond was measured to be 90.6 cm, which is much larger than previous values, ca. 20 cm. Surprisingly, a cubic-lattice (001) diamond crystal was epitaxially grown on a trigonal-lattice (112¯0) sapphire substrate. However, we found that the epitaxial relation is diamond (001) [110]//Ir (001) [110]//sapphire (112¯0) [0001]. Now, high-quality one-inch diamond wafers will be available as a substrate used for diamond electronic devices.

Thin-film epitaxy is critical for investigating the original properties of materials. To obtain epitaxial films, careful consideration of the external conditions, i.e. single-crystal substrate, temperature, deposition pressure and fabrication method, is significantly important. In particular, selection of the single-crystal substrate is the first step towards fabrication of a high-quality film. Sapphire (single-crystalline α-Al2O3) is commonly used in industry as a thin-film crystal-growth substrate, and functional thin-film materials deposited on sapphire substrates have found industrial applications. However, while sapphire is a single crystal, two types of atomic planes exist in accordance with step height. Here we discuss the need to consider the lattice mismatch for each of the sapphire atomic layers. Furthermore, through cross-sectional transmission electron microscopy analysis, we demonstrate the uniepitaxial growth of cubic crystalline thin films on bistepped sapphire (0001) substrates.

We performed atomic-scale surface patterning with a vertical resolution of approximately 0.3 nm on a poly(methyl methacrylate) (PMMA) polymer sheet (10 ' 10 mm 2 ) by thermal nanoimprinting using an atomically stepped sapphire template (α-Al 2 O 3 single crystal). The sapphire mold with (10 12) r-plane exhibited regularly arranged straight steps with a uniform height of approximately 0.31 nm. The template nanopattern could be transferred onto the surface of the PMMA sheet under the imprinting conditions of 0.2 MPa load for 300 s at 140°C. Atomic stairs with approximately 0.26-nm-high straight steps and approximately 600-nm-wide terraces were formed on the PMMA surface.