Special Issue for KSTP30 Years Anniversary: Recent Steel Research in Mechanics of Plastic Deformation and Materials Processing

Special Issue for KSTP30 Years Anniversary: Recent Steel Research in Mechanics of Plastic Deformation and Materials Processin

Robust Heteroepitaxial Growth of GaN Formulated on Porous TiN Buffer Layers

Gallium nitride (GaN) heteroepitaxial growth is widely studied as a semiconductor material due to its various benefits. Especially, development of a buffer layer between GaN and the substrate verifies to be an effective strategy to reduce high threading dislocation density. However, the buffer layer often impedes strong adhesion between the epilayer and foreign substrate because thermally induced residual stress often causes delamination of the epilayer during fabrication. Here, we developed a robust GaN heteroepitaxy employing a porous buffer layer formulated by hydride vapor phase epitaxy. A sufficiently low but completely coated thin Ti layer was deposited on the sapphire substrate, which led to a rough and porous TiN layer after nitridation. This porous structure enables the penetration of the GaN source into the porous structure, allowing GaN epitaxy initiation throughout the TiN layer. As a result, GaN crystal growth can fill the porous area during the GaN heteroepitaxy. Integrated visualization demonstrated that the voids were successfully removed by GaN infiltration, enabling the heteroepitaxial structure to show little deformation, confirmed by multiple indentations. Last, the void-free GaN heteroepitaxy with the porous TiN buffer layer displayed robust adhesion after delamination tests

Robust Heteroepitaxial Growth of GaN Formulated on Porous TiN Buffer Layers

Gallium nitride (GaN) heteroepitaxial growth is widely studied as a semiconductor material due to its various benefits. Especially, development of a buffer layer between GaN and the substrate verifies to be an effective strategy to reduce high threading dislocation density. However, the buffer layer often impedes strong adhesion between the epilayer and foreign substrate because thermally induced residual stress often causes delamination of the epilayer during fabrication. Here, we developed a robust GaN heteroepitaxy employing a porous buffer layer formulated by hydride vapor phase epitaxy. A sufficiently low but completely coated thin Ti layer was deposited on the sapphire substrate, which led to a rough and porous TiN layer after nitridation. This porous structure enables the penetration of the GaN source into the porous structure, allowing GaN epitaxy initiation throughout the TiN layer. As a result, GaN crystal growth can fill the porous area during the GaN heteroepitaxy. Integrated visualization demonstrated that the voids were successfully removed by GaN infiltration, enabling the heteroepitaxial structure to show little deformation, confirmed by multiple indentations. Last, the void-free GaN heteroepitaxy with the porous TiN buffer layer displayed robust adhesion after delamination tests

Hydrogen Back-Pressure Effects on the Dehydrogenation Reactions of Ca(BH₄)₂

The dehydrogenation reactions of Ca­(BH₄)₂ are investigated under different isobaric conditions using in situ synchrotron radiation powder X-ray diffraction and nuclear magnetic resonance measurements. Ca­(BH₄)₂ dissociates in multiple steps, and several intermediate phases, such as an amorphous phase(s), CaB₂H_<i>x</i>, and CaB₁₂H₁₂, are observed during dehydrogenation. Among the intermediate phases, it is known that CaB₂H_<i>x</i> is fully reversible, while the more stable CaB₁₂H₁₂ with an icosahedral structure hinders reversible reactions. Here, we try to control the dehydrogenation reaction pathway of Ca­(BH₄)₂ by applying different hydrogen back-pressures. The decomposition reaction of Ca­(BH₄)₂ in the absence of a catalyst was found to be sensitive to the H₂ back-pressure. At <i>p</i>(H₂) = 1 bar, Ca­(BH₄)₂ decomposes via two competitive dehydrogenation reaction routes to form CaB₂H_<i>x</i> or CaB₁₂H₁₂. At <i>p</i>(H₂) = 10 bar, the overall dehydrogenation reaction remains unchanged. However, the formation of CaB₂H_<i>x</i> is reduced, and amorphous elemental boron is observed as a final dehydrogenation product. At <i>p</i>(H₂) = 20 bar, the elemental boron formation is significantly increased, and the formation of the CaB₂H_<i>x</i> phase is suppressed. Possible routes to form CaH₂ and elemental boron are discussed

Robust Heteroepitaxial Growth of GaN Formulated on Porous TiN Buffer Layers

Gallium nitride (GaN) heteroepitaxial growth is widely studied as a semiconductor material due to its various benefits. Especially, development of a buffer layer between GaN and the substrate verifies to be an effective strategy to reduce high threading dislocation density. However, the buffer layer often impedes strong adhesion between the epilayer and foreign substrate because thermally induced residual stress often causes delamination of the epilayer during fabrication. Here, we developed a robust GaN heteroepitaxy employing a porous buffer layer formulated by hydride vapor phase epitaxy. A sufficiently low but completely coated thin Ti layer was deposited on the sapphire substrate, which led to a rough and porous TiN layer after nitridation. This porous structure enables the penetration of the GaN source into the porous structure, allowing GaN epitaxy initiation throughout the TiN layer. As a result, GaN crystal growth can fill the porous area during the GaN heteroepitaxy. Integrated visualization demonstrated that the voids were successfully removed by GaN infiltration, enabling the heteroepitaxial structure to show little deformation, confirmed by multiple indentations. Last, the void-free GaN heteroepitaxy with the porous TiN buffer layer displayed robust adhesion after delamination tests

Robust Heteroepitaxial Growth of GaN Formulated on Porous TiN Buffer Layers

Gallium nitride (GaN) heteroepitaxial growth is widely studied as a semiconductor material due to its various benefits. Especially, development of a buffer layer between GaN and the substrate verifies to be an effective strategy to reduce high threading dislocation density. However, the buffer layer often impedes strong adhesion between the epilayer and foreign substrate because thermally induced residual stress often causes delamination of the epilayer during fabrication. Here, we developed a robust GaN heteroepitaxy employing a porous buffer layer formulated by hydride vapor phase epitaxy. A sufficiently low but completely coated thin Ti layer was deposited on the sapphire substrate, which led to a rough and porous TiN layer after nitridation. This porous structure enables the penetration of the GaN source into the porous structure, allowing GaN epitaxy initiation throughout the TiN layer. As a result, GaN crystal growth can fill the porous area during the GaN heteroepitaxy. Integrated visualization demonstrated that the voids were successfully removed by GaN infiltration, enabling the heteroepitaxial structure to show little deformation, confirmed by multiple indentations. Last, the void-free GaN heteroepitaxy with the porous TiN buffer layer displayed robust adhesion after delamination tests

Elastic modulus of steel at various temperatures [20].

<p>Elastic modulus of steel at various temperatures <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0035987#pone.0035987-Wray1" target="_blank">[20]</a>.</p

Densities of austenite and ferrite phase as a function of temperature and chemical composition [21].

<p>Densities of austenite and ferrite phase as a function of temperature and chemical composition <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0035987#pone.0035987-Miettinen1" target="_blank">[21]</a>.</p

Comparison between measured [<b>18</b>] and calculated transformation plastic strains according to externally applied stress.

<p>(Yield stress of ferrite: 160 MPa, Yield stress of austenite: 200 MPa).</p

Calculated distribution of von-Mises stress during austenite-to-ferrite transformation.

<p>Calculated distribution of von-Mises stress during austenite-to-ferrite transformation.</p