Pressure profoundly alters all states of matter. The symbiotic development of ultrahigh-pressure diamond anvil cells, to compress samples to sustainable multi-megabar pressures; and synchrotron x-ray techniques, to probe materials' properties in situ, has enabled the exploration of rich high-pressure (HP) science. In this article, we first introduce the essential concept of diamond anvil cell technology, together with recent developments and its integration with other extreme environments. We then provide an overview of the latest developments in HP synchrotron techniques, their applications, and current problems, followed by a discussion of HP scientific studies using x-rays in the key multidisciplinary fields. These HP studies include: HP x-ray emission spectroscopy, which provides information on the filled electronic states of HP samples; HP x-ray Raman spectroscopy, which probes the HP chemical bonding changes of light elements; HP electronic inelastic x-ray scattering spectroscopy, which accesses high energy electronic phenomena, including electronic band structure, Fermi surface, excitons, plasmons, and their dispersions; HP resonant inelastic x-ray scattering spectroscopy, which probes shallow core excitations, multiplet structures, and spin-resolved electronic structure; HP nuclear resonant x-ray spectroscopy, which provides phonon densities of state and time-resolved Mossbauer information; HP x-ray imaging, which provides information on hierarchical structures, dynamic processes, and internal strains; HP x-ray diffraction, which determines the fundamental structures and densities of single-crystal, polycrystalline, nanocrystalline, and non-crystalline materials; and HP radial x-ray diffraction, which yields deviatoric, elastic and rheological information. Integrating these tools with hydrostatic or uniaxial pressure media, laser and resistive heating, and cryogenic cooling, has enabled investigations of the structural, vibrational, electronic, and magnetic properties of materials over a wide range of pressure-temperature conditions.

High-pressure studies with x-rays using diamond anvil cells

Prospects for the ammonothermal growth of large GaN crystal

Homoepitaxial growth of high structural quality and high-purity thick gallium nitride layers by crystallization from vapor phase (hydride vapor phase epitaxy (HVPE)) on 1, 1.5, and 2 inch substrates obtained by a solution (ammonothermal) growth method is presented. Advantages and disadvantages of both growth technologies are described in detail. Structural, optical, electrical, and thermal properties of gallium nitride grown from the vapor phase are demonstrated and compared to properties of ammonothermally grown material. It is shown that a synergy of these two methods can create new opportunities for an efficient production of bulk gallium nitride crystals and then substrates. It is also shown that free-standing (products of slicing procedures) gallium nitride crystals obtained from growth by vapor phase on ammonothermal substrates can be successfully used as seeds for the next growth process by both discussed methods. Factors limiting HVPE and making it a 'wafer to wafer' technology are presented, clarified, and analyzed. Intentional introduction of silicon to growth of gallium nitride by HVPE and crystals with a high free carrier concentration and high structural quality are demonstrated. First electronic and optoelectronic devices fabricated on the free-standing gallium nitride substrates are shown.

Challenges and future perspectives in HVPE-GaN growth on ammonothermal GaN seeds

Optoelectrical and microelectronic devices involving gallium nitride have become a challenge but their development is limited because of a lack of suitable substrates. This paper deals with the crystal growth of gallium nitride, the processes leading to GaN bulk crystals being significantly expanded during the last decade (the ones involving GaN thin films or nanocrystallites are not studied in this review). The main reviewed routes are: (i) the high pressure nitrogen solution growth (H.P.N.S.G.), (ii) the Na flux method, and (iii) the ammonothermal crystal growth.

Gallium nitride bulk crystal growth processes : a review

We present combined in situ X-ray diffraction and high-speed imaging to monitor the phase evolution upon cyclic rapid laser heating and cooling mimicking the direct energy deposition of Ti-6Al-4V in real time. Additive manufacturing of the industrially relevant alloy Ti-6Al-4V is known to create a multitude of phases and microstructures depending on processing technology and parameters. Current setups are limited by an averaged measurement through the solid and liquid parts. In this work the combination of a micro-focused intense X-ray beam, a fast detector and unidirectional cooling provide the spatial and temporal resolution to separate contributions from solid and liquid phases in limited volumes. Upon rapid heating and cooling, the β ↔ α' phase transformation is observed repeatedly. At room temperature, single phase α' is observed. Secondary β-formation upon formation of α' is attributed to V partitioning to the β-phase leading to temporary stabilization. Lattice strains in the α'-phase are found to be sensitive to the α' → β phase transformation. Based on lattice strain of the β-phase, the martensite start temperature is estimated at 923 K in these experiments. Off-axis high speed imaging confirms a technically relevant solidification front velocity and cooling rate of 10.3 mm/s and 4500 K/s, respectively.

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In situ investigation of phase transformations in Ti-6Al-4V under additive manufacturing conditions combining laser melting and high-speed micro-X-ray diffraction

The synthesis of large single crystals of GaN (gallium nitride) is a matter of great importance in optoelectronic devices for blue-light-emitting diodes and lasers. Although high-quality bulk single crystals of GaN suitable for substrates are desired, the standard method of cooling its stoichiometric melt has been unsuccessful for GaN because it decomposes into Ga and N2 at high temperatures before its melting point. Here we report that applying high pressure completely prevents the decomposition and allows the stoichiometric melting of GaN. At pressures above 6.0 GPa, congruent melting of GaN occurred at about 2,220 °C, and decreasing the temperature allowed the GaN melt to crystallize to the original structure, which was confirmed by in situ X-ray diffraction. Single crystals of GaN were formed by cooling the melt slowly under high pressures and were recovered at ambient conditions.

Congruent melting of gallium nitride at 6 GPa and its application to single-crystal growth.

To verify the high-pressure formation of the bulk metallic glass in elemental Zr and Ti, which Zhang and Zhao [Nature (London) 430, 332 (2004)] and Y. Wang et al. [Phys. Rev. Lett. 95, 155501 (2005)] recently reported, the high-pressure states were investigated by our newly developed in situ angle-dispersive x-ray diffraction using a two-dimensional detector and x-ray transparent anvils. Despite the disappearance of all the Bragg peaks in the one-dimensional energy-dispersive data, two-dimensional angle-dispersive data showed several intense Bragg spots even at the conditions where the amorphization was reported. This finding suggests that Zr and Ti do not transform into an amorphous state, but that their grain size becomes large, which causes the missing Bragg peaks in energy-dispersive data.

Does bulk metallic glass of elemental Zr and Ti exist

Decomposition of InN at high pressures and temperatures and its thermal instability at ambient conditions

GaN (Gallium Nitride) is an important material in optoelectronic devices for blue light-emitting diodes and lasers. Large-size single crystals of GaN are strongly desired, which can be used as substrates for epitaxial growth. We made in situ X-ray diffraction experiments using a multi-anvil high-pressure apparatus at the SPring-8 to confirm its decomposition and melting behavior under high pressures and temperatures. Congruent melting of GaN occurred around 2220 °C at pressures above 6.0 GPa, and decreasing the temperature allowed the GaN melt to crystallize to the original structure. This result leads to a new synthesis method of high quality single crystals of GaN by means of slow cooling of its stoichiometric melt under high pressure. Single crystal of GaN with a diameter of 100 μm has been obtained successfully that shows a sharp X-ray rocking curve smaller than 30 seconds.

Congruent Melting of Gallium Nitride under High Pressure and Its Application to Single Crystal Growth

A process for producing single-crystal gallium nitride comprising the steps of performing congruent melting of gallium nitride at a high pressure between 6×104 atm. and 10×104 atm. and at a high temperature between 2,200° C. and 2,500° C. and then slowly cooling the obtained gallium nitride melt at the stated high pressure.

Katsutoshi Aoki

Papers

Congruent melting of gallium nitride at 6 GPa and its application to single-crystal growth.

Does bulk metallic glass of elemental Zr and Ti exist

Decomposition of InN at high pressures and temperatures and its thermal instability at ambient conditions

Congruent Melting of Gallium Nitride under High Pressure and Its Application to Single Crystal Growth

Process for producing single-crystal gallium nitride